The History 

The XV-ResearcFrom Conce

The History of the XV-15 Tilt Rotor Research Aircraft: From Concept to Flig

* cover (Final) 2/18/04 2:27 PM Page 1

NASA SP-2000-4517

The History of the 

XV-15 Tilt Rotor Research Aircraft:

From Concept to Flight

Martin D. Maisel

Demo J. Giulianetti

and

Daniel C. Dugan

Monographs in Aerospace History #17

The NASA History Series

National Aeronautics and Space Administration

Office of Policy and Plans

NASA History Division

Washington, D.C.

2000

i

Prologue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii

Dedication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . vii

Forewords . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ix

List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xiii

List of Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xix

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Early Efforts. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

XV-3 Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

NASA-Army Cooperation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Building the Technology Base. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Tilt Rotor Research Aircraft Project Office. . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Aircraft Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

Navy Participation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

Flight Envelope Expansion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Flight Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

Incidents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

Paris Air Show . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

Evaluations and Demonstrations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

Crash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

The End of an Era . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

XV-15 Project Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

Epilogue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105

Appendix A Aircraft Descriptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

Appendix B Key Personnel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

Appendix C Chronology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140

Appendix D Awards and Records . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154

Appendix E Photo Gallery. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157

Appendix F Bibliography of Tilt Rotor Related Publications. . . . . . . . . . . . . 164

About the Authors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187

Contents

This monograph is a testament to the efforts of many people overcoming multiple technical challenges encountered while developing the XV-15 tilt rotor

research aircraft.

The Ames involvement with the tilt rotor aircraft began in 1957 with investigations of the performance and dynamic behavior of the Bell XV-3 tilt rotor aircraft. At that time, Ames Research Center was known as the Ames Aeronautical

Laboratory of the National Advisory Committee for Aeronautics (NACA).

As we approach the new millennium, and after more than 40 years of effort and

the successful completion of our initial goals, it is appropriate to reflect on the

technical accomplishments and consider the future applications of this unique

aircraft class, the tilt rotor.

The talented engineers, technicians, managers, and leaders at Ames have worked

hard with their counterparts in the U.S. rotorcraft industry to overcome technology barriers and to make the military and civil tilt rotor aircraft safer, environmentally acceptable, and more efficient. 

The tilt rotor aircraft combines the advantages of vertical takeoff and landing

capabilities, inherent to the helicopter, with the forward speed and range of a

fixed wing turboprop airplane. Our studies have shown that this new vehicle type

can provide the aviation transportation industry with the flexibility for highspeed, long-range flight, coupled with runway-independent operations, thus having a significant potential to relieve airport congestion. We see the tilt rotor aircraft as an element of the solution to this growing air transport problem.

I am proud of our past accomplishments and ongoing efforts in the development

of tilt rotor technology. Much remains to be done to continue to develop and further enable quiet, ultra-safe, cost-efficient flight for this class of vehicles. I am

convinced that Ames Research Center will continue to provide the leadership

necessary to be in the forefront of new developments leading to the introduction

of tilt rotor aircraft into the aviation transportation system of the 21st century.

Enjoy the aviation historical journey that unfolds on the following pages.

Harry McDonald

Director,

NASA Ames Research Center

Prologue

iii

The story of the successful development of the tilt rotor aircraft is not just about

technology, but also about the efforts of many capable people who dedicated

themselves to what they believed would be an important advancement in aviation. The tasks proved to be technically challenging and involved both high

financial and safety risks. This history, therefore, is dedicated to all of the people

who held on to the dream and made it possible, and especially to those who,

unfortunately, have passed on and are not able to witness the remarkable product

of their work. A partial list of these people includes: Bob Lichten and Lovette

Coulter of Bell Helicopter Textron Inc.; Pip Gilmore of the Boeing Helicopter

Company; Laurel (Shorty) Schroers and Gary Churchill of the U.S. Army; and

Jim Weiberg, Dr. Leonard Roberts, and Jerry Bree of the NASA Ames Research

Center. Their efforts have advanced aeronautical technology significantly and

made their mark on aviation history.

Dedication

v

The authors wish to acknowledge Frank J. Aguilera, manager of the Ames

Advanced Tilt Rotor Technology Office, who requested the writing of this history and who sponsored the work. This account of the history of the development

of tilt rotor technology is the product of valuable contributions from many people, each of whom played a key role in the long chain of events that led to the

successful accomplishment of the tilt rotor research aircraft program goals.

Firsthand recollections and comments were provided by Bell Helicopter 

personnel, both active and retired, including Dick Stansbury, Jerry Pickard,

Jose Caero, Claude Liebensberger, Jack DeTore, John Williams, Sam Ferguson,

and Hank Smyth. We are especially indebted for assistance provided by Ron

Reber of Bell Helicopter Textron Inc., who provided the link to other Bell personnel and who generously made available Bell’s large resource of XV-3 and

XV-15 still photographs. Former and current Government employees who provided important information included Paul Yaggy and Dave Sharpe of the Army

laboratory at Ames; Woody Cook, Dave Few, Mike Bondi, Don Reynolds, and

John Wilson of NASA Ames; John Ward of NASA Headquarters; and Hal

Andrews of NAVAIR. The authors also thank John Schneider (retired), Ken

Bartie and Hal Rosenstein of the Boeing Helicopter Company for photographs

and information, and Jay Hendrickson for background about the Platt-LePage

Aircraft Company. In addition, the authors wish to thank tilt rotor pioneer Mario

Guerrieri for his input regarding the Transcendental Aircraft Corporation. Finally,

the authors wish to acknowledge Roger W. Ashbaugh of the Documentation and

Technology Branch and Lynn L. Albaugh, Capitol Resource Management, both

at Ames, for their assistance in preparing this publication. 

The authors have made every effort to ensure that the historical accounts in this

document are reported accurately and that the people associated with these 

events are correctly identified. However, due to limitations of the available documentation, the names of some individuals may have been inadvertently omitted.

Also, the reader should be aware that some information is not presented chronologically because events that are important in telling this story often occurred

separately but during the same time period. 

There are also a number of people at NASA Headquarters who helped in various

ways. In the NASA History Office, M. Louise Alstork edited, proofread, and prepared the index while Stephen J. Garber and Nadine J. Andreassen also assisted

with editing and production. Roger D. Launius, the NASA Senior Historian, provided much useful overall guidance. The Printing and Design Office developed

the layout and handled the printing. Geoff Hartman and Joel Vendette handled

the editing and design, respectively, while Jeffery Thompson and Stanley Artis

saw this work through the publication process. Thanks are due them all. 

Acknowledgments

vii

The development of tilt rotor aircraft technology involved some of the same factors that led to other important aeronautical accomplishments of this century. The

vision of a few individuals in search of a practical and efficient new aircraft

design, commitment to their goals, and their willingness to continue to pursue

their objective while encountering major technical problems and programmatic

challenges were critical ingredients in this tale. However, the unique aspect of

the tilt rotor story was the combined Government and industry focused effort that

was sustained for over four decades to explore, comprehend, develop, and refine

this technology. The remarkable product of the investment of public and private

funds, and the efforts of the people dedicated to the concept, is an aircraft type

that will have an impact on civil and military aviation that will rival the introduction of the practical helicopter more than 60 years ago. 

As this book is being written, the first production V-22 Osprey tilt rotor aircraft

is being prepared for delivery to the U.S. Marine Corps and the Bell-Augusta

BA609 six- to nine-passenger civil tilt rotor aircraft is well into the development

phase. When these new vehicles enter service, I am confident that other visionaries will find new uses for this capability, both in the civil transport and military

arenas. The tilt rotor aircraft has come of age. 

I have had the good fortune to have been closely associated with a significant

element of this activity, the XV-15 tilt rotor research aircraft project, for several

decades. It is fitting that this adventure in aeronautical technology development

be recorded. I know firsthand that the success of the tilt rotor can be credited to

the capable industry and Government individuals whose story is told in the following pages. 

Hans Mark

Director of Defense Research and Engineering

Office of the Secretary of Defense

Director, NASA Ames Research Center, 1969-1977

Foreword

ix

The XV-15 tilt rotor research aircraft program resulted in part from earlier investigations by the U.S. military seeking new and more efficient concepts for air

support of field operations. The XV-3 tilt rotor emerged from the Army/Air

Force convertiplane program of the 50s as a strong contender. However, it faced

significant stability problems that discouraged many supporters and threatened to

swamp the program. The program was continued by those in industry and

Government who believed in the concept and its potential, and were willing to

risk their investment. They were rewarded by the discovery of new techniques

and the incorporation of new materials technology that made it possible to propose the XV-15 tilt rotor research aircraft project. It was my privilege to successfully advocate Army participation in the program with both funding and personnel. The unique aspects and synergistic values of the Army AARL/AMRDLNASA interagency participation made this possible. It demonstrated the value of

sharing resources in direct partnership toward common goals. Although not yet

integrated into Army field strategies, the tilt rotor aircraft holds significant potential for consideration in future missions. The soon-to-be deployed Marine V-22

Osprey demonstrates the rewards of the investment of defense dollars in the tilt

rotor research aircraft project. There likely will be more.

It was my good fortune to be directly associated with and participate in the

development and testing of the tilt rotor for almost three decades; first in

NACA/NASA working with the XV-3 and then as Director of AARL/AMRDL

with the XV-15 tilt rotor research aircraft project. I am grateful for the effort of

the authors to document and preserve this story of remarkable achievement. The

persistence and dedication of those who made it happen against many challenges

and discouragement demonstrates high standards that deserve to be acknowledged. It is a fitting tribute to their vision and expertise.

Paul Yaggy

Director,

Army Aeronautical Research Laboratory (AARL)/Army Air Mobility Research

and Development Laboratory (AMRDL), 1965-1974

Director,

Research, Development and Engineering

U.S. Army Aviation Systems Command, 1972-1974

Foreword

xi

Figure 1 Leonardo da Vinci 15th century helical air screw flying machine.

Figure 2 Illustration of vertical and short takeoff and landing (V/STOL)

aircraft developed by the McDonnell Aircraft Company in the

mid-1960s.

Figure 3 Hover vertical lift efficiency as a function of disc loading.

Figure 4 McDonnell XV-1 compound helicopter.

Figure 5 Sikorsky XV-2 convertible aircraft.

Figure 6 Bell Helicopter XV-3 tilt rotor aircraft.

Figure 7 Henry Berliner tilt-propeller helicopter. 

Figure 8 U.S. patent illustration of George Lehberger’s 1930 tilting propeller vertical takeoff “flying machine.”

Figure 9 The Baynes heliplane.

Figure 10 Three-view drawing of the Focke-Achgelis FA 269 convertiplane.

Figure 11 Platt-LePage XR-1A lateral-twin rotor helicopter.

Figure 12 Platt-LePage tilt rotor transport aircraft design. 

Figure 13 Illustration from the Haviland Platt patent of the tilt rotor concept.

Figure 14 Transcendental Model 1-G experimental tilt rotor aircraft.

Figure 15 Transcendental Model 2. 

Figure 16 York convertiplane. 

Figure 17 Bob Lichten (Bell), et al. in front of the XV-3. 

Figure 18 XV-3 at Bell, August 11, 1955. 

Figure 19 Crash of the XV-3 on October 25, 1956. 

Figure 20 Tiedown tests of the XV-3 with protective shields at Bell in 

July 1957. 

Figure 21 XV-3 in hover at Ames Research Center. 

xiii

List of Figures
Figure 22 XV-3 in airplane mode of flight near Ames Research Center.

Figure 23 XV-3 in the Ames Research Center 40- by 80-foot wind tunnel.

Figure 24 Boeing VZ-2 tilt wing research aircraft. 

Figure 25 Bell 25-foot diameter proprotor on semi-span wing in the Ames

Research Center 40- by 80-foot wind tunnel.

Figure 26 Boeing 26-foot diameter proprotor on semi-span wing in the

Ames Research Center 40- by 80-foot wind tunnel.

Figure 27 The Bell stop/fold tilt rotor in the Ames Research Center 40- by

80-foot wind tunnel. 

Figure 28 Performance tests of 5-foot diameter proprotor in the Army 7- by

10-foot wind tunnel at the Ames Research Center.

Figure 29 13-foot diameter proprotor in the ONERA S-1 wind tunnel, France.

Figure 30 13-foot diameter proprotor in the Ames Research Center 40- by 

80-foot wind tunnel.

Figure 31 Bell 25-foot diameter proprotor performance test in the Ames

Research Center 40- by 80-foot wind tunnel.

Figure 32 Members of initial Tilt Rotor Research Aircraft Project Office at

Ames, 1989.

Figure 33 Rotor Systems Research Aircraft (RSRA).

Figure 34 Illustration from 1974 Tilt Rotor Research Aircraft Project Plan.

Figure 35 Illustration of the Boeing Model 222 tilt rotor aircraft. 

Figure 36 1/5-scale XV-15 model in 7- by 10-foot wind tunnel, (a) small

landing gear housings, (b) large landing gear housings.

Figure 37 Tilt Rotor Research Aircraft WBSE. 

Figure 38 XV-15 nacelle arrangement.

Figure 39 Bell test apparatus used for transmission qualification testing.

Figure 40 Proprotor response to cockpit control input.

xiv
Figure 41 Simultaneous static test firing of XV-15 ejection seats.

Figure 42 Parachutes deployed during seat ejection test.

Figure 43 Bell XV-15 ground tiedown facility.

Figure 44 Initial Bell tiedown tests showing metal protective shields. 

Figure 45 XV-15 in the Ames Research Center 40- by 80-foot wind tunnel.

Figure 46 Government and Bell personnel at the Dryden Flight Research

Center, October 1981.

Figure 47 XV-15 plaque being presented to Government pilots by Bell pilots

at the acceptance ceremony. 

Figure 48 Ames Tiedown test facility showing rescue ramp.

Figure 49 Tiedown test facility at the Ames Research Center showing the

hydraulic lift.

Figure 50 XV-15 hovering in-ground-effect during 1984 performance and

downwash test.

Figure 51 Method used to position the aircraft for the downwash and

acoustics hover test at the Ames Research Center.

Figure 52 XV-15 proprotor on the propeller test apparatus at the Ames

Outdoor Aerodynamic Research Facility. 

Figure 53 Tilt rotor structural elastic modes.

Figure 54 Wing modes of the tilt rotor aircraft structure.

Figure 55 XV-15 during short takeoff performance test.

Figure 56 Flow visualization near the XV-15 wing tips.

Figure 57 Flow visualization near the XV-15 wing mid-span position.

Figure 58 Inboard flow visualization showing “fountain flow” above the

fuselage.

Figure 59 Hover acoustics tests during low wind conditions at sunrise.

xv
Figure 60 The XV-15 flying in close formation with the YO-3A for

acoustics data.

Figure 61 Typical cross section of the XV-15 metal blades.

Figure 62 XV-15 Advanced Technology Blades configuration variations.

Figure 63 Advanced Technology Blades proprotor mounted on the test apparatus at the Ames Research Center Outdoor Aerodynamic

Research Facility.

Figure 64 XV-15 N702NA colocated with XV-15 N703NA at the Dryden

Flight Research Center, October 1981. 

Figure 65 The XV-15 in enroute to the Paris Air Show in 1981.

Figure 66 Senator Goldwater in the XV-15 with Bell pilot Dorman Cannon.

Figure 67 Secretary of the Navy John Lehman after flying the XV-15. 

Figure 68 Nap-of-the-earth flight demonstrations at Ft. Huachuca, Arizona.

Figure 69 Shipboard evaluations of the XV-15 onboard the USS Tripoli.

Figure 70 XV-15 during nap-of-the-earth flight demonstrations at 

Ft. Rucker, Alabama. 

Figure 71 XV-15 at the New York Port Authority downtown heliport.

Figure 72 XV-15, QSRA, and AV-8B aircraft, in formation flight at Ames

Research Center.

Figure 73 XV-15 at the Dallas Convention Center Heliport/Vertiport. 

Figure 74 XV-15 at the 1995 Paris Air Show with the Bell-Boeing V-22

Osprey. 

Figure 75 The Bell-Boeing V-22 Osprey in hover flight.

Figure 76 The Bell tilt rotor eagle eye unmanned aerial vehicle. 

Figure 77 Mockup of the BA-Model 609 civil tilt rotor aircraft with BellBoeing markings. 

Figure A-1 Transcendental Model 1-G hovering in-ground-effect.

xvi
Figure A-2 Transcendental Model 2 three-view drawing.

Figure A-3 Transcendental Model 2 cutaway drawing.

Figure A-4 XV-3 three-view drawing.

Figure A-5 XV-3 inboard drawing, side view.

Figure A-6 Three-view drawing of the XV-15 tilt rotor research aircraft.

Figure A-7 Conversion corridor of the XV-15 tilt rotor research aircraft.

Figure A-8 General layout and major components of the XV-15 tilt rotor

research aircraft.

Figure A-9 Side view inboard profile of the XV-15.

Figure A-10 Top view inboard profile of the XV-15.

Figure A-11 XV-15 height-velocity flight envelope.

Figure A-12 XV-15 variation of power with airspeed.

Figure D-1 Jean Tinsley, first woman to fly the XV-15 tilt rotor aircraft.

Figure E-1 XV-3 at the Bell ramp, 1955.

Figure E-2 Bell XV-3 personnel in front of the XV-15 research aircraft.

Figure E-3 XV-15 flying by the Statue of Liberty.

Figure E-4 XV-15 flying near the Washington Monument.

Figure E-5 XV-15 flyby at the Jefferson Memorial.

Figure E-6 XV-15 landing at the Capitol Building.

Figure E-7 Bell test pilots Roy Hopkins and Dorman Cannon.

Figure E-8 XV-15 in executive transport markings.

Figure E-9 XV-15 in camouflage markings.

Figure E-10 XV-15 in Navy gray flying along side the USS Tripoli,

August 1982.

xvii
Figure E-11 Ken Wernicke, Bell tilt rotor design engineer, 1965.

Figure E-12 Ken Wernicke after flying the XV-15.

Figure E-13 XV-15 N703NA crew station (1982).

Figure E-14 Composite photograph showing V-22 Osprey in hover, conversion,

and airplane modes of flight.

xviii
AARL Army Aeronautical Research Laboratory

AATD Army Aeronautical Test Directorate

ADTA Aviation Development Test Activity

AEFA Army Engineering and Flight Activity

AFAPL Air Force Aero Propulsion Laboratory

AFSRB Airworthiness and Flight Safety Review Board

AGARD Advisory Group for Aerospace Research and Development,

North Atlantic Treaty Organization

AHS American Helicopter Society

AIAA American Institute of Aeronautics and Astronautics

AMRDL Army Air Mobility Research and Development Laboratory

APA Airport Planners Association

ARC Ames Research Center

ASME American Society of Mechanical Engineers

ASRO Advanced Systems Research Office

ASW Anti-Submarine Warfare

ATB Advanced Technology Blade

ATM Air Traffic Management

ATTT Advanced Tiltrotor Transport Technology

BHTI Bell Helicopter Textron Inc.

CAP Composite Aircraft Program

CNO Chief of Naval Operations

COD Carrier Onboard Delivery

CPIF Cost Plus Incentive Fee

CTR Civil Tilt Rotor

CTRDAC Civil Tiltrotor Development Advisory Committee

DCAA Defense Contract Audit Agency

DFRC Dryden Flight Research Center

DOD Department of Defense

DOT Department of Transportation

ECGB Engine Coupling Gearbox

EMD Engineering Manufacturing Development

EMI Electromagnetic Interference

EUROFAR European Future Advanced Rotorcraft

FAA Federal Aviation Administration

FAI Federation Aeronautique Internationale

FFS Force Feel System

FSAA Flight Simulator for Advanced Aircraft

FSD Full Scale Development

GPS Global Positioning System

HAI Helicopter Association International

HUM Health and Usage Monitoring

IAS Institute of Aeronautical Sciences

IGE In-ground-effect

IOC Initial Operating Capability

IR&D Independent Research and Development

xix

List of Acronyms
JAA Joint Aviation Authorities

JARG Joint Aeronautical Research Group

JTAG Joint Technology Assessment Group

JTR Joint Transport Rotorcraft

JVX Joint Vertical Experimental

LaRC Langley Research Center

LHX Light Helicopter Experimental

LPH Amphibious Assault Ship (helicopters)

MAC Military Airlift Command

MCAS Marine Corps Air Station

NACA National Advisory Committee for Aeronautics

NALF Naval Auxiliary Landing Field

NAS National Aerodynamic Simulation 

NASA National Aeronautics and Space Administration

NATC Naval Air Test Center

NATO North Atlantic Treaty Organization

NAVAIR Naval Air Systems Command

NAVMAT Naval Materiel Command

NOE Nap-of-the-earth

NRTC National Rotorcraft Technology Center

NTSB National Transportation Safety Board

OARF Outdoor Aerodynamic Research Facility

OART Office of Aeronautical Research and Technology

OEI One engine inoperative

OGE Out-of-ground-effect

ONERA Office National d’Etudes et de Recherches Aerospatiales

PCM Pulse Code Modulation

QSRA Quiet Short-Haul Research Aircraft

R&QA Reliability and Quality Assurance

RFP Request for Proposal

RMDU Remote Multiplex/Digitizer Unit

RPV Remotely Piloted Vehicle

RSRA Rotor Systems Research Aircraft

SAE Society of Automotive Engineers

SAR Search and Rescue

SAWE Society of Allied Weight Engineers 

SBA-MS Sea Based Air-Master Study

SCAS Stability and Control Augmentation System

SCS Sea Control Ship

SEB Source Evaluation Board

SETP Society of Experimental Test Pilots

SHCT Short-Haul Civil Tiltrotor

SNI Simultaneous non-Interfering

STO Short Takeoff

STOL Short Takeoff and Landing

TDT Transonic Dynamics Tunnel 

xx
TRENDS Tilt Rotor Engineering Database System

TRRA Tilt Rotor Research Aircraft

UAV Unmanned Aerial Vehicle

UT University of Texas

VDTR Variable Diameter Tilt Rotor

V/STOL Vertical or Short Takeoff and Landing

VDTR Variable Diameter Tilt Rotor

VMS Vertical Motion Simulator

VTOL Vertical Takeoff and Landing

WBSE Work Breakdown Structure Elements

WPAFB Wright-Patterson Air Force Base

xxi

For as long as can be remembered,

humans have always wanted to fly... to

be able to soar into the sky and alight

wherever their fancy takes them. One

such individual was Leonardo da Vinci

(1452-1519), who was the first person

to approach heavier-than-air-flight in a

somewhat scientific manner. Da Vinci

is credited with the design of the first

helicopter, basically a helical air screw

(figure 1), which was conceived to lift

off the ground vertically—no ground

roll required, no runway needed.

However, nearly four centuries later,

when technology advancements

allowed sustained, powered manned

flight, the practical solution demonstrated by the Wright brothers used a

fixed-surface to provide the lift. This required the aircraft to accelerate along the

ground until a sufficient speed was reached so that the necessary force could be

generated for the vehicle to become airborne. The da Vinci dream of vertical

liftoff was finally achieved with the development of the successful helicopter

more than 30 years after the first fixed-wing flight.1 While, in the second half of

this century, this remarkable machine has become an essential vehicle for numerous civil and military applications, because of its vertical lift capabilities, it

remains extremely limited in the speed and range that it can attain. By the middle

of this century, these limitations to the helicopter’s effectiveness and the demonstrated capabilities of the fixed-wing airplane had fostered a new dream... the

development of an aircraft with the vertical takeoff and hover capability of the

helicopter, and with the speed and range of the fixed-wing aircraft. This is the

story of the quest for a new type of aircraft that would make that dream a reality.

The search for an aircraft type with Vertical Takeoff and Landing (VTOL) capabilities triggered the imagination of designers and inventors to produce numerous

configurations using a wide variety of lifting and propulsion devices. A summary

of these configurations is shown in the V/STOL (Vertical or Short Takeoff and

Landing) concepts illustration2 prepared by the McDonnell Aircraft Company in

the 1960s (figure 2). For the various aircraft types considered, one of the key distinguishing features is associated with the device used for providing the vertical

Introduction

1

1 While several helicopters became airborne during the first three decades of the 20th century, the

Focke-Wulf Fw-61 is generally credited with being the first helicopter to demonstrate performance and precision control, essential characteristics of a successful helicopter. The first flight

occurred in Germany in 1937 and public flight demonstrations were made in 1938.

2 Seth B. Anderson, “Historic Overview of V/STOL Aircraft Technology,” NASA TM 81280,

March 1981.

Figure 1. 

Leonardo da Vinci 15th

century helical air screw 

flying machine.
lift. If the thrust for vertical takeoff is produced over a small area, such as the

exhaust nozzle area of a “direct-lift” jet engine, the lifting device is referred to as

a “high disc loading” type (where disc loading is defined as the thrust divided by

the area over which it is produced). On the other end of the spectrum, if a large

area is used to generate the vertical lift, as in the case of a helicopter with its

large diameter rotors, the system is called a “low disc loading” device. This singular parameter (disc loading) is a direct indicator of the achievable level of efficiency in the production of the required hover thrust, as shown in figure 3. Low

2

FLYING PROTOTYPE

Revised: September 1967

UNDER CONSTRUCTION WIND

TUNNEL TESTS OR MOCK UP

DESIGN CONCEPT STUDY

FREE

SLIPSTREAM

V/STOL

AIRCRAFT

SUMMARY

McDONNELL

AIRCRAFT

SURVEY

FIAT

G 95-4

FIAT

TAIL-SITTERS

ONE-MAN-LIFTS

FLYING JEEPS

FLYING

LOCKHEED

BRITISH

VTOL-P.D. 16

SHORT

PD 49

LOCKHEED

VTOL-F-104

SHORT

SC.1

FRENCH

VTOL MIRAGE III-V

BRITISH

P-148

GERMAN

VJ-101 C

DORNIER

De 31

EWR/REPUBLIC

US/FRG

HUGHES STOPPED

ROTOR

McDONNELL

MODEL 177

FAIREY

ROTODYNE-Y

McDONNELL

XV-1

LOCKHEED

XV-4A

"TRANSONIC"

MILLER

SIKORSKY

S-57(XV-2)

TR

N.A.A.

ANSPORT

G.E.-RYAN

XV-5A

BOEING

TRANSPORT

McDONNELL

FIGHTER

VANGUARD-2C

OMNIPLANE

HOOP

(RUSSIAN)

LOCKHEED

AAFSS

LOCKHEED STOWED ROTOR

PIASECKI COMPOUND

SIKORSKY STOWED ROTOR

CURTISS WRIGHT

X-19

CURTISS WRIGHT

X-100

CURTISS WRIGHT

300

BOEING VERTOL-76

BOEING VERTOL

VZ-2

NORD-CADET

500

BELL

X-22A

DOUGLAS-DOAK X-16

VOUGHT-RYAN

HILLER XC-142

HILLER

X-18

FAIRCHILD 224

VZ-5FZ

LOCKHEED

CL-379

ROBERTSON

VTOL

RYAN-02

VZ3-RY

COLLINS

AERODYNE

CHANCE-VOUGHT

ADAM

HAWKER P-1127

XV-6A

BELL

X-14

MIKOYAN V/STOL

VFW VC-400

CANADAIR CL-84

BELL XV-3

McDONNELL

TRANSPORT

LIFT+LIFT/CRUISE

VFW VAK-191B

XV-4B

GROUND EFFECT MACHINES

G 95-4

STOL AIRCRAFT

McDONNELL 188

BREGUET 941)

PLATFORMS

FLYING SAUCERS

(AVRO-CANADA)

AUGMENTED POWER

PLANT FOR HOVER

25 26 24 23 22

21

20

27 28 29 30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

46

47

48

49

50

51

52

11

12

13

14

15

16

17

18

19

1

2

3

4

5

6

7

8

9

10

H G D E F C B A

AND

FOR

HOVER

POWER PLANT

SPECIAL

SEPARATE

TYPES

HELICOPTERS

LIFT

ENG NI ES TIP JETS STATIC

COMBINED POWER

PLANT FOR

HOVER

JET

TIP

FAN

MECH

TIP TURBINE

FAN

ROTOR

FIXED POWER PLANT

SHAFT DRIVE

PNEUMATIC

TILTABLE MASS

DR

DYNAMIC IVE

FLOW GENERATION

TILT POWER

PLANT

PROPELLER

FAN

PROPELLER

RO

PROP

TOR

SAME PROPULS OI N SYSTEM

FOR HOVER & FORWARD FL GI HT

MASS FLOW GENERATOR AND DEFLECTOR

MASS

DEFLECTOR

FLOW

EXH.

ONLY

EXHAUST

AND

BLEED

DUCTED

STREAM

SLIP

FAN PROP

Figure 2. 

Illustration of vertical and

short takeoff and landing

(V/STOL) aircraft developed

by the McDonnell Aircraft

Company in the mid-1960s.
disc loading lifting systems are capable of delivering significantly more

thrust per horsepower than higher disc

loading devices. Therefore, for applications where extended-duration hover

or near-hover conditions are required,

or where lower installed power or

lower fuel consumption in hover is

important, low disc loading aircraft

concepts appear to be the right choice. 

The vertical lift efficiency of a VTOL

aircraft, however, is not the only area

of interest in the selection of a configuration. In addition to the need for

vertical liftoff, these aircraft need to be

designed to perform a cruise mission,

usually with certain speed and range

requirements. Performance in the

cruise flight regime, therefore, needs

to be assessed for each VTOL configuration. The challenge of finding an aircraft type that meets both the hover- and

cruise-mode performance criteria, while also meeting other operational, economic, and environmental requirements was the major task encountered by the developers of VTOL technology. 

3

Figure 3. 

Hover vertical lift efficiency

as a function of disc loading.
For one group of military planners in the late 1940s and early 1950s, the mission

requirements included significant hover duration, low speed maneuvering and

agility, and a speed and range greater than current helicopter capabilities. This, and

additional mission factors such as the need for moderate downwash velocities

below the hovering aircraft to enable safe rescue operations, led the planners to

specify low disc loading for the new VTOL vehicle. These considerations resulted

in the August 1950 initiation of the joint U.S. Army and U.S. Air Force

Convertiplane Program. This program was formulated to provide demonstrations

of different approaches to meeting the convertiplane requirements. The aircraft

selected from the design competition were the XV-1 compound helicopter, figure 4

(proposed by the McDonnell Aircraft Co.), the XV-2 stoppable rotor aircraft, figure

5 (proposed by Sikorsky Aircraft), and the XV-3 tilt rotor aircraft, figure 6 (submitted by the Bell Helicopter Company). A discussion of the aircraft concepts

addressed in the Convertiplane Program is provided by R. W. Prouty’s February

1984 Rotor and Wing International article “From XV-1 to JVX—A Chronicle of

the Coveted Convertiplane.”

Two designs, the XV-1 and the XV-3, survived the initial evaluation phase and

were developed as test aircraft for limited flight evaluations. While the XV-1

Early Efforts

4

Figure 5. 

Sikorsky XV-2 

Convertible Aircraft.

Figure 4. 

McDonnell XV-1 

compound helicopter. 

(Boeing Photograph 

AD98-0209-13)
achieved a speed of 200 mph in 1955

(as the XV-3 was encountering technical problems, to be discussed later), it

became apparent that the compound

helicopter, in high speed flight, would

still experience the type of severe

oscillatory load conditions that limits

the speed capability of the conventional helicopter. These vibrations are due

to the edgewise movement of the rotor

through the air during cruise flight.

Helicopter rotors operating in the

cruise mode are burdened with the

tasks of producing the required thrust

and lift while delivering the forces and

moments to maintain a balanced, or

trimmed, flight state. Because of the

essentially edgewise motion of the

rotor, the blades experience an aerodynamic acceleration and deceleration as they

“advance” into and “retreat” from the airstream. Although the compound helicopter uses a conventional fixed-wing to produce the required lift while in the cruise

flight mode, thereby unloading the rotor from the burden of producing lift and

trim moments, it still encounters the variations in rotor blade drag due to the

advancing and retreating airloads during each rotation. In addition, the edgewise

rotor limits maneuver capability at high speeds because of the extreme load oscillations that occur on the rotor. Also, the exposed rotor hub and control hardware

contribute significantly to drag in the high speed cruise mode, further limiting

maximum airspeed. The compound helicopter also suffered the weight penalty of

carrying the additional cruise mode propulsion system hardware. Collectively,

these issues inhibit the performance potential of the compound helicopter. The

compound helicopter was not the answer to the search for a viable low disc loading VTOL high performance aircraft.

According to advocates during the 1950s, the tilt rotor configuration was projected

to have the potential to overcome many of the limitations or deficiencies of the helicopter and compound helicopter. The XV-3 provided an opportunity to demonstrate the effectiveness of the tilt rotor aircraft and learn about unknown problems

of this aircraft type. However, before the story of the XV-3 program is told, the

evolution and early history of the tilt rotor aircraft will be briefly reviewed.

During the 1920s and 1930s, the numerous innovative flying machines devised

included several concepts that were expected to provide vertical takeoff capabilities. One of these was developed in the U.S. by Henry Berliner3 in the early

5

3 Jay P. Spenser, Whirlybirds, A History of U.S. Helicopter Pioneers, University of Washington

Press, 1998.

Figure 6. 

Bell helicopter XV-3 

tilt rotor aircraft. 

(Bell Photograph 209854)
1920s (figure 7). This design resembled a fixed-wing biplane of the period, except that it had a large diameter

fixed-pitch propeller mounted on a

vertical shaft near the tip of each

wing. For forward flight, the shafts

would be tilted forward. Reports indicate that the Berliner helicopter

achieved forward speeds of about 40

mph. While the propellers were not

designed to convert fully to the conventional airplane mode, the Berliner

side-by-side helicopter was an early

example of the rotor arrangement used

on current tilt rotor aircraft.

Another design conceived to provide vertical lift and forward flight is the “Flying

Machine” for which George Lehberger was issued a patent in September 1930

(figure 8). His approach contained the basic concept of the tilt rotor aircraft, that

is, the use of a relatively low disc loading thruster (propeller) that can tilt its axis

from the vertical (for vertical lift) to the horizontal (for propulsive thrust). While

the authors are not aware of any attempt by inventor George Lehberger to develop

this vehicle, it would be expected to encounter performance, loads, structural

dynamics, and control deficiencies if built as indicated in the patent illustration.

The vectored thrust low disc loading VTOL aircraft required many technology

advancements before it would be a practical aircraft type.

In the late 1930s, a British patent was issued for the Baynes Heliplane (figure 9)

which resembled the configuration of the current tilt rotor aircraft. Inadequate 

financial backing prevented development work, leaving the exploration of tilt

rotor technology to other engineers in the four decades that followed.

In Germany, the Focke-Achgelis FA-269 trail-rotor convertiplane project was initiated in 1942. This aircraft, illustrated in figure 10, followed the moderately successful lateral twin-rotor helicopter, the Focke-Wulf Fw-61 flown in 1937. The

FA-269 used pusher propellers that tilted below the wing for takeoff. This project

was discontinued after a full-scale mockup was destroyed during a bombing in

WWII. Years later, variants of the trail-rotor tilt rotor configuration would surface

again in design studies at Bell and McDonnell Douglas.

The accomplishments of the German Focke-Wulf activities did not go unnoticed by

the americans. Two enterprising engineers, Dr. Wynn Laurence LePage and Haviland

Hull Platt of the Platt-LePage Aircraft Company of Eddystone, Pennsylvania,

became intrigued with the success of the German helicopter and decided to pursue

the development of a viable helicopter in the U.S. with the same general arrangement

of the Fw-61. The product of this work was the 1941 Platt-LePage XR-1A lateral

6

Figure 7.

Henry Berliner tiltpropeller helicopter.

(National Air and Space

Museum–NASM–Photo)
7

Figure 8. U.S. patent illustration of George Lehberger’s 1930 tilting propeller vertical takeoff “flying machine.”
twin-rotor helicopter (figure 11). This

aircraft inspired the design of a large

(53,000-lb.) tilt rotor aircraft (figure 12),

which resembled the XR-1A configuration, but incorporated mechanisms that

permitted the rotors to be tilted for forward flight. While Platt-LePage never

developed their tilt rotor design,

Haviland Platt applied for a patent for

the concept on July 7, 1950, and was

granted U. S. patent 2,702,168 on

February 15, 1955 (figure 13). 

The next significant appearance of the

tilt rotor occurred in early 1947 when

the Transcendental Aircraft Corporation

of New Castle, Delaware, initiated work

that led to the development of the Model

1-G tilt rotor aircraft (figure 14). The

founding principals of Transcendental

were Mario A. Guerrieri and Robert

(Bob) L. Lichten, who had been

coworkers at the Kellett Aircraft

Company. Bob Lichten had earlier

worked for pioneer helicopter designers

LePage and Platt and had become

intrigued with the tilt rotor concept. His

experience at Platt-LePage provided him

a mission that he pursued for the rest of

his life.

While at Kellett, Guerrieri and Lichten

investigated the performance of a helicopter rotor acting as a propeller and, encouraged by the results, decided to

demonstrate tilt rotor technology by independently building and flying a small,

single-place experimental aircraft. Appendix A contains a brief description and

the general characteristics of this aircraft, the Transcendental Model 1-G.

Lichten left Transcendental in 1948, and, in September 1952, Guerrieri sold his

interests in the company to William E. Cobey, a Kellett Aircraft Corporation

vibrations expert who continued the development of the Model 1-G. With some

funding provided by a 1952 Army/Air Force contract for flight data reports and

analyses, hover testing of the 1750 lb. Model 1-G began on June 15, 1954. 

The Transcendental Model 1-G, however, met an unfortunate end. After successfully completing more than 100 flights in a period of just over one year, includ8

Top:

Figure 9. 

The Baynes Heliplane.

Bottom:

Figure 10. 

Three-view drawing of

the Focke-Achgelis 

FA-269 convertiplane.
ing partial conversions to within 10 degrees of the airplane mode, an inadvertent

reduction of the rotor collective pitch while flying with the rotors tilted forward

led to a crash into the Chesapeake Bay on July 20, 1955. Although the aircraft

was destroyed, the crash occurred near land in shallow water, which allowed the

pilot, who sustained minor injuries, to wade ashore.4

9

Figure 12. 

Platt LePage tilt rotor transport aircraft design 

(Boeing print-Ames 

AD98-0209-22)

Figure 11. 

Platt LePage XR-1A lateral

twin-rotor helicopter.

(Photograph courtesy of 

Jay Hendrickson)

4 This description of the Transcendental Model 1-G accident was based on an account by Mario

Guerrieri (Letters to the Editor, Vertiflite, Vol. 34, Number 5, Sept./Oct. 1988) in which he relates

information provided to him by William Cobey. This information disputes the account noted by

Mark and Lynn in “Aircraft Without Airports—Changing the Way Men Fly” (Vertiflite, May/June

1988) that states that the accident was fatal to the pilot.
10

Figure 13. Illustration from the Haviland Platt patent of the tilt rotor concept.

United States Patent Office 2,702,168

Patented Feb. 15, 1955

2,702,168

CONVERTIBLE AIRCRAFT

Haviland H. Platt, New York, N. Y.

Application July 7, 1950, Serial No. 172,507

15 Claims. (Cl. 244--7)
A second, improved, Transcendental

tilt rotor aircraft, the 2,249 lb., twoplace Model 2 (figure 15), was subsequently developed by Cobey but funding limitations resulting from the withdrawal of Air Force support prevented

the expansion of the flight envelope,

and the program was terminated in

1957. Transcendental became associated with the short-lived Helicopter

Division of the Republic Aviation

Corporation, Farmingdale, Long

Island, but the failure to gain

Government interest ended the venture.

The Transcendental Model 2 is

described in Appendix A.While never

attaining flight in the airplane mode

(but having flown within 10 degrees of

airplane mode), the Model 1-G is generally recognized as the first vehicle to

successfully explore the conversion

mode of flight of the tilt rotor aircraft.

Other tilt rotor aircraft designs

appeared during the 1950s but most

never left the drawing board. One aircraft, the York convertiplane (figure

16) was developed by C. H. York in

1956. However, no record of its operational experience was found.

11

Figure 14. Transcendental Model 1-G experimental tilt rotor aircraft. 

(Photograph courtesy of John Schneider)

Figure 16. York convertiplane. (John Schneider-Ames Photograph AD98-0209-16)

Figure 15. Transcendental Model 2. (John Schneider-Ames Photograph AD98-0209-19)
Long before Transcendental initiated

flight tests of the Model 1-G, Bob

Lichten had joined Bell Aircraft where

he was given the opportunity to further

the advancement of the tilt rotor with

the research and development resources

of a major rotorcraft company. At Bell,

Lichten began the task of developing a

new technology base associated with

the tilt rotor aircraft. In 1951, in

response to the Convertible Aircraft

Program Request For Proposal (RFP)

for the design of a “convertiplane,” the

Bell proposal offered Lichten’s tilt

rotor, the Bell Model 200. With the

subsequent award of a contract for two

full-scale “tilting-thrust-vector convertiplanes” in October 1953, and the infusion of Army and Air Force funds, the

exploration of this new technology was accelerated. The Bell Model 200, designated the XV-35 by the Army and Air Force, produced some interesting technical challenges for Lichten and his team during the next thirteen years. Figure 17 shows

Bob Lichten, the principal advocate of the tilt rotor concept, standing in front of

his creation, the XV-3. 

Instability

Following an extensive series of ground

tests by Bell, the initial hover trial of

the XV-3 was flown on August 11,

1955 (figure 18). After noting satisfactory characteristics during the beginning of the flight, Bell test pilot Floyd

Carlson experienced a high vibration in

hover. During a subsequent flight on

August 18, 1955, a reappearance of the

rotor dynamic instability problem

resulted in a hard landing that caused

minor airframe damage. A thorough

ground investigation was conducted to

XV-3 Program

12

Figure 17. 

Bob Lichten, extreme left,

et al. in front of the XV-3. 

(Bell Photograph 214838)

Figure 18. 

XV-3 at Bell,

August 11, 1955. 

(Bell Photograph 210021)

5 Interesting summaries of the early years of Bell’s development of tilt rotor technology are provided in Aerophile, Volume 2, Number 1, June 1979; “The Rebirth of the Tiltrotor - The 1992

Alexander A. Nikolsky Lecture” by Robert Lynn (Journal of the American Helicopter Society,

Vol. 38, No. 1, Jan. 1993); and “Aircraft Without Airports - Changing the Way Men Fly” by Hans

Mark and Robert Lynn (Vertiflite, May/June 1988).
understand and resolve the cause of the

dynamic instability. Flight testing

resumed on March 29, 1956, but on

July 25 the instability occurred again,

causing Bell to conduct another series

of ground tiedown tests which lasted

until late September of that year. 

It is important to note that the ability of

the rotorcraft dynamicists of that period

to analyze complex systems (such as

the rotor/pylon/wing of the tilt rotor)

was quite primitive compared to the

computational capabilities of the 1990s.

The attempts to correct the instability

that occurred on the XV-3 had to be done by combining the available analytical

methods with experimental data. Therefore, ground tiedown tests were needed to

expand the database documenting the fundamental characteristics of the tilt rotor as

well as to evaluate configuration changes.

Following the second ground test effort, flight testing continued with the goal of

expanding the speed and conversion envelope of the XV-3. On October 25, 1956, as

Bell test pilot Dick Stansbury moved the rotor shaft 17 degrees forward from the vertical, a severe rotor instability occurred that resulted in extremely high cockpit vibrations

and caused the pilot to black out. The subsequent loss of control caused the number 1

XV-3 ( aircraft tail number 4147) to crash, seriously injuring the pilot (figure 19). 

The XV-3 program faced a crisis. The inability to solve the instability using

traditional analyses, experimentation, and trial-and-error empirical methods

made even some of the tilt rotor’s most avid supporters question the readiness

of this technology. But the believers held on. A satisfactory solution to the

rotor/pylon/wing dynamic instability problem had to be found. Advocates of

the tilt rotor at Bell and the Government decided to continue the work and

authorized the initiation of a major design change as well as plans for testing

the XV-3 in the NACA Ames Aeronautical Laboratory6 40- by 80-foot wind

tunnel. The original three-bladed, 25-ft diameter articulated rotor was

replaced with a two-bladed stiff-inplane rotor. By July 18, 1957, with isolated

two-bladed rotor static tests and rotors-installed XV-3 tiedown tests completed

(figure 20), investigations of the performance and dynamic behavior of the

modified XV-3 began.

13

Figure 19. 

Crash of the XV-3 on 

October 25, 1956. 

(Bell Photograph 217259)

6 The NACA (National Advisory Committee for Aeronautics) was the predecessor to the NASA

(National Aeronautics and Space Administration). The NACA became the NASA in October

1958 and the Ames Aeronautical Laboratory was renamed the Ames Research Center (ARC).

ARC is located at Moffett Federal Airfield, formerly Moffett Naval Air Station, about 40 miles

south of San Francisco, California.
In the following 18 months, the XV-3

(tail number 4148) with its new rotor

system underwent two wind tunnel

entries in the 40- by 80-foot wind tunnel (September-October 1957 and

October 1958) and an additional series

of ground tiedown and flight tests.

During this period further changes

were made to improve stability,

including the reduction of the rotor

diameter to 23 feet, the addition of

external struts to stiffen the wing, and

a significant increase in the stiffness of

the rotor controls. The configuration

that emerged accomplished the elusive

goal of completing a dynamically stable full conversion to the airplane mode.

This occurred at Bell on December 18, 1958, with test pilot Bill Quinlan at the

controls. Subsequent flights explored the effect of wing stiffness (by modifying

the strut attachments) and expanded the flight envelope within the fairly narrow

range of the XV-3’s performance capabilities. 

Government Flight Tests

The XV-3 was transported to Edwards Air Force Base where, from May through

July 1959, Air Force Major Robert Ferry conducted a Government flight evaluation. The tests included handling qualities assessments, Short Takeoff and

Landing (STOL) operations, and

autorotation demonstrations. The Air

Force test report,7 authored by Project

Engineer Lt. Wallace H. (Wally)

Deckert, USAF, and Major Ferry,

noted numerous deficiencies in the

performance and flying qualities of the

aircraft. However, in spite of the deficiencies, the report concluded that “the

fixed-wing prop-rotor (i.e. the tilt

rotor) principle is feasible and should

be given serious consideration in

future Vertical or Short Takeoff and

Landing (V/STOL) aircraft design

competition.” “The XV-3 demonstrated that the fixed-wing prop-rotor con14

Figure 21.

XV-3 in hover at

Ames Research Center. 

(Bell Photograph 228649)

Figure 20. 

Tiedown tests of the XV-3 

with protective shields at 

Bell in July, 1957. 

(Bell Photograph 220955)

7 W. H. Deckert, R. G. Ferry, “Limited Flight Evaluation of the XV-3 Aircraft,” TR-60-4 ARDC

XV-3, May 1960.
cept is operationally practical with

safety and complexity comparable to

helicopters.”

After the conclusion of the flight program at Edwards AFB, the XV-3 was

transported to NASA ARC onboard an

Air Force C-130, where flight testing

continued until July 1962 (figure 21).

The first full tilt rotor conversion at

Ames was performed by test pilot Fred

Drinkwater on August 12, 1959 (figure

22). This flight program was followed

by an additional entry in the Ames 40-

by 80-foot wind tunnel (in June-July

1962, figure 23) to investigate the

effects of changes to the pitch-flap

coupling on rotor flapping and highspeed airplane mode stability.

Pitch-flap coupling refers to a feature

provided by the hub design wherein

the blade pitch angle is changed in a

manner that alters the amount of outof-plane flapping motion that occurs.

A standard stabilizing pitch-flap coupling, referred to as δ3, reduces the

flapping displacement by reducing

the pitch angle as flapping increases.

After another modification (this time

to increase the pylon/wing stiffness)

the XV-3 was able to reach a speed

of 155 knots before indications of

low damping, i.e. aeroelastic instability, were seen. While this was a definite improvement over the earlier stability limits of the XV-3, it would still be

inadequate for the intended military mission application of the tilt rotor aircraft and was substantially below the predicted performance capability of this

aircraft type.

Stability Validation

In 1965, after a period of model-scale testing and analytical studies, Bell funded

a ground test to continue its investigation of XV-3 tilt rotor dynamics. To further

pursue this work in a full-scale wind tunnel test, Robert (Bob) Lynn, Bell’s Chief

of Research and Development (who later was Bell’s Senior Vice President,

15

Top:

Figure 22.

XV-3 in airplane mode

of flight near Ames 

Research Center.

(Bell Photograph 028304)

Bottom:

Figure 23. 

XV-3 in the Ames Research

Center 40- by 80- ft. 

wind tunnel. 

(AMES Photograph A37017)
Research and Engineering), obtained support from C. W. (Bill) Harper, Chief of

the Aeronautics Division in the Office of Advanced Research and Technology

(OART) at NASA Headquarters, for another entry in the Ames 40- by 80-foot

wind tunnel. This test involved configuration variations that were predicted to

alter the rotor/pylon/wing aeroelastic stability. The test results were compared

with the pre-test predictions to determine if the evolving analytical methodology

adequately represented the aircraft’s structural dynamics. Without a speed capability well in excess of the helicopter’s maximum speed, the tilt rotor aircraft did

not fulfill the performance requirements of the VTOL mission. Lacking a valid

structural stability prediction method, the design of a new tilt rotor aircraft was

considered to have a high level of uncertainty and therefore an unacceptable

high-risk undertaking.

The planned test could have exposed the XV-3 aircraft, as well as the 40- by 80-

foot wind tunnel, to possible damage due to the potential for an explosively rapid

failure caused by instability. Could Ames accept this unusual risk? Showing great

confidence in the technical approach, the decision to accept the test was made by

Mark Kelly, NASA’s Chief of the Large Scale Aerodynamics Branch, and

Woodrow L. (Woody) Cook, Chief of the Advanced Aircraft Programs Office. 

The Bell test team was led by Kipling (Kip) Edenborough, who served as test

director, and included Claude Leibensberger, Flight Test Engineer for the XV-3

project. The test, which ran from October to November 1968, proceeded remarkably well for all of the planned test conditions. The level of damping (i.e. stability) was assessed by disturbing the pylon and measuring the resulting vibrations.

Decaying vibration amplitudes indicated a stable structure, constant amplitude

vibrations indicated neutral stability, and growing amplitudes revealed a dangerous unstable condition. Test results showed that configurations predicted to be

stable were in fact stable, and those predicted to be unstable showed signs of

decreasing stability as the stability limit speed was approached. With the aircraft

in its most stable condition, a run at maximum wind tunnel speed, recognized as

a high risk condition, completed the test activity. When the wind tunnel was

taken to its maximum airspeed capability (of nearly 200 knots), the vibratory

loads data once again verified the predicted stability.

Disaster Strikes

Suddenly both pylons separated from the wing and were blown down the tunnel.

The XV-3 was extensively damaged in what appeared to be the result of the

inability to design an aeroelastically stable tilt rotor aircraft. However, after

months of careful examination of the damaged structure and analyses of the incident, the test data revealed this was not the case. The failure was traced to a

fatigue crack and rivets working loose in the left wingtip spar. The progressing

crack and loose rivets reduced the stiffness of the pylon attachment to the level

where a resonance occurred, producing the high oscillatory loads that led to the

subsequent massive structural failure. The right rotor, exposed to extremely high

16
overloads as the aircraft was being shaken during the initial failure, failed under

a whirl divergence condition. In the final analysis, the wind tunnel investigation 

successfully accomplished its goals, but this wind tunnel entry would be the

final research activity conducted with the XV-3 experimental aircraft.8

XV-3 Legacy

At first look, an assessment of the results of 13 years of flight, ground, and wind

tunnel investigations with the XV-3 did not present a favorable prospect for the

future of the tilt rotor aircraft. The severely underpowered XV-3 had limited

hover capability and cruise performance. The maximum level flight speed of 115

knots (155 knots in a dive) was not adequate to prove that the tilt rotor had a useful airplane mode capability. However, it was fortunate that the airplane-mode

speed was so restricted since the aircraft would likely have been destroyed in

flight, due to the rotor/pylon/wing aeroelastic instability. The XV-3 also suffered

from handling qualities problems, including lateral and roll instabilities when

hovering in ground effect (IGE), and a directional divergent oscillation and poor

control responses in the longitudinal and directional axes at low airspeeds. In

addition, a complex gear shifting process, required to reduce rotor RPM after

converting to the airplane mode (to improve rotor efficiency), produced an unacceptably high pilot workload.

On the positive side, the significant achievement of the XV-3 project was clearly

the demonstration of the ability of the tilt rotor aircraft to perform in-flight conversion from the helicopter configuration to the fixed-wing (airplane) configuration and back to the helicopter mode in a safe, stable, controllable manner. This

was accomplished with sufficient airspeed margins and maneuverability and adequate tolerance to gusts and turbulence throughout the process. A total of 110 full

conversions were performed during the 125 flight hours logged by the 10 XV-3

test pilots (three Bell, three Army, two Air Force and two NASA). The proven

conversion capability, coupled with the predicted but unproven performance

potential in the hover and cruise flight regimes, provided the basis for continued

interest in the tilt rotor concept in the military and within the NASA Langley and

Ames Research Centers that were focusing on the search for new VTOL vehicle

technologies. A description of the XV-3 is provided in Appendix A.

Encouraged by the outcome of the flight and wind tunnel tests of the XV-3, Bell

17

8 After years of storage at Moffett Field, California, Tucson, Arizona, and the Wright-Patterson

AFB near Dayton, Ohio, the remains of the XV-3, tail number 4148, were found at the U.S.

Army Air Museum at Fort Rucker, Alabama, in 1984. This unexpected discovery occurred when

the Bell XV-15 flight test team visited the museum while conducting a demonstration tour with

the XV-15 tilt rotor research aircraft. The XV-3 had been stored outside and was in need of

extensive repair (including the damage from the wind tunnel accident). Claude Leibensberger, an

XV-3 engineer who at the time was retired from Bell, led the restoration accomplished with

Army support. The refurbishment was completed by December 1986 but the aircraft was not put

on display due to limited museum space. By late 1995, the XV-3 was again seen disassembled in

an indoor storage area where it remains as of the time of this writing.
management continued to show interest in pursuing the development of tilt rotor

technology. In 1966, to ensure they could legally proceed with the work, Bell

paid Haviland Platt for the rights to the convertible (tilt rotor) aircraft described

in his patent.

18
During the late 1960s, the U.S. Army established the Army Aeronautical

Research Laboratory (AARL) at the NASA Ames Research Center. In 1969,

a “Master Agreement” between the National Aeronautics and Space

Administration and the U.S. Army Materiel Command was authorized which

provided for the joint (Army/NASA) participation in the development of

advanced aircraft technology. The cooperative effort conducted under this agreement would be performed by NASA and Army personnel assigned to the Joint

Aeronautical Research Group (JARG). The NASA Ames Aeronautics and Flight

Mechanics Directorate and the Army Aeronautical Research Laboratory, later

designated the Army Air Mobility Research and Development Laboratory

(AMRDL), began a cooperative activity to address an array of tilt rotor aircraft

aeromechanics issues and deficiencies that had surfaced during the flight and

wind tunnel tests of the XV-3. The initial staff members of this joint tilt rotor

focused effort were James (Jim) Weiberg9 (NASA) and Martin D. (Marty)

Maisel10 and Gary B. Churchill11 of the AMRDL. During the early phases of this

cooperative effort, the Air Force joined the Army and NASA in funding some of

the research activities. The focused tilt rotor technology investigations included

in-house Government research, contracted work, and combined Government/

industry efforts. In addition, tilt rotor related work was conducted within the

rotorcraft industry under both company funded and Independent Research and

Development (IR&D) efforts. The following review will focus on the NASA

Ames/Army funded activities.

NASA-Army Cooperation

19

9 At the beginning of the tilt rotor activity, Weiberg had accrued nearly 30 years as an aerodynamics,

wind tunnel, and flight-test engineer at the Ames Research Center. 

10 Maisel had worked on the design and test of tilt wing propellers and tilt rotor proprotors at Boeing

Vertol before joining the AMRDL in 1970. 11 Churchill came to the AMRDL in 1971 from Boeing Vertol with an extensive background in

V/STOL flight control system development. Prior to working at Vertol, he participated in the 

development of the LTV XC-142 tilt wing aircraft.
Aeroelastic Stability

One of the principal areas of interest was the structural instability that plagued

the XV-3 when operating in the airplane flight mode. Although this condition

was found to occur on aircraft with wing-mounted propellers, such as the

Lockheed Electra, a complete understanding of the phenomenon and a validated

analysis capable of assessing the tilt rotor configuration did not exist in the late

1960s. Therefore, the rotor/pylon/wing aeroelastic instability12 subsequently

became the focus of analytical and experimental work initially at the NASA

Langley Research Center13 and then at NASA Ames.

A basic understanding of the physical phenomenon that causes the airplane mode

aeroelastic instability problem was developed by Earl Hall14 of the Bell

Helicopter Company in 1966. By 1968, this insight was applied by Troy Gaffey,

a Bell dynamicists (and later, Bell’s vice president for engineering) who developed an effective solution to provide the required high-speed airplane-mode

rotor/pylon/wing stability for the tiltrotor aircraft.15 His solution involved the use

of a hinged, or “gimbaled,” rotor hub design with a pitch change mechanism that

increased blade flapping when out-of-plane motion occurred. The increase of

blade flapping with out-of-plane motion is counterintuitive, which made the stability problem extremely difficult to solve. This pitch-flap coupling, called -δ3,

combined with a high wing stiffness and a reduced rotor-hub to wing torsional

axis distance, was predicted to provide stability up to and beyond the desired airspeeds. Small-scale wind tunnel test data cited in Gaffey’s paper demonstrated

Building the Technology Base

20

12 In this airplane-mode flight condition, when the rotor encounters small inflow variations,

large differences in the distribution of loads are generated at a sufficient distance from the

rotor hub to cause the pylon (or nacelle) structure and wing structure to bend or twist. These

dynamic deformations can build up to the point where the structure’s elastic, or spring, characteristics moves its shape back toward the original, undeformed configuration. The inertial

forces, however, continue the “unwinding” past the original shape and a repetitive cycle of

aeroelastic oscillation is initiated. Under certain conditions, once initiated, the deformations

continue to grow ultimately leading to a catastrophic structural failure. 

13 When the Army initiated the Composite Aircraft Program (CAP) in 1965, with the objective of

developing the technology for an aircraft that combines the characteristics of an airplane and a

helicopter, the Bell Helicopter Company proposed a tilt rotor design (the Bell Model 266) based

on their experience with the XV-3. Following the termination of the CAP in 1967, a 0.133-scale

semispan dynamic and aeroelastic wind tunnel model, representing the Model 266, was given to

the Langley Research Center (LaRC) by the Army. The availability of this model and NASA

Langley’s interest in exploring the aeroelastic characteristics of the tilt rotor aircraft led to a joint

NASA/Bell experimental investigation in the Langley Transonic Dynamics Tunnel (TDT). A

series of other tests in this facility, establishing an important aeroelastics/dynamics/loads data

base, continued into the early 1970s (using other Bell models and a Grumman Aircraft model).

See “A Historic Overview of Tiltrotor Aeroelastic Research at Langley Research Center” by

Raymond G. Kvaternik (NASA Technical Memorandum 107578, April 1992).

14 Earl Hall Jr., “Prop-Rotor Stability at High Advance Ratios.” Journal of the American

Helicopter Society, vol. 11, no. 2, April 1966. 15 Troy M. Gaffey, The Effect of Positive Pitch-Flap Coupling (Negative d3) on Rotor Blade

Motion Stability and Flapping. Paper No. 227, Presented at the 24th Annual Forum of the

AHS, Washington, D.C., May 8–10, 1968.
that satisfactory high-speed aeroelastic

stability was achievable.

Meanwhile, the Boeing Vertol

Company of Morton, Pennsylvania,

was also actively pursuing the development of VTOL aircraft technology. In

1956, they built a tilt wing research

aircraft, the Vertol Model 76, later designated the VZ-2 (figure 24). Although

the major focus at Vertol throughout

the 1960s remained on the higher disc

loading tilt wing vehicle, evaluations of

variants included lower disc loading tilt

wing aircraft, and the low disc loading

tilt rotor for certain applications. 

By 1967, preliminary designs for

transport-size tilt rotor aircraft had

been developed (Vertol had been producing at that time the heavy payload CH-46 and CH-47 helicopters) and a concentrated effort at Vertol to develop and validate methodology for all relevant

VTOL technologies had begun. The leading advocates for this work were

Kenneth B. (Pip) Gilmore, V/STOL Technology Manager, and David (Dave)

Richardson, Chief of Preliminary Design. To support these efforts, during the

mid-1960s, Boeing Vertol recruited engineers with technical expertise in the key

areas and toward the end of the decade had established a fully staffed Research

and Development organization devoted to the development of VTOL aircraft

technology. Appendix B presents the key technical personnel involved in these

activities at Boeing Vertol during the late 1960s and the early 1970s.

The Boeing Vertol Company’s technical approach to tilt rotor aeroelastic stability employed a hingeless rotor hub (i.e. with no blade flapping or lead-lag

hinges and no rotor-flapping gimbal) and structurally tailored blades. With

the appropriate wing stiffness, -δ3, and the short-coupled hub/wing distance,

wind tunnel tests would later show that this design approach allowed high

speed airplane mode flight free of aeroelastic instability. While Boeing’s

rotor would contain fewer parts and would provide higher helicopter mode

pitch and yaw control moments than the gimbaled rotor approach resulting in

increased aircraft control responses, it produced higher blade, hub, and main

transmission-component loads which could impose weight or life penalties

on these structures.

Nevertheless, both the Bell and Boeing technical approaches offered some desirable attributes and Government-funded analytical and experimental investigations were continued to compliment work being done by both companies.

21

Figure 24.

Boeing VZ-2 tilt wing

research aircraft.

(Boeing-Ames Photograph

AD98-0209-15)
Meanwhile, during the early

1970’s, Dr. Wayne Johnson at

Ames developed a comprehensive code16 that would evolve

into the accepted standard for

rotor dynamics and stability

analysis. This code would

prove to be an important tool

used by Ames and industry

engineers to predict the aeroelastic stability margins of

safety in later wind tunnel and

flight test programs. In the

same timeframe, a number of

small-scale wind tunnel tests

were conducted (largely by

LaRC and industry) to produce

the empirical databases for

validating the analyses being

developed. However, the

small-scale model tests did not

accurately represent the fullscale aircraft with respect to both the structural and the aerodynamic characteristics. Since the small-scale effects of these factors required analytical corrections to represent full-scale hardware, a large model test was deemed necessary.

Therefore, in 1969 a contract was awarded to the Bell Helicopter Company for

the Ames 40- by 80-foot wind tunnel tests of Bell’s 25-foot diameter proprotor,17 figure 25. This test was jointly sponsored by NASA, the Army, and the Air

Force. While wind tunnel speed limitations prevented operation at the actual

design maximum airspeed of the tilt rotor aircraft, the high speed operating condition was simulated by using a reduced-stiffness wing and by running the 25-

foot diameter Bell Model 300 rotor at reduced rotational speeds. The test results

confirmed the predicted stability margins and trends within the required accuracy

level, and provided the needed confidence in the ability to adequately predict

these critical tilt rotor aircraft characteristics. 

22

16 Between 1970 and 1974 independent work on analytical methodology to predict rotor motions,

forces, and stability was also conducted by Dr. Raymond G. Kvaternik at LaRC (reported in

“Studies in Tilt Rotor VTOL Aircraft Aeroelasticity,” Vol. 1, NASA TM-X-69497, June 1, 1973,

and Vol. 2, NASA TM-X-69496, June 1, 1973). The initial work by Dr. Wayne Johnson includes:

“Dynamics of Tilting Proprotor Aircraft in Cruise Flight,” NASA TN D-7677, May 1974, and

“Analytical Model for Tilting Proprotors Aircraft Dynamics, Including Blade Torsion and Coupled

Bending Modes and Conversion Mode Operation,” NASA TM X-62369, August 1974.

17 This proprotor was developed with IR&D funding for a small corporate tilt rotor aircraft called

the Bell Model 300. This configuration benefited from numerous in-house and Government sponsored preliminary design studies led by Bell’s John A. (Jack) DeTore and Kenneth (Ken) Sambell.

Figure 25.

Bell 25-ft. diameter proprotor on semi-span wing in the

Ames Research Center 40-

by 80-ft. wind tunnel. 

Left: David Koenig, Ames.

Right: Kip Edenborough,

Bell Helicopter Co. 

(Ames Photograph 

AC70-3476)
The Boeing technical approach

was also evaluated for dynamic

stability in the Ames 40- by 80-

foot wind tunnel. In August

1972, under Army funding,

Boeing conducted dynamics

tests of its 26-foot diameter

proprotor with the hingeless,

soft-in-plane hub on the same

semispan wing and rotor

nacelle used for the Bell fullscale aeroelastic stability test

(figure 26). Performance tests

of that proprotor in the 40- by

80-foot wind tunnel were completed in December 1972.

Performance and Control

In a related effort, a folding version of the Bell 25-foot diameter rotor (figure 27) was tested

in the Ames 40- by 80-foot wind tunnel in February 1972. The stop/fold tilt rotor

eliminated the rotor/pylon/wing aeroelastic instability by stopping the rotor while

in the airplane configuration. The aerodynamic drag of the stopped rotor blades

was then reduced by folding them back along the nacelle while a convertible

engine was used to produce the jet thrust required for airplane-mode flight up to

higher speeds than would be attainable with a rotor as the thrust-producer. This

23

Figure 26.

Boeing 26-ft. diameter proprotor on semi-span wing in

the Ames Research Center

40- by 80-ft. wind tunnel.

(Ames Photograph

AC72-5255)

Figure 27.

The Bell stop/fold tilt rotor 

in the Ames Research Center

40- by 80-ft. wind tunnel.

(Ames Photograph

AC85-0247-02)
test, also conducted with Bell

Helicopter as the hardware and technical support contractor (jointly funded

by the NASA, the Army, and the Air

Force), demonstrated the feasibility of

the airplane-mode rotor stopping and

blade folding, and of the blade deployment and spin-up process.18 The

stop/fold tilt rotor, however, had the

additional penalties of the increased

complexity and increased weight of the

stop/fold mechanism, and, with the lack

of a developed convertible engine, it

was put aside as a potentially feasible 

concept that would require further

advancements to be an effective 

contender. 

Another major deficiency revealed by

the XV-3 was the poor propulsive efficiency of the rotor (frequently referred to as a

“proprotor” when used on a tilt rotor aircraft) in the airplane (or cruise) mode as

well as poor performance in hover. The tilt rotor design philosophy that evolved

during this period was that the proprotor should meet stringent performance requirements in the hover and airplane modes of flight but should not be significantly compromised to meet helicopter-mode (edgewise flight) design conditions. This meant

that the proprotor blades could be designed with considerable twist, similar to that

of airplane propeller blades, instead of the moderate twist of helicopter rotor blades

(to accommodate the edgewise operation). While the opportunity to use twist more

freely as a design variable could improve performance, the significant differences in

blade loading (both in distribution and level) and in the distribution of air inflow to

the proprotor between the hover- and airplane-mode conditions provided a challenging problem for the design engineers. Furthermore, the large diameter (low disc

loading) proprotor which allowed the tilt rotor aircraft to hover at helicopter-like

low levels of horsepower, results in a proprotor that is much larger than is required

for maximum efficiency in the airplane mode. A search of prior experimental

reports for applicable airplane mode test results showed that insufficient empirical

data existed at this unusually light airplane-mode loading. NASA Ames and the

Army AMRDL, therefore, sponsored and conducted several analytical and test

activities to investigate both the hover performance level and airplane mode efficiency achievable with a properly designed proprotor.

In 1968, Boeing Vertol was awarded a contract by Ames to investigate the

effect of blade twist on the performance of model-scale proprotors. Under

24

18 Anon., “Large Scale Wind Tunnel Investigation of a Folding Tilt Rotor,” NASA CR 114464,

Bell Helicopter Co., May 1972.

Figure 28.

Performance tests of 5-ft.

diameter proprotor in the

Army 7- by 10-ft. wind tunnel at the Ames Research

Center. (Ames Photograph

AC98-0209-4)
this and an additional contract, Boeing conducted analytical design studies

and performance predictions for a range of tilt rotor hover and cruise operating conditions. A series of 5-foot diameter proprotors was tested in the Army

7- by 10-foot wind tunnel at Ames (figure 28). Also, to investigate the effect

of model scale on measured performance, 13-foot diameter proprotors of the

same blade configurations were fabricated. Between 1969 and 1973, these

proprotors (as well as others having additional twist configurations) were

tested in the ONERA (Office National d’Etudes et de Recherches

Aerospatiales) 8-meter (26 feet) diameter S-1 wind tunnel in ModaneAvrieux, France (figure 29), the Ames 40- by 80-foot wind tunnel (figure 30),

and at the Air Force Aero Propulsion Laboratory, Ohio. Test operations covered a range of axial-flow flight conditions including hover-mode and airplane-mode flight from slow speeds up to a high-speed flight Mach number

of 0.85. These experimental investigations also examined the changes in

blade twist due to the aerodynamic and rotational loads and the effect of this

“live twist” on cruise performance. The resulting data19 enabled the validation

of analytical proprotor performance codes by Government and industry engineers.

25

19 A summary of the results of these tests is provided in “A Summary of Wind Tunnel Research

on Tilt Rotors from Hover to Cruise Flight” by W. L. Cook and P. Poisson-Quinton, presented at

the AGARD- Fluid Dynamics Panel Specialists’ Meeting on the Aerodynamics of Rotary Wings,

Marseille, France, September 13-15, 1972.

Left:

Figure 29.

13-ft. diameter proprotor in

the ONERA S-1 wind tunnel,

France. (Ames Photograph

A98-0905-5)

Right:

Figure 30.

13-ft. diameter proprotor in

the Ames Research Center

40- by 80-ft. wind tunnel.

(Ames Photograph

ACD-98-0209-11)
For large-scale performance characteristics, the Bell

25-foot diameter proprotor was tested in the Ames

40- by 80-foot wind tunnel in November 1970 (figure

31) as part of an earlier contracted effort. Ames also

contracted with Bell and made arrangements with the

Air Force Aero Propulsion Laboratory (AFAPL) for

the March 1973 proprotor hover performance test at

Wright-Patterson Air Force Base.

While the fundamentals of tilt rotor aeromechanics

were being explored, another group of researchers

and engineers were investigating the flying qualities,

crew station, and control law aspects of this class of

VTOL aircraft. Model-scale wind tunnel tests, analytical modeling, and piloted simulations were used to

address these issues. 

A series of tests was conducted with a 1/5- scale powered aeroelastic model of the Bell Model 300 tilt rotor

aircraft design under an Ames contract. Hover tests

conducted in September, October, and December of

1972 with this model examined the performance and

dynamic characteristics for operations near the ground.

It was discovered that, in the helicopter mode, the downward flow from the rotors

impinging on the ground produced a strong upward-moving flow below the aircraft’s longitudinal axis. This upwash, known as the “fountain,” impacts the lower

surface of the fuselage with increasing strength as the aircraft descends to the

ground. Because this fountain is somewhat unsteady, the major portion of this air

mass is seen to skip from one side of the fuselage to the other (particularly on round

cross-section fuselages), causing this fountain-flow to impinge, alternately, on the

lower surface of the right or left wing. This condition can contribute to the lateral

darting observed during the XV-3 flight tests and lead to a considerably high pilot

workload during the landing operation. Also, the occurrence of the unsymmetrical

aerodynamic loading on the wing surfaces produces a rolling moment that increases

in magnitude, i.e. is statically destabilizing, as the aircraft descends toward the

ground.20 Recognition of these phenomena contributed to the development of

improved stability augmentation control algorithms for future tilt rotor aircraft. 

Subsequent wind tunnel tests, conducted in the Vought Aeronautics low speed

wind tunnel, Texas, from January through March 1973, documented the performance, static stability in yaw and pitch, and determined trimmed control positions

in all flight configurations. These data were critical for the flight dynamics ana26

Figure 31.

Bell 25-ft. diameter proprotor

performance test in the Ames

Research Center 40- by 80-ft.

wind tunnel. 

(Ames Photograph

AC70-5390)

20 R. L. Marr, K. W. Sambell, G. T. Neal, “Hover, Low Speed and Conversion Tests of a Tilt

Rotor Aeroelastic Model.” V/STOL Tilt Rotor Study, vol. VI, Bell Helicopter Co., NASA 

CR-114615, May 1973.
lytical models that were being developed in order to validate control systems

designed to meet the handling qualities requirements throughout the flight envelope. The tests also included flow surveys which revealed the presence of rotor

tip vortices in the vicinity of the tail surfaces. These vortices could influence the

effectiveness of the tail surfaces and produce oscillatory loads and disturbing

vibrations. 

Aircraft Design and Simulation

With the tilt rotor technology efforts producing positive results, the managers of

the joint AMRDL and NASA Ames activities could now justify the initiation of

the next step, the development of a new tilt rotor proof-of-concept aircraft. As

part of this plan, in August 1971 Ames awarded contracts to Boeing Vertol and

Bell to conduct preliminary tilt rotor aircraft design studies. These efforts defined

the characteristics and performance of a first generation military or commercial

tilt rotor aircraft using a hingeless (Boeing Vertol) or gimbaled (Bell) rotor system, provided a preliminary design for a minimum size “proof-of-concept” aircraft, developed a total program plan and cost estimates for the proof-of-concept

aircraft program, and developed a wind tunnel investigation plan for the aircraft. 

In January 1972, with Air Force funding, Ames extended an existing Boeing contract to produce a preliminary design on an advanced composite wing and to

define a gust and blade load alleviation feedback control system for the tilt rotor

aircraft. This study addressed the concern that the low-disc-loading proprotor

may experience significant thrust, torque, and blade load excursions due to a

high sensitivity to gusts and turbulence. 

Work under the Boeing and Bell contracts also included the development of a mathematical model for simulation and for participation by each contractor in a piloted

flight simulation investigation. These models allowed the test pilots to evaluate the

workload and the handling qualities of the basic aircraft, both without automatic control-enhancing systems and with various control configurations, employing Stability

and Control Augmentation System (SCAS) control-enhancing algorithms. The simulation also enabled the pilots to evaluate the thrust/power management characteristics, the Force-Feel System (FFS), and failure mode design philosophy and aircraft

behavior. The math models were developed not only as an evaluation tool for a particular aircraft control system design, but also as a device for the development of

improved generic tilt rotor control law and crew station configuration. Initial piloted

simulations were conducted in the Ames Flight Simulator for Advanced Aircraft

(FSAA) in November and December of 1973. The math model created by P. B.

Harendra and M. J. Joglekar of Bell during this period for the tilt rotor design selected for the flight program, through extensive development and refinement by Roger

Marr and Sam Ferguson, became the basis for the generic tilt rotor math model used

to evaluate various tilt rotor aircraft designs and related air traffic management issues

in the Ames Vertical Motion Simulator in the late 1990s.

27
Ames

In 1971, the Aeronautics and Flight Mechanics Directorate at NASA Ames, led

by Dr. Leonard Roberts, established the V/STOL Projects Office, headed by

Woody Cook, for the development and flight investigation of powered lift

V/STOL (Vertical or Short Takeoff and Landing) aircraft. Woody’s deputy and

manager of the Advanced VTOL Projects Office at that time was Wally Deckert

who, as an Air Force lieutenant, was the flight test engineer for the XV-3 evaluation conducted at Edwards Air Force Base in 1959. Deckert coauthored the XV-3

Flight Test Report with test pilot Major Robert Ferry. During the early 1970s,

flight research at Ames was being conducted with the Rotating Cylinder Flap

Aircraft (a modified North American YOV-10A), the Ryan XV-5B Fan-in-Wing

VTOL Aircraft, and the X-14B Jet-Lift VTOL Aircraft. Also, the Augmentor

Wing Aircraft (a modified deHavilland Buffalo) was under development for

STOL research. Since the tilt rotor presented technical issues or embodied technologies not found in these powered lift systems, NASA Ames and the Army

AMRDL set out to acquire both new employees as well as current Government

employees to staff the V/STOL Projects Office with personnel having the technical and managerial skills necessary to develop a new-technology rotorcraft.

David D. Few was selected to lead the Tilt Rotor Research Aircraft (TRRA)

Project Office on May 31, 1972. With a long background in experimental flight

testing, including the supersonic, rocket-powered X-15 project at NASA’s

Dryden Flight Research Center (DFRC), he had recently managed the development of the Augmentor Wing aircraft. Dean C. Borgman, of the AMRDL, was

appointed deputy project manager (technical), based on his demonstrated technical competence and leadership qualities. In later years, Borgman served as president of the McDonnell Douglas Helicopter Systems Division and then as president and chief operating officer of United Technologies’ Sikorsky Aircraft

Corporation. The Army and NASA personnel associated with the TRRA and

related supporting technology activities are identified in Appendix B.

U.S. Army LTC Daniel (Dan) Dugan, attached to the AMRDL, was designated

as the project pilot for the TRRA on December 18, 1972. While not assigned to

the TRRA Project Office, Dugan was responsible for providing technical guidance in areas related to flight management, flight safety, and crew station design.

A number of project management changes were made both at Ames and at Bell

during the course of the project. In 1974, Dean Borgman, Deputy Project

Manager, (Technical) left the NASA/Army TRRA Project Office and was

replaced by Army LTC James H. (Jim) Brown in September 1975, thereby maintaining the joint Army/NASA lead management positions. Wally Deckert was

appointed as chief of the V/STOL Aircraft Technology Division in 1977, and

Dave Few, formerly the TRRA project manager, was promoted to the position of

deputy division chief. At that time, LTC Jim Brown took over as the TRRA project office manager, with Mike Carness serving as deputy manager. When the

Tilt Rotor Research 

Aircraft Project Office

28
Helicopter Technology Division was

established at Ames in 1979, Kip

Edenborough, chief assistant, became

TRRA project engineer (deputy project manager, technical). 

Shortly after that, John P. Magee, who

had been a Principal Investigator in

numerous tilt rotor studies and experiments while at Boeing Vertol (including

the August 1972 tests of the 26-ft. diameter proprotor in the Ames 40- by 80-ft.

wind tunnel) before joining the

Government Project Office, became

chief engineer (deputy project manager,

technical) of the Tilt Rotor Aircraft

Office. Following LTC Jim Brown’s

retirement, Dave Few again became

project manager while remaining as deputy division chief until John Magee was

named as the project manager in October 1980, and LTC Clifford (Cliff) McKiethan,

who had served as deputy manager, Army liaison since 1978, became deputy project

manager, Army liaison. John Magee joined BHTI (Bell Helicopter Textron,

Incorporated) in 1984 where he became the manager of the Bell Eagle Eye

Unmanned Aerial Vehicle (UAV) project, and later the engineering director of the

609 program (Bell’s six- to nine-passenger executive tilt rotor aircraft). During the

last few years of the XV-15’s operation at Ames, the flight research was conducted

under the Rotorcraft Flight Investigations Branch led by William (Bill) Snyder.

In 1989, some of the initial TRRA Project Office staff and management who

were still at Ames gathered in front of the XV-15 for a group picture (figure 32).

Bell

Sadly, the person most responsible for promoting the development of tilt rotor

technology would not live to see the tilt rotor research aircraft project. Bob

Lichten, Bell’s director of advanced engineering and chief engineer for the XV-3

project, died on September 18, 1971, following an automobile accident. Through

his steadfast confidence in the ultimate success of the new technology, he provided the inspiration and kindled the dedication to the tilt rotor aircraft to Bell’s

management and research engineering staff that continues to this day.

Dick Stansbury, who survived the crash of the XV-3, became Bell’s IR&D manager and continued to advocate for the development of tilt rotor technology with

company funds. He also contributed to the development of tilt rotor crew station

configuration and flight controls. Many of his initiatives were encompassed in

Bell’s tilt rotor aircraft designs.

29

Figure 32.

Members of initial Tilt Rotor

Research Aircraft Project

Office at Ames, 1989.

(N=NASA, A=Army)

Left to right, front row,

Mike Bondi (N),

Dan Dugan (A),

Shorty Schroers (A),

Wally Deckert (N),

Marty Maisel (A),

Violet Lamica (N),

Robbie Robinson (N),

Demo Giulianetti (N),

Dave Chappell (A),

Duane Allen (N). 

Back row:

Jerry Bree (N),

Gary Churchill (A),

Dave Few (N),

Jerry Barrack (N),

Kip Edenborough (N),

Jim Lane (N),

Mike Carness (N). 

Not shown:

Dean Borgman (A),

Al Gaehler (N),

John Hemiup (N),

Jim Weiberg (N),

Jim Diehl (N).

(Ames Photograph

AC789-0048-13)
During the late 1960s and early 1970s, Bell’s Stanley (Stan) Martin (chief of

advanced design) and Richard (Dick) Spivey (manager of applications engineering) actively promoted the continuation of tilt rotor aircraft research and development to NASA and to the military services research organizations. This effort,

coupled with the progress made in related analytical and experimental areas,

helped to keep the tilt rotor alive during that period as a contender for future

Government-funded development programs.

With the loss of Bob Lichten, Ken Wernicke became the lead design engineer for

tilt rotor aircraft at Bell. When the RFP for the design of the tilt rotor research

aircraft was released by NASA, Bell Vice President for Program Management

Charles (Chuck) Rudning assigned Henry (Hank) Smyth as proposal manager

and Tommy H. Thomason as his deputy. Ken Wernicke was the chief engineer

during the proposal phase.

After contract award for the TRRA project, the Bell management team consisted of Hank Smyth, Jr. (program manager) and Tommy Thomason (deputy program manager). Troy Gaffey was the chief technical engineer for the project

from 1972 to 1975. In 1975, Hank Smyth was assigned to a major Bell international program and Tommy Thomason took over the top position. His new

deputy was Lovette R. Coulter. From 1974 until 1981, Mike Kimbell served as

the engineering administrator for the Bell Project Office. Thomason left the

project in 1981 to lead the new JVX military transport aircraft project (later

called the V-22 Osprey), and Lovette Coulter was appointed as program manager. When Coulter became deputy V-22 program manager in 1984, Ron Reber

was assigned as XV-15 program manager. In 1999, after serving in senior management posts at Bell and Rolls Royce Allison, Thomason became vice president of civil programs at Sikorsky Aircraft Corporation under President Dean

Borgman. In 1994, the XV-15 test activity at Bell was placed under the technical direction of Colby Nicks.

Getting Started

Initial activities of the Project Office at Ames focused on the previously

described Government-sponsored contractual efforts as well as several in-house

activities devoted to tilt rotor technology data base development and validation.

With increasing confidence in the ability to design a tilt rotor aircraft free of the

problems and limitations encountered with the XV-3, a new agreement for the

joint development and operation of tilt rotor proof-of-concept research vehicles

at the Ames Research Center was signed on November 1, 1971, by Robert L.

Johnson, Assistant Secretary of the Army, R&D, and Roy P. Jackson, NASA

Associate Administrator for Advanced Research and Development. This document would be the cornerstone in the development of the proof-of-concept tilt

rotor research aircraft project that was about to emerge and it came about

through the hard work and dedication of many Army and NASA managers. 

30
As the leader of the V/STOL Project

Office, Woody Cook recognized that

the tilt rotor would have a niche for

military and civil applications between

the helicopter (with good hover efficiency, low speed, and short range) and

higher disc loading concepts such as

the Harrier jet lift VTOL aircraft (with

poor hover performance, high speed

and longer range). With the critical

analytical tools for this concept being

honed and validated by the on-going

industry and Government work, he began to advocate the development of the

proof-of-concept aircraft to management at Ames and NASA Headquarters. 

Woody’s colleague on the Army side was Paul F. Yaggy, the director of the Army

Air Mobility Research and Development Laboratory (AMRDL). Yaggy provided

a high level of support for the development of tilt rotor technology by co-funding

the research and by sharing in the staffing requirements. While Woody promoted

the TRRA project to NASA management, Paul advocated the activity to his command organization, the U.S. Army Materiel Command. 

Dr. Irving C. Statler was appointed as the director of the Ames Directorate, U.S.

Army AMRDL in September 1974 and became an enthusiastic and effective supporter of the tilt rotor research aircraft project. In 1975 the tilt rotor project

acquired another important advocate when Dr. Richard (Dick) Carlson became

the Director of the Army AMRDL.

Project Advocacy

By late 1972, the Director of Ames Research Center, Dr. Hans Mark, recognized

that the technical “homework” had been done and done well, and that the tilt

rotor aircraft was a unique utility that could well serve the civil and military user.

Dr. Mark, therefore, strongly advocated continuing development of the tilt rotor

aircraft and carried this position to NASA Headquarters. During this time,

Langley Research Center, in a NASA/Army activity similar to the joint effort at

Ames, had been investigating the rotor systems research aircraft (RSRA).21 This

aircraft was a compound helicopter with a changeable configuration that was

flown with and without wings and auxiliary turbofan jet engines. Figure 33

shows the RSRA in flight with the rotors, the wings, and the turbofan engines

installed. It was also flown as a fixed-wing turbofan aircraft with the rotor

removed. The use of the additional lift and propulsion devices would enable

flight research to be conducted on the rotor system and airframe over a broad

31

21 C. White, Jr., G. W. Condon, “Flight Research Capabilities of the NASA/Army Rotor Systems

Research Aircraft,” NASA TM-78522, September 1, 1978.

Figure 33.

Rotor Systems Research

Aircraft (RSRA). 

(Ames Photograph

AC82-0089-17)
range of loading conditions and up to and beyond the high speed capability of

current helicopters. The leveraging of the equal sharing of both Army and NASA

financial and human resources made work on both projects at their respective

centers feasible. These two major NASA/Army efforts, the TRRA at Ames (West

Coast) and the RSRA at Langley (East Coast), satisfied the competitive interests

of both centers in working on the leading edge of rotorcraft technology. Also, the

tilt rotor project was consistent with NASA’s charter of maintaining world leadership in civil aeronautical vehicle technology and of advancing military aeronautical capabilities. The tilt rotor project was therefore an appropriate activity

for Ames. 

At NASA Headquarters, convinced of the validity of the approach taken by the 

tilt rotor advocates at Ames, C. W. (Bill) Harper, Director of the Aeronautics

Division, A. J. Evans, Director of Military Programs, and M. Adams, Associate

Administrator of OART (Office of Aeronautical Research and Technology) promoted the tilt rotor project to NASA Administrator Dr. James C. Fletcher and

Deputy Administrator Dr. George M. Low. In addition, the long association of

the former NACA aeronautics cadre at NASA Headquarters with their Army

counterparts in rotary wing research provided an important ingredient in the

advocacy of the tilt rotor project. It was decided that if Army support could be

obtained, approval would be granted for the project. 

Meanwhile, obtaining Army funding for this project was a formidable task. The

Army’s assistant chief of staff for force development, Lieutenant General Bob

Williams, who set policy for aviation research, had openly stated after the XV-3

activity was completed that he would not support the development of the tilt

rotor aircraft. With the favorable results of the technology activities in hand,

Dave Sharpe and Dean Borgman of the AMRDL prepared a briefing advocating

a joint Army/NASA tilt rotor research aircraft project. This briefing was then

presented to Lieutenant General Williams at the Pentagon by Paul Yaggy, who

was soon to receive a surprising response. Shortly after that meeting, Lieutenant

General Williams issued a letter stating that he was reversing his prior opposition

to tilt rotor research and requested his staff provide full support to the tilt rotor

effort. Funding for the Army’s participation in the tilt rotor research aircraft project was subsequently made available and additional funds were programmed into

succeeding year Army budget plans. This decision was the final gate which led

to the November 1971 Army/NASA agreement, cited earlier, for the joint development and operation of tilt rotor research aircraft.

Even with the groundwork established, NASA Headquarters required additional

documentation and planning prior to final approval of a new project of the complexity, risk, and magnitude of cost for the proposed tilt rotor effort. This documentation consisted of 20 items including a Project Development Plan, a Risk

Assessment, an Environmental Impact Statement, a Safety Plan, a Reliability and

Quality Assurance (R&QA) Plan, and a Procurement Plan.

32
Initial planning presented an activity with four elements. The first element consisted of establishing the technology base (essentially done by this time). The

next focused on program formulation and the third element was a competitive

design and proposal activity. In the fourth element, one contractor would fabricate and test two research aircraft. 

Project Plan Development

The TRRA Project Office began to prepare several of these key documents 

toward the end of 1971. The initial version of the NASA/Army Project Plan for

development of V/STOL tilt rotor research aircraft was released in April 1972. This

document described the technical objectives of the project and defined the program

elements, the management plan, the Government funding, facilities and manpower

requirements, and the schedules. While the term “dual use” had not yet come into

vogue, it was clear that the tilt rotor project would have to meet both civil and military needs to garner the necessary support. This dual use capability was highlighted

in the Project Plan with the first illustration to appear in the document (figure 34).

As the project took shape and underwent management reviews, the Project Plan

would undergo two major revisions (once in April 1973 and again in September

1974) to change the scope and cost of the project, and to include, among other

things, the review and reporting plan and the system and flight safety plan. 

33

Figure 34.

Illustration from 1974 

Tilt Rotor Research Aircraft 

Project Plan.
The initial projected cost of $48 million to complete two aircraft was rejected

by Ames and AMRDL management as unacceptably high, leading to a reduction of scope to achieve a projected $40 million program cost. After several iterations, the TRRA Project Plan was approved by Bruce K. Holloway, NASA’s

Acting Associate Administrator for the Office of Aeronautics and Space

Technology, and Norman R. Augustine, assistant secretary of the Army for

research and development.

The System Safety Plan document identified the safety objectives of the TRRA

project. It described the approach the Government and contractor organizations

were required to take to meet the airworthiness goals and to implement the

required industrial plant safety, reliability and quality assurance, and the groundand flight-test safety programs. The plan also called for extensive subsystem, system, and operating hazard analyses. The principal safety objective of the TRRA

project is concisely stated as: “the completion of all project activities without personnel injury or loss of life and without significant property damage or loss.” To

accomplish this, the following design philosophy was defined for the research aircraft: “As a goal, a single failure in any system or component should not result in

loss of the mission. Also, a double failure should not result in the loss of life. The

rotor blades and associated drive components are recognized exceptions to this

and special attention and conservative design will be applied to these elements.”

Crew safety would be enhanced through the inclusion of a crashworthy crew station structure, the use of crashworthy (damage-resistant) fuel cells, a bird-proof

windshield, and the installation of zero-zero crew ejection seats. 

The technical objectives cited in the Project Plan for the two research aircraft programs were presented in two groupings and are worth noting here. The first set of

objectives addressed fundamental tilt rotor proof-of-concept tasks. These were to:

a) verify rotor/pylon/wing dynamic stability and aircraft performance over the

entire operational envelope;

b) conduct an initial assessment of handling qualities;

c) investigate tilt rotor aircraft gust sensitivity; and 

d) investigate rotor downwash and noise.

In addition, advanced flight research program goals were defined as the secondary objectives and were not part of the contracted effort. These are summarized

as follows:

a) evaluate gust and load alleviation systems;

b) perform thorough handling qualities evaluations and identify where additional V/STOL research in this area is required;

c) develop and investigate terminal area operational methods and procedures to

reduce congestion and noise and increase safety;

d) determine V/STOL navigation/guidance requirements and evaluate automatic

landing systems;

34
e) evaluate potential benefits of applying tilt rotor capabilities to Army 

missions;

f) provide data for development of design and operational criteria for potential

civil and military tilt rotor aircraft; and

g) investigate alternate or advanced rotor concepts or configuration 

modifications.

Another key Project Office effort in late 1971 and early 1972 was the preparation

of the Statement of Work that would lead to the research aircraft program. As

noted earlier, this portion of the work was to be accomplished in two phases.

Phase I would fund two parallel “competitive” preliminary aircraft design studies

and the development of a program plan for a minimum-size tilt rotor research

aircraft that could meet the proof-of-concept objectives. The research aircraft

would be required to produce technology information that would be applicable

to, or could be reasonably extrapolated to, first generation military or commercial tilt rotor aircraft defined in the initial part of these studies. The results of this

effort were used as an industry-generated basis for determining the requirements

of the research aircraft and for detailed planning for the Phase II aircraft development program, performed by one of the two Phase I contractors.22

Phase I - Studies

Following an open solicitation for bids, four proposals were received for the 

Phase I effort. Submittals from Grumman Aerospace, Sikorsky Aircraft, Boeing

Vertol, and Bell were evaluated. On October 20, 1972, Ames awarded two fixedprice contracts of $0.5M each to the selected bidders, Boeing Vertol and Bell.

Phase I generated analytical studies of performance, noise, stability and control,

structural loads, and dynamics. Design efforts performed under these contracts

addressed major structural and dynamic system components and assemblies, and

included subsystem integration. In addition, outlines of the maintenance and

inspection plans, tooling and fabrication plans, and component and integrated

test plans were prepared. A model specification for the experimental aircraft

project was also created. On January 22, 1973, as a product of the Phase I

efforts, proposals from Bell and Boeing Vertol for the aircraft fabrication and

testing activity were delivered to Ames Research Center. Each set of proposal

documents consisted of 12 volumes weighing about 30 pounds.

The Bell proposal, as expected, was based on the IR&D-developed Model 300,

now called the Model 301. It utilized the 25-foot diameter gimbaled proprotor

design that had been extensively wind tunnel tested at Ames. The engines,

mounted in wing-tip nacelles, tilted with the proprotors as a unit. Also, as a

result of earlier wind tunnel test results, the Model 301 now incorporated an “H”

35

22 A description of the management approach used for the TRRA project is provided in “A Guide

to Management of Experimental Shop Programs at Ames Research Center (“Experimental Shop”

Concept),” NASA TM X-62,427 by David D. Few, April 1975.
empennage configuration to improve the directional stability characteristics. 

Boeing provided a proposal based on the new Model 222. This design used the

Boeing-developed 26-foot diameter soft-in-plane hingeless proprotor on nacelles 

that tilted only the proprotors. The engines of the 222 were mounted in fixed

horizontal wing-tip nacelles. To minimize research aircraft development costs,

the Boeing 222 was to use the fuselage, landing gear, and empennage of the

Mitsubishi Mu-2J twin turboprop executive transport aircraft. The 222 wing

incorporated leading edge “umbrella” flaps and large deflection trailing edge

flaps to reduce download. An artist’s illustration of the Boeing 222 in flight is

shown in figure 35.

A Source Evaluation Board (SEB) was convened to evaluate the proposals. In

accordance with procurement regulations, the outcome of the competition was to

be determined by the Evaluation Criteria defined in the RFP. These consisted of

the Mission Suitability Factors which were scored, and “other factors” which

were evaluated but not scored. The Mission Suitability Factors were comprised

of Design, Implementation, and Management components. The other factors that

were rated but not scored were: cost, company past performance, financial capability, and the new-technology plan.

36

Figure 35.

Illustration of the Boeing

Model 222 tilt rotor aircraft.

(Boeing-Ames Photograph

AC86-0140-1)
To support the SEB in determining the attributes and deficiencies of the proposals, a number of technical and management specialist committees were established to provide written assessments to the Board. Most of the Project Office

staff was involved in the SEB activity in the capacity of either Board members,

committee members, or consultants. While both proposals were found to be competitive, the Bell proposal offered significant technical and cost risk reduction

based on their successful demonstration of a flight simulation on the FSAA, their

development and demonstration of a flightworthy rotor system, and their development of a main transmission gearbox. The findings of the SEB were presented

to NASA and Army top management on April 12, 1973. The next day, the Bell

Helicopter Company, Fort Worth, Texas, was selected for negotiations which

would lead to a contract for the design and fabrication of two tilt rotor aircraft. It

is interesting to note that this procurement, for the first time in the selection of a

flight research aircraft, used a piloted simulation “fly-off” as part of the evaluation and selection criteria.

These negotiations, initiated in late April 1973, engaged the Government and Bell

in debates over a series of difficult issues for three months. One of the most contentious areas was the Government’s requirement for either a cost ceiling or a

negative fee approach to motivate the contractor to control costs. After a meeting

between Bell President James F. Atkins and Ames Director Dr. Hans Mark in

June, the possible use of company funds to share the cost of an overrun was

accepted by Bell. With that important decision made, other issues such as cost

reduction items were soon resolved and a contract for the Phase II-A effort was

awarded on July 31, 1973. This was to be a 60-day planning level of effort (not to

exceed $0.2M). Following a Government assessment of the plans presented at the

end of that period, a “go-ahead” for the Phase II-B for the design, fabrication, and

test of two V/STOL tilt rotor research aircraft was given on September 30, 1973. 

Phase II - Program Formulation

The work was to be performed under cost-plus-incentive-fee (CPIF) contract.

The incentive fee was based on the ability to meet the target cost of $26.415M. If

the contract was completed at the target cost, the contractor would earn a 6 percent fee. The fee would be increased to 12 percent if the final cost fell to

$23.2M, and would be decreased to a negative fee of about -5.6 percent if the

cost grew to $32.4M. This arrangement resulted in the contractor and the

Government sharing equally in any overrun or underrun from the target cost.

During the Phase II-A period, the Government Project Office worked with the

contractor to refine the Model Specification. This document defined the performance goals as well as the operational and design features and the structural

design standards for the new aircraft. With this Model Specification as the guide,

the TRRA would become the first experimental aircraft to be developed “from

scratch” to meet Ames research requirements. The Model Specification became

part of the Phase II-B contract and was revised, when necessary, to reflect

37
changes that evolved during the

detailed design process. 

Preparations for the Model

Specification and program planning

were made in late summer of 1972

when Shorty Schroers (a Project

Office member) and two other engineers from the Ames Aeronautics and

Flight Systems Directorate staff conducted a fact-finding mission to establish possible future military tilt rotor

research and technology requirements.

Their visits included the Aviation

Systems Command, St. Louis,

Missouri, the Naval Air Development

Command, Johnsonville,

Pennsylvania, the Army Electronic

Command, Ft. Monmouth,

New Jersey, and the Ft. Eustis

Directorate of the Army AMRDL at

Ft. Eustis, Virginia. The trip report

addressed takeoff, transition and cruise

requirements, descent and approach

issues, and precision hover requirements for military applications. The

findings identified both desirable or

required characteristics and areas of

research that would be included in the

Model Specification and future flight

test program of the TRRA.

The Ames Tilt Rotor Research Aircraft

Project Office requested an experimental designation for the new aircraft from the Air Force office that assigned

designation numbers for Department of Defense experimental aircraft. The

TRRA was to carry the prefix XV (for experimental, vertical takeoff). The initial

response from the Air Force was XV-14. This was perceived to be a problem,

since Ames was still operating the X-14B VTOL aircraft and the similar designations might cause confusion. The designation was therefore changed to XV-15

and a proof-of-concept aircraft that would make aviation history was named. 

In the 1970s, the tail numbers of aircraft flown by NASA under “public law” contained three digits, the first digit indicating the research center, with Ames being

assigned the 700 series. At the start of the aircraft development program tail numbers N702NA and N703NA were designated for the tilt rotor research aircraft.

38

Figure 36.

1/5 scale XV-15 model in 7-

by 10-ft. wind tunnel. 

Top:

Small landing gear housings.

(Ames Photograph

AC98-0204-2 (6-A))

Bottom:

Large landing gear housings.

(Ames Photograph

AC98-0202-1)
Shortly after the start of the Phase II-A work, Bell began to focus on TRRA

design issues and identified two options for the main landing gear configuration.

One approach retracted the gear into the fuselage. The retracted gear would

occupy a substantial amount of cabin space, but required only a modest-sized

housing (landing gear pod) to enclose the mechanism in flight. Another arrangement utilized the main landing gear hardware developed for the Canadair CL-84

Tilt Wing VTOL Aircraft. To achieve the necessary distance between the outboard wheels, the landing gear would be mounted so that it would retract outboard of the fuselage contour, resulting in a clear cabin space but requiring a

larger landing gear pod. While this approach offered a lower development risk

and could be implemented at a lower cost than the internally retracting configuration, it would produce a higher drag that would reduce the maximum airspeed

in the airplane mode. To evaluate and compare the drag of the two pod configurations, an unpowered 1/5 scale force-model was tested in the AMRDL 7- by 10-

foot wind tunnel at Ames in August 1973. Figures 36a and 36b show a front

view of the model installed in the wind tunnel with the different landing gear

housings. This brief test showed that using the larger pods would result in a 5-7

knot reduction in maximum airspeed. This performance loss was considered

acceptable in light of the cost and risk reduction benefits from using an already

flight-qualified landing gear. Therefore, the existing CL-84 landing gear design

was selected for the XV-15.

With the work on the aircraft design underway, a Government Resident Project

Office headed by Jim Lane (from the Ames TRRA Project Office) was established at the main Bell engineering plant in Hurst, Texas. The major function of

this Office was to interact with the Bell staff to monitor technical progress and to

understand technical issues. Additionally, this Office tracked the level of effort

and the periodic labor hour and cost reports, served as representatives of the

Ames Project Office in certain activities that required Government observation,

coordinated with the Army Defense Contract Audit Agency (DCAA) in matters

related to Government inspection activities, and coordinated pertinent information with the TRRA staff and management. The Resident Office issued weekly

reports which documented the key events of each week and the technical and

administrative status of the project from the time it initiated operation in 1973

until it shut down after completion of the aircraft proof-of-concept flight test at

Bell, nearly a decade later.

The tilt rotor research aircraft project (called Level I) was to be managed by a

Work Breakdown Structure approach. This management tool23 divided the planned

activity into its major parts, or “Elements” (Level II). These Work Breakdown

Structure Elements (WBSE) were further divided to the level required for adequate project management and control. For the TRRA project, the WBSE’s were

the major hardware subsystems and the major focused activities to be worked on

39

23 James J. Diehl, “Application of a Cost/Performance Measurement System on a Research

Aircraft Project,” NASA TM 78498, June 1, 1978.
under the contract. To accomplish this, a Statement of Work was prepared so that

these work areas and activities were clearly identified. It was also critical that the

WBSE’s be consistent with the work categories in the contractor’s internal operating system, and this was verified during the Phase II-A period. The TRRA project

utilized 50 key work elements as shown in figure 37.

In the Phase II-A effort, the Government Project Office and Bell each assigned

lead individuals, or element managers, to each Level II element. This was intended to assure that direct communications would be maintained between the

Government and Bell in every key work area. Initial Government and Bell

assignments to major WBSE lead positions are presented in Appendix B.

40
41

Air Vehicle Test and Evaluation Data and

 Documentation

Spares Systems Project

Management Plan

Training

V/STOL Tilt Rotor

Research Aircraft Project

Component

Acceptance Test

Systems

Test

Propulsion

Test

Structural

Test

Egress System

Test

Integrated

Systems Test

Ground, Tiedown

Aircr aftTests

Component

Development Tests

Test and Evaluation

WindTunnel

Aircraft Tests

PostTest Aircraft

Refurbishment

Government

Flight Test

Contractor Flight

Tests Simulations

Data and Documentation

System Safety

and R&QA Plan

Instrumentation and Data

Acquistion Manual

Flight Operations Manual

Inspection and

Maintenance

Manual

Design Reports

and

Specifications

Test Plans,

Procedures, and

Test Reports

Air Vehicle

Fuselage,

Landing Gear,

Empennage

Fuselage

Landing

Gear

Empennage

Wing, Nacelle

Wing

Nacelle

Rotors

Blade Assembly

Hub

Assembly

and

Controls

Transmission,

Cross Shafting

Interconnect

System

Engine

Coupling

Gearbox

Left/Right

Transmission

Fuel System

Engine

Power Plant

Installation

Power Plant Hydraulic

System

Electrical

System

Environmental

Control System

Emergency

Egress System

Flight Control Research

Instrumentation

Pr imary

Flight

Control

Secondary

Flight

Control

Thrust/Power

Management

System

Automatic

Flight

Control

Communication,

Navigation, and Flight

Instrumentation

Crew Station

Support Equipment

and Systems Aircraft No. 1

Final Assembly

Final

Assembly,

A/C No. 1

Analytical

Integration

Aircraft No. 2

Final Assembly Mock-Up

WBSE Level

 I

 II

 III

IV

 III

IV

 III

IV

 III

 III

Figure 37. Tilt Rotor Research Aircraft Work Breakdown Structure Elements.
The primary task of designing the TRRA was placed clearly and directly in the

hands of the very capable Bell team. The Bell chief engineer for the tilt rotor

research aircraft contract activity was Ken Wernicke. George Carter led the

design effort. Bell operated their aircraft programs with a matrix approach

wherein specialists from the various Bell technology and design organizations

were assigned to the project as required to meet the technical needs and schedule. These individuals were identified as the Bell Element Managers referred to

earlier. A few examples of the engineering and design activities they and the

Army/NASA TRRA Project Office staff were involved with during the development of the XV-15 are reviewed here. Appendix A provides a description of the

design characteristics and features of the TRRA.

Engines

One of the early areas of focus in the development activity was the qualification

of the engines. Prior to the award of the TRRA contract, the NASA/Army

Project Office had determined that the Government furnished powerplant would

be a variant of the Lycoming T53-L-13B turboshaft engine. The factors leading

to this decision were that the T-53 was available in large numbers in the Army

inventory, it produced more than the required power to handle the projected

weight growth of the XV-15 above that of the Bell Model 300 (i.e. 1550 SHP vs.

1150 SHP of the Model 300 Pratt and Whitney PT-6 engine), and it had previously been operated in the vertical mode on the Canadair CL-84 tilt wing VTOL

aircraft. The selection of the T-53 eliminated the need to conduct an extensive

engine development program required to qualify a vertical-running PT-6, and

therefore reduced the associated costs and program risk. To prepare the T-53

engines for the TRRA, Lycoming modified oil sumps and seals, changed the first

stage turbine disc to provide overspeed capability, replaced the first stage gas

producer turbine blades to provide a two-minute contingency rating, removed the

nose gearbox to provide for direct drive, and conducted ground qualification test

runs in the vertical and horizontal modes at their Stratford, Connecticut, facility.

The modified engine was designated the LTC1K-4K. 

Three negative aspects of the use of the T-53 to replace the PT-6 were the need to

redesign the nacelle configuration and the transmission interface, the increase in

engine weight (and its spillover effect on aircraft-structure installation weight),

and its higher fuel consumption. Nevertheless, the modified T-53 was the

Government’s choice to power the TRRA and this choice would prove to be a

good one.

Transmission

The Bell Model 300 technology demonstrator main-gearbox was considered by

the Government to be in an advanced state of development at the time the TRRA

contract was awarded. This transmission included a new gear design developed

by Bell to reduce weight and cost. The new technology gears, of a “herringbone”

Aircraft Development

42
tooth configuration, were fabricated by

grinding two halves of the herringbone

as separate components, and then joining them using a recently developed

electron beam welding technology.

The manufacturing process required

extremely accurate alignment, and distortions due to welding were unacceptable. When Bell started fabricating the

gears for the XV-15 TRRA, the

process proved to be more difficult

than expected. These difficulties which

led to schedule slippage and cost

increases were eventually resolved and

the required parts were produced.

The use of the Lycoming engine imposed significant changes to the transmission

arrangement. As previously noted, the Bell Model 300 main transmission had

been designed based on the use of the PT-6 engine. Since the PT-6 output drive

shaft operated at 30,000 RPM (at hover power) and the LTC1K-4K output speed

was about 22,000 RPM, an “engine-coupling gearbox” (ECGB) was required if

the designed main transmission was to be used. This engine-coupling gearbox

would have the unusual function of increasing the RPM provided by the engine

output to match the higher input speed of the existing design for the main gearbox. The main transmission then reduced the RPM to the rotor speed. An illustration of the new nacelle arrangement, showing the location of the engine-coupling gearbox is provided in figure 38.

Fabrication problems surfaced during the development of the engine coupling

gearbox. The ECGB case and cover plate were made of magnesium furnished by

a specialized casting vendor. Initially, the complex parts yielded from the casting

process had a level of porosity, flaws and voids that were not acceptable.

Ultimately, satisfactory parts were produced after incurring a further 

cost increase and a schedule slip.

By mid-1976, the problems that continued to occur during developmental testing

of the transmissions became a serious concern to the Government Project Office.

In May, NASA and Army Headquarters management established an ad hoc

review committee to provide an assessment of the design adequacy, manufacturing procedures, and qualification testing for the TRRA transmissions. The committee was composed of Government and industry transmission experts and was

chaired by John Wheatly, a renowned NACA rotorcraft pioneer and former Army

rotorcraft scientist and consultant. The final report issued by this committee validated the design and manufacturing approach but recommended a pre-flight

qualification test of not less than 50 hours duration. 

43

Figure 38.

XV-15 nacelle arrangement.
44

Extensive qualification test operations

were then conducted on the Bell transmission test rig illustrated in figure 39.

This apparatus placed the transmission

elements in a continuous drive linkage

that simulated the engine-input and the

proprotor drive-output shafts flight

loads. The test apparatus drive system

was assembled so that a prescribed

torque was applied to the XV-15

TRRA transmission which was then

operated for a specified number of

hours at a selected RPM. During these

qualification tests a range of torque

levels and RPM’s was applied to the

left and right main transmissions, the

engine coupling gearboxes, and the

center gearbox. 

Over the next two years the qualification test program revealed problems that

required modification of gear designs, gear and shaft welding processes, bearing

designs, and lubrication and cooling arrangements.

The transmission ground tests also included an evaluation and calibration of the output torque sensing system which was to provide the input to the torque indicator on

the instrument panel. This sensing system consisted of concentric cylindrical shafts

affixed to each other at one end. The inside shaft transmitted the torque while the

outside shaft remained unloaded. The torque was measured by determining the magnitude of the deflection of the loaded (inside) shaft and comparing it to the undeflected, un-torqued (outside) shaft. This torque sensing device, however, did not provide

output data of sufficient accuracy for a primary flight instrument. After considerable

effort to correct the problem, Bell suggested a rather unusual approach. This was to

make an exception to a standing XV-15 TRRA Project Office and Bell policy and

allow the use of research instrumentation system data for primary flight instrument

data. The Project Office agreed and the transmission output torque indication in the

cockpit was now to be obtained from research instrumentation strain gages mounted

on the proprotor drive shaft (called the proprotor mast). The research instrumented

proprotor mast had a calibration resolution of two to three percent, sufficient for the

management of the aircraft. Despite concerns by Bell and Government engineers

about the reliability and durability of this instrumentation-based torque indication

system, it served the XV-15 well during many years of flight operations. 

Fuel Cells

During the formulation of the TRRA Program Plan, a prime focus of many

discussions among members of the Government Project Office was the need

Figure 39.

Bell test apparatus used for

transmission qualification

testing.

Hydraulic drive

Torque

converter

Torque

converter

Test stand

high-speed

gearbox

Test stand

angle box

Test stand

low-speed

gearbox

XV-15

Center gearbox

XV-15 Main

transmission

Proprotor

mast
to build “safety” into the design. In the 1960s, the Army and civil rotorcraft

operators were experiencing loss of life and property due to post-crash fires.

Studies that examined the statistics from these crashes showed that injuries

and fatalities were significantly reduced when rupture and tear resistant fuel

cells were installed. The fuel cells, basically flexible rubberized fabric bladders that held the fuel, were less likely to burst and release fuel upon impact

with the ground than rigid metal tanks or fuel-containing wing structures that

did not include the bladders. By the early 1970s, the use of fuel cells, in particular in Army helicopters, had dramatically reduced the incidence of postcrash, fuel-fed fires. 

The original Bell Model 300 design (predecessor to the XV-15) incorporated a

“wet wing,” which used the volume within the wing to hold the fuel. While

crashworthy fuel bladders would significantly increase the cost and weight of the

fuel system and would reduce the available fuel volume by about five percent,

the potential safety benefits were believed to be high enough to accept penalties,

and the fuel cells were made part of the XV-15 design.

Bell then contracted with Uniroyal Inc., of Mishawaka, Indiana, the manufacturers of fuel cells for Army helicopters and Air Force fixed-wing fighters, to

fabricate the cells for the XV-15 TRRA. With no background in the design of

fuel cells for a research aircraft, a method for the selection of the thickness of

the rubberized fabric (i.e. the number of the rubberized fabric laminates used

in the bladder material) had to be defined. Thinner fabric would be lighter and

easier to install in the wing (through small openings in the aft wing spar) but

it would be more susceptible to impact damage than the thicker-wall material.

To resolve the issue, a standard test was conducted at the Uniroyal facility on

December 3, 1974. Two test bladders, in the shape of cubes measuring three

feet on each side were fabricated, one with a light gage material and one with

a thick wall material. The bladders were filled with water and dropped from a

height of 65 feet onto a concrete surface. The lighter-gage material bladder

ruptured on impact, while the thicker-walled bladder material did not. This

not-so-scientific method, along with the previously qualified seam and fitting

designs and validation of acceptable tear and puncture material characteristics,

provided the basis for the qualification of the thicker-wall fuel cells for use in

the XV-15.

In addition to the fuel bladders intended to provide fuel containment in the event

of damage to the wing structure, the interconnecting fuel lines between adjoining

cells (there are two cells in each wing) were provided with breakaway fittings

which sealed in fuel when the lines were broken on impact.

The fuel system, like all other critical XV-15 TRRA systems, was designed with

adequate redundancies (such as dual fuel pumps with the capability to feed both

engines) so that a single failure would not result in the requirement to terminate

the flight. 

45
Flight Controls

One of the more difficult technical challenges in the development of the XV-15

TRRA was the design of the flight control system. The XV-3 had revealed various degrees of flying qualities, handling qualities, and pilot work load deficiencies in nearly all flight modes. It was the job of the engineers to address these

problems and produce a flight control system that could meet existing and pending handling qualities and stability and control requirements from military and

Federal Aviation Administration (FAA) standards. While normal operations

would be conducted by a crew of two, the XV-15 control system was designed to

permit a single pilot to perform all normal and emergency procedures from either

seat.

The controls effort was divided into four categories: Primary Flight Controls,

Secondary Flight Controls, Thrust/Power Management System, and Automatic

Flight Controls.

Because the tilt rotor aircraft combines the flight characteristics of a conventional helicopter and those of a fixed-wing airplane, its flight control system had to

blend the basic elements of these two

vehicle types. The flight deck of the

TRRA was configured so that each

pilot station had complete controls for

pitch, roll, yaw, and thrust in all

modes of flight. They consisted of

control sticks, rudder pedals with

brakes, and power levers (for proprotor collective pitch and engine throttle

functions). A single set of airplanetype throttles, rpm governor, flap, and

landing gear controls were located in

the center console.

In the helicopter mode, the controls

apply collective or cyclic blade pitch

changes to the rotors to produce control moments and forces. Fore and aft

cyclic pitch (produced by moving the

center control stick fore and aft) provides longitudinal control, and differential cyclic pitch (in response to rudder pedal motion) produces directional

control. Collective pitch commanded

by collective lever input is used for

vertical control, and differential collective pitch, resulting from center stick

46

Figure 40.

Proprotor response to 

cockpit control input.

Collective Lateral Cyclic

Longitudinal Cyclic Directional
lateral input, controls roll. Figure 40 illustrates the helicopter mode control functions and the resulting proprotor forces that control aircraft motion.

Conversion or reconversion can be made within a corridor having a wide range

of airspeeds, conversion angles, and fuselage attitudes. While the fixed-wing

control surfaces (ailerons, elevator, and rudder) remain active in all flight configurations, the rotor controls are automatically phased out in two “mixing boxes,”

as the nacelles are tilted toward the airplane configuration. This system is

designed so that the need for control inputs during conversion is minimized, the

primary requirement being a longitudinal input to maintain attitude as the large

mass of the nacelles is tilted. The phasing of the controls through conversion is

smooth and not apparent to the pilot and the process effortless. 

In the area of automatic flight controls a stability and control augmentation system (SCAS) was incorporated in the aircraft design. It consisted of actuators

which were connected to the longitudinal, lateral, and directional fixed control

linkages in the fuselage. The SCAS makes automatic control inputs with these

actuators to effect rate damping, control augmentation, and pitch and roll attitude

retention. SCAS actuator motions are in series with the pilot’s control inputs.

Force-feel system (FFS) actuators prevent SCAS actuator motions from feeding

back motions or forces into the control stick or pedals. These actuators are

installed in parallel to the longitudinal, lateral, and directional control linkages 

and are effective in all flight modes. The SCAS and FFS control laws (i.e. the

equations built into the automatic control system) are hard-wired on circuit cards

which can be changed to alter the control characteristics of the aircraft. This feature would later be used for tilt rotor aircraft flight controls research. 

Another unique and flight-critical element of the TRRA was the conversion system. This electro/hydraulic/mechanical system was designed by Bell with functional redundancies to provide fail-operate and fail-safe features. After extensive

testing using production hardware, all operational and performance goals were

met and the system was qualified for flight.

Emergency Egress System

The safety goal of the TRRA project stated that two simultaneous failures should

not result in the loss of life. However, in the event of a catastrophic situation,

emergency protection for the crew was to be provided with the installation of

ejection seats. This was possible because the tilt rotor aircraft, unlike conventional helicopters, provides a clear crew ejection path without the need to remove the

proprotor blades. The Government-furnished LW-3B seats were developed by

North American Aviation (later Rockwell International) of Columbus, Ohio, for

the OV-10 aircraft used by the Marines and the Air Force. These seats, termed

“zero-zero” seats, were designed to be capable of ejecting a crew member and

deploying the parachute for a safe landing with the aircraft in a normal attitude

while on the ground and at zero airspeed. These seats were propelled out of the

47
cockpit by a rocket fired by a crew

member activated ignition system. The

inclusion of these seats dictated several aspects of the cockpit design. The

seats had to be oriented to allow

simultaneous ejection of the pilot and

copilot with adequate clearance from

the instrument panel, center console

and side panels, and the overhead window frame had to be large enough to

permit the seat and crew member to

pass through without interference. In

addition the flexible oxygen and communication lines had to have breakaway fittings to permit separation

upon ejection.

To verify the operation of these ejection seats in the XV-15 cockpit, a

functional test was conducted using

the forward section of the N703NA

fuselage, fabricated by Rockwell

International at their Tulsa, Oklahoma,

site. The test would determine if a

simultaneous (pilot and copilot) seat

ejection could be safely performed. Shorty Schroers was the principal

Government investigator for this evaluation and Rod Wernicke, brother of Bell

Program Manager Ken Wernicke, was the test director for the contractor.

Two 95 percentile (large-size) anthropomorphic test dummies were dressed in

flight suits and helmets and strapped into the seats. The cockpit interior structure,

control panels, and windows were marked with a pattern of various colors and

shades of lipstick so that any contact made by the simulated pilot and copilot

with the aircraft would be identified by the transferred markings. One of the project’s more unusual moments was when Schroers and his team of engineers and

technicians went to a local cosmetic store to purchase the large quantity of lipstick required for the test, being careful to select as many distinguishable colors

and shades as they could find. It is hard to imagine what the salesperson must

have been thinking.

In July 1975, the forward fuselage section mounted on a flatbed truck was moved

to the designated test area at the Tulsa International airport. An array of still- and

movie-cameras were set up around the site and two high-speed cameras were

mounted inside the cabin to capture the ejection in slow motion. Aerial movies of

the test were taken from a helicopter piloted by Ron Erhart, Bell’s XV-15 chief

test pilot. Figure 41 shows the nearly simultaneous ejection just after both seats

48

Figure 41.

Simultaneous static test firing of XV-15 ejection seats.

(Ames Photograph

AC75-1602)
left the fuselage and figure 42 shows

the parachutes deployed after seat separation. After the test it was concluded

that the ejection system operated satisfactorily (although prevailing winds

caused the deployed parachutes to contact each other) and that no XV-15

configuration changes were necessary.

In addition to the ejection seats, the

overhead and side windows were provided with an emergency removal system (employing a mild detonator cord

in the window frame) that could be

activated from within the cockpit or

from an external lever in the nose section. Markings were provided to indicate the location of the external emergency window release control lever.

Major Subcontractors

Critical to the development of the

TRRA was the design and fabrication

of numerous aircraft components and

subsystems. These required the development of specification documents which

detailed the configuration, performance, and functional definition during the

early design phase. Bell’s decision to “make or buy” based on Bell’s in-house

capability, the potential subcontractor’s ability to meet these requirements, as

well as the proposed cost and schedule, was submitted to the Government TRRA

Project Office for review and approval. Bell elected to fabricate in-house the critical dynamic components of the tilt rotor aircraft which included the proprotors,

the transmissions, and the wing. All components and subsystems, whether furnished by Bell or by subcontractors, had to be shown to be qualified in accordance with the requirements established by Bell and approved by the TRRA

Project Office. This qualification was established either by similarity to previously tested components, by tests, or in some cases, by analysis. The major subcontractors noted in this section provided unique components and subsystems and

were instrumental in making significant contributions to the development of the

XV-15 TRRA. 

The Rockwell International, Tulsa Division, was responsible for the detailed

design and fabrication of the fuselage and empennage of the TRRA. The flight

control hydraulic actuators and hydraulic reservoirs were provided by the

Hydraulic Research and Manufacturing Company (HR&M) of Valencia,

California, a long-standing and reliable supplier of hardware for many of Bell’s

49

Figure 42.

Parachutes deployed during

seat ejection test.

(Ames Photograph

AC75-1605)
helicopters. For the conversion system, SPECO (Steel Products Engineering

Company), a division of Kelsey-Hayes of Springfield, Ohio, modified Lockheed

P2V ball-screw jack actuator components to meet the requirements of the XV-15

and also provide the flap drive system for the TRRA. Components for the automatic flight control system were developed by Calspan Corporation, Buffalo,

New York, and the proprotor RPM electro-hydraulic governing system was

developed by Decoto Aircraft Inc. of Yakima, Washington.

A critical requirement was the need to carry high pressure hydraulic fluids, fuel,

and high temperature engine bleed air from the fixed airframe to the tilting

nacelles. This required the use of swivel fittings. Furthermore, because of the

design requirement to avoid the simultaneous loss of the critical hydraulic systems that provide power to the flight control actuators, each of the three

hydraulic systems used a differently designed swivel fitting at the wing/nacelle

joint. These components were designed by the Dumont Aviation Division of

Litton Industries, Lakewood, California. 

The later success of the TRRA is a tribute not only to the designers and engineers working directly on the project, but also to the subcontractors who were

able to meet the technical needs of the XV-15 within stringent cost and schedule

constraints.

Data Acquisition

Early in the design of the TRRA, attention was focused on defining the project’s data system. As it evolved, the system was required to have a capacity

for a large number of pressure, temperature, load, torque, and position data

parameters (containing a range of oscillatory levels from steady values to high

frequencies), an onboard flight-crew controlled data recorder to ensure the collection of complete, high quality data records, and the capability to transmit

critical data to a ground monitoring station. At the start of the development of

the data system for the XV-15, the use of state-of-the-art digital technology

was recommended by Ames instrumentation experts and accepted by the

TRRA Project Office. 

While Ames had been using digital data acquisition systems in its wind tunnels

for several years, the XV-15 would be the first research aircraft at Ames to use

such a system for primary (safety-of-flight) data acquisition. Furthermore, the

XV-15 would become the first new rotorcraft to utilize digital “pulse code modulation” (PCM) technology for the expansion of its flight envelope. 

By the early 1970s, the Teledyne Controls Division in El Segundo, California,

had completed the NASA DFRC-sponsored development of a versatile, high

speed, high capacity digital data acquisition system small enough to fit a

remotely piloted vehicle or a small-size piloted flight test vehicle.

Unfortunately, the DFRC-developed system did not meet the requirements of

50
the TRRA, so a larger, next-generation system was developed by Teledyne with

NASA Ames funding under the technical guidance of Herb Finger of the Ames

Instrumentation Branch. 

The advantages of the digital PCM system over prior state of the art analog

instrumentation were many. The onboard digitization of the measured values preserved data accuracy, the system was relatively compact, set-up and data calibration values were retained in the records, and the digital data allowed both rapid

“near-real-time” computerized data reduction (for safety or experiment monitoring), and efficient digital computer data processing after the flight. 

The timing for the application of a digital PCM system in the TRRA project

was right. Recent developments had demonstrated that these systems were

capable of handling the high frequencies (up to 10 Hz) required for the analysis of the proprotor dynamic data generated by the TRRA. For higher frequency needs (such as acoustic or engine vibration data) and for cockpit voice

recordings, a few channels of wideband FM (frequency modulated) tape

recording were provided.

The consideration of the TRRA’s instrumentation needs at the beginning of the

project resulted in a significant cost benefit by having access-limited sensors 

and instrumentation wiring installed as the aircraft was being built. As part of a

plan to facilitate major maintenance or modifications, the large components of

the data acquisition system were mounted on a pallet in the cabin. If required,

this allowed the entire pallet to be removed (with difficulty) and brought to the

instrumentation shop. 

For the planned wind tunnel test of the XV-15, provisions for connecting the

aircraft’s digital data to the control room were incorporated and consisted of

two coaxial cables (compared to hundreds of wires that would be required for

an analog instrumentation system), thus saving considerable installation time. A

further feature of the digital system that proved to be useful during the test was

the ability of the wind tunnel engineers to change the selection of the parameters being monitored in real time during the test runs. This provided the versatility needed to enable on-line trouble-shooting or anomaly assessments during the

running of the test. 

The XV-15’s data system consisted of two Remote Multiplexer/Digitizer Units

(RMDU’s), signal conditioning and filtering components to process various types

of sensors (such as strain gauges, pressure transducers, thermocouples, and

potentiometers), a wide-band twelve-channel flightworthy magnetic tape

recorder (that provided almost one hour of data acquisition), and a telemetry system. The wires from the various sensors were fed into the system using a large

patch-panel (similar to an old-fashioned telephone operator’s switchboard).

While the system was reasonably robust, this element proved to be the source of

many of the reliability problems that did occur.

51
Prior to the start of flight operations at Ames, the Government Project Office initiated the development of the data acquisition system for the TRRA. The requirement to monitor numerous structural load parameters in real-time at a ground

station dictated the need to improve the acquisition, processing, and display of

the digital data. This enhanced capability was developed, with the input of

TRRA project funding, by the Ames Avionics Systems Branch. Since some of

the test operations would be performed at the remote Crows Landing Naval

Landing Field, a duplicate data acquisition and processing capability was developed at that site. To allow the ground safety monitoring crew to be located at

either location during testing, a telemetry data “repeater” was installed on 3900-

foot Mount Diablo, one of the higher topographical features in the San Francisco

Bay area. For tests that called for the XV-15 to fly near Crows Landing or over

the adjacent San Joaquin Valley, while the monitoring crew remained at Ames,

the aircraft’s telemetered data would be initially received by the Crows Landing

data station. This signal was then sent to the repeater where it was transmitted to

the telemetry receiver at Ames. With the addition of video coverage, the remote

management of flight tests at Crows Landing became as routine and efficient as

if the aircraft was flying at Ames. 

The NASA Ames ground data acquisition group developed and maintained the

control rooms at Moffett Field and at Crows Landing, from where the Test

Director communicated with the flight crew and the ground test-support personnel. Each control room had a monitor that displayed aircraft status and critical

parameter values (such as loads or moments) and had four strip-chart recorders

that provided time histories of a total of thirty-two key items (selected for each

test) in engineering units in real time. These strip-chart recorders were monitored

by engineers or technicians familiar with the behavior of the items being monitored, and who would alert the Test Director if allowable-limit values were

approached or exceeded. In addition, the ground data acquisition group would

process all data from the flight tapes for post-flight review, and would calculate

the accumulated “fatigue damage” to structural components due to oscillatory

loads in excess of the “infinite-life” limit.

In addition to working with the group developing the ground data acquisition

and processing capabilities, Mike Bondi of the TRRA Project Office was also

responsible for the development of an interactive database program which could

be used by engineers to analyze the flight test results. This program would be

capable of storing a large volume of data, retrieving data sets within constraints

defined by the engineer, providing a variety of data processing options, and generating data tables or plots in a number of user-defined formats. This work was

contracted to Analytical Mechanics Associates of Sunnyvale California in 1980

and the resulting program was called the Tilt Rotor Engineering Database

System (TRENDS). In addition to the TRRA, TRENDS was also successfully

used for other Ames flight research activities such as the JUH-60A Black Hawk

Airloads project and the Quiet Short-Haul Research Aircraft (QSRA) JumpStrut project.

52
In 1979, the need to conduct XV-15

flight tests at the Dryden Flight

Research Center in southern California

presented logistics problems for the

TRRA Project Office engineering team

at Ames. With the small project staff

having responsibilities at both Ames

and Dryden within the same time period, it was apparent the team could not

be relocated frequently to meet the test

schedules. The solution was the use of

satellite communications technology

and the installation of a satellite dish at

Ames. With this equipment, flights at

DFRC were controlled and monitored

by the Army/NASA crew at Ames. 

The Ames-developed data systems

were installed in both XV-15 aircraft

and remained in use for several years.

For aircraft N702NA, Bell changed to a new onboard data acquisition system in

1988 to match the data acquisition system being used on the V-22 Osprey aircraft.

The original system remained in N703NA until 1994 when the aircraft was bailed

to Bell and was subsequently replaced with the Bell data acquisition system. 

Ground Tiedown Test

Following the completion of the Integrated Systems tests, each XV-15 was subjected to a series of operational tests at Bell on an elevated ground tiedown facility (figure 43) that Bell had constructed for that purpose. The facility had a

moveable tripod structure that attached to the wind tunnel “hard points” under

the wing to secure the aircraft while enabling the proprotors to be run in all

flight modes. The purpose of these tests was to evaluate the performance of all

systems within the limitations of the static operation. 

During the initial runs, for pilot protection, a set of thick steel shields were

placed along the sides of the cockpit (figure 44). Also, for early tiedown operations, a rescue ramp was moved into position over the nose of the aircraft to

facilitate exit of the crew through the overhead windows, if that should become

necessary. When sufficient running had been completed to gain confidence in the

proprotor structure and RPM control system, the shields were removed, but the

rescue ramp remained throughout the test series.

One of the primary elements examined was the transmission system. With an

external fuel supply, the tiedown facility permitted continuous operation of the

proprotors at various power levels and RPM’s (as in the initial qualification of the

53

Figure 43.

Bell XV-15 ground 

tiedown facility. 

(Ames Photograph 

AC76-1518-115)
transmissions in the Bell Transmission Test Laboratory). Functional checks of

the aircraft’s electrical system, hydraulic system, proprotor RPM governor, and

other systems were also conducted. 

First Flight

With the completion of the integrated systems test and with substantial progress

being made in the ground tiedown runs, the Bell and Government TRRA project

offices initiated discussions addressing the start of the flight test program. While

each was anxious to explore the new technology, the Government approach, as

54

Figure 44.

Initial Bell tiedown showing

metal protective shields.

(Bell Photograph 240178)
defined in the TRRA Project Plan, called for a wind tunnel test prior to flight to

“be certain all critical mode analyses are valid and that the (analytical) methods

properly assess the dynamic characteristics, capabilities, limitations, and operating behavior of the tilt rotor flight research aircraft.” Bell, however, argued for

the initiation of a limited flight program as soon as practical, which would have

been their usual practice, in order to reveal issues that may require further analytical investigations or additional attention during the wind tunnel test. The early

detection of problems would allow more time for their resolution and could ultimately accelerate the program. This rationale for an early flight evaluation had

sufficient technical merit for the Army/NASA TRRA Project Office to present

the plan to the Ames Airworthiness and Flight Safety Review Board (AFSRB).24

After establishing the readiness and airworthiness of XV-15 N702NA located at

the Bell Flight Test Center, and examining the test results from the ground-based

operations, the Ames AFSRB approved a limited hover and air-taxi flight test

activity. Since the Ames 40- by 80-foot wind tunnel could not produce steady

airflow at very low wind tunnel airspeeds, the AFSRB restricted flight speeds to

40 knots, where data at steady-state conditions could be obtained from flight

tests. In addition, this hover and air-taxi evaluation was limited to a maximum of

three flight hours and to low altitude operating conditions. 

On May 3, 1977, following a series of ground-taxi and systems tests, the XV-15

TRRA became airborne for the first time. The initial liftoff, conducted by Bell test

pilot Ron Erhart and copilot Dorman Cannon, was executed according to plan.

With Bell Test Director Shep Blackman and a team of XV-15 project engineers

carefully watching the critical flight parameters in the Bell Flight Test Center control room, the XV-15 was accelerated in helicopter mode along the runway by the

application of power and the forward movement of the center control stick. After

a short ground roll a slight increase in the power provided the additional thrust

needed for liftoff. As planned, the power was immediately reduced, the aircraft

settled back on the runway, and a rollout was performed. This first flight demonstrated satisfactory handling qualities and safe structural loads. In subsequent

operations, longitudinal and directional controllability were verified and a hover

over a fixed point was made. The initial flight pilot report stated “In general, the

aircraft hovered almost exactly as predicted, based on the simulator evaluations,”

a clear validation of the extensive TRRA simulation program. Further testing performed by the Bell pilots at the Arlington, Texas, Municipal Airport during this

initial test period included flights in the helicopter mode in hover and in forward,

sideward and rearward flight. Testing also included an assessment of the SCAS

and the FFS. For the first time in this flight program, the nacelles were tilted to 85

55

24 The Ames Airworthiness and Flight Safety Review Board was a panel of senior members of the

Ames staff, including technical/research specialists, who were charged with the evaluation of

planned aircraft activities to ensure that adequate design, testing, planning, and training had been

done in order to achieve proposed mission objectives safely. The AFSRB members were appointed by the Director of Ames Research Center and had no direct involvement in the project being

reviewed.
degrees25 and a brief assessment was made of the handling qualities. During this

period, NASA/Army project pilot Dan Dugan flew N702NA for his first tilt rotor

flight. After completing the authorized three flight test hours, the aircraft was configured with a remote control system for wind tunnel operation, and operationally

checked out on the Bell ground tiedown stand.

On March 23, 1978, after completing ground runs and repairs (to be discussed

later), XV-15 N702NA, with the proprotors and wing removed, was transported

to Moffett Field onboard an Air Force C-5A. At Ames, a Bell crew reassembled

the XV-15 and prepared it for the wind tunnel test. 

Wind Tunnel Tests

As noted earlier, prior to the expansion of the flight envelope, the TRRA was to 

be tested in the Ames 40- by 80-foot wind tunnel, both to check out the aircraft

and its systems and to collect data that accurately described the aerodynamic

characteristics of the XV-15. While the acquisition of a flight data base that

could be reliably used for the development of larger tilt rotor transport aircraft

clearly required the XV-15 to be as large as practical (considering factors such as

the available powerplant), the 40- by 80-foot wind tunnel test called for the consideration of two conflicting issues. One of these issues was due to the well

known effects that the wind tunnel walls have on the flow around the “test

model” (in this case the XV-15). The wind tunnel “wall effect” phenomenon

occurs because the flow at the wind tunnel walls is constrained to move along

the wall surfaces of the test section, whereas in free flight, induced velocities

occur that could have a component normal to the solid boundaries of the wind

tunnel. A test model is considered too large if the wall constraint on the flow has

a measurable influence on the desired test data such as the magnitude of wing

lift or proprotor performance. The other major consideration in selecting the

dimensions of a wind tunnel model is the effect that the size of a body moving

through viscous air has on the behavior of the flow, particularly in the vicinity

just over the surface called the “boundary layer.” On very small models the airflow, traversing through regions where local pressure changes occur, would not

develop the turbulence and resulting drag changes that would appear with larger 

models. To represent “full-scale” aerodynamic characteristics, therefore, the test

model needs to be large enough to adequately represent the viscous effects.26

While the size of the XV-15 was sufficient to properly represent “large-scale”

aircraft aerodynamics and proprotor performance in flight, it was about as large

56

25 The position of the proprotor shaft indicates the flight mode configuration of the tilt rotor aircraft. The convention adapted by the Government TRRA Project Office and Bell referred to the

airplane mode position as 0 degrees and the helicopter position as 90 degrees.

26 An aerodynamic term called “Reynolds Number” indicates the degree of influence that the flow

viscous effects have on the boundary layer for a selected model size and airspeed. For most fullscale aircraft, Reynolds Numbers exceed three million. The aerodynamic behavior of the wings

and propellers are usually significantly different for small scale models having Reynolds

Numbers below one million.
as could be accepted for tests in the 40- by 80-foot wind test section. 

The wind tunnel test of the XV-15 was made a required part of the TRRA project in

the first Project Plan that was issued in 1972. This requirement was carried forth in

later revisions of the Plan and through execution. The technical rationale for this test

was strong. It provided an opportunity to evaluate the loads, performance, and aerodynamic characteristics, as well as the function of the mechanical, electrical, and

hydraulic systems under operational conditions in the controlled environment of the

wind tunnel and without risk to a flight crew. Yet there were arguments against the

wind tunnel test. These detractors questioned the wisdom of exposing the aircraft to

the risks associated with a tied-down wind-on experiment.27 They were also concerned about the impact of the additional costs to conduct the test as well as delaying

flight activity. There were even discussions questioning whether the real motivation

was to show that the 40- by 80-foot wind tunnel was still a viable tool for developing

new types of aircraft. In any event, the wind tunnel test was a critical milestone that

needed to be reached before embarking on the flight evaluation program.

The ability to operate the XV-15 N702NA as an unmanned wind tunnel model

was provided as the aircraft was designed and constructed. Mounting locations

for the wind tunnel struts (called “hard points”) were built into the aircraft’s

structure at the lower surface of each wing and the tail. Provisions were made for

the installation of remote operation devices for the engines and flight controls.

The external supply source connections were installed for hydraulic and electrical power used to operate the control systems during wind tunnel testing with the

engines not operating. For tests with the engines running, the aircraft’s enginedriven electrical and hydraulic systems were used. 

Prior to entering the tunnel, the aircraft’s fuel tanks were drained and filled with

nitrogen (to reduce the risk of an explosion), and the fuel lines capped (the wind

tunnel “external” fuel supply was connected directly to the engines, bypassing

the fuel tanks). Actuators for the remote operation were installed. The landing

gear was retracted and the gear doors were closed during the test.

Figure 45 shows the XV-15 mounted on the three-strut support system in the

Ames 40- by 80-foot wind tunnel. To assure safe operation, crew training was

conducted during the ground tiedown tests at the contractor’s facility with the

remote control systems installed. At Ames, the TRRA simulation math model

was modified to represent operation in the wind tunnel and remote operations

were simulated to evaluate emergency operating procedures. The only failure

identified that could cause a dangerous condition was a simultaneous dual engine

failure in high-speed helicopter mode flight (with the nacelles above 85 degrees).

The emergency procedure required to avoid potentially destructive loads called

57

27 An aircraft constrained by a wind tunnel mounting system might be subjected to operating conditions not normally encountered nor sustained in flight. These unusual conditions could produce

airloads, moments, and torques that exceed allowable design limits and result in structural failure.
for the reduction of the nacelle incidence angle within five seconds of a

dual engine failure. 

During the two-month test period, 54

hours of wind-on time were logged. Of

this, 19 hours were with the rotors operating. Static aerodynamic forces and

moments data were acquired from the

wind tunnel balance (scales) system and

structural loads and aircraft systems data

were obtained from the XV-15’s onboard

instrumentation system. Critical temperature, pressure, and static and dynamic

load parameters were monitored in “realtime” in the control room. Testing was

conducted over a range of nacelle angles

and airspeeds and included the baseline

aircraft configuration and various combinations of vortex generators, pylon

strakes, and wing fences. A report titled “Wind Tunnel Tests of the XV-15 Tilt Rotor

Aircraft”28 containing summary data and photographs was issued in April 1980.

In general the aircraft’s components and systems performed well. The few exceptions to this were: a failed nacelle downstop29 (the “hard-point” that limits the

nacelle position when operating in the airplane mode), nose boom and antenna

vibration,30 and engine oil venting.31 The most significant issue, however, was the

high empennage loads that occurred in helicopter mode forward flight and in

portion of the conversion envelope. In helicopter mode flight, the loads were

caused by aerodynamic excitation of the vertical tail surfaces arising from the

close proximity of the inboard proprotor tip vortices. At the 60 degrees nacelleincidence conversion mode flight condition, a strong vortex emanating from the

nacelle/wing juncture also was swept near the tail, causing a high oscillatory

load condition. After the wind tunnel test program structural changes were made

to accommodate these loads.32

58

Figure 45.

XV-15 in the Ames Research

Center 40- by 80-ft.wind

tunnel. (Ames Photograph

A78-0579-3)

28 James A. Weiberg, M. D. Maisel, Wind-Tunnel Tests of the XV-15 Tilt Rotor Aircraft, NASA TM

81177 and AVRADCOM Technical Report TR-80-A-3, April 1980.

29 Following the wind tunnel test, the downstop bracket failure problem was resolved by a bracket

redesign and a change to the preload/rigging procedure to reduce impact loads.

30 The vibration problems were later resolved by increasing nose boom stiffness and by repairing

a structural failure (discovered after the wind tunnel test) at the antenna attach point.

31 The problem of seepage from the engine oil scavenge lines was addressed by providing an

angular (scarf) cut at the exposed end of the tube. The seepage, however, continued to occur in

subsequent flights.

32 Subsequent flight tests, however, showed that the empennage load problems were less severe

than indicated by the wind tunnel tests.
For a period of about eight years, starting in the mid-1970s, the USA experienced one of the longest sustained periods of high inflation of the century. This

occurred, unfortunately, while the TRRA project was in its most active phase 

and required high levels of contractor labor and expenditures of large amounts 

of funding for subcontractor work. The completion of the wind tunnel test of aircraft N702NA, in June 1978, left it in a non-flightworthy configuration and a

considerable effort was required to refurbish it for flight. In addition, resolution

of technical problems in most of the aircraft-development WBSEs was taking a

toll on the project’s financial resources. The “joint” commitment to the TRRA

project by the Army and NASA had, on numerous occasions, served to maintain

the support of each of the parties. Throughout the early years of the project, neither agency was prepared to be first to walk away from their funding obligation

while the other was seen to be preparing to step up to the challenge. However, by

early 1979 costs had grown to the point where the TRRA Project Office was

forced to reevaluate its plans for the completion of the project in light of its

resource limitations. Among the solutions considered was the reduction of the

contractor’s flight program, a reduction of the procurement of needed spare

parts, and the discontinuation of work on aircraft N702NA. Each of these

approaches could have serious adverse effects on the ability of the TRRA project

to complete its technical goals.

Meanwhile, starting in early 1978, the Naval Air Systems Command (NAVAIR)

had been engaged in a congressionally-authorized Sea Based Air-Master Study

(SBA-MS)33 to assess the Navy’s technology requirements. The report to

Congress was to “evaluate the capabilities and cost effectiveness of current and

future platform, aircraft and weapon system combinations.” The aircraft to be

considered included VTOL and V/STOL types (and, in fact, the original stimulus

for the SBA-MS was the Navy’s need to establish a basis for, or against, a future

commitment to V/STOL aircraft). For the Navy’s V/STOL Type “A” low speed

application, including the ASW (Anti-Submarine Warfare), COD (Carrier

Onboard Delivery), tanker, and SAR (Search and Rescue) missions, the tilt rotor

under development in the NASA/Army TRRA project was a candidate aircraft

type that warranted serious consideration. The XV-15 was seen by some

Navy Participation

59

33 The Navy’s SBA-MS evolved from the Sea Control Ship (SCS) initiative established by

Admiral Elmo Zumwalt after he became Chief of Naval Operations (CNO) in 1970. The SCS

was to employ high performance V/STOL aircraft to perform various elements of the sea control mission. Subsequent NAVAIR studies indicated that the tilt rotor aircraft had significant

advantages over other V/STOL concepts when applied to several Navy mission scenarios. In

1976, two years after relieving Admiral Zumwalt, CNO Admiral James Holloway initiated a

new V/STOL aircraft study under which the fighter/attack missions were to be performed by the

V/STOL “B” aircraft, and the assault/support (helicopter replacement) vehicles were designated

V/STOL “A.” During this period the Marine Corps was also investigating replacement aircraft

for their aging fleet of CH-46 helicopters under a program called HMX. Lieutenant General

Thomas Miller, USMC Deputy Chief of Staff, and Rear Admiral C. P. Ekas, Naval Materiel

Command (NAVMAT) Chief of Naval Development directed their subordinates to explore and

demonstrate, if feasible, new aircraft technology that could have potential for future Marine

assault transport applications. This provided the impetus for the Navy’s participation in the 

XV-15 project.
NAVAIR managers as a means of determining the tilt rotor aircraft’s readiness

and suitability for Navy missions. One of the leading advocates for Naval VTOL 

capability, and a strong proponent for the tilt rotor aircraft within NAVAIR at that

time, was Harold (Hal) Andrews.

By March 1978, discussions had been initiated between NAVAIR and the

NASA/Army TRRA Project Office for the participation of the Navy in the XV15 activity. This new collaboration provided for the infusion of $4.0M of Navy

funds into the program between 1979 and 1981. Of particular interest to the

Navy was the timely opportunity to conduct concept feasibility flight testing of

this unique aircraft type onboard a Navy carrier. The Navy funds permitted the

Army/NASA TRRA Project Office to refurbish aircraft N702NA for flight, purchase the required spare parts, continue the contractor flight test activity, and initiate the Government concept evaluation (proof-of-concept) flight testing as

planned. In addition, the use of the XV-15 for the Navy evaluation was consistent

with the NASA goal of making available advanced aeronautical technology to

the military. It was a “win-win” arrangement.

George Unger of NAVAIR was assigned to develop an agreement for Navy participation in the TRRA program with the Army and NASA. Coordination for this

between NASA Ames and the Navy was provided by Clark White, of the 

Ames Aeronautics and Flight Systems Directorate, who was on assignment to the

Naval Air Systems Command in Washington, D.C., John Ward, Rotorcraft 

Manager at NASA Headquarters, provided HQ support. The Army/NASA/Navy

Memorandum of Agreement signed in July 1978 led to a request from Rear

Admiral E. R. Seymour, Commander, Naval Air systems Command to the CNO

for a shipboard evaluation of the XV-15.

The request cited key areas of interest as “gust and turbulence sensitivity, deck

edge effect, handling qualities, pilot work load, and STOL performance.” Within 

a short time, the request was approved and arrangements were made to conduct

sea trials aboard an LPH class ship operating in the California coastal waters off

of San Diego. The story of that evaluation will be covered later.

The timely funding provided by the Navy enabled the TRRA to be put back on

its original plan.

60
With the initial hover/low-speed/low-altitude evaluation and the 40-by 80-foot

wind tunnel test completed and all identified technical issues addressed, authorization was provided by the Government to initiate Phase I of the Contractor

Flight Test activity. This phase involved the initial venture into the full flight

capabilities of the XV-15 TRRA. It became apparent that the Government

Project Office and the Ames Airworthiness and Flight Safety Review Board

(AFSRB) had a conservative view on the approach to envelope expansion. Bell,

on the other hand, having had more recent experience in the development of new

flight vehicles was anxious to more aggressively explore the flight capabilities of

the XV-15. The directives from the Ames TRRA Project Office prevailed and

Bell was required to accept the more cautious approach to envelope expansion.

Expansion would be performed in small airspeed and nacelle angle increments

and a thorough analysis of the test data would be conducted prior to the next

configuration and airspeed test condition.

The first flight of XV-15 N703NA (the aircraft available at Bell for the

Contractor Flight Tests) occurred on April 23, 1979, at the Bell Flight Test

Center, Arlington Municipal Airport, Texas. The Bell pilots assigned to the envelope expansion were Ron Erhart and Dorman Cannon, and the Bell test director

was Shep Blackman. 

In mid June, when the XV-15 had explored flight regimes from the helicopter

mode to a nacelle angle of 60-degrees, LTC Dan Dugan, the NASA/Army 

project pilot, made an evaluation flight. In his report to the Ames AFSRB, he

recommended that the envelope expansion be continued, and permission was

granted. 

After a total of 15 hours of flight testing and more than three months of expanding the flight envelope with carefully planned incremental steps, a major milestone was reached when, on July 24, 1979, the first full in-flight conversion from

helicopter-to-airplane mode was accomplished. During that initial airplane mode

flight lasting about 40 minutes, the crew evaluated climbs, descents, turns, accelerations, and decelerations and reached an airspeed of 160 knots. The Bell flight

crew and test engineers were quite pleased with the results and envelope expansion in the airplane mode continued.

The success of the Phase I effort now opened the door for the Proof-of-Concept

Flight Tests, Phase II of the contractor’s XV-15 flight activity. This phase of the

flight program involved a closer examination of the flight characteristics of the

XV-15 and of the performance and operation of its systems. It also presented an

opportunity for the Bell test pilots to train the Government flight crew and permitted an initial Government evaluation of the XV-15. 

In accordance with the Project Plan, the completion of the Proof-of-Concept

phase was to be conducted at a Government test site. Because of the level of risk

associated with the flight test of a low-time research aircraft, it was decided that

Flight Envelope Expansion

61
this work should be conducted at the NASA Dryden Flight Research Center

(DFRC) at Edwards Air Force Base near Mojave California, instead of the heavily populated area around Ames. To accomplish this, the proprotors and wing of

XV-15 N703NA were removed and the disassembled aircraft, along with support

equipment, were airlifted to DFRC from Bell onboard an Air Force C-130 on

August 13, 1980. Following reassembly and ground tests at DFRC, the proof-ofconcept flight activity resumed on October 6, 1980, and continued through May 

1981. During this period, the Government and Bell team members gathered in

front of N703NA for a group picture (figure 46).

Government Acceptance of N703NA

A formal Government acceptance ceremony for XV-15 N703NA was held on

October 30, 1980, at Dryden. The program schedule called for the arrival of

Ames senior NASA and Army personnel onboard the ARC/DFRC shuttle aircraft

at precisely 11:00 am, followed by the immediate XV-15 takeoff and fly-over

demonstration by a Government flight crew. The aircraft was then to land and 

taxi to the area where the presentations were to be made. The Project Office staff

had established a crisp schedule to reflect the precision and efficiency of the

TRRA project. 

Once again we learned that things do not always work as planned. The shuttle

flight landed on time, but as soon as the Ames dignitaries deplaned, one of them

anxiously asked where the nearest men’s restroom was located... and the “crisp”

schedule rapidly evaporated.

62

Figure 46.

Government and Bell 

personnel at the Dryden

Flight Research Center,

October 1981. 

(N=NASA, A=Army, B=Bell)

Left to right, standing,

back row:

Kit Boyce (N),

Benny Daniels (B),

unidentified,

Bob McClachety (B),

John Dial,

Ted Turner (B),

Dick Denman (N),

Marty Maisel (A),

Jerry Barrack (N),

“Tilt Rotor” Debbie (N),

Jim Bilger (B),

Jim Weiberg (N),

Roger Marr (B),

Dave Few (N),

Rich Young (N),

Jim Lane (N),

Wen Painter (N),

John Brown (N),

unidentified,

Jim Liu (N),

Al Morningtar (N),

John Weyers (N),

Doug Winniford (B),

unidentified. 

Front row:

Shorty Schroers (A),

Jerry Pickard (B),

Dan Dugan (A),

Joe Trentam (B),

unidentified,

Jerry Walker (B),

Mike Kimbell (B),

Ron Gerdes (N).

(Ames Photograph

AC81-0269)
At the end of the ceremony, Bell flight

test crew Ron Erhart and Dorman

Cannon provided a plaque to

Government pilots LTC Dan Dugan

and Ron Gerdes illustrating the XV-15

in various flight modes. This symbolized the turning over of the “keys” of

the new research aircraft to the

Government (figure 47). 

Ames Flight Preparations

Wherever the XV-15 was maintained

special facilities were required. In

preparation for planned future XV-15

operations at Ames, the Ames VTOL

tiedown pad, developed years earlier

for static tests of VTOL aircraft, was

modified to allow the proprotors of the

TRRA to be operated in the airplane

mode, providing similar ground run

capabilities as the one developed at

Bell. A hydraulic lift platform, under

the main and nose wheels (figure 48),

elevated the XV-15 so that hinged tripod structures could be attached to the

wing hard-points. In addition, a tubular strut pinned at ground level was

attached to the tail hard-point to stabilize the tail section. With the supports

in place, the hydraulic lift platform

under the main and nose wheels was

lowered to ground level, leaving the

area under the elevated aircraft clear

and resulting in a 6-ft wheel height.

When mounted on the tiedown stand, the proprotors could be operated in any

nacelle position. As a safety measure, for early operations of the XV-15 on the

Ames tiedown stand, a ramp was positioned over the nose of the aircraft to

enable emergency pilot egress and rescue (figure 49).

Prior to its first flight at Ames, it was necessary to remove modifications made to

the aircraft for the wind tunnel test in order to restore it to a flightworthy status.

This refurbishment was delayed several months while project funding issues

were being resolved. The provision of Navy funding at this point provided the

contractor support necessary to refurbish XV-15 N702NA. A Bell team of engineers and technicians arrived at Ames in mid-1980 to work with NASA and

63

Top:

Figure 47.

XV-15 plaque being presented to Government pilots by

Bell pilots at the acceptance

ceremony. (Bell Photograph

309835)

Bottom:

Figure 48.

Ames tiedown test facility

showing rescue ramp. (Ames

Photograph AC80-0686-1)
Army personnel on the restoration of

the aircraft to the flight configuration.

Following the reassembly, ground tiedown operations for XV-15 N702NA

were initiated in August 1980 and

completed in October 1980. The first

flight of this aircraft at Ames occurred

on November 20, 1980. 

With one XV-15 aircraft permanently

stationed at Ames, NASA contracted

with Bell to provide ongoing on-site

support. Bell’s Jerry Pickard performed this support and remained at

Ames, providing logistics between the

Government and Bell, until the task

was terminated in 1988. This support

was essential for the successful operation of the XV-15 at Ames. The long periods away from Bell presented an occasional dilemma (sometimes humorous) for Pickard. One example of such an

occurrence was when Pickard requested his manager at Bell to provide a few

Bell baseball caps to give to visiting dignitaries. After considerable time had

passed, and with no hats delivered to Pickard at Ames, his manager requested a

photograph of Pickard and the Government XV-15 pilot standing near the XV15. The picture sent back to Bell by Pickard was exactly what was requested,

except that Pickard was wearing a Hughes Helicopter Company hat and Dan

Dugan, the NASA pilot, wore a Sikorsky hat. Within one week the Bell hats

arrived at Pickard’s desk. His manager never again ignored his requests.

64

Figure 49.

Tiedown test facility at the

Ames Research Center

showing the hydraulic lift.

(Ames Photograph 

AC80-0686-3)
In 1981, after a number of maintenance

test flights, the Project Office began a

series of ground and flight investigations to acquire a comprehensive data

base to meet the fundamental and

advanced technical goals of the TRRA

project. These test activities would

eventually address structural loads, handling qualities, flight dynamics, structural dynamics and stability, acoustics,

performance, and proprotor downwash. 

Hover Performance

One of the first experiments at Ames explored several characteristics of the 

TRRA in the hover mode. The scope of this hover test included an evaluation of

performance, acoustics, and the documentation of the “outwash” (the flow parallel to the ground generated by the proprotor downwash) at various hovering

heights. These data were required by the Navy for the planned operational evaluation of the XV-15 onboard an aircraft carrier. To measure the proprotor wake

flow in the vicinity of the hovering aircraft, the Naval Air Test Center of

Patuxent River, Maryland, provided data acquisition equipment and a supporting

research team. The outwash test apparatus consisted of a remote-controlled

motorized cart that carried an array of sensitive electronic (ion-beam) anemometers (to measure the low-speed airflow) mounted on a 10-foot high pole. While

the aircraft hovered (figure 50) over a point on the hover pad at a selected height,

the instrumented cart was moved to various predetermined positions along a

track radiating from the point below the XV-15. To survey the region around the

hovering aircraft, the heading orientation of the TRRA was varied 180 degrees in

30-degree increments, thereby documenting the outflow from the region directly

forward of, to the region directly aft of the aircraft. 

The outwash test required that the aircraft hover at a precise height, heading, and

position for a 15- to 20-second data acquisition period. The method devised to

accomplish this involved the use of sets of visual targets mounted on tall poles

around the hover pad. By lining up two sets of selected targets, the aircraft was

positioned at the desired point in space (figure 51). Hover conditions for these 

tests ranged from an in-ground-effect (IGE) 2-foot wheel height to an out-ofground effect (OGE) 50-foot wheel height. In addition to the outwash data, these

steady hovering operations conducted in near-zero wind conditions enabled the

simultaneous acquisition of excellent performance data.34,35

Flight Research

65

34 M. Maisel, D. Harris, “Hover Tests of the XV-15 Tilt Rotor Research Aircraft.” Presented at the

1st Flight Testing Conference, Las Vegas, Nevada, AIAA Paper 81-2501, November 11-13, 1981.

35 D.J. Harris, R.D. Simpson, “Technical Evaluation of the Rotor Downwash Flow Field of the

XV-15 Tilt Rotor Research Aircraft.” NATC Report No. SY-14R-83, July 28, 1983.

Figure 50.

XV-15 hovering in-groundeffect during 1984 performance and downwash test.

(Ames Photograph 

AC81-0165-152)
Also during this period, an evaluation

of electromagnetic interference (EMI)

effects on the XV-15’s electronic systems was conducted at Ames to ensure

compatibility with Navy shipboard

operations.

An associated test to measure download performed during the same test

period was conducted with the XV-15

N702NA mounted on the tiedown

stand at Ames. Load cells placed

between the aircraft’s two wingsupport and one tail-support “hard”

points and the tiedown structure provided a means of determining the net

vertical force of the aircraft. This

information was then coupled with the

aircraft weight and the free hover performance data to determine the download, the downward force acting on the

aircraft due to the impingement of the

proprotor wake on its wing and fuselage surfaces. 

The magnitude of the download

deduced from this test series turned

out to generate a technical dilemma.

Previous estimates of the download for a tilt rotor aircraft using deployed plain

flaps ranged from 7 percent to 8 percent of the rotor thrust.36,37,38 This, combined

with the estimates of rotor hover efficiency obtained from earlier hover tests of

an isolated proprotor, appeared to properly account for the thrust produced and

the wing-in-proprotor wake (interference) losses. Now the download obtained

from the hover/tiedown tests indicated that the interference loss was twice the

expected value.

The question would not be completely resolved until nearly three years later

when several full-scale rotors were tested at the Ames Outdoor Aerodynamic

Research Facility (OARF, figure 52). Further investigations of the proprotor

wake interaction with the aircraft in 1985 provided a better understanding of the

66

Top:

Figure 51.

Method used to position the

aircraft for the downwash

and acoustics hover test at

the Ames Research Center.

Bottom:

Figure 52.

XV-15 proprotor on the propeller test apparatus at the

Ames Outdoor Aerodynamic

Research Facility.

(Ames Photograph

AC84-0473-100)

Cross hairs

marker

Wheel

height 39.6 m.

(130 ft.) Ground distance

61 m.

(200 ft.)

Vertical pole

Checkerboard

target

30°

36 R. L. Lichten, “Some Performance and Operating Characteristics of Convertiplanes,” SAE

National Aeronautical Meeting, Los Angeles, California, October 1957. 

37 Anon., “V/STOL Tilt Rotor Aircraft Study - Task I - Conceptual Design,” Bell Helicopter

Company, NASA CR-114441, Bell Helicopter Co., May 1972.

38 Anon., “V/STOL Tilt Rotor Aircraft Study - Volume I - Preliminary Design of Research

Aircraft,” NASA CR-114438, Boeing Vertol, March 1972.
flow phenomenon that caused the higher than expected download. These tests

involved the use of a new “balance” designed to provide highly accurate proprotor thrust and torque data. The balance, mounted between the proprotor and the

drive motors, was developed by Boeing Helicopters (previously Boeing Vertol)

under the contract that provided for the development of new composite-material

proprotors for the XV-15 aircraft. The original XV-15 metal blades obtained

from Bell for performance and stability wind tunnel tests in the early 1970s were

one of the full-scale configurations tested. Data obtained from this test showed

that the XV-15 proprotor performance was, in fact, better than the earlier estimates. The somewhat mixed blessing that came out of these investigations was

that highly twisted proprotor blades could be designed to produce high performance, but the high download generated by the proprotor wake consumed all of

the unexpected performance gains. It was clear that the hover performance, and

therefore the effectiveness of the tilt rotor aircraft, could benefit from an understanding and reduction of the download loss.

Aeroelastic Stability Evaluations

Of all of the technical areas to be explored in the TRRA test program, none

would be as important as the investigation of the aeroelastic stability of the XV15 in high-speed airplane-mode flight. The future of the tilt rotor aircraft depended on the outcome of these tests.

The instability problem encountered by the tilt rotor aircraft is caused by elastic

deformation of the wing, pylon, and proprotor which oscillate when disturbed.

The flexing of the wing and pylon imposes a pitching and/or yawing motion on

the proprotor. This produces a proprotor in-plane force acting in the same direction as the original motion. Under some circumstances these in-plane forces are

sufficient to make the displacements in amplitude grow with each oscillation, in

effect acting as a powerful negative spring, producing an aeroelastic instability.

Both Bell and the Army/NASA TRRA project offices produced predictions of

the structural dynamic stability of the XV-15. Bell used a company-developed

method and the Government used predicted values determined from the analysis

generated by Dr. Wayne Johnson. Both analyses indicated satisfactory stability

throughout the envelope of the XV-15 except for one operating condition. The

predicted instability occurred only at high airplane mode airspeeds and at the

high RPM that was used for the hover and helicopter mode flight. The solution

was to set an airplane mode speed limit above which the proprotor RPM had to

be reduced to a level where the “one-per-rev” excitation of the natural mode

could not occur. Fortunately, this RPM reduction was planned during the design

of the XV-15 to improve the performance of the proprotor so that it became standard procedure to reduce RPM just after converting to the airplane mode. 

To evaluate the aeroelastic stability of the TRRA in flight it was necessary to create

rotor/pylon/wing displacements at the frequencies that corresponded to the various

67
natural “modes” of the tilt rotor structure (as illustrated in figures 53 and 54)

and to measure the response of the aircraft’s structure to these deformations.

Diminishing oscillation amplitudes following the excitations occurred for a

stable system (called “positively

damped”), while potentially dangerous

increasing amplitude oscillations indicated an unstable (negatively damped)

structure at that operating condition.

The initial approach taken by

researchers at Ames and Bell involved

the installation of limited-authority

(i.e. limited-motion) electrohydraulic

actuators in the flaperon and collective-pitch control linkages on the right

side of the aircraft. These “excitation”

actuators were controlled from the

cockpit where amplitude and oscillatory rates (frequency) were set. 

The flight tests required special care.

While confidence was high in the predictions of stability within and beyond

the XV-15’s flight envelope, this evaluation was treated as having a significant risk because of the potential for a

catastrophic failure if the predictions

were wrong. Testing was initiated in

airplane-mode level flight. When

steady, level flight conditions were established, the crew activated the excitation

system in accordance with the test plan. To minimize hazard, the series of test

operations were initiated at lower airspeeds where the risk of encountering an

instability was very low. After a thorough analysis of the data and a projection

that the next test condition would be stable, the airspeed was increased in small

increments and the test cycle was repeated. 

Early flight tests involved oscillating the right-hand excitation actuators (one at a

time) at a fixed frequency to drive a selected structural mode at resonance. The

oscillations were then abruptly turned off and the resulting rate of decay of the

structural vibrations was measured to determine the level of damping (an indication of stability). Since the resonant frequency for each of the modes was not

precisely known in advance, the test had to be repeated several times to excite

the desired mode. Another early method used to excite the various structural

68

Top:

Figure 53.

Tilt rotor structural elastic

modes.

Bottom:

Figure 54.

Wing modes of the tilt rotor

aircraft structure.

Beam modes

Torsion modes

Chord modes

Flaperon

Flaperon

Strain gages

Symmetric

beam mode

Symmetric

chord mode

Symmetric

torsion mode

Antisymmetric

beam mode

Antisymmetric

chord mode

Antisymmetric

torsion mode
modes of the tilt rotor aircraft involved natural (or wake) turbulence excitation.

The results of these initial structural dynamic evaluations are presented in reports

by Bell and Government researchers.39,40 

An extensive series of airplane mode aeroelastic stability tests were conducted in

March and April of 1987 by Wally Acree, the Ames TRRA principal investigator.

The analysis of these test results revealed several problems. Many of the important

mode-shape natural frequencies were closely spaced and some modes were not easily excited, especially with the natural turbulence excitation. Most significantly, the

resulting damping-estimate scatter, although always indicating positive stability, was

too extensive for meaningful correlation with, and validation of, the analytical predictions. The addition of left-hand flaperon and collective-pitch actuators similar to

those on the right side of the aircraft enabled the excitation of specific symmetric

and anti-symmetric mode shapes but the damping level scatter remained too large.

Another modification to the excitation system provided the capability to input

“frequency sweeps,” the continuous variation of the excitation frequency from a

pre-selected low setting to a pre-selected high setting (over a period of 23 seconds), at a chosen amplitude. Each test point required the test pilot to maintain

the flight condition for about 30 seconds. Again, using the prior analytical methods, the damping level for many modes was poorly defined. 

The search for improved aeroelastic stability test and data analysis technology

led to the application of frequency-domain methodology by Dr. Mark B. Tischler

of the Army Aeroflightdynamics Directorate at Ames.41 This work improved the

quality of the flight test results, improved the identification of the modes and,

coupled with the frequency sweep excitation, was demonstrated to reduce the

total flight time required for flight envelope expansion stability evaluation.

The aeroelastic stability flight program at Bell, led by Jim Bilger, evaluated various experimental methods and conducted extensive investigations of two configurations of titanium proprotor hub yokes and one steel hub. No significant effects

on stability were detected for the three hub configurations.

An important result of the aeroelastic stability flight test evaluations42 done at

Ames and Bell was that positive damping (i.e. positive stability) was verified for

69

39 J. M. Bilger, R. L. Marr, Ahmed Zahedi, “Results of Structural Dynamic Testing of the XV-15 Tilt

Rotor Research Aircraft,” Presented at the 37th Annual AHS Forum, New Orleans, Louisiana, May 1981.

40 L. Schroers, “Dynamic Structural Aeroelastic Stability Testing of the XV-15 Tilt Rotor

Research Aircraft,” AGARD Paper No. 339; also NASA TM-84293, December 1982.

41 C. W. Acree, Jr., M. B. Tischler, “Using Frequency-Domain Methods to Identify XV-15

Aeroelastic Modes,” NASA TM-100033, Nov. 1987, and; C. W. Acree, Jr., Mark B. Tischler,

“Determining XV-15 Aeroelastic Modes from Flight Data with Frequency-Domain Methods,”

NASA TP-3330 and ATCOM Technical Report 93-A-004, 1993.

42 W. L. Arrington, M. Kumpel, R. L. Marr, K. G. McEntire, “XV-15 Tilt Rotor Research Aircraft

Flight Test Report,” Vol. I-V, NASA CR 177406 and USAAVSCOM TR-86-A-1, June 1985.
all identified elastic modes at all airspeeds and altitudes examined. The most significant and technically difficult objective of the TRRA project and the goal set

nearly 30 years earlier during the XV-3 project had finally been achieved.

Short Takeoff Investigations

In August, 1982, the Ames TRRA Project Office continued performance and handling qualities evaluations of the XV-15, aircraft N703NA. This included investigations of the tilt rotor’s short takeoff performance (STO) characteristics. To vary the

weight and center-of-gravity (c.g.), lead-shot-filled bags were placed in the fuselage and lead plates were affixed at the nose and tail of the aircraft. Following a

series of evaluations at various c.g. locations, a number of flights were conducted

to assess STO performance at high gross weights. Because of the high risk

involved, these tests were performed at the sparsely populated and remote Crow’s

Landing Naval Auxiliary Landing Field (NALF), located about sixty miles from

Ames. With the aircraft at or near the maximum takeoff gross weight, and the

nacelles positioned at a preselected angle, the pilot released the brakes as the proprotors were brought to the desired torque level. The aircraft was then rotated for

liftoff at a target ground speed and an attitude for maximum rate-of-climb was established (see figure 55). The aircraft position was measured using a laser operated by

Ames Flight Operations Division personnel and contractors. The tracker utilized a

laser retro-reflector mounted on the landing gear pods of the aircraft and the data

were recorded for later correlation with aircraft data. Even at the maximum gross

weight of the XV-15, the short takeoff operation was a rapid and very dynamic

maneuver. This investigation enabled the effect of nacelle angle on STO performance to be evaluated. Too high an angle (at reduced torque to simulate a condition

for which only STO and not vertical takeoff was possible) resulted in lower rates

of acceleration, therefore extending the ground roll before liftoff could occur. Too

low a nacelle angle provided improved ground roll acceleration, but the reduced

vertical lift vector from the proprotors delayed the liftoff. It was determined (for

the XV-15 at its maximum takeoff

gross weight, and at approximately 60

percent of the normal power) that the

optimum nacelle position for minimum

ground roll to clear a 50-foot obstacle

was 75 degrees. Evaluations of this

type verified the capability of the tiltrotor aircraft to perform short takeoffs at

gross weights well above its vertical

takeoff gross weight, adding an important performance capability to this new

aircraft type. 

As often happens in developmental

work, a totally unforeseen incident

involving a critical proprotor hub

70

Figure 55.

XV-15 during short takeoff

performance test.

(Ames Photograph

AC82-0723-22)
component occurred during the STO tests. This component, called the “yoke,”

to which the blades are attached, was manufactured of titanium because it

afforded valuable weight saving over steel while still providing the required

fatigue life. 

On October 1, 1982, while performing STO operations at the Crows Landing

NALF, at the XV-15’s maximum takeoff gross weight, a telephone call was

received by the Ames test director Shorty Schroers, from engineers at the Bell

facility in Texas. They informed Schroers that they had just discovered that

the strength of titanium material used for the rotor yokes was significantly

lower than that used in their design. The flight crew was informed about this

new and somewhat disturbing development while in flight. They landed the

XV-15 safely and removed the weights added for the STO tests. After further

consultation with Bell engineers, it was decided to “gingerly” fly the aircraft

back to Ames taking special care to keep the hub yoke oscillatory loads at a

low level.

The full story regarding the titanium fatigue strength anomaly emerged later.

While performing design work for another project, a Bell engineer came across a

published fatigue strength allowable load level for titanium that was lower than

that used for the design of the XV-15 yokes. Although the titanium identified by

the Bell engineer and the titanium used for the proprotor were the same, a difference existed in their fatigue strength because of heat treatment (a process by

which the strength and other properties of metals are altered by exposure to specific thermal conditions). As luck would have it, the heat treatment for the 

titanium used for the XV-15 yokes was the one which resulted in the lower

fatigue strength. This meant that aircraft N703NA had been operating at significantly higher loads than the lower strength titanium could bear for the duration

of the flight program.

Operation of both XV-15 aircraft was continued but with the installation of a

new set of titanium yokes and with the allowable loads reduced until a better

solution was found. The solution was replacement of the titanium yokes with

steel yokes of the same design. Steel yokes were installed on aircraft N703NA in

July of 1985 and have been used continuously since then without incident.

Flow Visualization Studies

In the early 1980s, a number of tilt rotor technical issues remained unexplained.

One of these was that acoustic measurements in the hover mode of flight

revealed that noise, rather than being at about an equal intensity around the aircraft, was greater behind the aircraft than at an equal distance along its sides.

Another issue was that, although the magnitude of the download was now

accepted as being greater than initially estimated (based on recent performance

investigations), verification of the reason for this was needed. In an attempt to

answer these questions and to better understand the airflow around the tilt rotor

71
aircraft in general, in-flight flow visualization studies were made using

tufts taped to the wing and flaperon

upper surfaces.43 Flow direction was

recorded in flight with a movie camera mounted at the tail of the XV-15.

These studies surprisingly showed a

spanwise inboard flow over the wing

instead of the expected chordwise

flow from hover through low-speed

helicopter flight mode. 

Another simple but unusual test was

set up on the Ames tiedown test stand

to investigate the flow conditions

above the wing. The approach involved

video taping smoke ejected over the

wing while the aircraft was operated in

the hover mode. Since the XV-15 was

full-scale with accompanying high airflow velocities through the rotor, a

high volume smoke source was

required. Nontoxic, non-corrosive,

smoke grenades of the type usually

used by downed aircrew were selected. 

The test apparatus consisted of a heatinsulated “smoke” box into which the

smoke grenade would be dropped, a

blower at the outlet of the box, and

ducting leading from the blower to the

top of the wing. Since this was a low

budget test operation, an electrically powered leaf blower, generously provided by

TRRA project engineer Jim Weiberg, was used to pump the smoke. To everyone’s

satisfaction, the first test of this system (without the aircraft in position) was a

resounding success. When a smoke grenade was ignited and dropped into the

smoke box everything looked fine. A thick jet of colored smoke emerged at high

speed from the duct exhaust accompanied by the comforting roar of the blower.

However, success was short lived. In very short order the sound of the leaf blower

changed from a roar to a high pitched squeal and smoke started flowing from the

box instead of from the end of the duct. Clearly something was not right.

Following a fast shutdown, it was discovered that the leaf blower was equipped

with a plastic fan which had melted from the heat generated by the smoke. Thus,

72

43 Tufting is a flow visualization technique using small lengths of yarn affixed to a surface to indicate directions and patterns of surface flow.

Top:

Figure 56.

Flow visualization near 

the XV-15 wing tips.

(Ames Photograph

AC85-0804-49)

Bottom:

Figure 57.

Flow visualization near 

the XV-15 wing mid-span

position.

(Ames Photograph

AC85-0804-35)
Jim Weiberg’s leaf blower became a

casualty in the quest for advancement

of tilt rotor aircraft technology. The leaf

blower was replaced with a commercial

blower having metal fan blades and an

electric motor. This new smoke generating system functioned well and provided the smoke needed for the flow

visualization study. 

The flow visualization data revealed

that near the wing tips, as expected,

the proprotor wake impinged on the

wing upper surface and spilled over

the leading- and trailing-edges of the

wing in a chordwise direction (figure

56). As the smoke was moved to the wing midspan position, it showed that the

proprotor wake was also moving in a spanwise direction toward the fuselage

(figure 57). With the smoke source moved further inboard, it was seen that the

flows from the two proprotors moved spanwise toward each other and combined

above the fuselage centerline, turning vertically upwards to form a “fountain

flow” above and along the aircraft’s longitudinal plane of symmetry (figure 58). 

These observations confirmed the inboard flow observed from the tuft study

mentioned earlier. Furthermore, the large air mass involved in the over-fuselage

fountain flow created a large downward force which accounted for the higher

than expected download in the hover mode of flight. As explained later, this

fountain flow was also found to contribute to the nonuniform distribution of

noise around the hovering tilt rotor aircraft.

Sidestick Controller

Among the many decisions made early in the development of the TRRA was the

cockpit control configuration. Simulation and flight evaluations by Bell and

Government pilots resulted in the selection of a helicopter-type power lever for

rotor control and a conventional center stick and rudder pedals for longitudinal,

directional, and pitch control inputs. The tall center stick, however, with its masscenter several inches above its pivot point, introduced undesirable dynamic

effects (called “bobweight” motions) during maneuvers. This issue, coupled with

the possible interference of the center stick with crew station structure (instrument panel), problems with cockpit ingress or egress, and the general interest in

conserving limited cockpit “real estate,” led researchers to investigate the use of

a sidestick controller as the principal flight control for the developing military

JVX tilt rotor aircraft (later called the V-22 Osprey). The principal concerns with

this type controller were whether it would be able to provide the same level of

control as the conventional center stick, and whether it could perform adequately

73

Figure 58.

Inboard flow visualization

showing “fountain flow”

above fuselage.

(Ames Photograph

ACD-0804-3.1)
during “degraded” flight control system conditions (such as a malfunctioning or

battle-damaged control system).

To answer these questions, it was decided to perform a piloted simulation evaluation and a full flight investigation of a 3-axis sidestick controller on an XV-15

TRRA for both normal and “degraded” flight control system conditions. Gary

Churchill, senior controls engineer with the TRRA Project Office, developed the

control laws and was the primary investigator. 

The XV-15 TRRA was ideal for the installation of the sidestick controller

because it had bulging side windows (designed into the aircraft for better visibility) and an uncluttered side console which provided room for a functional installation, including an adjustable arm rest. A control and status panel for the sidestick controller was added to the instrument panel. 

Initial sidestick control system gains and sensitivities were established using the

Ames Vertical Motion Simulator (VMS) and a refined simulation math model

based on the one originally developed in the early stages of the XV-15 project.

These control law parameters were adjusted during XV-15 installation/hangar

checks, and the resulting configuration was taken into the flight program.

In July 1985, an intensive flight evaluation of a three-axis sidestick controller

was performed in XV-15 N703NA. During a nine day period, a total of 13 flights

were flown with eight pilots from six agencies (the NASA, Army, Navy, Marine

Corps, Bell Helicopter Textron, and Boeing Helicopters). Control characteristics

of the center stick and the sidestick controller were compared. Each pilot

received a familiarization flight in the left seat using a conventional center stick

control and flew an evaluation flight in the right seat which was equipped with a

sidestick controller. Without exception, all of the evaluation pilots found the sidestick to be a viable controller and that the aircraft was safe to fly with a degraded

control system (i.e. with the SCAS turned off). The pilots even reported that

some tasks could be performed with more precision with the sidestick controller

than with the conventional center stick. 

While the sidestick investigation successfully achieved its objectives, the V-22

Osprey was nonetheless configured with a center stick control. However, the

sidestick controller continues to be considered by the V-22 Project Office for

future application to the tilt rotor aircraft. 

Acoustics

By the late 1970s, communities adjacent to airports and heliports had become

quite sensitive to the noise generated by aircraft operations, in particular, to the

disturbing character of the sound of rotorcraft noise. Therefore, if the tilt rotor

were to be used as a civil transport aircraft, it was important to document its 

noise in the terminal area. In addition, it was necessary to establish a tilt rotor

74
noise database for various flight modes and operating conditions for use in the 

development of prediction methodology. The XV-15 became the test bed for a

wide range of tilt rotor acoustics studies.

Some very limited initial noise data were obtained with the XV-15 at Bell and

consisted of only a few data points acquired during early hover tests. The next

opportunity to measure tilt rotor noise occurred during hover performance testing

at Ames in February and March 1981. An array of 16 microphones was distributed around a selected hover point to fully document the noise around the aircraft. The resulting acoustic data (refer to footnote 34) surprisingly showed that

the noise varied by a few decibels around the aircraft, rather than remaining

nearly constant. An explanation was later provided by Professor Al George of

Cornell University who postulated that this was caused by the reingestion of the

turbulent fountain flow (revealed during the flow visualization test) into the proprotor over the root end of the wing.44 

Several subsequent tests were conducted to explore the sound generated during flyover or terminal approach conditions. The first was conducted at Crows Landing in

September 1982 by a NASA/Army team and again in April 1986 with support

from Bell. The NASA operated radar-coupled laser tracker was used at the isolated

Crows Landing NALF to measure the track of the XV-15 during approach and flyover operations. This allowed the researchers to relate the exact position of the aircraft with respect to each microphone with the recorded noise data. The initial

evaluation45 of these data was reported by John Brieger, et al. Later analysis of this

and other acoustic data was reported46 by Bell’s Bryan Edwards.

Another area of interest was the proprotor noise at the external fuselage walls

of the aircraft (which would affect cabin acoustics). The cabin noise, especially

for civil transports, would have to be at or below acceptable comfort levels.

Furthermore, if large amounts of noise-reducing insulation were required, it

would impose a significant weight penalty and impact the economic viability

of the civil tilt rotor aircraft. Measurements of the distribution of sound pressure along the side of the XV-15 fuselage and at two locations within the cabin

were obtained for various flight conditions during tests of N703NA at Ames.

Later tests focusing on cabin interior noise were conducted by Suzanna Shank47

of Bell.

75

44 C. D. Coffen, Albert. R. George, Analysis and Prediction of Tilt Rotor Hover Noise, AHS 46th

Annual Forum and Technology Display, Washington D.C., May 21-23, 1990.

45 John T. Brieger, Martin D. Maisel, Ronald Gerdes, External Noise Evaluation of the XV-15 Tilt

Rotor Aircraft, AHS National Specialists’ Meeting on Aerodynamics and Aeroacoustics,

Arlington, Texas, February 25–27, 1987.

46 Bryan D. Edwards, “XV-15 Tiltrotor Aircraft Noise Characteristics.” Presented at the AHS 46th

Annual Forum and Technology Display, Washington, D.C., May 21–23, 1990.

47 Suzanna S. Shank, “Tiltrotor Interior Noise Characteristics.” Presented at the AHS and Royal

Aeronautical Society, Technical Specialists’ Meeting on Rotorcraft Acoustics/Fluid Dynamics,

Philadelphia, Pennsylvania, October 15–17, 1991.
A further series of noise measurements was made during hover tests at

Ames in December 1990, and during

terminal area and flyover tests at the

Crows Landing NALF in August and

September 1991, with the new composite blades installed on XV-15

N703NA. These were the first such

experimental measurements from

flight data with a proprotor blade configuration other than the original metal

blades. The data were acquired to validate acoustics analyses being developed by researchers at the Langley

Research Center, under the NASA

Short-Haul Civil Tiltrotor (SHCT) program. These tests were a joint effort between the Langley acoustics engineers

and technicians and the Army/NASA TRRA team at Ames. Operations were conducted just after sunrise (shown in figure 59) to ensure low wind conditions (usually less than 3 knots) during noise data measurements. 

Additional investigations of the terminal area noise generated by the XV-15

with metal blades were conducted by Bell at a remote site near Waxahachi,

Texas, in October and November of 1995. The relatively level, undeveloped terrain, far from major roads and undesirable background noise, provided an ideal

environment for this work. A large microphone array was set up around the target landing point while a mobile laser tracker from Ames was placed nearby to

measure the position of the XV-15 during the tests. This study focused on the

effect of approach profile on the intensity of the noise propagated to the ground,

and utilized approach conditions examined earlier during simulation evaluations

of terminal area operations in the Ames Vertical Motion Simulator. Bill Decker,

the NASA Ames principal investigator for the simulation studies, participated in

the terminal area test planning and test operations. To provide flight path guidance, the XV-15 used a Global Positioning System (GPS) monitoring research

flight director which was developed by Mark Stoufflet and Colby Nicks of Bell.

A Langley team acquired acoustic data from an array of 33 microphones covering an area of five miles long and 1.25 miles wide. The test results confirmed

that appropriate combinations of aircraft configuration and flight path profile

could be used to significantly reduce the noise level and footprint area during

tilt rotor approaches.

In December 1995, with plans being developed for an acoustics test of the XV15 metal-bladed proprotor in the acoustically treated test section of the Ames 80- 

by 120-foot wind tunnel, a special flight investigation was required to obtain

comparable free flight noise data to determine the effect of the wind tunnel walls

on the measured sound. The evaluation involved flying the XV-15 behind, and in

76

Figure 59.

Hover acoustics tests 

during low wind conditions

at sunrise.

(Ames Photograph

AC90-0448-31)
close formation to a quiet research aircraft (the Lockheed YO-3A) which was

equipped with microphones and recording equipment. By maintaining the YO3A microphone location at a fixed distance and position with respect to the XV15 proprotor (shown in figure 60) corresponding to a microphone location in the

test section of the wind tunnel, and by operating at the same proprotor operating

condition, a direct comparison (with corrections for the second proprotor)

between the flight data and wind tunnel test data was obtained. This experiment

was conducted by Ames researchers. The tests48 involved a Bell flight crew in the

XV-15, and a NASA flight crew in the YO-3A. 

Composite Proprotor Blades

From the very beginning of the TRRA project the proprotor blades were of special concern to the Government Project Office. The metal blades used on the XV15 were designed in the late 1960s under Bell’s IR&D funding for the predecessor tilt rotor aircraft, the Bell Model 300. This aircraft had a design gross weight

of 12,400 pounds, 600 pounds lighter than that of the XV-15. The concern was

77

Figure 60.

The XV-15 flying in close

formation with the YO-3A

for acoustics data. 

(Ames Photograph

AC95-0438-15.1)

48 The results of the 1995 terminal area and in-flight acoustics tests are presented in: Michael A.

Marcolini, Casey L. Burley, David A. Conner, C. W. Acree, Jr., “Overview of Noise Reduction

Technology in the NASA Short Haul (Civil Tiltrotor) Program,” SAE International Powered Lift

Conference, Jupiter, Florida, November 18-20, 1996.
that the proprotors would be too highly

loaded, i.e. operating too close to aerodynamic stall, to provide adequate

reserve thrust for control when operating in hover at high gross weights.

This could result in a reduction of control effectiveness or the need for a substantial increase in power when operating at the high gross weight condition. 

Flight tests of the XV-15, however, did

not indicate deficiencies. The metal

bladed proprotor, although sized for a

smaller aircraft, performed well at all

XV-15 operating weights and flight

conditions. While performance was

satisfactory, another problem emerged

that could threatened the future of the

XV-15. This was the possibility that

one or more blades could become

unserviceable or unflightworthy due to

mishandling or deterioration of the

blade’s structural integrity.

Concern centered on the aft blade section, an aerodynamic fairing constructed of a lightweight aluminum honeycomb core covered with a thin steel skin

(figure 61). Over the first few years of aircraft operations, minor surface damage

was incurred due to ground handling. More significantly, small areas of separation of the bond between the skin and the honeycomb was detected on several

blades. While the size of these “voids” was monitored during frequent inspections, the discovery of a rapid growth in size or an unacceptably large separation

area could render the blade unusable for flight. The limited number of spare

blades (two right and one left) meant that the loss of two left flightworthy blades

would ground an aircraft.

Part of the TRRA Project Office advanced flight research program goals was the

“investigation of alternate or advanced proprotor configurations.” This was consistent with the Project Office’s perceived need to replace the blades, both to assure the

continuation of flight testing and to explore the application of new materials technology. The activity, to design, build, and flight test a new set of proprotor blades

for the XV-15, was known as the Advanced Technology Blade (ATB) project.

Although there were no immediate prospects for funding an upgraded transmission that would allow a larger amount of the installed engine power to be used

(providing a significant enhancement of the XV-15’s performance), the ATB proj78

Top:

Figure 61.

Typical cross section of the

XV-15 metal blades.

Bottom::

Figure 62.

XV-15 Advanced

Technology Blades 

configuration variations.

Trailing edge block

Nose weight Aluminum honeycomb core

Steel spar Stainless steel skin

Basic cuff

Basic tip

Swept tip

Square tip

Extended cuff

Cuff extension

Cuff off

Eccentric bushings
ect was considered the first step in this

direction. Therefore, on August 12,

1980, an RFP was issued by the TRRA

Project Office for the procurement of

the ATB’s. The design objectives called

for the development of “a blade design

compatible with the XV-15 tilt rotor

research aircraft which improves static

stall margin and cruise speed performance using advanced structural materials and design techniques to improve

the strength and service life of the tilt

rotor blades.” Proposals in response to

this RFP were received from Bell and

Boeing Helicopters, and were evaluated by an SEB comprised of NASA and

Army technical and procurement specialists. While both proposals were determined to be acceptable, the decision was

made to award the contract to Boeing. Among the factors that influenced this

decision was the significant experience Boeing had acquired with composite

rotor blades provided for the Army’s fleet of CH-47 helicopters. Also, the Boeing

blade design provided the ability to alter blade sweep and incorporate removable

tip and cuff (inboard fairing) sections which allowed them to propose alternate

blade configurations for research purposes. These features are illustrated in figure 62. It was noted that the Boeing blade had a larger solidity (effective area)

than the Bell blade which contributed to the desired improvement in the stall

margin. This would prove to have an unexpected effect on the XV-15/ATB flight

program. A contract to develop the composite proprotor blades was awarded to

Boeing Helicopters on July 9, 1982.

As part of the ATB qualification and evaluation program, a series of hover performance tests were conducted on the OARF at the Ames Research Center

between February and April of 1985. These tests evaluated three tip configurations

and two cuff configurations on the ATB, as well as the XV-15 metal bladed proprotor, and an approximate 2/3-scale model of the proprotor designed for the JVX

military tilt rotor aircraft. Figure 63 shows the ATB on the OARF Prop Test Rig. 

This test series produced a large amount of high quality performance data.49, 50

The isolated proprotor hover data validated the predicted ATB performance and

showed that the XV-15 metal blades actually performed slightly better than previously expected.

79

49 F. F. Felker, M. D. Maisel, M. D. Betzina, “Full-Scale Tilt-Rotor Hover Performance.”

Presented at the AHS, 41st Annual Forum, Fort Worth, Texas, May 15–17, 1985.

50 K. Bartie, H. Alexander, M. McVeigh, S. Lamon, H. Bishop, “Hover Performance Tests of

Baseline Metal and Advanced Technology Blade (ATB) Rotor Systems for the XV-15 Tilt Rotor

Aircraft,” NASA CR-177436, 1986.

Figure 63.

Advanced Technology

Blades proprotor mounted

on the test apparatus at the

Ames Research Center

Outdoor Aerodynamic

Research Facility.

(Ames Photograph

AC84-0498-2)
Following the completion of controllability flight evaluations at Ames with modified SCAS components installed in N703NA, efforts began to prepare the ATB

for flight tests. XV-15/ATB ground runs on the ramp and on the tiedown stand

were conducted between September and early November of 1987 and the first

hover flight with the new blades was performed on Friday, November 13, 1987.

From the first operations with the ATB there were problems. The initial difficulties surfaced during the runs required to obtain a satisfactory proprotor track and

balance. Balance of the two interconnected proprotors presented problems on the

XV-15 since a change on one proprotor provided an excitation that resulted in a

change in the dynamic behavior of the other proprotor. Obtaining a proper balance with the ATB presented a special problem which stemmed from the frequent addition or removal of small weights from a fiberglass weight block located at the tip of each blade within a removable tip cover. The frequent removal of

the tip covers to alter the weights resulted in the failure of the metal screw-retention inserts installed in the fiberglass weight blocks. Other problems included the

deformation of the skin material under the retention screws at the fiberglass tip

requiring the installation of metal washers, the failure of the bonds within the

tip-weight assembly, and the delamination (unbonding) of the blade skins from

the underlying nomex honeycomb material. Many of these material issues continued to cause problems during operations with the ATB.

When the expansion of the flight envelope in the helicopter mode with the ATB

began in June 1989, higher than expected oscillatory blade control loads were

measured at airspeeds as low as 40 knots. These loads increased with airspeed

and reached the allowable limit at about 65 knots, too low to allow a safe envelope for initiating conversion. At that point, efforts were intensified to analyze

test results and initiate analytical studies in order to determine the cause of the

high loads. In addition, the loads investigation, headed by John Madden from

Ames, included a series of tests on the XV-15 control system to determine stiffness characteristics as a function of the rotational (azimuthal) position of the proprotor. The results of this evaluation revealed that a major mechanical rotor control component, called the swashplate inner ring, did not provide uniform stiffness at all azimuthal positions. The lower than expected stiffness, coupled with

the increased blade mass and inertia of the ATB (due to the larger solidity than

the metal blades) resulted in lowering the natural frequency of the control system

to the 3/rev (3 vibrations per proprotor revolution). When the three-bladed proprotor was flown in forward helicopter mode flight, the 3/rev aerodynamic excitation coupled with the system’s natural frequency to produce high structural

loads. 

A temporary remedy was proposed by John Madden and was subsequently

implemented. A set of shims was installed between the inner ring and the transmission housing which locked out the lateral cyclic input to the rotor (used for

flapping reduction in helicopter mode flight) and provided the required increase

in the control system stiffness. A permanent modification to change the inner

80
swashplate ring material from aluminum to steel was planned if the shims proved

effective. 

After another series of ground runs, tiedown tests, envelope expansion flights

and tip repairs, the XV-15 with the ATB achieved airplane mode flight on

December 14, 1990. The oscillatory control loads were sufficiently reduced by

the shims to allow full conversion. Then another problem appeared.

The ATB, having a larger solidity than the metal blades which the control system was designed for, required greater steady control forces to hold the blade

at the collective blade angles required for high-speed airplane mode flight. The

dual hydraulic collective actuator was, in fact, capable of providing this force,

but since only one of the dual units was equipped with an automatic switchover

to the backup hydraulic system in case of a primary hydraulic system failure,

flight operations had to be limited to loads within the capability of one half of

the dual actuator. This imposed a restriction on the maximum airplane mode

airspeed with the current control system configuration. To correct this limitation, Bell was tasked to develop a design for the automatic hydraulic backup

for the unprotected side of the dual collective actuator. The task order, under

the XV-15 support contract, also required Bell to provide steel swashplate

inner rings to correct the low control system stiffness and restore the lateral

cyclic control.

With the dynamics and loads issues associated with the ATB understood and

with corrective actions taken, the Army/NASA TRRA team once again focused

on tilt rotor research. In a cooperative program with acoustics experimenters

from Langley Research Center, ATB noise surveys in the hover mode were conducted at the Ames Research Center in December 1990. Starting on August 21,

1991, a series of flyover and terminal area noise measurements were also performed at Crows Landing. 

On September 6, at Crows Landing, while NASA test pilots George Tucker and

Rickey Simmons were on a downwind leg of the traffic pattern prior to setting

up another test approach, they heard a loud noise in the cockpit followed by a

sudden and violent increase in the vibration level. At the same time, in the control room at Ames, the normally narrow traces on the strip chart recorder showing safe, within-limit, oscillatory loads and moments instantly blossomed to the

full width of the bands, indicating that the safe load levels had been greatly

exceeded. In the cockpit, the vibration was so severe that the instruments were

not readable. Rickey Simmons reduced power and turned toward the runway

while George Tucker contacted the control tower requesting an immediate landing. The tower asked if emergency vehicles were required and the response was

affirmative. With fire and rescue trucks rolling, the aircraft was brought to a safe

landing about 80 seconds after the high vibration started, followed by a rapid

shut-down.

81
After the proprotors stopped, the problem became obvious. The cuff fairing on

one of the left proprotor blades had moved outboard about eight inches. Analysis

revealed that the displacement of the cuff was due to the failure of metal retaining clips to carry the cuff’s centrifugal loads to the blade structure, as intended

by the design. Instead, because of tolerance buildup and poor workmanship and

assembly, the loads were borne by the fiberglass flange rather than the metal

retaining clips. This eventually led to the failure of the fiberglass flange.

Following inspection, the aircraft was disassembled with the assistance of a Bell

crew, and transported to Ames onboard flatbed trucks.

After reassembly, a structural dynamics “shake test” was performed at Ames

with aircraft N703NA. This activity was conducted by Wally Acree to provide

accurate aircraft resonant frequency characteristics for aeroelastic stability analyses. Upon completion of the shake test in January 1991, the aircraft entered a

100-hour major inspection. 

Meanwhile the high oscillatory loads imposed on the aircraft’s structure were

analyzed to determine the amount of fatigue life consumed by the sliding cuff

incident. While the fatigue damage was considerable for a single event, it was

determined that aircraft N703NA was safe to fly again.

Before the ATB could be used again, however, the cuff retention configuration

would have to be redesigned to prevent a reoccurrence of the failure. An

improved cuff retention was designed and fabricated by Ames and successfully

proof tested. Changes in NASA Ames’ role in flight research soon occurred and

altered plans for further flight testing of the XV-15.

82
Door Strike

On September 2, 1977, during the start of a ground run the test pilots noticed

that the door lock caution light was illuminated as the normal engine start procedure was initiated. This caution light, which was connected to a sensitive

microswitch, indicated the position of the door bolt. This had previously been the

source of numerous nuisance indications which were suspected to be due to the

improper setting of the microswitch position. Because of this, the position of the

door handle was visually checked, found to appear to be in the correct position,

and the engine start procedure was continued.

After less than three minutes following engine start, with the proprotors in the

airplane mode and while increasing power, the cabin door opened and was struck

by the proprotor blades, scattering pieces of aluminum over the aircraft. 

The crew immediately began to convert back to the helicopter mode, shut down

the engines, and cut off the fuel supply. There were no injuries.

The incident which damaged the three right hand blades (one beyond repair) and

destroyed the cabin door, was a classic example of ignoring a troublesome caution light that was, in fact, providing valid information. 

Because the XV-15 airframes were essentially hand crafted, the door from

N703NA would not fit the N702NA fuselage satisfactorily, so a new door was

fabricated. Modifications to reduce the probability of a repeat of this incident

included a small window in the frame to allow visual inspection of positive

latching of the door bolt and the installation of a short cable inside the cabin for

the crew to connect to the door after entry. 

In-Flight Engine Failures

After the initial XV-15 flight envelope expansion activities demonstrated the predicted favorable handling qualities and performance capabilities, the Government

Project Office began to plan for the second phase of flight tests with the tilt rotor

aircraft in support of potential military applications. To better understand military application requirements, and to inform key civil and military aviation planners of the capabilities offered by tilt rotor aircraft, a Tilt Rotor Experiments

Planning Workshop was held on December 4 and 5, 1979, in Arlington, Texas.

The meeting was attended by about 100 senior military and civilian personnel

from each of the U.S. armed services. Presentations were made by Government

Project Office personnel, senior Government and Bell managers and test pilots.

During the second day of the meeting, the managers were informed that XV-15

aircraft N702NA, on approach to Arlington Municipal Airport, only a few miles

away from the meeting site, experienced an engine failure. It was reported that

the chase aircraft had observed smoke trailing from the right nacelle at about the

same time that the XV-15 flight crew heard a screeching noise, followed by the

Incidents

83
sudden stoppage of the right engine. The flight crew declared an emergency and

were cleared for immediate landing as emergency vehicles were positioned along

the runway. The failed engine was immediately disengaged from the drive system by the automatic clutch, allowing the operating left engine to drive both proprotors by transmitting power through the cross-shaft to the right proprotor. The

drive system and flight controls worked as planned and a single engine run-on

landing was completed with the nacelles set at 70 degrees without further complications. This unplanned event, although poorly timed because of the workshop, demonstrated both the benign impact that an isolated, single engine failure

would have on the tilt rotor aircraft, and the proper functioning of the crossshafting during an engine-out emergency. 

A subsequent analysis and evaluation by Lycoming determined that the cause for

the failure was a fatigue crack in one of the aft bearing support struts which

caused interference of concentric counterrotating shafts. This resulted in the sudden engine stoppage. The strut failure was brought about when, during numerous

startups over the life of the engine, it was operated at a particular RPM that corresponded to the resonant frequency of that structure, momentarily producing

excessively high loads. The approach employed to avoid this problem in the

future was to monitor engine housing vibrations during startup to ensure that

when high “g” vibration levels occurred that could lead to a fatigue failure of the

aft bearing support, the high loads would be reduced by changing RPM.

Another engine failure occurred on September 7, 1983, while XV-15 N702NA

was hovering at a wheel height of about 15 feet. A fuel control unit failure

induced fuel starvation, and the low initial height did not allow sufficient time

for the power on the operating engine to be brought to the necessary level to

arrest the rapid descent. In spite of the hard landing that followed, the cross-shaft

system again worked as it was designed to and no damage to the aircraft was

incurred.

Tree Strike

The XV-15, like all aircraft of any type, at one time or another, was exposed to

inadvertent hazardous conditions, even while in the hands of the very capable

pilots. One such incident occurred on July 30, 1979, at the Arlington, Texas,

Municipal Airport, the base for Bell’s flight test operations. The flight plan called

for a maximum acceleration from a low hover over the runway. As the nacelles

were continuously tilted forward at the maximum conversion rate, the aircraft

speed began to increase. But when the end of the airfield was reached the XV-15

was still below the level of the tree tops just beyond the perimeter of the airport.

With barely enough speed to climb, the XV-15 passed through the tops of the

trees. After circling back and landing, the ground determined that the aircraft had

survived the “tree strike” essentially unscathed. The only damage being the green

stains on the proprotor blade tips, the shattered nerves, and the embarrassment of

the crew.

84
It turned out that once again a set of unusual circumstances caught the experienced pilots by surprise. The maximum rate conversion was previously performed at a greater altitude, usually from a helicopter mode with some airspeed.

A conversion from hover to airplane mode can be completed in 12 seconds.

While this routinely does not present a problem at altitude, it did in this case.

The rapid conversion lowered the rotor thrust vector before adequate speed for

wing lift was achieved, resulting in a slight settling of the aircraft. However, the

settling was not detected by the flight crew until they approached the end of the

airfield boundary. The XV-15 on that flight came within a few feet of disaster.

Good fortune is a wonderful thing to have.

Gear Down Conversion

The NASA flight crew continued to have good luck on their side. During a 

busy flight test at the Crows Landing NALF, the XV-15 was converted from the

helicopter mode to the airplane mode in view of the photo/chase helicopter. The

ground monitoring station personnel at Ames noticed that many of the critical

structural loads that were usually reduced following conversion remained seriously high. In the cockpit, the pilots were alerted by the unusually high noise

level. After an anxious call from the Test Director to the XV-15 to report the high

load levels, the loads suddenly were reduced to the expected levels. It turned out

that the conversion to the airplane mode was inadvertently made with the landing

gear down (and, of course, with the landing gear doors open). Besides resulting

in increased aerodynamic drag that could have adversely affected the control

characteristics of the aircraft, the landing gear doors were structurally limited to

flight speeds below 90 knots. The loss of these doors in flight could have damaged the aircraft’s tail surfaces. On that flight, the landing gear doors were inadvertently “test-qualified” to an airspeed of nearly 160 knots without a failure.

While the XV-15 was definitely the product of good, sound engineering, it did

benefit from a large measure of good luck on that day.

Oil Vent Incident

Sometimes lessons are learned the “hard way.” This was the case when what

appeared to be a minor configuration change turned out to have a major effect.

During the initial flights of the XV-15 in 1977 and the 40- by 80-foot wind tunnel test in 1978, seepage from the engine oil vent was noticed. After putting up

with the annoying, but unimportant seepage for about eight years, on December

19, 1986, Bell engineers decided to make a minor modification to the left engine

oil vent tube in an attempt to reduce or eliminate the problem. If the modification

worked, then they would apply it to the right engine vent tube. The modification

consisted of nothing more than rotating the beveled end of the vent tube by 90

degrees. During the first flight with the modification in place, and after converting to airplane mode, the chase aircraft reported seeing excessive oil venting

from the left engine while the flight crew simultaneously noted fluctuating left

engine oil pressure. These conditions precipitated a shut down of the left engine.

85
A reconversion to the helicopter mode was followed by a safe single engine rollon landing within three minutes of shutting down the left engine. The engine oil

system was thoroughly checked and nothing amiss was found. The excessive oil

venting could only be attributed to the “minor” change made to the oil vent tube

resulting in a change in vent pressure. The engine oil level was restored, the oil

pressure transducer was replaced, and the vent tube returned to its original configuration and position. The important lesson re-learned was that all aircraft configuration changes, no matter how minor, should be thoroughly evaluated before

implementation.

Bird Strike

The “unexpected” can be expected at any time while flying in an aircraft. This

occurred on May 1, 1991, when Bell pilot Ron Erhart was demonstrating aircraft

XV-15 N702NA to USAF Gen. Schmaltz. Shortly after converting to the airplane

mode, while performing a high speed flyby at Bell’s Arlington Flight Research

Center, a large hawk passed through the right proprotor disc and collided with

the leading edge of the right wing at midspan. The impact was so severe that it

collapsed the leading edge fairing and cracked the wing’s aluminum forward

spar. No damage was incurred by the proprotor blades. The aircraft was slowed

and a reconversion and an uneventful landing was made immediately after the

impact incident. The crack in the spar seriously damaged the integrity of the

wing. The repair took over five months and the aircraft was returned to flight on 

October 8, 1991. 

How a bird the size of a large hawk could have passed through the proprotor

turning at about 500 rpm and not touch any of the blades was nothing short of

miraculous. Such an occurrence could easily have seriously damaged the proprotor with catastrophic results.

This event was remembered by Ron Erhart’s friends from the Federal Bureau of

Investigation (FBI) when he retired in October of 1998. At his retirement party a

bogus but official-looking “Wanted by FBI” poster was displayed that claimed

Ron Erhart was sought for allegedly “stalking, hitting, and killing a red tailed

hawk,” leaving its chicks to starve, all casualties of the tilt rotor project.

Blade Cracks

Because of the limited budget of the TRRA project, the critical structural components of the XV-15 were subjected to the minimum number of structural tests

required to establish safe operating loads and to define the fatigue life. During

ground and flight operations, these critical parameters were constantly monitored

and the data analyzed to assess the portion of the allowable fatigue life that was

consumed due to high loads. For many components, such as the metal proprotor

blades, the requirement for frequent inspections was also established. 

86
The metal blades were constructed of a steel spar, the main load-carrying component, with light-weight aluminum honeycomb aft of the spar to provide the airfoil trailing-edge fairing. The entire surface of the blade was covered with a thin

steel skin as illustrated in figure 61. Early in the flight test program evidence 

of delamination (i.e. the loss of the bond) between the honeycomb and the skin

was detected in small areas on several blades. The method for determining the

delamination area was called the “tap test” and was performed by a skilled

inspector or engineer with a “good ear.” A small tool or coin was lightly tapped

along the surface of the blade and the characteristics of the sound revealed the

integrity of the bond. Delamination areas were outlined on the blade and any

growth in the size of this area was documented. Ernie Schellhase, the Bell blade

designer, defined acceptable (i.e. “not-to-exceed”) delamination areas and a frequent blade tap test was instituted as part of the standard XV-15 inspection program.

On October 22, 1987, during a routine inspection, Bell technicians discovered

chordwise hairline cracks in the skins of two left proprotor blades near the 60

percent blade radius. These cracks, which ran from just aft of the spar to just forward of the trailing edge, were at first thought to be a defect in the surface paint,

but were confirmed by use of an electrical “eddy current” testing device and by

x-ray analyses to be a complete skin separation. 

Metallurgical studies of skin samples from the vicinity of these cracks revealed 

the presence of severe intergranular corrosion. It was determined that the cause

of the corrosion occurred during the manufacture or processing of the blade skin

material. An inappropriate “pickling” or chemical surface cleaning treatment produced the corrosion on some of the material used in the manufacture of the

blades. The severity of the intergranular corrosion on the two damaged blades

rendered them unrepairable, leaving only two flightworthy left blades (the

remaining undamaged flight blade and a spare) for N702NA. Further analyses

determined that the fatigue failures were due to the reduced effective skin thickness resulting from the corrosion combined with the local stress increase caused

by the proximity to the edge of an internal doubler (below the skin).

Shortly after the determination of the probable cause of the cracks, Bell and the

Government TRRA Project Office initiated an inspection of all XV-15 metal

blades. The blades from N703NA at Ames were removed from the aircraft and

shipped to Bell. While all of the remaining blades did not show signs of the

severe intergranular corrosion (probably because of variations in the pickling

process), it was decided to install the N703NA blades on the N702NA XV-15 at

Bell and to initiate the investigation of the composite Advanced Technology

Blades on XV-15 N703NA at Ames. The use of the N703NA shipset by Bell was

driven by the fact that these blades had about half of the flight time as those of

the N702NA blades, and by the desire to maintain a balanced set of blades. On

May 16, 1988, N702NA was returned to flight.

87
By early 1981, the XV-15 had sufficiently explored the flight envelope in all

flight modes to provide at least a first-order verification of the validity of all

design-critical analytical methods. This included performance, loads, and structural-dynamic stability. While much work remained to be done to document the

accomplishment of the initial proof-of-concept goals and to complete the

advanced research objectives of the TRRA project, it was becoming apparent

that the significance of the technical success of the project was recognized only

by a small core of people close to the activity. The NASA/Army Project Office

and Bell, therefore, decided that the emerging tilt rotor technology should be

demonstrated before a wider aviation community. The venue for this public

debut would be the renowned Paris Air Show.

The successful participation of the XV-15 at the Paris Air Show at Le Bourget

from June 4 to June 14, 1981, would prove to be one of the key nontechnical

moments in the history of this aircraft. While operation of the XV-15 at the Paris

Air Show was primarily managed by Bell, the presence of this aircraft at this

event marked two “firsts” for NASA. It was the first time that NASA has ever

participated in flight demonstrations of any experimental aircraft at an event having public international exposure such as the Paris Air Show in France. It also

marked the first public flight demonstration of an aircraft of this type. Its success

paved the way for participation of yet another NASA Ames aircraft, the QSRA,

at the 1983 Paris Air Show. 

Participation of the XV-15 TRRA at the Paris Air Show was a cooperative effort

between the NASA, the Army, the Air Force, and Bell. Dr. Irving Statler, director of the Ames Directorate, AMRDL, enthusiastically supported this activity.

Army top management support came from Dr. Carlson, Director, AMRDL, and

from Dick Ballard, Department of the Army, who advocated the showing of the

XV-15 at the Paris event to Dr. Hans Mark. Dr. Mark (previously Director of the

Ames Research Center when the TRRA project was initiated and, at that time,

Secretary of the Air Force) arranged for USAF Military Airlift Command

(MAC) transport of the XV-15 and support equipment. The logistics of such a

venture were complex. 

The schedule for the next few weeks was carefully planned to meet the critical

requirement to arrive at Le Bourget within the assigned time period. This

involved not only the preparations required to return the XV-15 to flight after

shipment to Europe, but the movement of high-value items (including the XV-15,

the spare engine, sophisticated support equipment, tools, and instrumentation)

into and out of foreign countries. The achievement of this important task, tightly

controlled by foreign regulations, was handled by Demo Giulianetti of the Ames

TRRA Project Office. 

Paris Air Show

88

51 The Carnet documents the items to be shipped and provides an official permit to cross international borders.
Working with an international freight

company, the necessary documentation

(including a U.S. Department of

Commerce-issued International

Carnet51) was prepared that permitted

the transport of this equipment into

and back out of England and France.

As a result, transport of the XV-15 and

all associated equipment into and out

of England and France was accomplished as planned and the Paris Air

Show arrival schedule was met without a problem.

On March 6, 1981, just after the completion of the hover, tiedown, and outwash investigation, XV-15 N702NA

was ferried from Ames to the DFRC.

From the time of the arrival of

N702NA at DFRC, until it departed

less than two months later, both XV-15

aircraft, in flight status, were stationed

at the same site (figure 64). This rare

event would occur only one more time

during the TRRA project. At DFRC,

the XV-15 TRRA, along with a complete set of spares (including an

engine), ground support equipment,

and a mobile telemetry data van, was

prepared for shipment to Europe. On

April 28, 1981, the aircraft (with its wing and proprotor blades removed) and

related equipment were flown to Farnborough, England, onboard a MAC C-5A

and a C-141 aircraft.

At Farnborough, the XV-15 TRRA was reassembled and the flight routine

designed to exhibit the unique capabilities of the aircraft at the Paris Air Show

was practiced by Bell pilots Ron Erhart and Dorman Cannon, and by NASA

Ames pilot Dan Dugan. This flight routine demonstrated vertical takeoffs and

landings, hovering turns, transitions to and from the cruise mode of flight, backward and lateral translations, rapid climb-outs, and airplane- and helicopter-mode

fly-bys. The 10-minute demonstration consisted of flight conditions selected to

minimize fatigue damage to the aircraft’s structural components in order to preserve the structural life of the XV-15 for further flight experiments. On May 27,

the aircraft was ferried from Farnborough, with a refueling stop at Manston,

England, to a staging airfield at Melun, France, near Paris. Figure 65 shows the

XV-15 with the assigned Paris Air Show number “53” on its fuselage while

89

Top:

Figure 64.

XV-15 N702NA co-located

with XV-15 N703NA at the

Dryden Flight Research

Center, October 1981.

(Ames Photograph

AC84-0498-2)

Bottom:

Figure 65.

The XV-15 enroute to the

Paris Air Show in 1981.

(Bell Photograph 027781)
enroute from Farnborough to France. After clearance was received from the

French authorities, the aircraft was flown from Melun to Le Bourget on June 1.

During the 10-day air show, the XV-15 was the only aircraft to perform flight

demonstrations daily and on schedule. Inclement weather caused the cancellation

of all flight demonstrations on one day, except during the time period assigned to

the tilt rotor aircraft. On another morning, the wheels-up landing of a transport

aircraft resulted in the closure of the airfield’s only runway for the remainder of

the day. The only aircraft that could perform its demonstration routine with the

restricted operating conditions was the XV-15 which simply took off vertically

from the ramp. This high level of operational readiness was the result of a sustained cooperative effort by the Government and Bell teams. When not flying,

the aircraft was on static display. To say that it “stole the show” would be an

understatement. The successful first public appearance of this new aircraft type

before a crowd which included U.S. and foreign dignitaries, journalists, and people representing the international aircraft community, greatly enhanced U.S.

prestige around the world. 

Aircraft N702NA was then ferried back to Farnborough on June 15 where a

demonstration flight was performed for British Government officials and members of the Royal Aeronautical Society on June 17. The aircraft was disassembled and airlifted to the Ames Research Center onboard a MAC C-5A transport

on July 1, 1981. On arrival and for the last time, aircraft N702NA was co-located

with aircraft N703NA which had been ferried to Ames after completion of

Government flight tests at the DFRC.

Turning Point

By this time it was clear to the Army/NASA Project Office that it would not be

possible to continue with a funded two-aircraft flight test program. During the

next few weeks, negotiations were conducted between the Government and Bell.

A contract modification provided for the operation of aircraft N702NA by Bell

and performance of military evaluations at no cost to the Government. Co-location of the two XV-15 aircraft was therefore ended on October 26, 1981, when

aircraft N702NA was ferried back to the Bell Flight Test Center at Arlington,

Texas. The operation of N702NA by Bell under contract was later converted to a

bailment agreement. This was motivated by the Government’s concern that while

it still retained airworthiness responsibility for the aircraft, it had no direct oversight of its use. Under the bailment, ownership of the aircraft remained with the

Government, while the day-to-day airworthiness accountability was Bell’s. With

this arrangement, Bell would be able to demonstrate the capabilities of the tilt

rotor aircraft to military and civil aviation decision makers in an attempt to seek

and develop potential markets. These demonstrations of the XV-15 may well

have been the catalyst that turned a successful proof-of-concept research aircraft

program into a cornerstone for a new type of future civil and military aircraft.

90
Evaluations

Under the agreement for Navy participation in the Army/NASA XV-15 TRRA

program, the first military pilot familiarization and preliminary assessment of

this aircraft type for Navy and Marine Corps applications was arranged at the

Bell Flight Research Facility. The first military pilot flight in the XV-15 was

made by Major William S. “Bill” Lawrence, USMC (later Commander of the

Rotary Wing Division, Naval Air Test Center) on June 5, 1980. From June 5

through June 9, 1980, Major Lawrence evaluated the characteristics of the tilt

rotor aircraft. The final Naval Air Test Center technical report (No. RW-44R-80)

concluded that “... the tilt rotor concept exhibited excellent potential for a variety

of Navy/Marine Corps V/STOL missions.” During that test period, the first Army

evaluation flight was conducted by Major Ron Carpenter (later Director of the

Army Aeronautical Test Directorate (AATD), Ft. Eustis, Virginia). 

A subsequent Navy flight test was conducted in May 1983 by LCDR John C.

Ball, USN (who later joined Bell as a test pilot), to evaluate the potential of the

tilt rotor aircraft to perform the combat search and rescue (SAR) and external lift

applications. This was done to determine the suitability of the tilt rotor aircraft

for the projected JVX (V-22 Osprey) missions. In the final Naval Air Test Center

technical report for these tests (No. RW-29R-83), it was concluded that “the tilt

rotor concept, as represented by the XV-15, exhibited excellent potential to perform the combat search and rescue and external lift missions.”

One of the notable XV-15 events occurred when Ames test pilot Fred Drinkwater

flew N703NA for the first time on January 13, 1984, almost 25 years after he

became the first NASA pilot to perform a full conversion in the XV-3. Fred

Drinkwater later participated in control system test flights in the XV-15. 

Civilian pilot evaluation flights were also conducted. On March 12, 1985, pilot

and writer David L. Green flew the XV-15 at Ames and issued the first widely

distributed pilot report in the June 1985 issue of Rotor and Wing International.

Other familiarization flights were performed by Sikorsky Aircraft test pilot Frank

Gallagher on October 22, 1982, and by Boeing Vertol test pilots Dick Balzer on

July 18, 1985, and A. Lynn Freisner on October 28 and 31, 1986. Later, with the

initiation of a civil tilt rotor aircraft program in the late 1990s, a pilot evaluation

of the XV-15 was conducted by Clay Lacy, an experienced test pilot and author,

to assess the characteristics of this aircraft type from the commercial pilot’s point 

of view.52

Over the 21 years since its first hover flight, the XV-15 has had more than 300

military and civilian guest pilots.

Evaluations and Demonstrations

91

52 Clay Lacy, “Tiltrotor Technology,” Professional Pilot, September 1998.
Guest Pilots

One of the influential U.S. dignitaries

who observed the performance of the

XV-15 at the 1981 Paris Air Show and

recognized its potential as a military

aircraft was Senator Barry Goldwater

(R-AZ), then Chairman of the Senate

Armed Services Committee. Senator

Goldwater, a former military pilot,

requested and was granted a flight

demonstration in the XV-15. On

October 30, 1981, immediately after

N702NA returned to Bell, Senator

Goldwater became the first nontest

pilot to fly in Bell’s tilt rotor aircraft

guest pilot program (figure 66).

Following his flight he said that “the

tilt rotor is the biggest advance in aviation in a quarter of a century.”

On September 28, 1981, just prior to

the Senator Goldwater flight, the

Army/NASA TRRA team at Ames

provided their first guest pilot demonstration for General Story Stevens,

Commander, U.S. Army Aviation and

Troop Command. 

Another important early flight demonstration occurred on March 26, 1982,

when Secretary of the Navy John

Lehman, who also witnessed the

demonstration at the Paris Air Show,

flew the XV-15 at the Quantico USMC Air Station, Virginia (figure 67). The

experiences of both Senator Goldwater and Secretary Lehman were instrumental

in obtaining the support of Congress and of the administration for the future

acquisition of the military tilt rotor aircraft, the V-22 Osprey.

Initial Demonstrations

Following return to the U.S. after a successful performance at the Paris Air Show,

the XV-15 N702NA, painted in a desert camouflage color scheme, conducted napof-the-earth flight evaluations at Ft. Huachuca, Arizona, during June and July

1982 (figure 68). The evaluations were flown against simulated “enemy” groundto-air threats employing then-current procedures for the location, identification,

92

Top:

Figure 66.

Senator Goldwater in the

XV-15 with Bell pilot

Dorman Cannon. 

(Bell Photograph 02727)

Bottom:

Figure 67.

Secretary of the Navy 

John Lehman after flying

the XV-15.

(Bell Photograph 023970)
and “lock-on” of enemy aircraft. The

simulated ground-to-air threats were

located at ranges of 10, 20, and 30

kilometers from a simulated “enemy”

command post. The procedure followed for these evaluations allowed a

radar lock-on to the XV-15 at the simulated command post after which the

XV-15 would attempt to break the

radar lock through a combination of

maneuvers which included hover, rapid

altitude changes, quick transitions to

cruise speeds and back again to hover

or near-hover flight, and nap-of-the

earth flying. In every case, the XV-15

was able to break radar lock and avoided further detection and lock-ons by

the ground-to-air radar trackers. The results indicated an impressive capability of

the tilt rotor aircraft to avoid being captured by the ground-to-air tracking systems

used for these flight tests.

In July of 1982, immediately following the Ft. Huachuca evaluation, a flight

demonstration was performed at a Marine Harrier flight facility in Yuma,

Arizona. Once again, the XV-15 performed faultlessly. It exhibited precision

hover control, transition and cruise capability, and maneuvering capability in all

flight modes. Demonstrations such as this were effective in convincing potential

users that the tilt rotor aircraft was ready for serious consideration. 

One of the significant military applications issues was the ability of the

TRRA to perform shipboard operations. With the tilt rotor surfacing as a candidate for Navy and Marine shipboard vertical assault and replenishment

missions, questions had arisen concerning what was known as the “deck-edge

effect.” The issue addressed the condition that occurred as the tilt rotor aircraft moved laterally on to or away from the deck of an aircraft carrier.

During that operation, one proprotor would be “in the ground-effect” (IGE)

of the deck while the other proprotor would be in an “out-of-ground–effect”

(OGE) condition. The concern was that this would cause an unacceptable

control problem or a high pilot workload issue. While the results of the Ames

hover test conducted in 1981 indicated that a slight lateral displacement of

the control stick would be adequate to compensate for the “deck-edge effect”

(so small that it was expected that the pilots would probably not notice the

required control motion), it was clear that only a flight demonstration would

satisfy tilt rotor critics. In addition, proprotor noise and downwash effects on

flight deck personnel during launch and retrieval operations of tilt rotor aircraft needed to be assessed. 

93

Figure 68.

Nap-of-the-earth flight

demonstrations at Ft.

Huachuca, Arizona.

(Ames Photograph

AC82-0601)
Therefore, in order to evaluate tilt rotor aircraft suitability for operations on an

aircraft carrier, arrangements were made with the U.S. Navy under the terms of

the NASA/Army/Navy agreement, to perform shipboard evaluations with the

XV-15 on the U.S. Navy amphibious assault ship USS Tripoli (LPH 10) during

the first week of August 1982. This class of ship is designed to launch and

retrieve helicopters in support of the vertical assault phases of amphibious landing operations. The Navy’s purpose in performing these tests was to “...evaluate

the XV-15 as a representative future tilt rotor aircraft in the Navy shipboard environment... .” To accomplish this, the XV-15 was to launch daily during the test

period from North Island Naval Air Station near San Diego and rendezvous with

the USS Tripoli to perform the shipboard operational assessments.

The XV-15 TRRA arrived at North Island on July 9, 1982 ,after completing evaluations at Ft. Huachuca in Yuma, Arizona. While at North Island, the aircraft was

prepared for the upcoming sea trials.53 Takeoffs and landings were practiced in a

designated area marked to represent the flight deck of an LPH. LCDR John Ball,

94

Figure 69.

Shipboard evaluations of 

the XV-15 onboard the 

USS Tripoli.

(Ames Photograph

AC82-0612)

53 The XV-15 was painted in a desert camouflage color scheme with water soluble paint for the 

Ft. Huachuca, Arizona, flight tests. After arrival at North Island, California, and prior to the shipboard evaluations, the camouflage colors were washed off to reveal a Navy gray paint scheme.
from the Naval Air Test Center,

Patuxent River, Maryland, and Dorman

Cannon of Bell, performed the Navy’s

flight evaluations of the XV-15.

During the week of the evaluations (figure 69), the XV-15 TRRA once again

performed faultlessly. It completed a

total of 54 operations which included

short- and vertical-landings and takeoffs

with the LPH headed into the wind as

well as with crosswinds over the deck.

It successfully performed all shipboard

operations that helicopters would normally have performed including shipside hover at various distances over

water during simulated air-sea rescue.

There were no adverse effects with one rotor positioned OGE and the other IGE.

Pilot evaluations during such conditions were that the XV-15 was stable and easily

controllable as predicted based on data from the prior ground-effect performance

evaluation at Ames. Jim Lane, Demo Giulianetti, and Mike Bondi were the Ames

project personnel assigned to the support the operations onboard the USS Tripoli

during all phases of the XV-15 shipboard evaluation flights. Following the carrier

operations, LCDR Ball reported that he “was struck by how easy and just plain fun

it was to control.”54 Postoperation interviews with deck personnel indicated that

deck-handling of the XV-15 was quite manageable; that tiedown operations after

retrieval and preparations prior to launch were no worse than, or, as some deck personnel reported, were easier and quicker than with helicopters.

Later Flight Demonstrations

While Ames continued flight tests to expand the evaluation and documentation

of the XV-15’s characteristics with aircraft N703NA in accordance with the

objectives of the TRRA project, Bell flew aircraft N702NA for a wide range of

missions as was permitted by the contract modification executed in October

1981. These included assessments of engineering enhancements, aircraft evaluations by guest pilots, and flights demonstrating military and civil tilt rotor applications. Therefore, in September of 1984, the Bell XV-15 team embarked on a

tour to demonstrate the civil and military potential of this aircraft type. This

would become one of the highlights of the project and was to be known as the

“Eastern U.S. Tour.” This three-week adventure was managed by Ron Reber,

Bell’s Program Manager and LTC Cliff McKiethan, Department of Defense liaison for the Government Project Office.

95

54 John C. Ball, “Tilt-Rotor Memories,” Naval Helicopter Association Rotor Review, Number 19,

November 1987.

Figure 70.

XV-15 during nap-of-theearth flight demonstration at

Ft. Rucker, Alabama.

(Ames Photograph

AC86-0140-25)
The first destination was Ft. Rucker, Alabama,

where the XV-15 was evaluated in nap-of-the-earth

(NOE) flying, that is, following the contour of the

terrain at low flight levels (figure 70). In addition to

the Army Aviation Development Test Activity

(ADTA), pilots (including one marine pilot) who

flew the XV-15 NOE course, several other Army

aviators, including the Commanding General of 

Ft. Rucker, Major General “Bobby” Maddox, were

given guest pilot evaluation flights. The timing of

this series of evaluations was important in that it

allowed the attributes of the tilt rotor aircraft to be

considered for the Army’s new LHX program,

although a later decision influenced by aircraft cost

and weight resulted in the selection of a conventional helicopter for the program.

After the Ft. Rucker demonstrations, the XV-15 was

ferried to the U.S. Navy flight test center at Patuxent

River, Maryland, where it was evaluated in air-to-air

maneuvers against a jet (A-4 Skyhawk), a turboprop

(OV-1 Mohawk), and a helicopter (CH-46). In each

case the XV-15, with its ability to maneuver by vectoring the proprotor thrust independently of the fuselage attitude, demonstrated that the tilt rotor aircraft

had effective evasive maneuver capability against these aircraft types at a range of

speeds up to 450 knots. Also while at Patuxent River, the XV-15 made sloped

landings at up to 14-degree nose-up attitude and simulated refueling operations,

all without difficulty. 

The next series of demonstrations used the Quantico Marine Corps Air Station

(MCAS), Virginia, as a base for operations. Flight demonstrations for the

Washington, D.C., area military and congressional personnel were then performed

at Bolling Air Force Base, Ft. Belvoir, Virginia, the Pentagon, and at Quantico. 

The XV-15 was then flown to the New York Port Authority downtown heliport at

the base of the World Trade Center in Battery Park, Manhattan (figure 71), where

it was placed on static display. The return flight, from downtown New York to

downtown Washington, D.C. was completed in 66 minutes with Bell’s President

Jack Horner flying co-pilot. This demonstrated tilt rotor city center-to-city center

capability as a civil transport. After landing at the Bolling AFB heliport, Bell

conducted a briefing for congressional and news media personnel gathered for

the event. The XV-15 was subsequently ferried back to Arlington, Texas.

In all, 55 flights in 20 flying days were accomplished, including 21 evaluation

flights by five military pilots, 19 guest pilot flights, and demonstration flights in

96

Figure 71.

XV-15 at the New York 

Port Authority 

downtown heliport.

(Bell Photograph 02704)
five different locations. All flights

were made on schedule, without difficulty, and with a “proof-of-concept”

research airplane. The XV-15 performed faultlessly.

The success of the “Eastern U.S.

Tour” proved to be typical of later

events of this type conducted by Bell

and supported introduction of the tilt

rotor concept to military planners and

a curious public. In following years,

Bell demonstrated and displayed the

XV-15 at locations such as Chicago,

Illinois; Dayton, Ohio; McDill AFB,

Tampa, Florida (Special Operations);

and to commercial users such as Petroleum Helicopters International, Lafayette,

Louisiana. The exposure generated the desired widespread interest in the potential of this unique aircraft.

Air Shows and Demonstrations

The first opportunity to show the XV-15 to the public in the U.S. occurred on

September 1, 1981, when N703NA, stationed at Ames, was demonstrated at the

NASA/APA (Airport Planners Association)/HAI (Helicopter Association

International) Community Benefits Conference in Monterey, California. Because

of the low noise level produced during the takeoff and landing operations, the

XV-15 was seen to be a community-friendly aircraft by the many conference participants.

For many years during the Navy’s operation of Moffett Field, an open house and

air show was held annually during the spring or summer months to allow the community to enjoy an interesting close look at local aviation activities. By 1982, the

XV-15 was becoming a familiar sight in the sky over Moffett Field. On May 15

and 16, 1982, the NASA/Army TRRA performed air show demonstrations on each

day of the event and was then placed on static display before an overflow crowd of

approximately 200,000 people. This popular exhibition of the XV-15 was repeated

at the July 1983 and the July 1985 Moffett Field air shows. The last public appearance of XV-15 N703NA at Moffett Field occurred on March 9, 1987, when NASA

pilots Ron Gerdes and Grady Wilson flew in formation with the NASA QSRA and

the NASA AV-8C Harrier during the dedication of the NASA Ames Numerical

Aerospace Simulation (NAS) computational facility (figure 72).

Public flight demonstrations of the tilt rotor aircraft by Bell included appearances of XV-15 N702NA at the Wright-Patterson AFB air show (July 19-22,

1990), the Ft. Worth air show (October 9 and 10, 1991), and the Dallas air show

97

Figure 72.

XV-15 in formation flight at

Ames Research Center with

the NASA QSRA and AV-8B

aircraft.

(Ames Photograph

AC87-0180-454.1)
(October 16 and 17, 1991).

For about 10 years, Bell effectively promoted the tilt rotor aircraft concept by

using the bailed XV-15 N702NA to demonstrate flight capabilities to guest pilots

from various military, civil, and governmental organizations. One of these guest

pilot flights was conducted on August 20, 1992, for M. Guy Dabadie, the chief

experimental test pilot for Eurocopter, France.

Prior to the flight, Bell test pilots Ron Erhart and Tom Warren performed a short

flight with the XV-15 both as a check of the aircraft, which had not been flown

for a while, and to maintain pilot proficiency. During the check flight each pilot

made three uneventful landings after which the aircraft was declared ready for

the demonstration flight. The pilot in command for that flight was Ron Erhart,

chief pilot for BHTI.

A briefing for the guest pilot was conducted the day before by Bill Martin, the

Bell flight test engineer, and was continued by Ron Erhart just prior to the flight.

The briefing covered the configuration and operation of the aircraft’s systems,

flight controls and safety features, as well as emergency procedures. The planned

flight profile and communications procedures were also reviewed.

The demonstration, with Guy Dabadie at the controls, lasted about 30 minutes

and included hover, conversion, and airplane mode operations. After returning to

the Bell ramp at the Arlington Municipal Airport, the XV-15 was lifted to a low

hover by Dabadie to perform another landing before terminating the flight. As he

initiated the descent for landing, the aircraft began to roll to the right. Dabadie

put in correcting roll control as Ron Erhart reached up to provide the input. By

that time the control stick was on the stop but the aircraft continued to roll to an

inverted attitude and struck the ground. The rapid roll, inverted attitude, and

proximity to the ground prevented the use of the ejection seats.

The right nacelle engine exhaust ejector and the steel proprotor blades broke 

upon contact with the ground. The impact also fractured the left wing at the root,

swinging the still rotating spinner with only blade stubs attached against the inverted left side window. The right nacelle separated at the wing tip and came to rest

next to the wing. Except for a crushed nose section, collapsed vertical fins and a

gash from a rotor blade, the fuselage and cockpit area received little damage. The

flight crew, Ron Erhart and Guy Dabadie, hanging upside-down by the seat harnesses, initiated emergency egress procedures. However, the cabin door was

blocked by the damaged wing and attempts by Ron Erhart to activate the side window removal system failed. By this time, the Bell ground personnel who had

reached the aircraft, were able to remove broken sections of the thick side window

plexiglass in order to rescue the crew. Both pilots received only minor injuries.

No fire occurred after the crash, due in part to the low fuel level onboard at the

time of the accident, but also attributable largely to the crashworthy fuel cells

which contained most of the remaining fuel, thereby providing the added safety

Crash

98
for which they were designed.

An accident investigation was conducted by the National Transportation Safety

Board (NTSB). Their findings revealed that the loss of control was due to 

human error and was not due to any inherent characteristics of the tilt rotor aircraft. It was found that the proprotor blade collective angle mechanical linkage

had disconnected from the hydraulic actuator when a critical nut backed off

because it had not been secured by a cotter pin as required.

Further, failure of the window removal system to operate properly was found to

be due to improper procedures noted in the XV-15 tilt rotor research aircraft

flight manual. These procedures had been extracted directly from the flight manual for the Army AH-1S which used the same device. Following the discovery of

this major procedural problem, both the XV-15 and AH-1S manuals were

changed to include the correct operating instructions for the window removal

system. 

99
By the mid 1990s, the flight research situation in the Government was changing.

NASA Administrator Daniel Goldin, in an attempt to reduce the operating costs 

of the Agency, directed that all flight activities were to be consolidated at one

research center, the Dryden Flight Research Center (DFRC). For nearly two

years following that directive, heated debates raged between NASA Headquarters

(for consolidation) and the Centers, local officials, and members of Congress

(from affected congressional districts) who argued for the continuation of flight

activities at the Centers. Ultimately the decision was made to move all NASA

flight test activity to DFRC by January 1, 1998. 

Meanwhile, XV-15 N703NA was undergoing a major inspection at Ames. In

addition to the scheduled transmission overhaul, the composite blade cuffs were

modified to prevent a reoccurrence of the failure that resulted in the emergency

landing in September 1991, and modifications to the proprotor control system

were designed to correct the dynamic problems and load limitations encountered

during the initial flights with the advanced technology blade (ATB). When the

final decision was made by NASA Headquarters to terminate XV-15 operations

at Ames in early 1994, the inspection and modification work ceased and the XV15 TRRA was placed in temporary storage at Ames.

Starting Over

The availability of XV-15 N703NA was seen as an unexpected opportunity by

Bell. Having lost N702NA in the unfortunate accident in August 1992, and with

an ongoing requirement to conduct tilt rotor research, demonstrations, and applications evaluations in support of the V-22 program, Larry Jenkins, Bell’s director

of technology, requested that a new bailment be established for N703NA. The

NASA/Army owners of the XV-15, with no funding and no prospects for continuing the XV-15 flight test program, consented to a new agreement. The terms of

the new bailment, described in a Memorandum of Agreement effective April 21,

1994, were approved by the NASA Ames Director, Dr. Ken Munechika; the

NASA Associate Administrator for Aeronautics, Dr. Wesley Harris; the Director

of the Army Aeroflightdynamics Directorate, Andrew W. Kerr; the Executive

Director of the Army Aviation Research, Development, and Engineering Center,

Thomas L. House; and Bell President Webb Joiner. The aircraft was again disassembled and was shipped from Ames to Bell’s flight test center onboard flatbed

trucks immediately after the bailment was authorized. 

The delivery of that aircraft with the Boeing Helicopters composite blades (ATB)

presented a problem for Bell. The unplanned cost of refurbishing the aircraft during the current fiscal year represented a large portion of Bell’s tilt rotor discretionary research funds. With inadequate funding for the development of the control system improvements, and with low confidence in compatibility of the ATB

with the XV-15 and in the integrity of some of the ATB’s composite components,

Bell sought an alternate approach. Bell’s preferred solution was to return to the

use of their metal blades fabricated for the XV-15.

The End of an Era

100
A review of the availability of flightworthy metal blades brought the plan to an

immediate halt. It turned out that when the NASA/Army TRRA team at Ames

initiated the ATB evaluations on the XV-15, the metal blades from N703NA were

sent to Bell to replace the higher flight-time blades on N702NA, some of which

had become unserviceable due to skin cracks. When N702NA crashed, all of its

proprotor blades (from N703NA) were destroyed leaving only two usable left

metal blades and four usable right blades. Short of fabricating a new blade for

the left proprotor at great expense, there was no hope of obtaining the required

blade. 

At this point another of those remarkable events that have periodically rescued

the program from a seemingly unsolvable situation occurred. The Ames longterm hardware storage facility, located at Camp Parks near Oakland, California,

requested the aircraft projects to remove or dispose of the aircraft-related items

in storage since, under the new flight activities consolidation plan, the Ames aircraft assets were now to be moved away, primarily to DFRC. While searching

through the large warehouse, a crate containing a left XV-15 metal blade was

discovered. The blade, still in primer paint was apparently unused. The documents indicated that this blade was fabricated for structural fatigue testing under

the TRRA contract, but funding limitations at that time caused that test to be

eliminated. After inspection at Bell by Ernie Schellhase, who had designed these

blades in the late 1960s under Bell’s IR&D funding, the blade was declared

flightworthy. It was refinished and installed on N703NA, completing the required

proprotor “shipset.” Once again, lady luck smiled on the TRRA.

With the proprotor blade problem question resolved, Bell stepped up the refurbishment of N703NA. At the completion of this refurbishment, the aircraft had

the original metal blades, a new data acquisition system (similar to the system

being used on the V-22), and had the Ames-modified automatic flight control

system restored to Bell’s control laws. Because of the extended time period since

the last operation of the XV-15, arrangements were made for the Bell pilots to

perform training sessions in the Ames simulator as the aircraft was nearing flight

readiness. On March 3, 1995, test pilots Ron Erhart and Roy Hopkins returned

this aircraft to flight at the Bell Flight Test Center, Arlington Texas, nearly 16

years after its initial flight at the same location. 

Deja Vu

After a brief checkout period, XV-15 N703NA was pressed into action as the tilt

rotor technology demonstrator, this time in executive transport colors including

painted-on simulated windows. On April 21, 1995, it became the first tilt rotor to

land at the world’s first operational vertiport, the Dallas Convention Center

Heliport/Vertiport (figure 73). In June, when the Bell-Boeing V-22 Osprey made 

its first international public debut at the forty-first Paris Air Show, the XV-15

was also present (figure 74), marking a triumphant return 14 years after its

initial appearance. With both aircraft performing flight demonstrations on the

101
six days of the air show, the tilt rotor aircraft, once again, was the star of the

event. 

Onward/Back to Work

Following the return of the XV-15 to the United States (the round trip this time

was made onboard a cargo ship) Bell performed additional demonstrations

before flying N703NA back to its base. These included the first tilt rotor flight

demonstration in Canada, made on July 10, 1995, at the Bell Helicopter Mirabel

facility near Montreal, Quebec, and the first tilt rotor operation at the

Indianapolis Heliport on the following day. During the next few years, in addition to the continuation of the guest

pilot program, the aircraft was utilized

in numerous tilt rotor research activities sponsored by Bell, Boeing

Helicopters, NASA Ames and NASA

Langley, and the National Rotorcraft

Technology Center (NRTC) located at

Ames. At the time of this writing,

investigations of tilt rotor flight controls, crew station displays, terminal

area operations, certification issues,

and other technical areas continue to

be planned for this aircraft.

102

Top:

Figure 73.

XV-15 at the Dallas

Convention Center

Heliport/Vertiport.

(Bell Photograph 042869)

Bottom:

Figure 74.

XV-15 at the 1995 Paris Air

Show with the Bell-Boeing

V-22 Osprey.

(Bell Photograph 042900)
This review of approximately 45 years of Government and industry efforts to

develop a VTOL tilt rotor aircraft reveals the unique research activities that were

accomplished and the magnitude of the technical challenges that had to 

be overcome.

The large diameter rotor required for the desired low speed characteristics produced serious dynamic instabilities at moderate cruise speed conditions. Initially,

the lack of understanding of the cause of these instabilities, as well as the

absence of a valid mathematical model to analyze the multiple degree-of-freedom elastic tilt rotor aircraft structure, made the search for a solution a slow and

costly trial and error process. For about 20 years, starting with the XV-3 program

in the early 1950s, extensive experimental work ultimately resulted in breakthrough analyses which made possible the identification of solutions to the high

speed aeroelastic stability problem. 

Other significant problems that surfaced in the early XV-3 tilt rotor flight work

included poor performance and handling qualities. Once again, many hours of

wind tunnel tests provided the empirical data required to support the development of new analytical codes necessary to address these problems. By the late

1960s, the results of these methodology developments were being applied to the

design of tilt rotor aircraft that could effectively perform various civil and military missions. Also, improvements in the flight control system, which provided

the desired handling characteristics, were demonstrated through the use of realtime piloted flight simulations.

This new understanding of the tilt rotor’s complex problems was achieved

through focused studies conducted by Government and industry researchers,

largely directed by a single joint Army/NASA office. Each major problem was

addressed and solved by a planned series of experimental and analytical investigations leading to the highest level of confidence possible, short of actual flight

test validation. 

This ever-expanding technology base, coupled with the validation of new analytical codes completed in the early 1970s, provided the evidence needed to proceed

confidently with the development of a new proof-of-concept tilt rotor research

aircraft.

A Tilt Rotor Research Aircraft Project Office was created at Ames in the early

1970s to develop and flight-test two tilt rotor research aircraft. The Office was

staffed by both Army and NASA personnel knowledgeable in the critical disciplines needed to develop such an aircraft. The contractor, Bell Helicopter Textron

Inc., likewise provided the necessary personnel and facilities to design and build

the aircraft. Furthermore, a Government Resident Office was established at the

contractor’s facility to provide a high level of communication between both parties as well as close monitoring of technical status and costs. Despite significant

technical and cost problems encountered during the conduct of the work, the

XV-15 Project Summary

103
management system was very effective for controlling costs and resolving technical issues and was instrumental in contributing to the overall success of the

project.55

A lesson learned was the advantage of multiple or joint participation, in this

case the Army and NASA. This became an important factor in maintaining the

continuation of project funding when one agency was able to provide funds during a period that the other agency was experiencing a temporary funding shortfall. This was further emphasized when the Project Office was able to accommodate a request by the Navy for sea trial evaluations of the XV-15 tilt rotor

research aircraft (TRRA) to evaluate it for Navy ship board applications. As it

happened, this provided further funding at a time of critical need. Yet another

important “funding” lesson learned was to include the contractor as a participant in the project funding. In the case of the XV-15 TRRA, this was accomplished contractually by an incentive fee arrangement tied to contractor cost

performance. While the incentive fee did not prevent cost increases, it did provide for significant funding participation by the contractor through a negative

fee arrangement.

Most important are the results of the TRRA flight test program. Within just over

two years after the first full conversion of the XV-15 TRRA, sufficient data had

been collected to determine that the primary proof-of-concept objectives were

successfully completed. This included validation of rotor/pylon/wing dynamic

stability, performance, and noise. The XV-15 met its predicted characteristics in 

all critical areas and was determined to be suitable for advanced flight research

investigations, including evaluations of the tilt rotor aircraft’s suitability of civil

and military applications. These investigations and related advanced research

have been conducted with the XV-15 for nearly twenty years after the completion of the proof-of-concept flight testing, demonstrating the tilt rotor aircraft’s

versatility and potential in many VTOL aircraft applications. Without question,

the XV-15 TRRA has met or exceeded the goals as specified in the original

Program Plan. Although the TRRA was built by the Bell Helicopter Company, it

is significant to note this was the first time the Government (the Army and

NASA) successfully wrote the specifications for and fostered the introduction of

a new aircraft type into the U.S. aviation market.

104

55 A discussion of the approach and key considerations of the TRRA and other flight research projects at Ames is contained in: David D. Few, “A Perspective on 15 Years of Proof-of-Concept

Aircraft Development and Flight Research at Ames-Moffett by the Rotorcraft and Powered-Lift

Flight Projects Division, 1970-1985,” NASA Reference Publication 1187, August 1987.
The XV-15 continues to contribute to the advancement of aeronautical technology through its flight test activity at Bell, thereby further increasing the benefits

derived from the TRRA project. It is appropriate, however, to note the costs

incurred by the Government in the performance of this work. By September

1981, sufficient data had been acquired in the two research aircraft flight test

program for the Government to declare that the primary proof-of-concept objectives had been successfully completed. At that time, the cost of the TRRA contract was $39.5M. An additional $5.0M was used during this period for supporting research and technology. Research and support work continued with the

prime contractor (Bell) for several years under the same contract, and when it

was terminated in August 1993, the final cost to the Government was $50.4M.

Bell had contributed over $1.5M to the effort in accordance with the incentive

fee arrangements of the contract. In recent years it has become apparent that the

Government’s investment in tilt rotor aircraft technology, through the new programs now under development, will likely provide thousands of new jobs and

may even improve the U.S. balance of trade. The key events leading to the validation of tilt rotor technology by the XV-15, and the subsequent development of

production tilt rotor aircraft (discussed later in this section) are listed in the

chronology provided in Appendix C.

In addition, the contributions of many people associated with these projects over

the years have been recognized by leading U.S. technical organizations and societies. A summary of the key awards and new speed and climb records set with

the XV-15 are described in Appendix D. A collection of pictures showing the tilt

rotor aircraft during the flight program is provided in the photo-gallery,

Appendix E. Also, Appendix F contains a comprehensive bibliography of tilt

rotor related publications.

The remarkable achievements, both technical and operational, of the XV-15

TRRA were directly responsible for the introduction of the world’s first military

and civil tilt rotor aircraft. Without the technology validation and the demonstrations provided by the TRRA, it would not have been possible for the leaders of

industry and the Government to be confident enough to launch these new aircraft

production programs. Thumbnail sketches of these programs, as well as brief

summaries of the Government activities spawned by the TRRA project are provided here.

JVX/V-22 Osprey

Beginning in the late 1960s, and continuing for more than a decade, the Marines

studied the options available for their future vertical assault role and transport

needs. However, because of the relatively small number of vehicles required,

coupled with the specialized missions, they could not establish the necessary

level of support in the Department of Defense (DoD) and in the Congress to initiate acquisition of a new purpose-built aircraft. By the end of 1981, the DoD

identified additional vertical lift missions for the Army and the Air Force which

Epilogue

105
could make use of the same flight vehicle that would satisfy the Marine’s

requirements. If a common aircraft could be designed to fill the operational

needs of these three services, the aircraft procurement might then be large

enough to justify development and unit acquisition costs. Therefore, an assessment of the feasibility for identification of a single vehicle which could satisfactorily perform these diverse missions and the identification of the most suitable

vehicle type for these applications was directed by the DOD. This study was

conducted by a Joint Technology Assessment Group (JTAG) consisting of

Government engineers and military specialists at the Ames Research Center,

between February and May of 1982. Col. Jimmie Creech, USMC, was the study

manager. The study was to include both current and advanced VTOL aircraft.

Four vehicle types were selected for this investigation, with a team leader and a

technical staff appointed to assess each type. The helicopter and compound helicopter teams were led by Dr. Michael Scully of the Army Advanced Systems

Research Office (ASRO), the fan-in-wing team was headed by Sam Wilson of

NASA Ames, and John Magee, also of NASA Ames, directed the tilt rotor study

team. The latest design methodology and performance data were applied to

develop a credible and practical design configured and sized to meet, to the best

degree possible, the various and often conflicting mission requirements.

The results of the study made it clear that the tilt rotor aircraft was best suited to

meet diverse missions. These included the Marine vertical assault, Navy rescue

and logistics, Air Force long-range special operations, as well as the Army medical evacuation, long-range combat logistics support, and combat air assault support missions. With a single technical approach identified that could satisfy the

requirements of the three military services, advocacy of the multiservice tilt rotor

aircraft to the Congress and to the administration was initiated. The XV-15

proof-of-concept and flight research programs had established that performance,

loads, and structural dynamics of the military tilt rotor transport could be predicted with high confidence. However, major changes occurred that affected the

course of the JVX (Joint Vertical Experimental) program.

First, at the time of the advocacy of the new aircraft, the Army was engaged in

the initiation of another major, high cost weapons system procurement, the LHX

(Light Helicopter, Experimental). With a commitment to develop a world-class

fighting machine that would use state-of-the-art structures, propulsion, avionics

and weapon systems technology, it was not economically or politically feasible

for the Army to simultaneously advocate and manage the development of a new

technology transport rotorcraft. Since the primary user of the JVX aircraft would

be the Marines, the task of managing this program was handed to the Navy, the

weapon systems procurement agency for the Marine Corps.

Second, as the flight tests of the JVX aircraft, now called the V-22 Osprey, were

about to get under way, a new administration came into office under President

George Bush. With a focus on reducing DoD expenditures, Secretary of Defense

Richard Cheney identified major procurements selected for cancellation. Since

106
the JVX activity had recently begun

and relatively little funding had been

invested at this point, it became a target for elimination. The battle for the

survival of the advanced rotorcraft

transport aircraft would be waged for

several years. Advocates included the

potential military users, members of

Congress, and elements of the rotorcraft industry. The opposition was the

administration and the upper management of the DoD. 

Other issues surfaced. In accordance

with a longstanding DoD procurement

policy, contractors for major new

acquisitions were selected from competitive bids. In this case only two rotorcraft

companies had sufficient technical expertise to bid. These were Bell and Boeing,

and only Bell had extensive flight test experience with the tilt rotor aircraft.

Furthermore, in the early 1980s, there was a DoD mandate for prime contractor

teaming arrangements seen as a means of sharing Research and Development

costs by the prime contractors, thus reducing the financial risk to any one company, as well as permitting the development of a broader technology base.

To satisfy the teaming requirement, two companies that had been competitors,

Bell and Boeing, joined forces. Although this would bring together the world’s

greatest resources of tilt rotor technology, it left no credible competitors in the

U.S. rotorcraft industry. When the RFP for the V-22 was issued, only the BellBoeing team responded. While this presented a dilemma for advocates of competitive procurements, the qualifications of the team, coupled with strong political advocacy from the powerful Texas and Pennsylvania congressional representatives, provided the support needed to proceed.56 The successful advocacy of this

program is credited to strong congressional support, confirming the observation

by political analyst Brenda Foreman that “if the politics don’t fly, the hardware

never will.”

On March 19, 1989, the first flight of the Osprey was conducted at Bell’s Flight

Research Center at Arlington Texas, the site of the first XV-15 flight twelve

years earlier. Bell test pilot Dorman Cannon (who was also onboard the XV-15

during its first test flight) and Boeing Helicopter test pilot Dick Balzer were at

the controls. The first full conversion to airplane mode was flown on September

14, 1989. Figure 75 shows one of the V-22 EMD (Engineering Manufacturing

107

Figure 75.

The Bell-Boeing V-22

Osprey in hover flight.

(Ames Photograph

AC89-0246-3)

56 An account by Brenda Forman of the factors leading to the approval of V-22 program funding

is presented in “The Political Process in Systems Architecture Design,” Program Manager,

March-April 1993.
Development) aircraft during early

flight tests.

The flight test program of the V-22,

however, was not without serious

problems. Of the six Full Scale

Development (FSD) aircraft planned

for the flight test efforts (of which

only five were completed), two

crashed and were destroyed, with one

crash taking the lives of all seven people on board. However, it was determined that these accidents were not

due to the inherent characteristics of

this vehicle type and the program survived. 

As of late 1999, the V-22 Osprey is undergoing operational testing by the 

U.S. Navy and initial operational capability (IOC) is planned for the year 2001. 

On September 8, 1999, the first production V-22 delivered to the U.S. Marine

Corps landed at the Pentagon for a tilt rotor demonstration hosted by Secretary

of Defense William S. Cohen. A CH-46 (the helicopter that will be replaced by

the V-22) and XV-15 proof-of-concept aircraft, in Coast Guard colors, landed

along side the Osprey. After several members of Congress flew in the new V-22

tilt rotor transport, Secretary Cohen described it as a “revolution in military

affairs.” Based on the technology demonstrated by the XV-15 TRRA, the V-22

will bring capabilities to the U.S. armed services that are not available in any

other vehicle.

UAV

Tilt rotor aircraft technology also offers performance and operational capabilities

that are highly desirable for unmanned aircraft being developed for military

applications. The ability to takeoff and land from a very small area, such as a

landing pad onboard a ship, coupled with a large radius of action, high altitude

performance, and a high cruise speed to get to the target area quickly provides a

combination of attributes that meet the needs of the military users. 

To demonstrate the readiness of tilt rotor technology for this application, Bell

Helicopter Textron developed the Eagle Eye Unmanned Aerial Vehicle (UAV),

(figure 76). This aircraft performed flight evaluations at the Naval Air Test

Center, Patuxent River, Maryland, in 1995 and at the Proving Grounds in Yuma,

Arizona, in 1998. The later activity demonstrated the ability to takeoff from and

land within a 24-foot landing spot (and consistently touch down within a 10-foot

square area), hover with the required fuel and payload, fly at over 200 knots, and

cruise at 14,600-foot altitude with the 200-pound payload. The Eagle Eye uses a

108

Figure 76.

The Bell tilt rotor eagle eye

unmanned aerial vehicle.

(Bell-Ames Photograph

ACD99-0209-24)
highly automated, command based

flight control system, that includes two

inertial navigation systems and a GPS

(Global Positioning System).

As of this writing, Bell continues to

explore missions and applications for

the tilt rotor UAV.

Model 609

In November 1996, Bell and Boeing

announced that they had agreed to

jointly design and build the world’s

first production civil tilt rotor aircraft,

the Bell Boeing 609 (BB 609). This

major and multiyear commitment of

company resources represented the culmination of the early research and technology efforts begun with flight tests of the XV-3 in the mid-1950s and completed with the technology validation provided by the XV-15 proof-of-concept tilt

rotor research aircraft in the 1980s and 1990s. 

In addition to the fundamental engineering and design capabilities provided by

the joint Government and industry research programs, the model 609 will incorporate many features developed for the V-22 Osprey. This technology transfer

will include state-of-the-art fly-by-wire flight controls and avionics, advanced

composites in the rotors and structure, and Health and Usage Monitoring (HUM)

systems. The 609 aircraft will have a crew of two and carry six to nine passengers. It is designed to cruise at 275 knots (316 miles per hour) and have a range

of 750 nautical miles (863 statute miles), which is nearly twice the speed and

range capability of current helicopters of the same payload class. Takeoff gross

weight will be about 16,000 pounds with an approximate useful load of 5,500

pounds, which means that it can carry a full complement of passengers and plenty of cargo and/or baggage, an important consideration for civil aircraft. The

fuselage will be pressurized to 5.5-psi pressure differential providing a passenger

cabin altitude of 8,000 feet at a 25,000-foot ceiling. Although the BB 609 has

VTOL capability, it is anticipated to be utilized as a fixed wing, turboprop airplane using rolling takeoffs during more than 90 percent of its operations. This

will give it the ability to increase payload and/or range when VTOL operations

are not required, thus lowering operating costs. Efforts are underway with the

Federal Aviation Administration (FAA) and the European Joint Aviation

Authorities (JAA) to establish certification for this aircraft type, anticipated by

early 2001, followed by first deliveries of the aircraft later that year. 

A full-scale mockup of the aircraft (figure 77) was displayed at the June 1997

Paris Air Show where the V-22 Osprey with the XV-15 TRRA flew daily flight 

109

Figure 77.

Mockup of the BA-Model

609 civil tilt rotor aircraft

with Bell-Boeing markings.

(Bell-Ames Photograph

ACD97-0133-3)
demonstrations two years earlier. The interest generated by the mockup and

flight demonstrations was such that Bell received 36 advanced orders at that time

for the new aircraft. Bell President Webb Joiner, speaking of the early customers

for the Model 609, said that “These are not just customers, these are visionaries,”

noting their commitment to a new aircraft type two years before design freeze

and four years in advance of first delivery. Bell further anticipates a market of up

to 1000 Model 609’s over the next 20 years, serving needs such as executive

transport, offshore oil operations, search and rescue, emergency medical service,

drug enforcement and border patrol.

In March of 1998, shortly after the Boeing Company purchased McDonnell

Douglas Helicopters, and subsequently made the decision to focus on military

helicopters only, Boeing removed itself as a major contributing partner in the 

BB 609 program. However, at the Farnborough Air Show in September of 1998,

Bell announced a joint venture with the Agusta Helicopter Company of Italy

wherein Agusta will participate in the development, manufacture, and final

assembly of 609s delivered in Europe and other parts of the world. The 609 was

now renamed the BA 609 (for Bell Agusta 609). 

Agusta has had a long history of joint programs with Bell and also worked with

other European aerospace companies on the development of tilt rotor technology

under a program called EUROFAR (European Future Advanced Rotorcraft).

Following the Bell-Agusta teaming announcement, Eurocopter, a French-German

company, stated that it too was seeking funding for a civil tilt rotor project.

As a commuter aircraft operating in a growing worldwide short-haul commuter

market, the BA 609 can operate to/from vertiports or conventional airports and

will go a long way toward relieving congestion and delays at many of the

world’s major airport hubs. The BA 609 will be breaking new ground (or should

we say “new air”) in aviation.

CTRDAC

The development of the V-22 Osprey and the initiation of flight testing provided the encouragement needed by tilt rotor advocates to press for a civil

application of this new aircraft type. Earlier FAA- and NASA-funded studies,57 managed by Dr. John Zuk of NASA Ames, showed that the tilt rotor aircraft had potential worldwide market application and could be economically

beneficial to the manufacturers as well as the operators. In late 1992, results

were brought to the attention of members of Congress who directed Secretary

of Transportation Samuel (Sam) Skinner to establish a Civil Tiltrotor

Development Advisory Committee (CTRDAC) to examine the costs, technical

feasibility, and economic viability of developing civil tilt rotor aircraft

110

57 Anon., “Civil Tiltrotor Missions and Applications, Phase II: The Commercial Passenger Market,”

NASA CR 177576, February 1991.
(CTR).58 The CTRDAC was to also consider issues associated with the integration of CTR aircraft into the national transportation system and assess the

resulting national economic benefits. Furthermore, the Committee was

charged with determining the required additional research and development,

the needed regulatory changes to integrate the CTR into the transportation

system, and how the CTR aircraft and related infrastructure development costs

should be allocated between Government and industry.

The members appointed to the CTRDAC represented a broad spectrum of private

and public sector agencies, companies, and associations, as well as the

Department of Transportation (DOT), the National Aeronautics and Space

Administration, and the Department of Defense. The chair of the CTRDAC was

Frank E. Kruesi, Assistant Secretary for Transportation Policy (DOT). Among

the 31 committee members were Dr. Hans Mark of the University of Texas (UT)

(previously Director of the NASA Ames Research Center and later Chancellor of

the UT), and Webb Joiner, president of Bell Helicopter Textron, Inc. 

The findings of the Committee issued in December 199559 stated that the CTR is

technically feasible and can be developed by the U.S. industry. However, additional research and development and infrastructure planning are needed before

industry can make a CTR production decision. Furthermore, under the assumptions made during the study, it was concluded that a CTR system could be economically viable and could operate profitably without Government subsidies in

heavily traveled corridors. The CTR, the Committee found, could reduce airport

congestion, create jobs, and have a positive impact on the balance of trade.

The Committee recommended the creation of a public/private partnership to

address CTR infrastructure issues and the initiation of associated planning. Work

should begin, they stated, on regulatory and certification issues and on changes

to the air traffic control system to safely and effectively use the capabilities of

the CTR. In addition, the CTRDAC recommended that an integrated CTR aircraft and infrastructure research, development, and demonstration program

should be conducted and the costs for this should be shared by the Government

and industry.

In response, elements of work suggested by the CTRDAC have been included in

the NASA rotorcraft program that are consistent with the NASA aeronautics

technical thrusts.60

111

58 The CTRDAC used a 40-passenger CTR as a baseline for its analysis. Earlier studies (see footnote 57) indicated that this size CTR has the greatest initial potential to provide societal benefits

and achieve commercial success.

59 Anon., “Civil Tiltrotor Development Advisory Committee Report to Congress in Accordance

with PL102-581,” December 1995.

60 Anon., Aeronautics and Space Transportation Technology: Three Pillars for Success, National

Aeronautics and Space Administration, Office of Aeronautics and Space Transportation

Technology Brochure, March 1997.
Future Tilt Rotor Aircraft

By the early 1990s, an extensive tilt rotor data base had been developed from the

Bell and Government XV-15 flight test activities. The larger military V-22 tilt

rotor aircraft, which was designed using methodology validated with the XV-15

data, was well under way and was showing promise of meeting important performance goals. Also at that time, NASA’s investigation of technical solutions to

the growing air transport system congestion problems led to the identification of

the tilt rotor aircraft as a part of the solution. However, significant advancements

in several technology areas would be required before the tilt rotor aircraft could 

be accepted as a civil transport. To address these “barrier issues,” researchers at

the Ames, Langley, and Lewis Research Centers, led by Bill Snyder of Ames,

developed a comprehensive effort called the Advanced Tiltrotor Transport

Technology (ATTT) Program to develop the new technologies. 

The research, started in 1994, was to be conducted as an element of NASA’s

Advanced Subsonic Technology Program. Due to funding limitations, the initial

research activity, the Short-Haul Civil Tiltrotor (SHCT) Program (a subset of the

ATTT Program), was restricted to issues of primary concern, noise and safety.

The noise investigations focused on the reduction of the sound levels generated

by transport-size tilt rotor aircraft while operating to and from downtown vertiports of major metropolitan areas. Community and regulatory acceptance

requires much lower noise levels for this environment than is generated using 

V-22 technology. The research activity included the development of refined

acoustics analyses, the acquisition of wind tunnel small- and large-scale proprotor noise data to validate the new analytical methods, analytical and wind tunnel

investigations of innovative proprotor and blade configurations designed to

reduce the most disturbing content of the noise signature, and flight tests to

determine the effect of different approach profiles on terminal area and surrounding community noise. The Boeing and Bell Helicopter Companies, McDonnell

Douglas Helicopter Systems, and the Sikorsky Aircraft Company, participated in

the noise investigations. Mike Marcolini was the lead researcher at Langley

Research Center for many of these efforts.

The safety effort was related to the projected need to execute approaches to and

departures from confined vertiports. For these conditions the capability to operate safely with one engine inoperative (OEI) would be required and a safe/low

pilot workload (referred to as Level 1 handling qualities by the FAA) must be

maintained under adverse weather conditions. This area was addressed by conducting engine design studies seeking the ability to produce high levels of emergency power in the event of an OEI condition without adversely impacting

weight, reliability, maintenance, or normal operation fuel economy. These studies

were conducted by Allison, Allied Signal and General Electric under the technical guidance of Joe Eisenberg of Lewis Research Center (LeRC). Further safety

investigations involved piloted simulations at the Ames Vertical Motion

Simulator (VMS) to assess crew station issues, control law variations, advanced

112
configurations such as the variable diameter tilt rotors, and terminal area

approach path profiles including nacelle position variations. Bill Decker of Ames

was the principal investigator for the simulation efforts.

As the SHCT Program nears the scheduled 2001 completion date, a new follow-on

research effort is being developed by NASA to apply and evaluate relevant technologies that emerged during the SHCT activity. One key area of interest is the

feasibility evaluation of Simultaneous Non-Interfering (SNI) terminal area operations. SNI operations are expected to increase the capacity of existing airports by

allowing VTOL tilt rotor transport aircraft to takeoff and land using terminal area

flight paths separate from that used by the fixed-wing transports. Furthermore, if

short-haul aircraft utilize the SNI operations and are thereby removed from the

runway queue, the larger capacity long-range aircraft would occupy the limited

slots, thereby increasing the number of passengers that can be transported on existing airport runways. The planned research would identify the technologies and procedures needed for the aircraft and Air Traffic Management (ATM) system to

obtain maximum aviation system benefits. The evaluations would involve the use

of piloted simulations and flight tests, employing helicopters to represent the tilt

rotor aircraft in near-terminal area operations. A separate program element includes

ATM systems integration work and addresses adverse weather operations (such as

icing conditions). This effort also deals with the automated cockpit and will examine methods of maintaining safe control during emergencies.

A new element of this follow-on activity is focused on Variable Diameter Tilt

Rotor (VDTR) technology. This tilt rotor variant, being developed by Sikorsky,

employs a proprotor system that provides a larger diameter and lower disc loading for higher efficiency in hover and low speed helicopter mode flight and, by

the use of a blade retraction mechanism, a smaller diameter “prop” for airplane

mode flight. The lower disc loading also contributes to safety by improving OEI

performance and, if lower tip speeds were employed, would reduce the noise

level. The planned five-year VDTR effort would address full-scale system

design, system integration and reliability and would be conducted with shared

funding by the Government and the contractor. 

Additional investigations planned for this initiative address the application of

conformable proprotor blade technology or other advanced proprotor designs to

improve performance and reduce noise. The selected system would be wind tunnel and flight tested to validate predictions. The last major element deals 

with economic viability and passenger comfort issues. These issues include the

improvement of high speed performance by reducing wing thickness while maintaining the required stability margins, the reduction of proprotor/airframe interaction losses, and the development of methods to control interior noise and reduce

cabin vibrations.

In a more aggressive effort developed in response to the CTRDAC recommendations, NASA planners have proposed the advanced technology demonstrator

113
tiltrotor aircraft program. This program carries some of the vehicle technology

proposed in the SHCT follow-on program to flight demonstration with a highly

modified V-22 Osprey. To accomplish this high cost program, it is expected that

Government and industry would participate and cost share in order to make it

affordable. While support for funding major new programs is usually difficult to

obtain, Army planners have cited possible applications for the large tilt rotor aircraft technologies being considered here in their joint transport rotorcraft (JTR)

program (for a CH-47 helicopter replacement) and in the recent “Army After

Next” study of future Army tactics and related technology. 

The rest of the tilt rotor aircraft story begins now. The dream has become 

a reality.

114
Transcendental Model 1-G

The Transcendental Aircraft Corporation Model 1-G61 (figure A-1), was a singleseat convertible rotorcraft with two counter-rotating three-bladed rotors located

at the tips of its fixed wing. The high-wing Model 1-G incorporated a conventional empennage and a fixed tricycle landing gear. When the rotors were placed

in the horizontal plane, they acted as normal helicopter rotors. The rotors could

also be tilted forward 82 degrees to perform as traction propellers of a conventional fixed-wing aircraft. In this configuration lift was provided by the wing. 

The four-cylinder Lycoming O-290-A, located in the fuselage behind the cockpit,

powered the rotors through a two-speed reduction main gear box, spanwise drive

shafts, and outboard gearboxes. In the helicopter mode, at the engine’s maximum

output of 3000 RPM, the rotors rotated at 633 RPM. 

The rotor blade-angle control system employed two concentric tubes around the

rotor shaft emanating from the outboard gearboxes. One of these tubes controlled

cyclic blade pitch and the other the collective pitch. The change of the rotor position from the helicopter to the aircraft configuration was accomplished in

approximately three minutes.

Appendix A—Aircraft Descriptions

115

61 Model 1-G information from Janes, All the World’s Aircraft, 1953-1954.

Figure A-1.

Transcendental Model 

1-G hovering in ground

effect.

(Ames AD98-0209-14)
Model 1-G Characteristics

Powerplant

Lycoming O-290-A

Brake horsepower . . . . . . . . . . . . . . . 160 HP @ 3000 RPM

Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 ft

Wing 

Span. . . . . . . . . . . . . . . . . . . . . . . . . . 21 ft

Area. . . . . . . . . . . . . . . . . . . . . . . . . . 63 sq ft

Chord. . . . . . . . . . . . . . . . . . . . . . . . . 3 ft

Loading . . . . . . . . . . . . . . . . . . . . . . . 27.7 lb/sq ft

Airfoil section . . . . . . . . . . . . . . . . . . NACA 23015

Width. . . . . . . . . . . . . . . . . . . . . . . . . 38 ft (to outer tip of rotor disc)

Height . . . . . . . . . . . . . . . . . . . . . . . . 7 ft

Rotor

No. of rotors . . . . . . . . . . . . . . . . . . . 2, interconnected

Type. . . . . . . . . . . . . . . . . . . . . . . . . . articulated

Blades/proprotor . . . . . . . . . . . . . . . . 3

Diameter . . . . . . . . . . . . . . . . . . . . . . 17 ft 

Chord. . . . . . . . . . . . . . . . . . . . . . . . . 4 in

Disc loading . . . . . . . . . . . . . . . . . . . 3.6 lb/sq ft

Rotational speed

Helicopter mode . . . . . . . . . . . . . . . . 633 rpm

Weight

Empty . . . . . . . . . . . . . . . . . . . . . . . . 1450 lb

Gross . . . . . . . . . . . . . . . . . . . . . . . . . 1750 lb

No. of seats . . . . . . . . . . . . . . . . . . . . . . . . . . 1 

116
Transcendental Model 2

The Transcendental Model 2

Convertiplane62 (figures A-2 and A-3)

was a high-wing monoplane with a

helicopter-type rotor mounted at each

wing tip. It was configured with a conventional airplane empennage and a

fixed-position tricycle landing gear. In

addition to vertical takeoff capability,

the Model 2 Convertiplane was

designed to perform rolling takeoffs

combining the lift from the rotors and

the wing at gross weights above the

maximum vertical takeoff weight. 

In hovering and slow flight, control

was achieved by the use of conventional helicopter rotor cyclic and collective blade angle controls. At higher

airspeeds the conventional fixed-wing

airplane tail control surfaces and

ailerons provided the means for flight

path control. 

A Lycoming O-435-23 opposed sixcylinder reciprocating engine was

mounted vertically in the fuselage

behind the crew area. Power was

transmitted through a main transmission located above the engine to drive

shafts which provide input to the outboard gearboxes. The spanwise shafts

incorporated flexible couplings to

accommodate the angular misalignments of the drive system. The outboard gearboxes and the shafts that

drove the rotors could be tilted by the

pilot through the use of electrical actuators. This permitted the rotors to be moved from a horizontal plane (for the

hover and low speed flight mode) to a vertical plane (for higher speed forward

flight) when sufficient airspeed was attained for the wings to support the aircraft.

117

62 Transcendental Model 2 information from Janes, All the World’s Aircraft, 1955-1956.

Figure A-2.

Transcendental Model 

2 three-view drawing.
Model 2 Characteristics

Powerplant

One Lycoming . . . . . . . . . . . . . . . . . O-435-23

Brake horsepower . . . . . . . . . . . . . . . 250 HP@ 3200 RPM 

Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 ft 1 in

Wing 

Span. . . . . . . . . . . . . . . . . . . . . . . . . . 22 ft 9 in

Area. . . . . . . . . . . . . . . . . . . . . . . . . . 100 sq ft

Airfoil section . . . . . . . . . . . . . . . . . . NACA 23015

Width. . . . . . . . . . . . . . . . . . . . . . . . . 39 ft (to outer tip of rotor disc) 

Height . . . . . . . . . . . . . . . . . . . . . . . . 9 ft 5 in

Rotor

No. of rotors . . . . . . . . . . . . . . . . . . . 2, interconnected

Type. . . . . . . . . . . . . . . . . . . . . . . . . . articulated

Blades/proprotor . . . . . . . . . . . . . . . . 3

Diameter . . . . . . . . . . . . . . . . . . . . . . 18 ft 

Chord (constant) . . . . . . . . . . . . . . . . 0.356 ft

Disc loading . . . . . . . . . . . . . . . . . . . 4.4 lb/sq ft

Weight

Empty . . . . . . . . . . . . . . . . . . . . . . . . 1579 lb

Gross . . . . . . . . . . . . . . . . . . . . . . . . . 2249 lb

Useful load . . . . . . . . . . . . . . . . . . . . 670 lb

No. of seats . . . . . . . . . . . . . . . . . . . . . . . . . . 2

118

Figure A-3.

Transcendental Model 

2 cutaway drawing.
Bell XV-3

The XV-363 (figures A-4 and A-5) is a fixed mid-wing VTOL research aircraft

developed to explore the flight characteristics of the tilt rotor aircraft. A two-bladed

proprotor is mounted on a shaft assembly at each wing tip. The proprotors can be

rotated over a range of 90 degrees to permit hover and helicopter-, conversion-, and

airplane-modes of flight. The aircraft has a conventional empennage plus a ventral

fin below the vertical fin and rudder and uses a skid type landing gear. Small

wheels can be attached for rolling takeoff and landing tests. The wing incorporates

half-span 20 percent wing chord flaps and 22.5 percent wing chord ailerons.

Fuselage-to-wing tip struts are incorporated to increase the stiffness of the wing.

A supercharged Pratt and Whitney R-985-AN-1 reciprocating radial engine is

mounted in the fuselage aft of the wing center section. The standard rating for

this engine is 450 BHP at 2300 rpm but, to increase performance for test purposes, the manufacturer authorized operation at 2400 rpm for takeoff, hovering

and STOL flight conditions. Power is transferred from the engine to the proprotors through a short flexible-coupled drive shaft to the main (center) transmission located between the wing center section spars. From the main gear

box, flexible-coupled shafts extend spanwise to the outboard wing tip (outboard) transmissions. The proprotor shafts extend from the outboard transmissions normal to the input drive shaft. The outboard transmissions are mounted

on yokes which enable the proprotor shafts to be pivoted from the vertical to

the horizontal.

The cockpit contains helicopter-type controls: a cyclic control stick, a collective

pitch stick with a twist grip throttle, and rudder pedals. The longitudinal cyclic

proprotor controls are mechanically reduced as the airplane mode configuration

is approached. In the airplane mode no longitudinal cyclic response is provided.

The differential collective controls (lateral stick inputs) are also reduced but onethird of the helicopter mode control is retained in airplane flight to increase

maneuverability. The collective pitch is also automatically increased in the airplane mode to correspond to the required operating range. The longitudinal, lateral, and collective controls are hydraulically boosted. There is no stability or

control augmentation installed in the XV-3 and it does not have lateral cyclic

proprotor controls.

The conversion system is powered by an electromechanical linear actuator 

connected to each outboard transmission yoke and is controlled by a “beep”

switch on the cyclic stick. An interconnect shaft links the right and left conversion systems to ensure synchronization and to permit the conversion cycle to be

119

63 The XV-3 configuration described in this section (from Deckert and Ferry, footnote 7) reflects

the characteristics of aircraft number 2 (tail number 4148) as flown during evaluations at the

NASA Dryden Flight Research Center and the NASA Ames Research Center (from 1959 

through 1962).
completed with either actuator inoperative. The proprotors can be returned to the

helicopter position by means of a hydraulic emergency reconversion clutch in the

event of a complete electrical failure. 

To accommodate the requirement for a high proprotor RPM for the high-thrust

helicopter flight, and a lower RPM for airplane flight, the XV-3 incorporates 

a two-speed gear reduction capability in the center transmission. The gear shift,

performed while flying in the airplane mode, is accomplished by means of an

electrically controlled hydraulic clutch actuated by a switch in the cockpit.

120

27'

11' 1"

50'

S.ARMY U.

Struts on 4148 only

Attachable

wheels on 4148 only

Rotors in VTOL mode

4147 (Early configuration with 3-bladed proprotor) 4148 (Final configuration with 2-bladed proprotor)

U.S. ARMY

4148

XV-3

U.S. ARMY

4147

13' 3"

30' 4"

Figure A-4.

XV-3 three-view drawing.
XV-3 Characteristics

Powerplant

One Pratt and Whitney R-985-AN-1 reciprocating radial engine

Brake horsepower . . . . . . . . . . . . . . . 450 @ 2300 RPM 

Length 30 ft 4 in

Wing 

Span. . . . . . . . . . . . . . . . . . . . . . . . . . 31 ft 2 in

Area. . . . . . . . . . . . . . . . . . . . . . . . . . 116.0 sq ft

Chord (constant) . . . . . . . . . . . . . . . . 3.75 ft

Airfoil section . . . . . . . . . . . . . . . . . . NACA 23021

Sweep/dihedral . . . . . . . . . . . . . . . . . 0 degrees

Aspect ratio. . . . . . . . . . . . . . . . . . . . 8.4

Width. . . . . . . . . . . . . . . . . . . . . . . . . 50 ft (to outer tip of proprotor disc)

Height . . . . . . . . . . . . . . . . . . . . . . . . 13 ft 3 in

Width. . . . . . . . . . . . . . . . . . . . . . . . . 39 ft (to outer tip of rotor disc)

Horizontal tail

Span. . . . . . . . . . . . . . . . . . . . . . . . . . 11ft 1 in

Area. . . . . . . . . . . . . . . . . . . . . . . . . . 32.6 sq ft

Chord. . . . . . . . . . . . . . . . . . . . . . . . . 3 ft 10 in

Airfoil section

root . . . . . . . . . . . . . . . . . . . . . . . . . . NACA 0015

tip . . . . . . . . . . . . . . . . . . . . . . . . . . . NACA 0012

Aspect ratio. . . . . . . . . . . . . . . . . . . . 3.8

Leading edge sweep . . . . . . . . . . . . . 9.5 degrees

Vertical tail

Area. . . . . . . . . . . . . . . . . . . . . . . . . . 32.8 sq ft

Airfoil section . . . . . . . . . . . . . . . . . . NACA 0012

Aspect ratio. . . . . . . . . . . . . . . . . . . . 1.33

Leading edge sweep . . . . . . . . . . . . . 20 degrees

Proprotor

No. of proprotors . . . . . . . . . . . . . . . 2, interconnected

Blades/proprotor . . . . . . . . . . . . . . . . 2

Diameter . . . . . . . . . . . . . . . . . . . . . . 23 ft

Chord (constant) . . . . . . . . . . . . . . . . 11.0 in

Solidity . . . . . . . . . . . . . . . . . . . . . . . 0.051

Disc loading . . . . . . . . . . . . . . . . . . . 5.66 lb/sq ft

Airfoil section . . . . . . . . . . . . . . . . . . NACA 0015

Twist (linear). . . . . . . . . . . . . . . . . . . 1 degree, 36 sec/ft

Delta 3 angle. . . . . . . . . . . . . . . . . . . 20 degrees

Rotational speed 

Helicopter mode . . . . . . . . . . . . . . . . 532 rpm

Airplane mode. . . . . . . . . . . . . . . . . . 324 rpm

Weight

Design . . . . . . . . . . . . . . . . . . . . . . . 4700 lb

Empty (actual). . . . . . . . . . . . . . . . . . 4205 lb

121
Actual gross (at engine start) . . . . . . 4890 lb

Standard fuel . . . . . . . . . . . . . . . . . . 280 lb (600 lb capacity)

Instrumentation . . . . . . . . . . . . . . . . . 160 lb

No. of seats . . . . . . . . . . . . . . . . . . . . . . . . . . 1

122

Instrument panel

Cooling fan

Pilot Oil tank

Radio

Battery

Skid type gear

Cyclic stick

Rudder pedals

Pilot's 

collective 

stick

R-985

piston engine

Center

transmission

Figure A-5.

XV-3 inboard drawing, side

view.
XV-15 Tilt Rotor Research Aircraft

The XV-15 tilt rotor research aircraft

was designed to be representative of

the class of VTOL aircraft that

employs large diameter, low disc loading, wingtip mounted proprotors that

provide the thrust for vertical lift and

forward flight. The XV-15 was sized

to meet two requirements. First, it had

to be large enough to properly demonstrate the performance, flight- and

structural-dynamics, acoustics, and

handling qualities of this vehicle class.

Second, it had to be small enough to

be accommodated in the test section of

NASA Ames 40- by 80-foot wind 

tunnel for aerodynamics, loads, and

systems performance evaluations.

The XV-15, shown in figure A-6, has

25-foot diameter proprotors and a

design gross weight of 13,000 pounds.

The proprotor axes rotate from 5

degrees aft of vertical for rearward

flight or autorotation, to 90 degrees

(vertical), the normal position for

hover and helicopter flight, and to 0

degrees (horizontal) for airplane mode

flight. The TRRA can also operate

over a broad range of airspeeds at proprotor (or “nacelle”) positions in

between the helicopter and airplane

modes. This flight region is referred to

as the conversion mode and is depicted on the conversion envelope shown

as figure A-7.

Two Lycoming T-53-L-13B engines, modified for vertical starting and running

(designated the LTC1K-4K) are installed in the wingtip nacelles. These engines

are rated at 1,550 shp (shaft horsepower) for takeoff, with a normal (continuous

operation) rating of 1,250 shp. The engines drive the proprotors through main

transmissions also located in each nacelle. The two proprotors are also linked by

a cross shaft system that allows both rotors to continue to be powered after the

shut down of one engine. Upon the loss of a single engine during flight, it is disengaged from the drive system by an automatic clutch. Because of continuous

123

Figure A-6.

Three-view drawing of the

XV-15 tilt rotor research 

aircraft.
torque transmission limitations, the

engines on the XV-15 do not operate

above 1,160 shp in the helicopter

mode. A cross-shaft center gear box

located below the wing in the fuselage

accommodates the 6.5-degree forward

wing sweep which is required to

obtain proprotor-to-wing clearance in

airplane mode flight. The free turbine

engines permit the proprotor speed to

be reduced during airplane mode flight

to improve performance and reduce

cruise noise.

The flight controls in the hover and

helicopter modes resemble those of a

lateral-tandem rotor helicopter. While

the fixed-wing control surfaces remain

active at all times, the primary low speed control forces and moments are provided by proprotor collective- and cyclic-blade angle (pitch) changes. Differential

collective pitch produces aircraft roll and differential cyclic pitch results in yaw

motions. The proprotor rpm is regulated by automatic control of the collective

pitch. To reduce the hover performance loss resulting from the proprotor’s wake

impinging on the surface of the wing, the inboard flaps can be lowered to preset

deflection positions. The outboard wing control surfaces are also deflected down

when the flaps are deployed, but to a displacement less than two thirds of the

flap position. The outboard wing control surfaces serve as ailerons in high speed

flight and are referred to as “flaperons.”

During conversion from helicopter flight to airplane mode flight, the helicoptertype control inputs to the proprotor are mechanically phased out and the conventional airplane control surfaces provide all flightpath-control forces and

moments. By the time the nacelles are in the airplane position, the power lever

inputs to the proprotor are nulled and the total control of the collective pitch is

transferred to the automatic rpm governor. 

A stability and control augmentation system (SCAS) is provided with a threeaxis (pitch, roll, and yaw) rate system that includes a pitch and roll attitude retention feature. SCAS characteristics are continuously varied from the helicopter to

the airplane modes as a function of conversion angle to provide appropriate rate

damping and control augmentation. The pitch and roll axes have dual channels

and the yaw axis has a single channel system. SCAS-off flight has been routinely

evaluated and demonstrated and, although damping and control are degraded, the

XV-15 is still quite safe to fly, albeit with a higher pilot workload. A force feel

system (FFS) provides stick and pedal forces proportional to control displacements while isolating the pilot’s controls from SCAS feedback forces. Force gra124

Figure A-7.

Conversion corridor of the

XV-15 tilt rotor research 

aircraft.

120

90

60

30

0 40 80 120

Calibrated airspeed (knots)

Nacelle angle (degrees)

160 200 240 280 320

Wing

stall

Airplane mode 

(0° Nacelle angle)

Helicopter mode
dients are increased and trim rates are decreased with airspeed through an airspeed sensor. With the FFS off, pitch trim is available at a reduced rate and control forces are high but manageable.

The XV-15 aircraft has three independent transmission-driven 3000-psi hydraulic

systems. The pump for each system is geared to the rotor side of the transmission clutch so that full hydraulic power can be provided with both engines shut

down, as long as the rotors are turning within the normal speed range. Automatic

shuttle valves are provided on some critical flight control actuators which switch

the utility hydraulic system onto the critical actuator in the event of the loss of

one of the primary hydraulic systems. The tricycle landing gear, operated by the

utility hydraulic system, is automatically switched to a 3,000-psi pneumatic

backup system for a one-time deployment when the normal hydraulics source

becomes dedicated to the flight controls.

The electrical system includes two engine-mounted 300-ampere starter-generators. Each generator provides sufficient power to accommodate the aircraft’s

peak electrical load requirements. The XV-15’s electrical system consists of two

28-volt dc busses and two 600 VA solid state inverters for ac power. Automatic

dc bus interconnection is provided with pilot-controlled override switches in the

cockpit. A 13-ampere-hour battery is connected to each dc bus during normal

operation to prevent the bus voltage from dropping excessively during bus

switching operations. The batteries also provide a self-contained engine-start

capability.

The nacelles are tilted by ball-screw-jack actuators with hydraulic motors and

electrically-powered servo valves. A triply redundant hydraulic power supply is

provided for the conversion system because the XV-15 cannot be landed in the

airplane mode without destroying the proprotor system. In the event of total electrical failure, the pilot still has mechanical access to hydraulic power to convert

to the helicopter mode. The conversion system interconnect shaft provides a

means to maintain both nacelles at the same angle and to provide 

power to drive the nacelle conversion in the event of a total power failure on one

side. For flight operations, the pilot can select the normal 7.5-degree/per second

rate continuous conversion (which completes the conversion in 12.5 seconds) or

a slower rate of 1.5-degree/per second. The conversion can be stopped and

steady flight performed at any point in the conversion envelope. 

Fuel is supplied to each engine by separate fuel systems contained in each 

wing. Each system has two lightweight crash-resistant fuel cells which are interconnected to form a single tank. An electrically driven submerged boost pump is

located at the lowest point of each tank. Interconnect valves and lines permit fuel

transfer between tanks or supplying fuel to both engines from the same tank.

With a complete loss of electrical power to both boost pumps, adequate fuel flow

would be maintained by the engine-driven pumps up to an altitude of 10,000

feet. The 1,475 pounds of fuel carried in the wing allows a flight of one hour.

125
For ferry operations, Bell developed a removable internal tank that extends the

duration of flight to more than two hours.

An environmental control system provides ventilation and temperature control. An

air-cycle environmental control unit mounted in the aft fuselage is powered by

bleed-air from the right engine. During hot day operation of the XV-15, the cooling

capacity was found to be inadequate. To reduce crew station heating, the overhead

windows were covered with an opaque coating during the late 1990s. Each crew

seat is equipped with an oxygen system supplied from a 1,800-psi oxygen cylinder.

Adequate oxygen for a one hour flight with a 20 percent reserve is carried onboard.

The design of the XV-15 TRRA incorporates many features and system redundancies intended to enhance the safety of this vehicle. Some of these are not

expected to be included in civil aircraft of its weight class and, therefore, must

be considered when evaluating the XV-15’s weight and payload capacity. Among

the additional items are the Rockwell International Model LW-3B ejection seats,

capable of removing the crew members in flight or from a zero airspeed, groundlevel (zero-zero) normal attitude condition. The ejection seats can be triggered

independently or simultaneously by pilot command. In addition, the overhead

and side windows can be removed by a mild detonator cord placed around the

window frames. The window removal can be initiated from within the cockpit or

from an external lever located under a door on the nose cone.

126

Figure A-8.

General layout and major

components of the XV-15 tilt

rotor research aircraft.

38

36

37

35

22

21

24 25

19

32

2

1 4

8

10

7 5

12

33 11

34

20

13

18

17

14

27

28 30

31

29

23

39

40

1. Instrument boom

2. Pitot static probe

3. Door for emergency window release handle

4. Nose landing gear from Canadair CL-84

5. Bird-proof windshield

6. Rockwell LW-3B ejection seats

7. Avionics installation

8. Entry door, 2 ft 8 in opening

9. Phasing actuator for tilting mechanism

10. Cross-shaft between tilting actuators

11. Center gearbox for transmission cross-shaft

12. Bonded splice doublers

13. Upper and lower wing skins, 3/4 in honeycomb sandwich

14. Aluminum alloy extrusion front spar

15. Lightweight crash-resistant fuel tanks, total fuel 1,509lb

16. Aluminum alloy extrusion rear spar

17. Flaps, max deflection 75°

18. Flaperons, max deflection 47°

19. Actuator fairing

20. Navigation lights

21. Collective-pitch lever

22. Collective crosshead

23. Cyclic swash plate

24. Cyclic tube

25. Pitch-change lever

26. Gravity refueling point

27. Transmission cross-shaft

28. Rotor-tilt actuator, controlled by thumb switch on pilots 

collective lever

29. 25 ft diameter rotors (14 in chord)

30. Modified 1,550 h.p. Lycoming LTCIK-4K turboshafts

31. Main transmission

32. Main landing gear from Canadair CL-84

33. Cyclic mixing box

34. Collective mixing box

35. Controls to cockpit via cabin floor

36. Anti-collision light

37. VOR localizer aerial

38. VHF aerial

39. UHF aerial

40. Navigation light

3

15

9

16

15 6

26
A cutaway drawing showing the key components of the XV-15 is shown in 

figure A-8. Inboard profile drawings are provided in figures A-9 and A-10.64

As an example of the overall performance capabilities of the XV-15, the heightvelocity flight envelope is shown in figure A-11. The variation of power with

airspeed indicating the ability of the tilt rotor aircraft to operate over a broad 

range of airspeeds at power levels well below that required for hover is shown in

figure A-12.65

Although only two XV-15 aircraft were built, the Government Project Office

took the unusual step of directing the Bell Helicopter Company, under the TRRA

Contract, to develop a complete flight manual. These manuals66 became a valuable source of systems information for the flight and ground crews during 

the flight program and served as a training and familiarization tool for the many

guest pilots.

127

64 Martin D. Maisel, et al, “Tilt Rotor Research Aircraft Familiarization Document,” NASA TM

X-62.407, January 1975. 

65 W.L. Arrington, “Flight Test Report,” Vol. I-V, NASA CR 177406 and USAACSCOM TR-86-

A-1, June 1985.

66 Anon., “XV-15 Flight Manual,” Bell Helicopter Textron Report TP-78-XV-15-1, August 15, 1980.
128

Pylon Assembly

Blade Assembly Rotor

Ejection Seat

Rotor and Controls Inst

Standby Compass

Window Release Handle

Instrument Panel

Birdproof Windshield

Rudder Pedal Instl

Power Level Instl

Avionics andElectronics Inst

DME Antenna

Marker Beacon Antenna

Anti-Collision Light

XPNDR Antenna

Radar Altimeter Antenna

Main Landing Gear

AntennaRadar Altimeter Hydraulic Systems Ground Power Door

Fuselage Assy

Battery (2)

Cabin Deck WL 33.0

DC Relay J-Box

Electrical Ground Power Door

VOR Localizer Antenna

Compass XMTR Magnetic Flux Compensator

Position Light

ArrangementStructural Empennage 

Oxygen Cylinders

Nitrogen Bottle

Window Release Handle

Nose Landing Gear

Instr. Boom

Glideslope Antenna

39.00

W.L.

Emergency Escape Hatch

Emergency Escape and Rescue Window

Mode Flight PositionPylon Shown in Airplane

Controls Installation Anti-Collision Light

Empennage Controls

and Distribution DuctsEnvironmental Control Unit

STA

112.00 STA

240.00

STA

397.00

603.8

STA

VHF Antenna-Right Tailfin

UHF Antenna-Left Tailfin

Figure A-9.

Side view inboard profile of

the XV-15.
129

Figure A-10.

Top view inboard profile of

the XV-15.
130

32

28

24

20

16

12

8

4

0 40 80 120 160 200

True airspeed, knots

Density altitude, ft × 103

240 280 320 360

335 knots True

Normal rated

power limit

Level flight

5° Dive

Torque limit

2400

2000

1600

1200

800

400

0 40 120

Calibrated airspeed (knots)

Rotor shaft horsepower

80 160 200 240

Single engine

emergency power

available

Conversion corridor

torque limit

Estimated

Predicted

Flight test data

Hover

Symbols

Nacelle angle = 90°

Flap angle = 20°

60°

20°

30°

20°

0°

20°

0°

0°

Airplane

torque limit

Vtrue ~= 300 knots @ 16,000 ft

Helicopter mode Airplane mode

Figure A-11.

XV-15 height-velocity

envelope.

Figure A-12.

XV-15 variation of power 

with airspeed.
XV-15 Characteristics

Powerplant

Two Lycoming LTC1K-41K turboshaft engines (modified T53L13B)

Horsepower ratings

Contingency (2 minutes). . . . . . . . . . 1802 SHP

Takeoff (10 minutes). . . . . . . . . . . . . 1550 SHP

Normal (max. continuous) . . . . . . . . 1250 SHP

Length . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 ft 1 in (not including nose boom)

Wing 

Span. . . . . . . . . . . . . . . . . . . . . . . . . . 32 ft 2 in

Area. . . . . . . . . . . . . . . . . . . . . . . . . . 169.0 sq ft

Chord (constant) . . . . . . . . . . . . . . . . 5.25 ft

Airfoil section . . . . . . . . . . . . . . . . . . NACA 64A223

Sweep . . . . . . . . . . . . . . . . . . . . . . . . -6.5 degrees

Dihedral. . . . . . . . . . . . . . . . . . . . . . . 2.0 degrees

Aspect ratio. . . . . . . . . . . . . . . . . . . . 6.12

Width . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 ft 2 in (to outer tip of 

proprotor disc)

Height . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 ft 8 in

Horizontal tail

Span. . . . . . . . . . . . . . . . . . . . . . . . . . 12ft 10 in

Area. . . . . . . . . . . . . . . . . . . . . . . . . . 50.25 sq ft

Chord. . . . . . . . . . . . . . . . . . . . . . . . . 3 ft 11 in

Airfoil section . . . . . . . . . . . . . . . . . . NACA 64A015

Aspect ratio. . . . . . . . . . . . . . . . . . . . 3.27

Vertical tail

Area. . . . . . . . . . . . . . . . . . . . . . . . . . 50.5 sq ft

Airfoil section . . . . . . . . . . . . . . . . . . NACA 0009

Mean Aerodynamic Chord . . . . . . . . 3.72 ft

Aspect Ratio . . . . . . . . . . . . . . . . . . . 2.33

Proprotor

No. of proprotors . . . . . . . . . . . . . . . 2, interconnected

Blades/proprotor . . . . . . . . . . . . . . . . 3

Diameter . . . . . . . . . . . . . . . . . . . . . . 25 ft

Chord (constant) . . . . . . . . . . . . . . . . 14.0 in

Solidity . . . . . . . . . . . . . . . . . . . . . . . 0.089

Disc loading . . . . . . . . . . . . . . . . . . . 13.2 lb/sq ft

Twist, geometric (spinner to tip). . . . 36 degrees

Delta 3 angle. . . . . . . . . . . . . . . . . . . -15.0 degrees

Rotational speed

Helicopter mode . . . . . . . . . . . . . . . . 589 rpm

Airplane mode. . . . . . . . . . . . . . . . . . 517 rpm

131
Weight

Design . . . . . . . . . . . . . . . . . . . . . . . 13,000 lb

Empty (actual). . . . . . . . . . . . . . . . . . 10,083 lb

Actual gross (at engine start) . . . . . . 13,248 lb

Fuel . . . . . . . . . . . . . . . . . . . . . . . . . 1,436 lb

Research instrumentation . . . . . . . . . 1,148 lb

No. of seats . . . . . . . . . . . . . . . . . . . . . . . . . . 2

132
Air Vehicle

Fuselage, Landing Gear, Empennage

Fuselage

Landing Gear 

Empennage

Wing, Nacelle

Wing

Nacelle

Rotors

Blade Assembly

Hub Assembly and Controls

Transmission, Cross Shafting

Left/Right Transmission

Engine Coupling Gear box

Interconnect System

Power Plant

Engine

Power Plant Installation

Fuel System

Hydraulic System

Electrical System

Environmental Control System

Emergency Egress System

Flight Control

Primary Flight Controls

Secondary Flight Controls

Thrust/Power Management System

Automatic Flight Control

Crew Station

Communication, Nav. Flight Instrument’s

Research Instrumentation

Support Equipment and Systems

Aircraft No. 1 Final Assembly

Analytical Integration

Aircraft No. 2 Final Assembly

Mock-up

Appendix B—Key Personnel

133

67 Tilt rotor research aircraft project assignments, both at Bell and in the Government Project

Office, changed during the activity. This list reflects the staff assignments during the project’s

first four years (1973-1977) when a large portion of the design, fabrication, and component testing was accomplished. The inclusion of a second name indicates where WBSE reassignments

were made.

Key Tilt Rotor Research Aircraft Project Personnel67

WBS LEVEL: Bell Army/NASA 

I II III IV

V/STOL Tilt Rotor Research 

Aircraft Project

Ken Wernicke

George Carter

George Carter

Ed Broman/Jerry Pickard

George Carter 

George Carter

Pete Smith 

Pete Smith

Ed Covington

Ernie Schellhase

Ernie Schellhase

Charlie Bradocks

J. D. Mooney

J. D. Mooney

J. D. Mooney

Norm Busbee

Norm Busbee

Norm Busbee

Norm Busbee

Jose Caero

Marvin Willis

Ed Broman/

Claude Leibensberger

Rod Wernicke/Ray Conrad

Ken Wernicke

Paul Keefer

Paul Keefer

Paul Keefer

Marvin Willis

Ken Wernicke

Dave Glass/Doug Winniford

Mike Kimbell/Jerry Pickard

George Carter/Pete Smith 

Troy Gaffey/Roger Marr

Pete Smith

Pete Smith

Kip Edenborough

Robby Robinson

Robby Robinson

Robby Robinson

Robby Robinson

Robby Robinson 

Kip Edenborough 

Kip Edenborough

Kip Edenborough

Kip Edenborough

Kip Edenborough

Jim Weiberg

Jim Weiberg

Jim Weiberg

Jim Weiberg

Jim Weiberg

Jim Weiberg

Jim Weiberg

Marty Maisel

Marty Maisel

Marty Maisel

Marty Maisel

Shorty Schroers

Gary Churchill

Gary Churchill

Gary Churchill

Gary Churchill

Gary Churchill

Shorty Schroers

Shorty Schroers

Al Gaehler/Mike Bondi

John Hemiup

John Hemiup

Dean Borgman/Marty Maisel

John Hemiup

Shorty Schroers
Test and Evaluation

Component Acceptance Test

Component Development Tests

Systems Test

Structural Tests

Propulsion Tests

Egress System Tests

Integrated Systems Tests

Ground Tiedown Aircraft Tests

Wind Tunnel Aircraft Tests

Post Test Aircraft Refurbishment

Contractor Flight Tests

Government Flight Tests

Simulations

Data and Documentation

Test Plans, Procedures, and Test 

Reports

Design Reports and Specifications

Inspection and maintenance 

Manual

Flight Operations Manual

Instrumentation and Data Acquisition

Manual

System Safety and R&QA Plan

Spares

Systems Project Management Plan

Training

Rod Wernicke

Mike Kimbell

Rod Wernicke

Mike Kimbell

Mike Kimbell

Mike Kimbell

Rod Wernicke/

Ray Conrad

Claude Liebensberger

Bill Martin

Roger Marr

Pete Smith

Roger Marr/

Shep Blackman

Roger Marr

Tommy Thomason

Mike Kimbell

Tommy Thomason

Jerry Pickard

Mike Kimbell

Aaron Whitner/

Dave Glass

Tommy Thomason

Mike Kimbell/

Jerry Pickard

Lovette Coulter

Jerry Pickard

Kip Edenboroug

Jim Lane

Kip Edenborough

John Hemiup

Kip Edenborough

Jim Welberg

Shorty Schroers

Al Gaehler

Jim Weiberg

John Hemiup

Shorty Schroers

Gary Churchill

Dean Borgman

John Hemiup

Dean Borgman

John Hemiup

Shorty Schroers

Al Gaehler

Mike Bondi

Mike Carness

John Hemiup

Jim Diehl

Jim Diehl

134

WBS LEVEL: Bell Army/NASA 

I II III IV

V/STOL Tilt Rotor Research 

Aircraft Project
Key Industry and Government Tilt Rotor Technology Personnel

Boeing Vertol (late 1960s to early 1970s)

V/STOL Technology Manager Kenneth (Pip) Gilmore

Chief of Preliminary Design David Richardson

Tilt Rotor Research Engineers

Aerodynamics Methodology Allen Schoen

Preliminary Design and Performance Harold (Hal) Rosenstein

Aeroelastics and Structural Dynamics Harold (Alex) Alexander

Rotor Performance Methodology John Magee

VTOL Propeller Design and Test Marty Maisel

Aerodynamics M. A. (Tony) McVeigh

Aerodynamics Ross Clark

Aerodynamics S. Jon Davis

VTOL Aircraft Design Unit Chief Paul Dancik

Preliminary Design Bernard (Bernie) Fry

Flight Controls Gary Churchill

Bell Helicopter Company (late 1960s to early 1970s)

Director of Advanced Engineering, 

XV-3 Chief Engineer Robert (Bob) Lichten

Chief of Advanced Design Stanley (Stan) Martin

Manager, Applications Engineering Richard (Dick) Spivey

IR&D Manager Richard (Dick) Stansbury

Tilt Rotor Lead Design Engineer Kenneth (Ken) Wernicke

Tilt Rotor Proposal Manager Henry (Hank) Smyth

Tilt Rotor Research Engineers

Dynamics Troy Gaffey

Dynamics Jim Bilger

Dynamics Kip Edenborough

Preliminary Design Jack DeTore

Preliminary Design Ken Sambell

Stress Bob Gunderson

Design Pete Smith

Fatigue Analysis D. J. Reddy/Will Broekhuizen

Rotor Design Ernie Schellhase/Ed Covington

135
US Army Air Mobility Research and Development Laboratory (late 1960s to

early 1970s)

Director Paul Yaggy

Deputy, Technical Andrew (Andy) Morse

Test Pilot Daniel (Dan) Dugan

Tilt Rotor Research Engineers

Preliminary Design Methodology Michael Scully

Dynamics Dave Sharpe

Project Management Dean Borgman

Aerodynamics, Performance Marty Maisel (from 1970)

Flight Controls Gary Churchill (from 1971)

Flight Test Shorty Schroers (from 1972)

Dynamics Kip Edenborough (from 1972) 

NASA Ames Research Center (late 1960s to early 1970s)

Director Hans Mark

Director, Aeronautics and Flight Mechanics Leonard Roberts

Deputy Director, Aeronautics and Flight Mechanics John Boyd

Chief, V/STOL Projects Office Woodrow L. (Woody) Cook

Manager, Advanced VTOL Projects Office Wallace H. (Wally) Deckert

Chief, Full-Scale and Systems Research Division Brad Wick

Research Engineers

Dynamics Wayne Johnson

Rotor Dynamics Test Jim Biggers

V/STOL Wind Tunnel Test Demo Giulianetti

V/STOL Wind Tunnel Test David Koenig

V/STOL Wind Tunnel Test William (Bill) Tolhurst

Flight Data Acquisition Herb Finger

Instrumentation Don Reynolds

Test Pilot Ronald (Ron) Gerdes

136
Tilt Rotor Research Aircraft Project Office 

Bell

Program Manager (1973-1975) Henry (Hank) Smyth

(1975-1981) Tommy Thomason

(1981-1984) Lovette Coulter

(1984-1990) Ron Reber

Tilt Rotor IR&D Manager (1990-1993) William (Bill) McKinney

(1993-1994) Don Ferguson

XV-15 Project Manager (1994- ) Colby Nicks

Deputy Program Manager (1973-1975) Tommy Thomason

(1975-1981) Lovette Coulter

(1987-1989) Jerry Pickard

Chief Project Engineer (1973-1987) Ken Wernicke

Chief Design Engineer (1973-1978) George Carter

Chief Technical Engineer (1973-1975) Troy Gaffey

Engineering Administrator (1981-1990) Mike Kimbell

Technology Manager Roger Marr

Project Dynamicist Jim Bilger

Test Pilots Ron Erhart

Dorman Cannon

Roy Hopkins

Tom Warren

John Ball

Robert Williams

Dwayne Williams

Army/NASA

Project Manager (1972-1977) David Few

(1977-1979) Jim Brown

(1979) David Few (Acting)

(1980-1984) John Magee

(1985-1994) Bill Snyder

Deputy Project Manager (1972-1974) Dean Borgman

(1975-1977) Jim Brown

(1977-1981) Mike Carness

Deputy Project Manager,

(Technical) (1974-1979) Kip Edenborough

(1979-1980) John Magee

Deputy Project Manager,

(Army Liaison) (1978-1981) Clifford McKiethan

Resident Manager (1973-1984) Jim Lane

137
Army/NASA (continued)

Staff (1972-1981) Jerry Barrack

Mike Bondi

Dave Chappell

Gary Churchill

Kip Edenborough

Al Gaehler

John Hemiup

Violet Lamica

Jim MacDonnell

John Magee

Marty Maisel

Robbie Robinson

Shorty Schroers

Victor (Tory) Stevens

John Weyers

Test Pilots Dan Dugan

Ron Gerdes

Grady Wilson

George Tucker

Rickey Simmons

XV-15 Principal Investigators

Discipline Bell Army/NASA

Flight Test Director Shep Blackman Shorty Schroers

Bill Martin Marty Maisel

Colby Nicks Brent Wellman

Flight Controls and Dynamics Roger Marr Gary Churchill

Marvin Willis Shorty Schroers

Jim Weiberg

Structural Loads Bob Gunderson Dave Chappell

D. J. Reddy Shorty Schroers

Jim Weiberg

Aeroelastic Stability M. J. Joglekar Wally Acree

Jim Bilger Mark Tischler

Performance Roger Marr Marty Maisel

Jim Weiberg

Jerry Barrack

Acoustics John Breiger Marty Maisel

Bryan Edwards Dave Conner

Suzanna Shank Mike Marcolini

Arnold Mueller

Gloria Yamauchi

Megan McCluer

138
XV-15 Ground Crew

Assignment Bell Army/NASA

Aircraft Manager/Engineer Bill Martin John Weyers

Ted Turner Jim McDonnell

Jerry Pickard Paul Espinosa

Crew Chief Jerry Bree

Dick Denman

Assistant Crew Chief Dick Denman

Kit Boyce

Jim Lesko

Inspector John Brown

Jerry Bree

Instrumentation Engineer Dave Glass Tony Ogden

Doug Winniford John Lewis 

Instrumentation Technician Al Morningstar

Duane Allen

Alex Macalma

John Wilson

139
1452-1519 Leonardo da Vinci credited with the design of the first helicopter, basically a helical airscrew.

1924 Henry Berliner flies fixed-wing biplane with large diameter fixed-pitch propeller mounted on a tiltable vertical

shaft near the tip of each wing.

September 1930 “Flying Machine” patent, issued to George Lehberger,

employs use of a relatively low disc loading thruster (propeller) that can tilt its axis from the vertical (for vertical

lift) to the horizontal (for propulsive thrust).

Late 1930s Baynes Heliplane patent issued in England.

1937 Focke-Wulf Fw-61 (Germany) lateral-twin rotor helicopter

flown.

1942 Focke-Achgelis FA-269 trail-rotor convertiplane project

initiated. 

Early 1940s Platt-LePage Aircraft Company conduct preliminary

design work on tilt rotor aircraft. 

January 1947 Mario Guerrieri and Robert Lichten establish

Transcendental Aircraft Company to develop convertiplane

(tilt rotor aircraft).

August 1950 U. S. Army and U. S. Air Force announce competition to

design a Convertiplane. Bell Helicopter and

Transcendental Aircraft submit bids.

May 1951 USAF Air Research and Development Command awards

contract to Bell to build mockup and begin detailed design

of a flight tilt rotor aircraft.

October 1953 Award of U.S. Air Force contract for development, prototype construction, and testing of two XH-33 (later designated XV-3) full-scale “tilting-thrust-vector convertiplanes” to Bell Helicopter Company.

15 June 1954 First flight of Transcendental Aircraft Corporation 

Model 1-G tilt rotor aircraft.

15 February 1955 Platt patent of tilt rotor aircraft, No. 2,702,168 granted.

February 1955 First XV-3 rolled out.

Appendix C—Chronology

140
20 July 1955 Transcendental Model 1-G crashes. Had previously flown

from helicopter configuration to within 10 degrees of airplane configuration.

11 August 1955 First hover flight of XV-3. Piloted by Bell Chief

Helicopter Test Pilot Floyd Carlson.

25 October 1956 XV-3 (tail number 4147) crashes due to a severe rotor

instability injuring Bell test pilot Dick Stansbury.

1957 Transcendental Model 2 program terminated as

Government funding shifts to Bell XV-3.

18 December 1958 XV-3 achieves first full in-flight conversion from helicopter to airplane mode and from airplane to helicopter mode.

World’s record event. Piloted by Bell XV-3 project test

pilot Bill Quinlan.

6 February 1959 USAF Captain Robert G. Ferry first military pilot to execute full conversion of XV-3.

1 May 1959 XV-3 shipped to Edwards Air Force Base for Phase II

Flight Test Program. Flight testing begins 15 May 1959.

12 August 1959 First hovering, altitude, and full conversion flight of XV-3

by a NASA test pilot Fred Drinkwater.

8 August 1961 U.S. Army test pilot Major E. E. Kluever conducts first

flight evaluation of the XV-3 by an Army pilot.

June/July1962 XV-3 (tail number 4148) with new rotor system tested in

ARC 40- by 80-foot wind tunnel.

April 1966 Analysis explaining the tilt rotor aircraft rotor/pylon/wing

aeroelastic instability issued by Dr. Earl Hall of Bell.

14 June 1966 NASA Ames Research Center announces completion of

XV-3 testing. Total of 250 flights accomplished, 125 flight

hours, 110 full conversions.

May 1968 Aeroelastic stability obtained for a gimbaled proprotor

using positive pitch-flap coupling (negative δ3) 

documented by Troy Gaffey of Bell.

October/November Ames wind tunnel test of modified XV-3 validated predicted

1968 rotor/pylon/wing aeroelastic stability.

141
1968 Boeing Vertol awarded contract NAS2-5025 from Ames to

investigate the effect of blade twist on the performance of

model-scale proprotors. Several performance tests were

conducted between 1969 and 1973 in the Army 7- by 10-

foot wind tunnel at Ames, the Ames 40-by 80-foot wind

tunnel, and the ONERA (Office National d’Etudes et de

Recherches Aerospatiales) 8-meter (26-foot) diameter S-1

wind tunnel in Modane-Avrieux, France. 

1969 Contract NAS2-5386 awarded to Bell for the Ames 

40-by 80-foot wind tunnel aeroelastic stability tests of 

25-foot diameter gimbaled proprotor.

November 1970 Performance tests conducted with Bell 25-foot diameter

proprotor in the Ames 40- by 80-foot wind tunnel as part

of contract NAS2-5386.

1971 Woodrow L. Cook appoints manager of the V/STOL

Projects Office (for the development and flight investigation of powered lift V/STOL aircraft) by Dr. Leonard

Roberts, Director of the NASA Ames Aeronautics and

Flight Mechanics Directorate. Cook’s deputy and manager

of the Advanced VTOL Projects Office was Wally Deckert. 

August 1971 Contracts awarded to Boeing Vertol (NAS2-6598) and Bell

(NAS2-6599) to conduct preliminary tilt rotor aircraft

design studies. 

18 September 1971 Robert Lichten, Bell’s director of advanced engineering

and chief engineer for the XV-3 project, dies following an

automobile accident.

12 October 1971 NASA Ames Research Center awarded Bell contract for

engineering study and a report to define future military

and commercial applications of tilt rotor vehicles.

1 November 1971 “An Agreement Between the National Aeronautics and

Space Administration and the Department of the Army for

the Joint Development and Operation of Tilt Rotor Proofof-Concept Research Vehicles at the Ames Research

Center” signed.

January 1972 Boeing contract NAS2-6598 was extended to include a

preliminary design of an advanced composite wing and to

define a gust and blade load alleviation feedback control

system for tilt rotor aircraft. 

142
February 1972 Bell’s tilt-fold-stowed tilt rotor tested in the Ames 40- by

80-foot wind tunnel under contract NAS2-5461. 

August 1972 Boeing conducts dynamics tests of its 26-foot diameter

hingeless, soft-in-plane proprotor in the Ames 40- by 80-

foot wind tunnel under Army-funded contract NAS2-6505.

September, October, Hover tests of 1/5 scale powered aeroelastic model of the 

December 1972 Bell Model 300 tilt rotor aircraft conducted under Ames

contract NAS2-6599 to examine performance and dynamic

characteristics for near-ground operations.

20 October 1972 Two fixed-price contracts of $0.5M each awarded by Ames

to Boeing-Vertol (contract NAS2-7259) and Bell (contract

NAS2-7260) for preliminary aircraft design studies and the

development of a program plan for a minimum-size tilt rotor

research aircraft that could meet proof-of-concept objectives.

December 1972 Performance tests of Boeing 26-foot diameter hingeless,

soft-in-plane proprotor conducted the Ames 40- by 80-foot

wind tunnel.

22 January 1973 Proposals received at Ames from Boeing Vertol and Bell

for the design, fabrication, and testing of two Tilt Rotor

research aircraft.

January-March 1973 Low speed wind tunnel tests conducted by Bell to document the performance and static stability of a 1/5 scale

powered aeroelastic tilt rotor model.

March 1973 Ames contracted with Bell (NAS2-7308) and made arrangements with the Air Force Aero Propulsion Laboratory

(AFAPL) at Wright-Patterson Air Force Base for the hover

performance test of the Bell 25-foot diameter proprotor. 

13 April 1973 Bell Helicopter Company, Fort Worth, Texas, selected for

negotiations leading to a contract for the design, fabrication, and testing of two tilt rotor aircraft.

31 July 1973 Phase II-A 60-day planning limited level of effort activity

for the development of the tilt rotor research aircraft

awarded to Bell.

30 September 1973 “Go-ahead” given to Bell for the design, fabrication, and

test of two V/STOL tilt rotor research aircraft (Contract

NAS2-7800).

143
November, Initial piloted simulations conducted in the Ames Flight 

December 1973 Simulator for Advanced Aircraft (FSAA) employing simulation math models developed by Boeing Vertol and Bell. The

math model created by P. B. Harendra and M. J. Joglekar of

Bell became the basis for the generic tilt rotor math model

used for Ames piloted simulations into the late 1990s.

May 1974 Initial publication of aeroelastic stability analysis developed by Dr. Wayne Johnson was issued.

September 1974 Dr. Irving Statler becomes director, U.S. Army Air

Mobility Research and Development Laboratory following

retirement of Paul Yaggy.

July 1975 Demonstration of simultaneous XV-15 ejection seat 

operation.

14 October 1975 XV-15 tilt rotor research aircraft entered final assembly

stage.

22 October 1976 Official roll-out ceremony for XV-15, N703NA at Bell.

1 May 1977 XV-15 N702NA completes ground tiedown development

tests at Bell.

3 May 1977 First hover and low speed flight with XV-15 N702NA

piloted by Bell Experimental Test Pilots Ron Erhart and

Dorman Cannon.

23 March 1978 XV-15 N702NA transported to Ames Research Center,

Moffett Field, California, onboard an Air Force C-5A.

4 May thru Ames 40- by 80-foot wind tunnel test of XV-15 N702NA.

23 June 1978 

July 1978 Army/NASA/Navy Memorandum of Agreement established providing Navy funding and for shipboard evaluation of TRRA. 

27 August 1978 XV-15 N703NA begins ground run tests at Bell.

23 April 1979 First flight of XV-15 N703NA performed at the Bell Flight

Test Center, Arlington Municipal Airport, Texas.

24 July 1979 First full in-flight conversion from helicopter-to-airplane

mode accomplished by Bell with XV-15, aircraft N703NA.

144
5 December 1979 Uneventful landing of XV-15 N702NA following sudden

in-flight engine failure.

5 June 1980 Major William S. “Bill” Lawrence, USMC becomes first

military pilot to fly the XV-15. 

9 June 1980 Major Ron Carpenter becomes the first U.S. Army evaluation pilot to fly the XV-15. 

17 June 1980 XV-15 N703NA flown in the airplane mode, level flight at

a record true airspeed of 301 knots (346 mph).

13 August 1980 XV-15 N703NA airlifted to DFRC from Bell.

30 October 1980 Formal Government acceptance ceremony of XV-15

N703NA held on at Dryden Flight Research Center.

28 April 1981 XV-15 N702NA transported to Farnborough, England,

onboard an Air Force MAC C-5A and a C-141 aircraft.

4-14 June 1981 XV-15 performs daily flight exhibitions at the Paris Air

Show, Le Bourget, France. First tilt rotor aircraft public

demonstration.

September 1981 Army/NASA TRRA Project Office declares that the primary proof-of-concept project objectives have been successfully completed.

28 September 1981 Army/NASA TRRA team at Ames provides first guest

pilot demonstration for General Story Stevens,

Commander, U.S. Army Aviation and Troop Command. 

26 October 1981 XV-15 N702NA ferried back to the Bell Flight Test Center

at Arlington, Texas, from California to allow Bell to continue flight research and to conduct military applications

evaluations. Longest cross-country flight to date covered

1700 statute miles with an average ground speed of 

334 mph.

30 October 1981 U.S. Senator Goldwater becomes the first non-test pilot to

fly in Tilt Rotor aircraft in Bell’s guest pilot program. At

end of flight he said, “The tilt rotor is the biggest advance

in aviation in a quarter of a century.”

December 1981 Deputy Secretary of Defense establishes Joint Services

Advanced Vertical Lift Aircraft (JVX) Program.

145
24 March 1982 XV-15 demonstrated at NASA Langley, Virginia.

26 March 1982 Secretary of the Navy John Lehman flies XV-15 at

Quantico USMC Air Station, Virginia.

29-30 March 1982 XV-15 demonstrated at Davison Army Air Field,

Fort Belvoir, Virginia.

31 March 1982 XV-15 displayed at Pentagon.

7 June 1982 Bell Helicopter and Boeing Vertol announces teaming

agreement to propose a tilt rotor aircraft for the Joint

Services Advanced Vertical Lift Aircraft (JVX) Program

competition.

8 July 1982 XV-15 low level nap-of-the-earth and evasive maneuver

flight evaluations at Ft. Huachuca, Arizona.

July 1982 XV-15 flight demonstration at Yuma, Arizona.

2, 4, 5 August 1982 XV-15 demonstrates tilt rotor shipboard operations

onboard USS Tripoli (LPH 10) in waters off the coast of

southern California... . Fifty-four takeoffs and landings

completed.

14 December 1982 Ten service-specific missions established for the proposed

joint services aircraft in a Joint Services Operational

Requirement (JSOR) document. This led to the establishment of the joint services tilt rotor, or JVX program. The

JVX program marked the first time that an aircraft had

been assigned a multimission role to serve all four services.

17 February 1983 Bell-Boeing Vertol proposes a tilt rotor aircraft to the U.S.

Navy for the JVX Program.

19-24 May 1983 LCDR John C. Ball, USN, evaluates the potential of the

tilt rotor aircraft to perform combat SAR and external lift

applications. Demonstrated over-water rescue and cargo

hookup capabilities at Dallas Naval Air Station, Texas.

10 October 1983 Completion of XV-15 military mission evaluation tests at

Marine Corps Air Station, Yuma, Arizona.

8 June 1984 Naval Air Systems Command awarded Bell and Boeing

contract for second stage of JVX tilt rotor preliminary

design.

146
15 July 1984 Bell-Boeing submitted a joint Full-Scale Development

JVX proposal to Naval Air Systems Command.

10-13 September XV-15 flown over the nap-of-the-earth course at 

1984 Fort Rucker, Alabama.

18-26 September XV-15 demonstrates air-to-air evasive maneuvers,

1984 slope landings and aerial refueling capabilities at Patuxent

River Naval Air Station, Maryland.

28 September- XV-15 demonstrated at USMC Air Station, Quantico,

2 October 1984 Virginia.

2 October 1984 XV-15 demonstrated flight from downtown New York City

to downtown Washington, D.C., in 45 minutes.

5 October 1984 Bell completed a 3500-mile demonstration tour with 

XV-15... 54 flights in 20 flying days, 21 evaluation flights,

five military pilots, and 16 guest pilots.

15 January 1985 Navy Secretary John Lehman announced that the official

name for the JVX aircraft is “Osprey.”

July 1985 Flight evaluation of a three-axis sidestick controller performed in XV-15 N703NA by Ames. 

2 May 1986 U.S. Naval Air Systems Command awards Bell-Boeing

Vertol contract for seven-year Full Scale Development

Program for V-22.

21 May 1986 As part of the Bell “guest pilot” program Colonel Harry

M. Blot, USNAVAIRSYSCOM V-22 program manager,

flew the XV-15 for his first official tilt rotor flight.

June 1986 A new contract is awarded to the Bell-Boeing V-22 team

by NAVAIR following a year of program reassessment and

negotiations. The new contract called for a fixed-price

development for the first three production lots, totaling

228 aircraft. Six prototype aircraft were to be built under

the full-scale development contract.

18 December 1986 Department of Defense approved the full scale development program for the V-22 Osprey.

147
30 July 1987 FAA/NASA/DOD Tilt Rotor Applications Forum

announces results of U.S. Government study on civil use

of tilt rotor aircraft titled “Civil TiltRotor Missions and

Applications: A Research Study.”

30 July 1987 Port Authority of New York and New Jersey released results

of their civil tilt rotor study that assesses the feasibility of

tilt rotors in commercial air transportation infrastructure.

30 July 1987 U.S. Department of Transportation, Transportation

Systems Center, released “Civil TiltRotor Industrial Base

Impact Study.”

31 August 1987 FAA Administrator T. Allan McArtor flew XV-15 and

made TiltRotor certification a top priority of his agency.

22 October 1987 Cracks in the skins of two left proprotor blades grounded

XV-15 N702NA. 

13 November 1987 First hover flight of composite proprotor blades on XV-15

N703NA.

18 November 1987 Congressional hearing on “Civil Application of Tilt Rotor”

sponsored by Subcommittee on Transportation, Aviation &

Materials of the House Committee on Science, Space, and

Technology.

18 November 1987 U.S. Army announces withdrawal from V-22 program.

16 May 1988 XV-15 N702NA returns to flight status with metal blades

from N703NA.

23 May 1988 V-22 roll-out takes place in ceremonies at Bell’s Flight

Research Center.

20 July 1988 Bell-Boeing V-22 Joint Program Office formally applies to

FAA for commercial certification of the V-22.

12 August 1988 Memorandum of Understanding signed between FAA and

DoD allowing FAA participation in DoD’s V-22 flight test

program.

19 March 1989 First flight of V-22 aircraft no. 1.

19 April 1989 Revised DoD budget for FY90; deleted all V-22 funding.

148
14 September 1989 V-22 aircraft no. 1 achieved first full conversion to airplane mode.

9 November 1989 FAA published “National Civil TiltRotor Initiative

Implementation Plan” which initiated civil tilt rotor 

activity in the Department of Transportation and other

Government agencies.

1 December 1989 Deputy Secretary of Defense instructed Navy Secretary to

terminate all contracts funded with FY89 advanced procurement funds which effectively would end the V-22 program.

February 1990 Department of Defense budget submission for FY91 contained no funds for V-22.

4 February 1990 XV-15 flew in opening ceremony of Helicopter

Association International (Heli-Expo ‘90) in Dallas, Texas.

7 March 1990 General Accounting Offices charged Department of

Defense acted improperly in terminating V-22 contracts

funded with FY89 advanced procurement funds.

15 March 1990 XV-15 set five new Federation Aeronautique Internationale

records:

1. Attained 3,000 meters altitude in four minutes,

24.5 seconds.

2. Attained 6,000 meters altitude in eight minutes,

29 seconds.

3. Attained altitude of 22,600 feet.

4. Cruised in horizontal flight at 22,600 feet.

5. Flight altitude with payload (1,000 kg) record reached.

25 April 1990 XV-15 N702NA landed on the east lawn of the Capitol during the time that the House Aviation Subcommittee held

hearings on civil applications of tilt rotor technology.

May 1990 First formal evaluation of V-22 by Government pilots completed. The V-22 demonstrated excellent potential for its

intended missions.

6 May 1990 V-22 aircraft no. 2 conducts first cross-country flight from

Arlington, Texas, to Wilmington, Delaware (1,210 nm, 1,392

miles) in 5.2 hours with a refueling stop in Atlanta, Georgia.

149
21 August 1990 V-22 reached 340 knots (391 mph) in level flight and 

349 knots (402 mph) true air speed in a 1,200 feet per

minute dive.

29 October 1990 Secretary of Transportation, Samuel Skinner, flew XV-15 and

proclaimed the tilt rotor a commercial aircraft of the future.

November 1990 FAA publishes “Rotorcraft Master Plan” that includes the

civil tilt rotor as a major Agency initiative.

4-7 December 1990 V-22 sea trials aboard USS Wasp.

4 February 1991 FY92 and FY93 Department of Defense budget submission contains no funding for the V-22.

19 February 1991 NASA publishes “Civil Tilt Rotor Missions and

Applications Phase II: The Commercial Passenger

Market.” Press conference held in Washington.

1 May 1991 XV-15 N702NA wing was damaged by an in-flight collision with a bird. Aircraft was repaired and returned to

flight 8 October 1991.

11 June 1991 First flight of V-22 aircraft no. 5 terminated in a crash at

Wilmington, Delaware, which result in a suspension of the

V-22 flight test program.

6 September 1991 Failure of composite blade cuff retention causes emergency landing of XV-15 N703NA.

10 September 1991 V-22 full scale development flight tests resumed with

flight of aircraft no. 3 at BHTI Flight Research Center.

9 October 1991 A bill is introduced in the House of Representatives that

would establish a Civil TiltRotor Development Advisory

Committee. A similar bill was introduced in the Senate on

26 November.

22 November 1991 Congress submitted National Authorization and

Appropriation Bills for FY92 to the President that provided the V-22 program with $790 million which included

$165 million from prior year funds for the development,

manufacture, and operational test of three production representative aircraft and an additional $15 million for a special operations variant of the V-22.

150
26 November 1991 President signs Appropriations Act. Authorization Act

signed 5 December 1991.

February 1992 The FY93 Department of Defense budget submission contained no funding for the V-22.

20 July 1992 V-22 aircraft no. 4 crashes in the Potomac River during

ferry flight from Eglin Air Force Base to Quantico Marine

Corps Air Station. All V-22 flight tests suspended. Later

investigations found the cause of the crash was a correctable mechanical problem. No basic flaws were found

in tilt rotor design or concept.

4 August 1992 Department of Defense awards a contract to Bell for the

construction and flight demonstration of two short-range

unmanned aerial vehicles (Bell Eagle Eye UAV).

20 August 1992 XV-15 (N702NA) crashes at Arlington, Texas, airport on

completion of a demonstration flight as the result of a bolt

that worked loose in a flight control connecting rod.

2 October 1992 U.S. Navy Mishap Board releases findings that a fire in

the right-hand nacelle, coupled with a fire-induced failure

of the interconnect drive shaft connecting the proprotor

gearbox and the tilt-axis gearbox caused the crash of V-22

aircraft no. 4 on 20 July 1992.

23 October 1992 A new Engineering and Manufacturing Development letter

contract was awarded to the Bell-Boeing Team for the V22 Osprey program. The letter contract provided initial

funding of $550 million on a contract that would total

more than $2 billion.

31 October 1992 The President signs the Aviation Reauthorization Act that

established a Civil TiltRotor Development Advisory

Committee under the Department of Transportation to

evaluate the technical feasibility of developing civil tilt

rotor aircraft and a national system of infrastructure to

support the incorporation of tilt rotor aircraft technology

into the national transportation system.

18 May 1993 The U.S. Navy Air Systems Command cleared the V-22 to

continue flight tests following the U.S. Navy Mishap

Board findings of 2 October 1992.

151
14 June 1993 The Department of Transportation issues an order establishing the Civil TiltRotor Development Advisory

Committee.

10 July 1993 The Bell eagle eye tilt rotor unmanned aerial vehicle successfully completed its first flight.

September 1993 President Bush reinstates production V-22 program.

21 April 1994 New Memorandum of Agreement signed between

Army/NASA and Bell for bailment of XV-15 N703NA to

Bell.

9 December 1994 Secretary of Defense, William Perry, announces that the 

V-22 will be produced for the United States Marine Corps

and Special Operations Forces. Low rate initial production

was announced for 1996 through 2001.

December 1994 Bell begins Model D-600 commercial tilt rotor program.

(Later designated BB-609 and then BA-609)

Early 1995 NASA Administrator Daniel Goldin announced termination of flight operations at NASA ARC.

3 March 1995 XV-15 N703NA returns to flight at the Bell Flight Test

Center, Arlington, Texas.

21 April 1995 XV-15 becomes first tilt rotor to land at the world’s first

operational vertiport, the Dallas Convention Center

Heliport/Vertiport.

June 1995 XV-15 (N703NA) joins the Bell-Boeing V-22 Osprey at

the 41st Paris Air Show.

10 July 1995 XV-15 puts on first tilt rotor flight demonstration in

Canada at Bell Helicopter Mirabel facility near Montreal,

Quebec.

11 July 1995 XV-15 is first tilt rotor to land at the Indianapolis Heliport.

December 1995 Findings of CTRDAC state that the civil tilt rotor transport

is technically feasible and can be developed by the U.S.

industry. However, additional research and development

and infrastructure planning are needed.

152
August 1996 Boeing becomes partner with Bell in BB609 program.

February 1997 V-22 production contract awarded to Bell-Boeing team.

February 1997 First flight of first V-22 EMD (Engineering Manufacturing

Development) aircraft (Ship Number 7).

June 1997 First public showing of full-scale BB-609 mockup at Paris

Air Show.

March 1998 Boeing withdraws from BB-609 program. 

September 1998 Bell announces a joint venture with the Agusta Helicopter

Company of Italy in the now renamed BA-609 commercial

tilt rotor aircraft program.

14 May 1999 Delivery of first production V-22 Osprey rotor aircraft to

the U.S. Marine Corps.

Post script:

11 July 2003 Completion of the last research flight to be conducted by

the XV-15. The test concluded 26 years of research testing

(1977–2003)—longest research span for any X-plane.

3 September 2003 Mr. Mike Redenbaugh, the new CEO of Bell Helicopter,

becomes the last guest pilot, making him the 419th person

to fly in the XV-15—greatest number of pilots for any Xplane.

10–16 September XV-15 N703NA ferried to the National Air and Space 

2003 Museum (NASM) Udvar-Hazy Center at Dulles

International Airport, VA. Its arrival was celebrated by a

reception sponsored by Bell and the NASM. The aircraft

was subsequently placed on permanent display at the new

NASM facility.

December 2003 XV-15 recognized by the National Geographic Society as

one of the 47 most important aircraft of the first century of

flight by its inclusion in a fold-out insert in the December

2003 issue of the National Geographic periodical.

153
Over the years the engineers, designers, and test pilots of the tilt rotor aircraft

have been recognized by leading American aviation technical organizations and

societies for having made important contributions to the state of the art. These

awards, including the most prestigious symbols of aeronautical achievement in

the United States, indicated the high level of technical competence, perseverance,

and commitment that the award recipients, and the entire tilt rotor “supporting

cast” demonstrated to accomplish the project’s goals. Looking back, it seems that

the tilt rotor’s technical problems were solved not by engineering alone, but by

the magical effects of the positive spirit exhibited by the industry and

Government tilt rotor team.

XV-3

8 May 1959 Dr. Alexander Klemin Award (American Helicopter

Society) presented to Robert L. Lichten, Bell chief experimental project engineer, for development of the tilt rotor

type convertiplane.

10 October 1959 Iven C. Kincheloe Award (Society of Experimental Test

Pilots) presented to XV-3 test pilot USAF Major Robert G.

Ferry, designating him as test pilot of the year.

XV-15

13 December 1976 Laurels (Aviation Week and Space Technology) to Tommy

Thomason, Textron Bell Helicopter Div., and David Few,

NASA/Army program director for driving the XV-15 tiltrotor research program through budgetary knotholes to the

prospects for technical success that open a promising line

for rotary-wing vehicle future applications.

15 May 1978 Paul E. Haueter Memorial Award (American Helicopter

Society) presented to Ken Wernicke, Bell Helicopter technical manager of tilt rotor programs.

16 November 1979 NASA Group Achievement Award awarded to the Bell and

Army/NASA XV-15 tilt rotor research aircraft team.

16 November 1979 NASA Exceptional Service Medal awarded to David Few

for leadership of the augmentor wing jet STOL research 

aircraft project and the tilt rotor research aircraft project.

13 May 1980 Frederick L. Feinberg Award (American Helicopter

Society) presented to Bell Helicopter pilots Ron Erhart

and Dorman Cannon for the most outstanding helicopter

piloting achievement during 1979 testing of the XV-15.

Appendix D—Awards and Records

154
1 October 1980 Iven C. Kincheloe Award (Society of Experimental Test

Pilots) presented to Bell XV-15 test pilots Ron Erhart and

Dorman Cannon, designating them test pilots of the year.

20 May 1981 Grover P. Bell Award (American Helicopter Society) presented to the NASA/Army/Bell XV-15 Project Team for

“outstanding achievement in successfully demonstrating

the feasibility and potential of TiltRotor technology.”

10 September 1981 Kelly Johnson Award (Society of Flight Test Engineers)

presented to the XV-15 flight test team.

18 October 1983 Aircraft Design Award (American Institute of Aeronautics

and Astronautics) presented to Ken Wernicke, Bell

Helicopter technical manager of tilt rotor programs.

29 May 1986 Harmon Trophy presented by President Ronald Reagan to

XV-15 pilot Dorman Cannon for the most outstanding

achievement in the art of flying in 1983.

V-22

7 May 1990 Paul E. Haueter Memorial Award (American Helicopter

Society) presented to the Bell-Boeing and NAVAIR V-22

team for “significant contributions to development of

VTOL aircraft other than helicopters.”

17 May 1991 1990 Collier Trophy (National Aeronautics Association)

presented to the V-22 tilt rotor team for the greatest

achievement in aeronautics demonstrated by actual use

during 1990.

Model 609

9 February 1998 Laurels (Aviation Week and Space Technology) to Webb

Joiner and John P. Magee of Bell and Anthony M. Parasida

of Boeing Aircraft and Missiles Systems for the “development of the 609 and the foresight to launch a unique mode

of civil transportation using tilt rotor technology.”

155
Records

The XV-15 TRRA provided an opportunity for setting new performance records

for rotorcraft and Bell stepped up to the challenge. On June 17, 1980, XV-15

N703NA was flown in the airplane mode at a true airspeed of 301 knots (346

mph), exceeding the speed of any prior low disc loading rotorcraft not having a

separate cruise-mode propulsion device, and establishing an unofficial world’s

speed record. In addition, the following six new FAI (Federation Aeronautique

Internationale) official records were set with the XV-15, aircraft N702NA. On

March 15, 1990, in the “without payload” category, the XV-15 achieved a timeto-climb record of 4 minutes and 24 seconds to reach an altitude of 3000 meters

(9842 feet) and 8 minutes and 28 seconds to reach an altitude of 6000 meters

(19684 feet). A record maximum altitude of 6,907 meters (22,660 feet) was

reached and a record sustained horizontal flight altitude of 6,876 meters (22,560

feet) was recorded (also without payload). On the second flight of that day, a

record altitude for this category with 1,000 kg (2,205 lb.) of payload was

obtained (6,879 meters, 22,560

feet). The official record for

speed of 247.56 knots (284.89

mph) over a recognized course

was set on April 4, 1990, during

a flight from Arlington

Municipal Airport, Texas, to

Baton Rouge, Louisiana, a distance of 611.5 kilometers (380

statute miles).

Another interesting first

occurred on April 23, 1990,

when Jean Tinsley, an accomplished aviatrix and member of

the renowned “Whirly Girls”

club, became the first woman to

pilot a tilt rotor aircraft (figure

D-1). This “first” added to several rotorcraft records already

credited to her. 

156

Figure D-1.

Jean Tinsley, first woman 

to fly the XV-15 tilt rotor

aircraft.

(Ames Photograph

A90-0218-1)
Appendix E—Photo Gallery

157

Figure E-1.

XV-3 at Bell ramp, 1953.

(Bell Photograph XV-3-35)

Figure E-2.

Bell XV-3 personnel in 

front of the XV-15 

research aircraft.

(Bell Photograph 308597)
158

Figure E-3.

XV-15 flying by the 

Statue of Liberty.

(Ames Photograph

AC86-0410-4)

Figure E-4.

XV-15 flying near the

Washington Monument.

(Ames Photograph

AC86-0410-2)
159

Figure E-5.

XV-15 flyby at the

Jefferson Memorial.

(Ames Photograph

AC86-0410-3)

Figure E-6.

XV-15 landing at the 

Capitol Building.

(Bell Photograph 037868)
160

Figure E-7.

Bell test pilots Roy Hopkins

and Dorman Cannon.

(Bell Photograph 037868)

Figure E-8.

XV-15 in executive 

transport markings.

(Bell Photograph 043100)
161

Figure E-9.

XV-15 in camouflage 

markings.

(Bell Photograph 024741)

Figure E-10.

XV-15 in Navy gray 

flying over the USS Tripoli,

August 1982.

(Ames Photograph

AC82-0612-25)
162

Figure E-11.

Ken Wernicke, Bell tilt rotor

design engineer, 1965.

(Bell Photograph 262938)

Figure E-12.

Ken Wernicke after 

flying XV-15.

(Bell Photograph 05590)
163

Figure E-13.

XV-15 N702NA crew 

station (1982).

(Ames Photograph

AC82-0493-6)

Figure E-14.

Composite photograph

showing V-22 Osprey in

hover, conversion, and 

airplane mode flight.

(Bell Photograph 039956)
Tilt Rotor Related Publications

Acree, C. W.; Jr. Tischler, M. B. “Using Frequency-Domain Methods to Identify

XV-15 Aeroelastic Modes.” NASA TM-100033, November 1987. 

Acree, C. W., Jr. “An Improved CAMRAD Model for Aeroelastic Stability Analysis

of the XV-15 With Advanced Technology Blades.” NASA TM-4448, 1993.

Acree, C. W., Jr.; Tischler, M. B. “Identification of XV-15 Aeroelastic Modes Using

Frequency Sweeps.” Journal of Aircraft, vol. 26 no. 7, July 1989, pp. 667-674.

Acree, C. W., Jr.; Tischler, Mark B. “Determining XV-15 Aeroelastic Modes

from Flight Data with Frequency-Domain Methods.” NASA TP-3330 and

ATCOM Technical Report 93-A-004, 1993.

Acree, C. W., Jr.; Tischler, Mark B. “Frequency-Domain Identification of

Aeroelastic Modes.” NASA Tech Brief ARC-12407, 1991.

Acree, Cecil W., Jr.; Tischler, Mark B. “Identification of XV-15 Aeroelastic

Modes Using Frequency-Domain Methods.” NASA TM-101021, 1989.

Ahuja, K. K. “Tilt Rotor Aircraft Noise: A Summary of the Presentations and

Discussions at the 1991 FAA/Georgia Tech Workshop.” DOT/FAA/RD-91, 1992.

Alexander H. R.; Amos, A. K.; Tarzanin, F. J.; Taylor, R. B. “V/STOL Dynamics

and Aeroelastic Rotor-Airframe Technology.” AFFDL-TR-72-40, vol. 2, Boeing

Vertol Co., September 1972.

Alexander, H. R.; et al: “Preliminary Design Study of Advanced Composite Blade

and Hub and Nonmechanical Control System.” NASA CR-152336, November 1979.

Alexander, H. R.; Hengen, L. M.; Weiberg, J. A. “Aeroelastic Stability

Characteristics of a V/STOL Tilt Rotor Aircraft with Hingeless Blades:

Correlation of Analysis and Test.” AHS Preprint 835, Presented at AHS 30th

National Forum, Washington, D.C., May 1974.

Alexander, H. R.; Kuntz, W. H.; Wasserman, L. S. “Dynamically Similar Model

Tests of Rotary Wing and Propeller Types of VTOL Aircraft.” U.S. Air Force

V/STOL Technology and Planning Conference, Las Vegas, NV, September 1969.

Alexander, Harold R.; Maisel, Martin D.; Giulianetti, Demo J. “The

Development of Advanced Technology Blades for Tilt-Rotor Aircraft.” 11th

European Rotorcraft Forum, London, England, September 10–13, 1985.

Amos, A. K.; Alexander, H. R. “Simulation Study of Gust Alleviation in a Tilt

Rotor Aircraft.” Vols. I and II, NASA CR-152050/1, June 1977.

Appendix F—Bibliography

164
Anon. Bell’s XV-3. AEROPHILE, vol. 2, no. 1, ISSN0147-7668, June. 1979.

Anon. “Civil TiltRotor Development Advisory Committee Report to Congress in

Accordance With PL102-581.” December 1995.

Anon. “Civil TiltRotor Missions and Applications, Phase II: The Commercial

Passenger Market.” NASA CR 177576, February 1991.

Anon. “Full-Scale Hover Tests 25-Foot Low Disk Loading Tilt Rotor.” NASA

CR-114626, May 1973.

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177
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181
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182
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184
Martin D. Maisel

After graduating in 1960 from the Polytechnic Institute of Brooklyn with a BS in

aeronautical engineering, Martin Maisel worked on propeller and rotor aerodynamic design and technology development at the Hamilton Standard Division of

United Technologies Corporation, Windsor Locks, Connecticut, and then at the

Boeing Helicopter Company, Riddley Park, Pennsylvania, where he was involved

in advanced VTOL aircraft design. While at Hamilton Standard, he did additional

graduate work at the Rensselaer Polytechnic Institute, East Hartford, Connecticut,

Graduate Center. In 1970, Mr. Maisel moved to the Army Air Mobility Research

and Development Laboratory (now the Army Aeroflightdynamics Directorate) at

the NASA Ames Research Center, Moffett Field, California, and became a member of the NASA/Army project team that developed the XV-15 tilt rotor research

aircraft. His project responsibilities included aerodynamics integration, subsystem

development, and systems, wind tunnel, and flight testing. After managing the

XV-15 for the last two years of flight test activity at Ames, he continued to participate in the development of tilt rotor technology for civil transport application as

part of the NASA Short-Haul Civil Tilt Rotor (SHCT) Project Office. Since early

1998, Mr. Maisel has been serving as the airworthiness officer for Army flight

operations at Ames, in support of the joint Army/NASA rotorcraft technology

activity. He has authored over 20 papers and technical reports on the design,

performance, acoustics, and operations of the tilt rotor aircraft. With a lifelong

interest in aviation, he is also a licensed private pilot.

Demo J. Giulianetti

Demo Giulianetti’s 42-year career as a Government research engineer began in

1956 at the NACA Ames Aeronautical Laboratory (later, the NASA Ames

Research Center), Moffett Field, California. For the first 15 years, his work

focused on small- and large-scale wind tunnel investigations of conventional and

advanced aircraft, including new V/STOL configurations. In 1975, he joined the

Tilt Rotor Research Aircraft Project Office and participated in the development

of two XV-15 aircraft. His technical efforts in this Office included analytical

evaluations, aircraft systems and hardware development, and wind tunnel and

flight test investigations. He also participated in the development and testing of

the first all-composite rotor blades for this aircraft type. Prior to his retirement in

1998, he was a senior staff engineer in the Advanced Tiltrotor Transport

Technology Office, responsible for the management of major rotorcraft task

order contracts with four major U.S. rotorcraft companies which concentrated on

supporting the short-haul civil tilt rotor program. He graduated from San Jose

State University with a BS in general engineering followed by one year of graduate work. He has authored and co-authored numerous NASA technical reports.

He is currently a consultant to the Eloret Institute in Sunnyvale, California.

185

About the Authors
Daniel C. Dugan

Daniel Dugan graduated from West Point in 1955 and began his Army aviation

career with primary flight training at Gary AAF, San Marcos, Texas. Following

various military aviation assignments, he attended Virginia Polytechnic Institute

at Blacksburg, Virginia, earning a master’s degree in aerospace engineering in

1963. After serving in Korea, he attended the U.S. Naval Test Pilot School at

Patuxent River, Maryland, graduating in 1965. He was then assigned to the Army

aviation test activity at Edwards AFB, California, as an experimental test pilot.

There he was assigned as the project pilot on the YUH-1C gunship (XM-30), the

ACH-47A armed and armored Chinook, and the YCH-47B. After a tour in

Vietnam (1968-1969), he was assigned to the Army’s aviation laboratory at

Ames Research Center, after completing a year at the Command and General

Staff College. At Ames, Mr. Dugan flew the XV-5B lift fan, YOV-10A rotating

cylinder flap research aircraft, and was the project pilot for the UH-1

V/STOLAND research helicopter and the XV-15 tilt rotor research aircraft. He

remained the NASA project pilot on the XV-15 for 14 years and accumulated

200 hours in this unique aircraft. From 1990 through 1995, Mr. Dugan was

assigned to Patuxent River as the NASA member of the V-22 test team. He is

currently on the staff of the rotorcraft division at Ames Research Center and is

assigned to a civil tilt rotor team for the development of tilt rotor displays, symbology, and operational procedures. Dan has authored over 20 technical reports

and papers, many of which are tilt rotor related. He holds airline transport pilot

certificates for both airplanes and helicopters and has accumulated over 7,000

hours of flight time.

186
Acree, C. Wally, 69, 77n, 82

Adams, M., 32

Advanced Subsonic Technology

Program, 112

Advanced Technology Blade, ATB,

78, 79, 80, 81, 82

Advanced Tiltrotor Transport

Technology, ATTT, 112

Agusta Helicopter Company, 110

Air Traffic Management, ATM, 113

Allied Signal, 112

Allison, 30, 112

Anderson, Seth B., 1n

Air Force, U.S., 4, 8, 10, 12, 14, 15,

17, 19, 24, 27, 38, 45, 47, 62, 88,

105, 106

Air Force Aero Propulsion Laboratory,

Ohio, 25, 26

Airport Planners Association, APA, 97

Alexander, H., 79n

Ames Aeronautical Laboratory, Ames

Research Center, ARC, 13n, 14, 15,

16, 17, 19, 20, 22, 23, 24, 25, 26,

27, 28, 29, 31, 32, 35, 38, 51, 52,

53, 55, 56, 60, 61, 63, 64, 66, 68,

70, 71, 75, 76, 80, 81, 82, 85, 88,

90, 91, 93, 95, 97, 100, 101, 102,

103, 106, 111, 112, 113, 119n, 123

Andrews, Harold, 60

Arlington, Texas, 83, 84, 90, 96, 101,

107

Army, U.S., 4, 8, 12, 17, 19, 20n, 23,

24, 25, 28, 29, 30, 31, 32, 34, 35,

37, 42, 43, 45, 53, 55, 56, 59, 60,

63, 64, 67, 74, 75, 76, 79, 81, 88,

90, 92, 94, 97, 99, 101, 103, 104,

105, 106

Army Advanced Systems Research

Office, ASRO, 106

Army Aeronautical Research

Laboratory, AARL, 19

Army Air Mobility Research and

Development Laboratory, AMRDL,

19, 24, 27, 28, 31, 32, 39, 88

Army Electronic Command, 38

Army Materiel Command, 19, 31

Arrington, W.L., 69n

Atkins, James F., 37

Augmentor Wing Aircraft,

deHavilland Buffalo, 28

Augustine, Norman R., 34

Aviation Systems Command, 38

Ball, John C., 91, 95n

Balzer, Dick, 91, 107

Bartie, K., 79

Baynes Heliplane, 6, 8

Bell, 21, 26, 27, 28, 29, 30, 35, 39, 42,

44, 53, 54, 55, 56, 61, 63, 64, 68,

69, 71, 75, 76, 77, 79, 81, 83, 86,

87, 88, 89, 90, 91, 92, 95, 96, 97,

98, 100, 101, 102, 105, 107, 109,

110, 112

Bell Aircraft, 6, 12, 13, 14, 15, 17, 18

Bell Flight Research Center, 107

Bell Helicopter Company, 4, 20, 22,

24, 26n, 37, 49, 74, 104, 111

Bell Helicopter Textron, Incorporated,

BHTI, 29, 103

Berliner, Henry, 5, 6

Betzina, M.D., 79

Bilger, Jim, 69

Bishop, H., 79n

Blackman, Shep, 55, 61

Boeing, 21, 23, 25, 27, 35, 67, 74, 79,

100, 107, 109, 110, 112

Boeing Vertol Company, 19n, 21, 24,

27, 29,91

Bolling Air Force Base, 96

Bondi, Mike, 52, 95

Borgman, Dean C., 28, 30, 32

Brieger, John, 75

Brown, LTC James H., 28, 29

Burley, Casey L., 77n

Bush, George, 106

Calspan Corporation, 50

Canadair, 39, 42, 92

Cannon, Dorman, 55, 61, 63, 89, 95,

107

Carlson, Floyd, 12

Carness, Mike, 28

Index

187
Carnet, 88

Carpenter, Ron, 91

Carter, George, 42

Cheney, Richard, 106

Chesapeake Bay, 9

Chicago, Illinois, 97

Churchill, Gary B., 19, 74

Civil Tiltrotor Development Advisory

Committee, CTRDAC, 110, 113

Cobey, William E., 8, 10, 11

Coffen, C.D., 75

Cohen, William S., 108

Columbus, Ohio, 47

Commerce, U.S. Department of, 88

Composite Aircraft Program, CAP, 20

Condon, G.W., 31n

Congress, U.S., 92, 100, 105, 106, 107

Conner, David A., 77n

Cook, Woodrow L., 16, 25n, 28, 31

Cornell University, 75

Creech, Jimmie, 106

Dabadie, M. Guy, 98

da Vinci, Leonardo, 1

Dayton, Ohio, 97

Decker, William, 76, 113

Deckert, Wallace H., 14, 28, 76,

Decoto Aircraft Inc., 50

Defense Contract Audit Agency,

DCAA, 39

Defense, U.S. Department of, DOD,

105, 106, 107, 111

deHavilland Buffalo, Augmentor Wing

Aircraft, 28

DeTore, John A., 22n

Diehl, James J., 39

Drinkwater, Fred, 91

Dryden Research Center, DFRC, 28,

50, 53, 62, 100, 101, 119n

Dugan, LTC Daniel, 28, 56, 61, 63,

64, 89

Eddystone, Pennsylvania, 6

Edenborough, Kipling, 16, 29

Edwards Air Force Base, 14, 15, 28

Edward, Bryan D., 75

Eisenberg, Joe, 112

Ekas, C.P., 59n

Electromagnetic interference, EMI, 66

Engine-coupling gearbox, ECGB, 43

England, 89

Erhart, Ron, 48, 55, 61, 63, 86, 89, 98,

101

EUROFAR, European Future

Advanced Rotorcraft, 110

Europe, 88

Evans, A.J., 32

Experimental vertical takeoff, XV, 38

FA-269, Focke-Achgelis convertiplane, 6, 8 

Farnborough Air Show, 110

Federal Aviation Administration, FAA,

46, 109, 110, 112

Federal Bureau of Investigation, FBI,

86

Felker, F.F., 79

Ferguson, Sam, 27

Ferry, Robert, 14, 28

Few, David D., 28, 29, 104n

Finger, Herb, 51

Fletcher, James C., 32

Flight Simulator for Advanced

Aircraft, FSAA, 27

Force-Feel System, FFS, 27, 47, 56

Foreman, Brenda, 107

France, 88, 89

Freisner, A. Lynn, 91

Ft. Belvoir, Virginia, 96

Ft. Eustis, Virginia, 38, 91

Ft. Huachuca, Arizona, 93, 94

Ft. Monmouth, New Jersey, 38

Ft. Rucker, Alabama, 17n, 96

Ft. Worth, Texas, 97

FW-61, Focke-Wulf helicopter, 6

Gaffey, Troy, 20, 30

Gallagher, Frank, 91

General Electric, GE, 112

George, Albert R., 75

Gerdes, Ronald, 63, 75n, 97

Gilmore, Kenneth B., 21

188
Giulianetti, Demo J., 88, 95, 185

Global Positioning System, GPS, 76,

109

Goldin, Daniel S., 100

Goldwater, Barry, 92

Green, David L., 91

Grumman, 20, 35

Guerrieri, Mario A., 8

Hall, Earl, 20

Harendra, P.B., 27

Harper, C.W., 16, 32

Harrier, aircraft, 31, 97

Harris, D.J., 65n

Harris, Wesley, 100

Health and Usage Monitoring, HUM,

109

Helicopter Association International,

HAI, 97

Holloway, Bruce K., 34

Holloway, James, 59n

Hopkins, Roy, 101

Horner, Jack, 96

House, Thomas L., 100

Hughes Helicopter Company, 64

Hydraulic Research and Manufacturing

Company, HR&M, 49

Independent Research and

Development, IR&D, 19

Indiana, Mishawaka, 45

Jackson, Roy P., 30

Jenkins, Larry, 100

Joglekar, M.J., 27

Johnson, Robert L., 30

Johnsonville, Pennsylvania, 38

Joiner, Webb, 100, 110, 111

Joint Aeronautical Research Group,

JARG, 19

Joint Aviation Authority, JAA, 109

Joint Technology Assessment Group,

JTAG, 106

Joint Vertical Experiment, JVX, 106,

107

Johnson, Wayne, 22, 67

Kellett Aircraft Company, 8

Kelly, Mark, 16

Kelsey-Hayes, 50

Kerr, Andrew W., 100

Kimbell, Mike, 30

Kruesi, Frank E., 111

Kumpel, M., 69n

Kvaternik, Raymond G., 20n

Lacy, Clay, 91

Lafayette, Louisiana, 97

Lamon, S., 79

Lane, Jim, 39, 95

Langley Research Center, LaRC, 17,

20, 22, 31, 32, 76, 81, 102, 112

Lawrence, William S., 91

Lehberger, George, 6, 7

Lehman, John, 92

Leibensberger, Claude, 16, 17n

LePage, Wynn Lawrence, 6

Lewis Research Center, LeRC, 112

Lichten, Robert L., 8, 12, 29, 30, 66n

Litton Industries, 50

Lockheed, 20, 50, 77

Low, George M., 32

Lycoming, engine, 42, 43

Lynn, Robert, 12n, 15

Madden, John, 80

Maddox, Bobby, 96

Magee, John P., 29

Maisel, Martin D., 19, 58, 65n, 75, 79

Marcolini, Michael A., 77n, 112

Marine Corps, U.S., USMC, Marines,

47, 74, 91, 93, 105, 106, 108

Mark, Hans, ix, 12n, 31, 37, 88, 111

Marr, R.L., 26, 27, 69n

Martin, Bill, 98

Martin, Stanley, 30

McDill Air Force Base, 97

McDonnell Aircraft Company, 1, 4

McDonnell Douglas, 6, 28, 110, 112

McEntire, K.G., 69n

McKiethan, Clifford, 29, 95

McVeigh, Michael A., 79n

Miller, Thomas, 59n

189
190

Moffett Airfield, Moffett Field, Moffett

Naval Air Station, 13n, 17n, 52, 97

Mohawk, 96

Monterey, California, 97

Montreal, Quebec, 102

Morton, Pennsylvania, 21

Mount Diablo, 52

Munechika, Ken, 100

National Advisory Committee for

Aeronautics, NACA, 13n, 32

National Aeronautics and Space

Administration, NASA, 17, 19, 20,

24, 28, 30, 31, 32, 37, 38, 42, 43,

50, 53, 55, 56, 59, 60, 63, 64, 74,

75, 77, 79, 81, 82, 85, 88, 90, 91,

94, 97, 104, 110, 111, 112, 113

National Rotorcraft Technology Center,

NRTC, 102

National Transportation Safety Board,

NTSB, 99

Naval Air Development Command, 38

Naval Air Systems Command,

NAVAIR, 59, 60

Naval Air Test Center, 65, 91, 108

Naval Auxiliary Landing Field, Crows

Landing, NALF, 70, 71, 75, 76, 81,

85

Naval Materiel Command, NAVMAT,

59n

Navy, U.S., 60, 63, 66, 74, 76, 91, 93,

94, 95, 96, 104, 106, 108

Neal, G.T., 26n

New Castle, Delaware, 8

New York Port Authority, 96

North American Aviation, 47

North Island Naval Air Station, 94

Nicks, Colby, 30, 76

Nikolsky, Alexander A., 12n

Numerical Aerospace Simulation,

NAS, 97

Office National d’Etudes et de

Recherches, ONERA, France, 25

Paris Air Show, 88, 89, 92, 101, 109

Patuxent River, Maryland, 95, 96

Pentagon, 96,108

Petroleum Helicopters International, 97

Pickard, Jerry, 64

Platt, Haviland Hull, 6, 7, 10, 18

Platt-LePage Aircraft Company, 6, 8

Poisson-Quinton, P., 25

Prouty, R.W., 4

Pulse code modulation, PCM, 50, 51

Quantico Marine Corps Air Station, 96

Quiet Short-Haul Research Aircraft,

QSRA, 52

Quinlan, Bill, 14

Reber, Ron, 95

Remote Multiplexer/Digitizer Units,

RMDU’s, 51

Republic Aviation Corporation, 11

Richardson, David, 21

Roberts, Leonard, 28

Rockwell International, 47, 48, 49

Rotor and Wing International, 4, 91

Rotor systems research aircraft, RSRA,

31

Royal Aeronautical Society, 90

Rudning, Charles, 30

Sambell, Kenneth, 22n, 26n

San Francisco, California, 13n

San Joaquin Valley, 52

Schroers, Shorty, 38, 48, 69n, 71

Scully, Michael, 106

Seymour, E.R., 60

Shank, Suzanna, 75

Sharpe, Dave, 32

Short Haul Civil Tiltrotor, SHCT, 76,

112, 113, 114

Short Takeoff and Landing, STOL, 14,

60

Sikorsky Aircraft Corporation, 28, 30,

35, 91, 112

Simmons, Rickey C., 81

Simpson, R.D., 65n

Skinner, Samuel, 110

Skyhawk, 96
Smyth, Henry, 30

Snyder, William, 29, 112

Spivey, Richard, 30

Stability and Control Augmentation

System, SCAS, 27, 55, 74, 80, 124

Stansbury, Dick, 13, 29

Statler, Irving C., 31, 88

Steel Products Engineering Company,

SPECO, 50

Stevens, Story, 92

Stoufflet, Mark, 76

Tampa, Florida, 97

Teledyne Controls Division, 50, 51

Texas, Hurst, 39

Thomason, Tommy H., 30

Tilt Rotor Engineering Database,

TRENDS, 52

Tilt Rotor Research Aircraft, TRRA,

28, 29, 30, 32, 34, 37, 38, 39, 40,

42, 47, 49, 50, 51, 52, 59, 60, 63,

65, 67, 69, 70, 72, 73, 76, 77, 78,

81, 86, 87, 88, 89, 91, 92, 95, 97,

100, 101, 104, 105, 108, 109

Tischler, Mark B., 69

Transcendental Aircraft Corporation, 8,

10, 11, 12

Transonic Dynamics Tunnel, TDT, 20

Transportation, U.S. Department of,

DOT, 111

Tucker, George, 81

Tulsa, Oklahoma, 48

Unger, George, 60

United States, U.S., 6, 90, 102, 105,

111

Unmanned Aerial Vehicle, UAV, 29,

108, 109

USS Tripoli, 94, 95

V-22 Osprey, 106, 108, 109, 110, 112,

113

Variable Diameter Tilt Rotor, VDTR,

113

Vertical Motion Simulator, VMS, 27,

74, 76, 112

Vertical or Short Takeoff and Landing,

V/STOL, 1, 2, 14, 28, 31, 34, 37, 59

Vertical Takeoff and Landing, VTOL,

1, 3, 4, 5, 16, 17, 21, 26, 28, 31, 59,

60, 63, 103, 109

Ward, John, 60

Warren, Tom, 98

Waxahachi, Texas, 76

Weiberg, James, 19, 58, 72, 73

Wernicke, Ken, 30, 42, 48

Wheatly, John, 43

White, Clark, Jr., 31n, 60

Williams, Bob, 32

Wilson, Grady, 97

Wilson, Sam, 106

Work Breakdown Structure Elements,

WBSE, 39, 40, 59

World Trade Center, 96

World War II, WWII, 6

Wright brothers, 1

Wright-Patterson Air Force Base, 17n,

26, 97

XV-1, 4

XV-2, 4

XV-3, 4, 5, 12, 13, 14, 15, 16, 17, 18,

19, 20, 24, 28, 30, 32, 46, 70, 91,

103, 109, 119

XV-14B, 38

XV-15, 39, 42, 44, 45, 46, 48, 50, 51,

53, 55, 56, 57, 58, 59n, 60, 61, 62,

63, 64, 65, 66, 67, 68, 70, 72, 74,

75, 76, 77, 78, 79, 80, 81, 82, 83,

85, 86, 87, 88, 89, 90, 92, 93, 95,

96, 97, 99, 101, 102, 103, 104, 105,

106, 107, 108, 109, 112

Yaggy, Paul, xi, 31, 32

Yakima, Washington, 50

York, C.H., 11

Yuma, Arizona, 93, 94, 108

Zahedi, Ahmed, 69n

Zuk, John, 110

Zumwalt, Elmo, 59n

191

Launius, Roger D., and Gillette, Aaron K. Compilers. The Space Shuttle: An

Annotated Bibliography. (Monographs in Aerospace History, No. 1, 1992). 

Launius, Roger D., and Hunley, J.D. Compilers. An Annotated Bibliography of

the Apollo Program. (Monographs in Aerospace History, No. 2, 1994).

Launius, Roger D. Apollo: A Retrospective Analysis. (Monographs in Aerospace

History, No. 3, 1994). 

Hansen, James R. Enchanted Rendezvous: John C. Houbolt and the Genesis of

the Lunar-Orbit Rendezvous Concept. (Monographs in Aerospace History, No. 4,

1995).

Gorn, Michael H. Hugh L. Dryden’s Career in Aviation and Space. (Monographs

in Aerospace History, No. 5, 1996). 

Powers, Sheryll Goecke. Women in Aeronautical Engineering at the Dryden

Flight Research Center, 1946-1994. (Monographs in Aerospace History, No. 6,

1997).

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A Chronology of Extravehicular Activity (EVA). (Monographs in Aerospace

History, No. 7, 1997).

Logsdon, John M. Moderator. The Legislative Origins of the National

Aeronautics and Space Act of 1958: Proceedings of an Oral History Workshop.

(Monographs in Aerospace History, No. 8, 1998). 

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Achievement, 1961-1998. (Monographs in Aerospace History, No. 9, 1998). 

Portree, David S.F. NASA’s Origins and the Dawn of the Space Age.

(Monographs in Aerospace History, No. 10, 1998). 

Logsdon, John M. Together in Orbit: The Origins of International Cooperation

in the Space Station Program. (Monographs in Aerospace History, No. 11,

1998).

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Those monographs still in print are available free of charge from the 

NASA History Division, Code ZH, NASA Headquarters, Washington, D.C.

20546. Please enclose a self-addressed 9x12” envelope stamped for 15 ounces

for these items.

194

The History 

The XV-ResearcFrom Conce

The History of the XV-15 Tilt Rotor Research Aircraft: From Concept to Flig

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