NASA Facts

National Aeronautics and 

Space Administration 

Dryden Flight Research Center 

P.O. Box 273

Edwards, California 93523 

AC 661-276-3449 

FAX 661-276-3566 

pao@dfrc.nasa.gov FS-2003-08-002 DFRC 

F-18 High Angle-of-Attack (Alpha)

Research Vehicle

The F-18 HARV in flight (NASA photo EC91 495-15) 

NASA’s Dryden Flight Research Center, Edwards, Calif., used an F-18 Hornet fighter aircraft as its High Angleof-Attack (Alpha) Research Vehicle (HARV) in a three-phased flight research program lasting from April 1987 until 

September 1996. The aircraft completed 385 research flights and demonstrated stabilized flight at angles of attack 

between 65 and 70 degrees using thrust vectoring vanes, a research flight control system, and (eventually) forebody 

strakes (hinged structures on the forward side of the fuselage to provide control by interacting with vortices, 

generated at high angles of attack, to create side forces). 

This combination of technologies provided carefree handling of a fighter aircraft in a part of the flight regime that 

was otherwise very dangerous. Flight research with the HARV increased our understanding of flight at high angles 

of attack, enabling designers of U.S. fighter aircraft to design airplanes that will fly safely in portions of the flight 

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envelope that pilots previously had to avoid. 

Background 

Angle of attack (alpha) is an aeronautical term that describes 

the angle of an aircraft’s body and wings relative to its 

actual flight path. During maneuvers, pilots often fly at 

extreme angles of attack — with the nose pitched up 

while the aircraft continues in its original direction. This 

can lead to conditions in which the airflow becomes 

separated over large regions of the lifting surfaces (airfoils). This can result in insufficient lift to maintain 

altitude or control of the aircraft and a corresponding 

increase in drag — a condition known as stall. (In an ideal 

situation, the airflow would remain attached to the airfoil 

surface from leading to trailing edge; this would reduce 

the drag that impedes the movement of the airfoil through 

the atmosphere. When the airflow separates from the 

surface, this increases the drag and can lead to a stall.) 

About 1985, NASA began a formal program to explore 

and understand aircraft flight at high angles of attack, 

although the agency has always been interested the 

subject. NASA’s Langley Research Center in Hampton, 

Va., managed this High Angle-of-Attack Technology 

Program (HATP) in partnership with Ames Research 

Center at Moffett Field, Calif.; Dryden Flight Research 

Center (a name it regained in 1994); and Lewis Research 

Center (now Glenn Research Center) Cleveland, Ohio. 

Besides managing the program, Langley performed 

subscale wind-tunnel testing, advanced control law synthesis, and computational fluid dynamics (CFD — use of 

computers to predict aerodynamic behavior). Ames contributed further CFD work and its 80- by 120-foot wind 

tunnel. Lewis worked on inlet and engine integration. 

Dryden performed the flight research. Other partners 

came from industry, academia, and the Department of 

Defense (U.S. Navy, U.S. Air Force, and U.S. Marine 

Corps) plus some NATO participants. Between 1990 

and 1996, NASA was the host for conferences every two 

Diagram of strake 

creating a side force 

(yawing moment) 

years dealing with high angle-of-attack research and 

technology. 

The NASA HATP has produced technical data from 

actual flight at high angles of attack to validate computer 

codes (CFD) and wind tunnel research. Successful validation of these data has given engineers and aircraft 

designers a better understanding of aerodynamics, effectiveness of flight controls, and airflow phenomena at 

high angles of attack. Motivated by the tactical advantages of enhanced high angle-of-attack agility and maneuverability, they have used this understanding to design features providing better control and maneuverability in future high performance aircraft that will make 

them safer to fly. This is partially exemplified in designs 

already implemented in the F-22 Advanced Tactical 

Fighter and prototypes of the Joint Strike Fighter. Thrust 

vectoring research also continued at Dryden with the 

Enhanced Fighter Maneuverability X-31 and the F-15 

Advanced Controls Technology for Integrated Vehicles 

(ACTIVE) research aircraft. These research efforts led to 

revolutionary technological leaps in design tools and to 

increased confidence resulting from actually flying experimental aircraft at high angles of attack or under 

supersonic cruise conditions. 

The primary objectives of HATP were to provide flightvalidated aircraft design tools and to improve the maneuverability of aircraft at high angles of attack. The 

program has placed particular emphasis on the areas of 

aerodynamics, propulsion, control law research, and 

handling qualities. To this end, program personnel determined that full-scale flight research was essential to 

address the inherent shortcomings of subscale model 

testing in wind tunnels and CFD. The aircraft NASA 

selected for a large part of this flight research was a 

McDonnell Douglas F-18, which became known as the 

F-18 HARV after it was rebuilt and modified for HATP. 

The F-18 was an exceptionally fine high angle-of-attack 

aircraft in its production form. It had no angle-of-attack 

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Dye from small holes in the nose of the F-18 HARV showing the flow of air. (NASA photo EC88-0015-79) 

restrictions at normal center-of-gravity positions. It was 

this characteristic that made it NASA’s choice as a 

research vehicle. 

Phase One 

research pilot Einar Enevoldson made the first (functional check) flight on April 2, 1987, and three succeeding flights before turning the piloting duties over to 

NASA research pilots Bill Dana and Ed Schneider. The 

purpose of this phase was to obtain experience with 

aerodynamic measurements at high angles of attack and 

to develop the flight research techniques needed for this 

measurement. This was something that had not previously been done. 

Researchers conducted visual studies of the airflow over 

various parts of the aircraft. Special tracer smoke was 

released through small ports just forward of the leading 

edge extensions near the nose; on-board video and still 

cameras captured images of the smoke as it followed 

airflow patterns around the aircraft. Also photographed 

in the airflow were short pieces of yarn (tufts) taped on 

the aircraft and anti-freeze with dye in it, released onto 

the aircraft surfaces from hundreds of small orifices 

around the vehicle’s nose. 

Researchers used the film and videotape images of the 

airflow patterns from the smoke, dye, and tufts for a 

comparison with computer and wind tunnel predictions. 

Additional data that they obtained included air pressures 

The first phase of “high alpha” flights began in April 

1987 and lasted through 1989. It consisted of 101 research flights in the specially instrumented F-18 at angles 

of attack as high as 55 degrees. During this phase, there 

were no external modifications to the aircraft, but technicians equipped it with extensive instrumentation. NASA 

Flow visualization at 30 

degrees angle of attack 

using tufts. (NASA photo 

EC89 0096-205 with text 

added) 

Note the leading-edge 

extension from next to the 

cockpit extending back to 

the wing and the location of 

the point where the LEX 

vortex broke down and 

caused buffeting of the tail. 

recorded by sensors located in a 360-degree pattern 

around the nose and at other locations on the aircraft. 

Researchers paid particular attention to the location of 

strong vortices (masses of air in circular motion) that 

formed off the forebody and wing-body-strake (leadingedge extension or LEX) at high angles of attack and their 

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role in inducing tail buffeting (beating by unsteady flow, 

gusts, etc.). 

Phase Two 

Phase Two flights examined the benefits of using vectored thrust to achieve greater maneuverability and control at high angles of attack while continuing the correlation of flight data with wind-tunnel and CFD data 

begun in Phase One. The initial portion of the Phase Two 

flight program was completed in January 1993. 

Phase Two featured major hardware and software modifications to the HARV — a multiaxis thrust-vectoring 

control system featuring vanes attached to the aft end of the 

aircraft and a research flight control system. Three paddle-like 

vanes, made of Inconel®1 metal (a nickel alloy containing 

chromium and iron that is heat resistent), were mounted 

around each engine’s exhaust. They provided both pitch (up 

and down) and yaw (right and left) forces to enhance maneuverability when the aerodynamic controls were either unusable or less effective than desired. The engines had the 

divergent portion of the exhaust nozzles removed to shorten 

the distance the vanes had to be cantilevered by about two feet. 

Four-view drawing of thrust-vectoring control system. 

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Thrust-vectoring control system. (NASA photo EC90-177-4 

with text added) 

The subsonic performance of the engines, including 

afterburning, was largely unaffected by the modifications, but 

supersonic flight was no longer possible — a penalty unique 

to this experimental project. The thrust vectoring control 

system added 2,200 pounds to the total weight of the aircraft. 

In addition, the inclusion of a spin parachute for safety plus an 

emergency power system and ballast added a further weight 

penalty of 1,500 pounds. The HARV also carried 419 pounds 

of other equipment and wiring not directly associated with the 

thrust-vector-control system. 

The research flights began in July 1991 using the thrust 

vectoring system for control. The system could deflect the 

exhaust flow from the two turbofan engines to provide enhanced maneuverability and control in areas where conventional aerodynamic controls are ineffective. (These controls 

included ailerons, rudders, stabilators [stabilizers + elevators 

— all-movable horizontal stabilizers in the tail area], and

leading-edge flaps.) The system resulted in significantly increased maneuverability at moderate angles of attack and 

some degree of control at angles of attack up to roughly 70 

degrees. It also allowed researchers to collect a greater amount 

of data by remaining at high angles of attack longer than they 

could have done without it. 

The modified flight control computers used a PACE 1750A 

computer (made by Performance Semiconductor Corp., Sunnyvale, Calif.) and specially written flight control laws to provide the research flight control capability. These laws commanded the optimum combination of aerodynamic control 

and vectored thrust to satisfy pilot demand. Program engineers integrated all controls into these flight control laws. The 

pilot used standard cockpit controls and no special pilot action 

was required after the system was engaged in flight. In 

addition to the research flight control computers, pilots also 

used the original F-18 flight control system both as a backup 

to the research system and to perform take-offs and landings. 

Dryden research pilots Bill Dana and Ed Schneider completed 

the envelope expansion flights in February l992. Demon-
strated capabilities included stable flight at approximately 70 

degrees angle of attack (previous maximum was 55 degrees) 

and rolling at high rates at 65 degrees angle of attack. Controlled rolling would have been nearly impossible above 35 

degrees without vectoring. 

Between January 1993 and January 1994 the aircraft was 

modified with additional instrumentation, including a sophisticated engine inlet pressure measurements system between 

the inlet entrance and the engine face. The inlet instruments 

could measure pressure fluctuations up to 250 Hz at over 2000 

samples per second. This information provided unprecedented 

understanding of what happens to engine airflow under extreme maneuver conditions. Flights resumed in January 1994 

and continued through June 1994, with Ed Schneider and Jim 

Smolka as the Dryden research pilots, joined for short periods 

by U.S. Navy pilots. In Phase Two there were a total of 193 

flights, including some transition flights to Phase Three. 

Phase Three 

A Phase Three effort began in March 1995 to evaluate moveable strakes on both sides of the aircraft’s nose to provide yaw 

control at high angles of attack where conventional rudders 

became ineffective. These strakes, 4 feet long and 6 inches 

wide, were hinged on one side and mounted to the forward 

sides of the fuselage. At low angles of attack, they were folded 

flush against the aircraft skin (see diagram page 2). At higher 

angles of attack, they were extended to interact with the strong 

F-18 HARV with strakes deployed. (NASA photo EC95-

43249-15) 

vortices generated along the nose and thereby produce large 

side forces for control. Wind tunnel tests indicated strakes 

could be as effective at high angles of attack as rudders are at 

lower angles. Flights with active moveable strakes began in 

July 1995. The availability of the strakes enabled pilots to 

employ three separate flight modes. One used thrust vectoring 

alone. Another used thrust vectoring for longitudinal (pitch) 

control and a blend of thrust vectoring and strakes for lateral 

(side-to-side) control. A third mode used thrust vectoring 

solely for longitudinal control, with strakes alone for controlling lateral motion. These three options were a unique feature 

of the HARV project. They afforded a great deal of flexibility 

in research into control power requirements at high angles of 

attack. They were also a means of achieving detailed investigation of handling qualities at high angles of attack. 

The strake project concluded in September 1996 with flight 

385 of the HARV program. It yielded a great deal of information about the operation of nose strakes, which were effective 

in providing control above 35 degrees angle of attack. Phase 

Three included 109 flights. Besides Schneider and Smolka, 

the research pilots included Mark Stucky from Dryden, Phil 

Brown from Langley, and a number of guest pilots from the 

U.S. Navy and Marine Corps, the Canadian Air Force, the

(British) Royal Air Force, McDonnell Douglas, CalSpan and 

NASA Dryden. 

Program Management 

As previously mentioned, the high angle of attack technology 

program was a joint effort of NASA’s four aeronautical 

centers. Joe Chambers and Billy Gilbert at Langley started the 

program. The project managers at Dryden were, successively, 

Don Gatlin, Jenny Baer-Riedhart and Denis Bessette. The 

project engineers were Bruce Powers, Dave Fisher, John Del 

Frate, Bob Meyer, Neil Matheny, Vicki Regenie and Al 

Bowers. Operations engineers included Art Tanaka, Dick 

Klein, and Brad Flick. Flight test engineers were Jim Cooper 

and Mike Muratore. The crew chiefs were Al Grieshaber, 

John McKay, and Darryl Townsend. 

Aircraft Specifications 

The HARV was developed from a pre-production model of 

the F-18, a single-seat fighter aircraft built by the McDonnell 

Aircraft Co., St. Louis, Mo. This particular F-18, built before 

the redesignation of the aircraft to F/A-18, was borrowed from 

the U.S. Navy for high angle of attack research because it was 

equipped with a spin chute, having been used by the Navy for 

spin testing. Bearing Bureau Number 160780, this F-18 was 

the sixth full-scale developmental F-18 built for the Navy by 

McDonnell Douglas Corporation (St. Louis, Mo.). F/A-18s 

are currently in service with the U.S. Navy and Marine Corps. 

The particular F-18 used for the HARV had unfortunately 

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Research Pilot Ed Schneider in the cockpit of the HARV, 

showing the “Silk Purse” designation for the aircraft. (NASA 

photo EC89-0032-2 ) 

been heavily “cannibalized” (used as a source of spare parts) 

by the Navy, which never expected the aircraft would fly 

again. It arrived at Dryden in pieces aboard a semitrailer in 

October 1984, missing 400 parts and having very little documentation on its existing wiring system. Dryden mechanics 

and technicians had to find substitute parts, cut out all the 

existing wiring, then assemble the aircraft and rewire it. When 

they completed the job, they painted the 

words “Silk Purse” on the side 

of the fuselage to indicate what 

they had made from the “Sow’s 

Ear” of cannibalized aircraft 

parts with which they had begun. 

The aircraft had a wing span of 37 feet 5 inches. The fuselage 

was 56 feet long and 10 feet 6 inches high at the canopy. 

The HARV was powered by two General Electric F404-GE400 turbofan engines, each producing 16,000 pounds of thrust 

in afterburner. 

The weight of the HARV was 31,980 pounds, with 6,480 

pounds of internal fuel, in the Phase One configuration. It was 

36,099 pounds with the same amount of fuel, configured for 

Phases Two and Three, including the thrust vectoring and spin 

recovery parachute systems. 

The aircraft is on permanent display in the Virginia Air & 

Space Center, Hampton, Va. 

Sources 

Flight logs compiled by Joe Wilson and Karen Richards, 

available in the Dryden Historical Reference Collection. 

Kenneth W. Iliff and Kon-Sheng Charles Wang, FlightDetermined Subsonic Longitudinal Stability and Control 

Derivatives of the F-18 High Angle of Attack Research Vehicle (HARV) With Thrust Vectoring (Edwards, Calif.: NASA 

TP-97-206539, 1997). 

Three-view drawing of 

the F-18 HARV. 

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Albion H. Bowers, et al., An Overview of the NASA F-18 High 

Alpha Research Vehicle (Edwards, Calif.: NASA TM-4772, 

1996). 

David F. Fisher with Daniel G. Murri, Forebody Flow Visualization on the F-18 HARV With Actuated Forebody Strakes 

(Edwards, Calif.: NASA TM-1998-206556, 1998). 

Victoria Regenie, Donald Gatlin, and Robert Kempel, “The F18 High Alpha Research Vehicle: The High-Angle-of-Attack 

Testbed Aircraft,” paper delivered at the AIAA Biennial 

Flight Test Conference, August 24-26, Hilton Head Island, 

S.C. (AIAA-92-4121).

David F. Fisher, John H. Del Frate, and David M. Richwine, 

In-Flight Flow Visualization Characteristics of the NASA F18 High Alpha Research Vehicle at High Angles of Attack 

(Edwards, Calif.: NASA TM 4193, 1990). 

1. Inconel is a trade name belonging to Huntington Alloy

Products Division, International Nickel Company, Huntington, W.Va. 

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