NASA 

National Aeronautics and 

Facts

Space Administration 

Dryden Flight Research Center 

P.O. Box 273

Edwards, California 93523 

Voice 661-258-3449 

FAX 661-258-3566 

pao@dfrc.nasa.gov FS-2000-02-009 DFRC 

X-31 Enhanced Fighter Maneuverability Demonstrator

X-31 #1 flying over Edwards. Notice the three thrust-vectoring paddles at the rear of the aircraft. (NASA photo EC 93-42152-8) 

Two X-31 Enhanced Fighter Maneuverability (EFM) demonstrators flew at the NASA Dryden Flight Research Center, 

Edwards, CA, and at Palmdale, CA, to obtain data that may apply to the design of highly maneuverable next-generation 

fighters. The program ended in June 1995. 

The X-31 program demonstrated the value of thrust vectoring (directing engine exhaust flow) coupled with advanced 

flight control systems, to provide controlled flight during close-in air combat at very high angles of attack. The result of 

this increased maneuverability was a significant advantage over most conventional fighters. 

Background 

“Angle of attack” (AoA or alpha) is an engineering term used to describe the angle of an aircraft’s wings relative to the 

incoming wind direction. 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 loss of control and result in the loss of the aircraft, 

pilot, or both. 

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Three thrust vectoring paddles mounted on the X-31’s 

airframe adjacent to the engine nozzle directed the exhaust 

flow to provide control in pitch (moving the nose up or 

down) and yaw (moving it right or left) to improve control. 

Made of graphite epoxy, the paddles could sustain temperatures of up to 1,500 degrees centigrade for extended periods 

of time. In addition the X-31s were configured with movable forward canards and eventually with fixed aft strakes. 

The canards were small wing-like structures set on a line 

between the nose and the leading edge of the wing. The 

strakes were set on the same line between the trailing edge 

of the wing and the engine exhaust. Both supplied additional pitch control in tight maneuvering situations. 

In-flight line drawing 

emphasizing thrustvectoring paddles. 

The X-31 operated with a digital fly-by-wire flight control 

system. It included four digital flight control computers with 

no analog or mechanical back-up. Three synchronous main 

computers drove the flight control surfaces. The fourth 

computer served as a tie-breaker in case the three main 

computers produced conflicting commands. 

The X-31 research program produced technical data at high 

angles of attack. This information gave engineers and aircraft 

designers a better understanding of aerodynamics, effectiveness of flight controls and thrust vectoring, and airflow 

phenomena at high angles of attack. This may lead to design 

methods providing better maneuverability in future highperformance aircraft and make them safer to fly. 

The X-31 program was divided into three phases: 

Phase 1 

Phase 1 was the conceptual design phase. During this phase, 

program personnel outlined the payoff expected from the 

application of EFM concepts in future air battles and defined 

the technical requirements for a demonstrator aircraft. 

Phase 2 

Phase 2 carried out the preliminary design of the demonstrator and defined the manufacturing approach. Three 

governmental design reviews took place during this phase 

to examine the proposed design. Technical experts from the 

U.S. Navy, German Federal Ministry of Defense, and

NASA contributed to the careful examination of all aspects 

of the design. 

Phase 3 

Phase 3 initiated and completed the detailed design and 

fabrication of two aircraft, which were assembled at the 

Rockwell International (now Boeing) facility at Air Force 

Plant 42, Palmdale, CA. This phase required that both 

aircraft fly a limited flight test program. The first aircraft 

was rolled out on Mar. 1, 1990, followed by a first flight on 

Oct. 11, 1990, piloted by Rockwell chief test pilot Ken 

Dyson. The aircraft reached a speed of 340 mph and an 

altitude of 10,000 feet during its initial 38-minute flight. 

The second aircraft made its first flight on Jan. 19, 1991, 

with Deutsche Aerospace chief test pilot Dietrich Seeck at 

the controls. Despite the fact that the number one and two 

aircraft had identical external dimensions, X-31 number 

two experienced stronger yaw asymmetries than aircraft 

number one. For this reason, X-31 team members began to 

refer to aircraft number two as the “evil twin.” The team 

tested aircraft number two with varying lengths of extended 

nose strakes and found that it could get the two aircraft to 

fly identically with 8 1/2 inches of strake length on the 

second X-31, making it an evil twin no longer. 

Flight Summary 

During the program’s initial phase of operations at 

Rockwell International’s Palmdale facility, pilots flew the 

aircraft on 108 test missions. They achieved thrust vectoring in flight and expanded the post-stall envelope to 40 

degrees angle of attack before flight operations were moved 

to Dryden in February 1992 at the request of the Defense 

Advanced Research Projects Agency (DARPA). (Stall is a 

condition of an airplane or an airfoil in which lift decreases 

and drag increases due to separation of airflow. Thrust 

vectoring compensated for the loss of control through 

normal aerodynamic surfaces that occurred during a stall. 

Post-stall refers to flying beyond the normal stall angle of 

attack, which in the X-31 was at 30 degrees angle of 

attack.) 

Because of the basic stability of the aircraft at high angle of 

attack, it exhibited low tolerance for sideslip. This became 

a problem at higher angles of attack. Below 30 degrees 

AoA, the nose boom updated the inertial navigation unit 

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with air data. When the pilots began flying for extended 

periods above 30 degrees AoA, the inertial navigation unit 

began calculating large but fictitious values of sideslip as a 

result of changes in wind direction and magnitude. 

To solve this problem, the X-31 team incorporated an 

unusual noseboom called a Kiel probe instead of the 

standard NASA pitot tube to calculate air flow. The Kiel 

probe was bent 10 degrees downward from the standard 

pitot configuration. The team also had to rotate the sideslip 

vane down 20 degrees from the noseboom to counter an 

oscillation that occurred at 62 degrees AoA. With these 

modifications, the air data to the inertial navigation unit 

was accurate throughout the AoA envelope, eliminating the 

problem of false readings of sideslip drift at high AoA. 

Side view of the Kiel probe on an X-31 showing the 10-degree 

bend downward. (NASA photo EC93-42024-1) 

At Dryden an international team of pilots and engineers 

(ITO: International Test Organization) expanded the 

aircraft’s flight envelope, including military utility evaluations that pitted the X-31 against comparable but non-thrust 

vectored aircraft to evaluate the maneuverability of the X31 in simulated air combat. The ITO included participation 

by NASA, the U.S. Navy, the U.S. Air Force, Rockwell 

Aerospace, the Federal Republic of Germany, and DaimlerBenz (formerly Messerschmitt-Bolkow-Blohm and 

Deutsche Aerospace). 

The first flight from Dryden under the ITO was in April 

1992, and by July 1992 the X-31 program was continuing 

the initial stage of post-stall envelope expansion. 

Throughout the process of envelope expansion, the team 

learned more about the aircraft and had to make many 

modifications to the control laws (aircraft equations of 

motion that governed the control of the airplanes through 

the flight control computers). This was necessary because 

the actual aerodynamics of the aircraft were slightly 

different from what the wind tunnels had predicted. The X31 team was able to make these changes quickly, so they 

rarely delayed the process of envelope expansion. 

When the pilots started flying above 50 degrees AoA, they 

encountered kicks from the side that they called lurches. 

The international team added narrow 1/4-inch-wide strips 

of grit to the aircraft’s noseboom and radome to change the 

vortices flowing from them. The grit strips reduced the 

randomness of the lurches caused by the vortices, enabling 

the pilots to finish envelope expansion to the design AoAlimit of 70 degrees at 1 g of acceleration. 

As the pilots began flying entries to post-stall angles of 

attack, they experienced unintentional departures from 

controlled flight at 58 degrees AoA during maneuvers 

known as a split-s. Analysis by engineers indicated that the 

departures were caused by very large asymmetries in yaw 

(movement of the nose to the right or left)—so large that 

they overcame the power of the available thrust vector 

controls. 

Previous AoA research and new wind-tunnel testing with 

an X-31 model led the research team to add strakes to the 

nose that were 6/10 of an inch wide by 20 inches long. The 

team also blunted the nose tip slightly (increasing the 

radius of curvature from essentially zero to 3/4 of an inch). 

The strakes forced more symmetric transition of forebody 

vortices and the blunted nose tip reduced yaw asymmetries. 

Both changes constituted significant improvements to the 

aircraft’s aerodynamics, but the blunting of the nose tip 

was the more significant of the two. 

The No. 2 X-31 achieved controlled flight at 70 degrees 

angle of attack at Dryden on Nov. 6, 1992. On that same 

day, it performed a controlled roll around the aircraft’s 

velocity vector at 70 degrees angle of attack. 

On April 29, 1993, the No. 2 X-31 successfully executed a 

minimum radius, 180-degree turn using a post-stall maneuver, flying well beyond the aerodynamic limits of any 

conventional aircraft. The revolutionary maneuver has been 

dubbed the “Herbst Maneuver,” after Wolfgang Herbst, a 

German proponent of using post-stall flight in air-to-air 

combat. The maneuver has also been described as a “J” 

turn when flown to an arbitrary heading change. 

Diagram illustrating Herbst maneuver. 

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X-31 at high angle of attack—Herbst maneuver. (NASA photo 

EC94-42478-4) 

During the final phase of evaluation, with the X-31’s engaged 

in simulated air combat scenarios against adversaries flying F/ 

A-18’s and some other tactical aircraft, the X-31’s were able 

to outperform other aircraft without thrust vectoring through 

use of post-stall maneuvers. 

The X-31 constituted a revolution in air combat in the poststall region. The pilots in the program did not support trading 

off other important fighter characteristics just to acquire the 

EFM capabilities the X-31 possessed. But they did conclude 

that the improved pitch pointing and velocity-vector maneuvering permitted by thrust vector control did provide new 

options for the pilot to use in close-in combat. Post-stall 

maneuvering allowed the pilots to rotate and point the nose of 

the vehicle at the adversary aircraft in such a way that the 

adversary pilot could not counter the maneuver. But this was 

true only when used selectively and rapidly. The X-31 also 

greatly improved flight safety since it was fully controllable 

and flyable in the post-stall region, unlike other fighter aircraft 

without thrust vectoring. 

Despite this greater safety, the No. 1 X-31 aircraft was lost in 

an accident Jan. 19, 1995. The pilot, Karl Heinz-Lang, of the 

Federal Republic of Germany, ejected safely before the 

aircraft crashed in an unpopulated desert area just north of 

Edwards. The crash resulted from an unexpected single-point 

failure in the noseboom airspeed system. 

The X-31 program logged an X-plane record of 580 flights 

during the program, 559 research missions and 21 in Europe 

for the 1995 Paris Air Show. A total of 14 pilots representing 

all agencies of the ITO flew the aircraft. 

Quasi-Tailless Demonstration 

In 1994, software was installed in the X-31 to simulate the 

feasibility of stabilizing a tailless aircraft at supersonic 

speed using thrust vectoring. Tests also included subsonic 

speeds. During the flights the pilot destabilized the aircraft 

with the rudder to stability levels that would be encountered if the aircraft had a reduced-size vertical tail. The X31 quasi-tailless flight test experiment demonstrated the 

feasibility of tailless and reduced-tail fighters. Had the 

capability been part of the airplane’s design from the 

beginning, the benefits had the potential to outweigh the 

complexity the concept entailed. 

1995 Paris Air Show 

The X-31’s enhanced maneuvering capabilities were 

demonstrated to the international aerospace industry during 

daily flights at the 1995 Paris Air Show. These flights 

featured post-stall maneuvers at low altitudes. The aircraft 

flew to Europe aboard a U.S. military C-5 transport. A 

small team of NASA and industry personnel supported it 

there. 

Program Management 

An international test organization of about 110 people, 

managed by the Defense Advanced Research Projects 

Agency (DARPA—redesignated ARPA from March 1993 

to 1996), conducted the flight tests at Dryden and 

Palmdale. NASA was responsible for flight test operations, 

aircraft maintenance, and research engineering after the 

project moved to Dryden. As the research flight program 

matured, the test organization declined in size to approximately 60 persons. 

The X-31 was the first international experimental aircraft 

development program administered by a U.S government 

agency and was a key effort of the NATO Cooperative 

Research and Development Program. 

The ITO director and NASA’s X-31 project manager at 

Dryden was Gary Trippensee. 

Aircraft Specifications 

Designed and constructed as a demonstrator aircraft by 

Rockwell International Corporation’s North American 

Aircraft and Deutsche Aerospace, the X-31 had a wing 

span of 23.83 feet. The fuselage length was 43.33 feet. 

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The X-31 was powered by a single General Electric F404-

GE-400 turbofan engine, producing 16,000 pounds of 

thrust in afterburner. 

Typical takeoff weight of the X-31 was 16,100 pounds 

including 4,100 pounds of fuel. 

The X-31 design speed was Mach 0.9 with an altitude 

capability of 40,000 feet. For specific tests to determine 

thrust vectoring effectiveness at supersonic speeds the 

aircraft was flown to Mach 1.28 at an altitude of 35,000 

feet. 

Three-view of X-31. 

References 

Bosworth, John T. and P. C. Stoliker, “The X-31A QuasiTailless Flight Test Results” (Edwards, CA: NASA TP3624, 1996). 

Canter, Dave, “X-31 Post-Stall Envelope Expansion and 

Tactical Utility Testing,” paper delivered on Jul. 13, 1994, 

at the Fourth High Alpha Conference, NASA Dryden 

Flight Research Center, Jul. 12-14, 1994 (NASA CP10143), Vol. 2. 

Flight logs, X-31 Nos. 1 and 2, NASA Dryden Historical 

Reference Collection, Location L1-7-4B-2. 

Groves, Al; Fred Knox; Rogers Smith; and Jim Wisneski, 

“X-31 Flight Test Update,” Society of Experimental Test 

Pilots, Thirty-Seventh Symposium Proceedings (Lancaster, 

CA: SETP, 1993), pp. 100-116. 

Interview, Gary Trippensee by Lane Wallace, Aug. 24, 

1995, NASA Dryden Historical Reference Collection. 

Stoliker, Patrick C. and John T. Bosworth, “Evaluation of 

High-Angle-of-Attack Handling Qualities for the X-31A 

Using Standard Evaluation Maneuvers” (Edwards, CA: 

NASA TM-104322, 1996). 

“X-31 carries out revolutionary turning maneuver,” XPress, May 21, 1993. 

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