Dedicated to all those who served with or supported the 456th Fighter Squadron or 456th Fighter Interceptor Squadron or the UNITED STATES AIR FORCE

 

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The Rockwell X-31

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In the early 1980’s, an awareness of the benefits of thrust vectoring for dramatically improved control at high angles of attack surfaced. In addition to studies of advanced engine concepts with vectoring nozzles, interest arose over the use of simple thrust-vectoring paddles in the engine exhaust to deflect the thrust for control augmentation. As discussed in Langley Contributions to the F-14, the Navy, with Langley’s assistance, had taken the lead in this area with flight tests on an F-14 modified with single-axis yaw-vectoring paddles. In addition, during a cooperative program with Rockwell led by Langley researcher Bobby L. Berrier, Langley provided design data for multi-axis thrust-vectoring paddle configurations using the Jet Exit Test Facility in 1985. Based on these fundamental research studies, Rockwell incorporated multi-axis thrust-vectoring paddles into the SNAKE configuration. Free-flight tests of the modified SNAKE model in the Full-Scale Tunnel by Croom’s team in 1985 provided an impressive display of the effectiveness of thrust vectoring at extreme angles of attack.

In West Germany, Dr. Wolfgang Herbst of Messerschmitt-Bolkow-Blohm (MBB) aggressively touted the advantages of post-stall technology (PST) for increased effectiveness during close-in air combat. Herbst’s conclusions were based on wind-tunnel tests of a German advanced canard fighter configuration known as the TKF-90 and piloted simulator studies during which the application of simulated thrust vectoring resulted in rapid directional turns at high angles of attack had increased the turn rate by over 30 percent. Technical discussions between the Rockwell SNAKE Program managers and Herbst were initiated in 1983, and planning for a mutual program on PST ensued. Discussions with the Defense Advanced Research Projects Agency (DARPA) were very positive. When funding for collaborative international activities became available from the U.S. (the Nunn-Quayle research and development initiative in 1986) and West German governments, the technical expertise of Rockwell and MBB were joined under DARPA sponsorship in the X-31 Program. In view of Langley’s extensive experience in high-angle-of-attack technology, unique test facilities, and contributions to the Rockwell SNAKE Program, DARPA requested in 1986 that Langley become a participant in the X-31 development program.

Langley Flight Test Center NASA

 

 

X-31
Type Experimental
Manufacturer Rockwell
Messerschmitt-Bölkow-Blohm
Maiden flight 1990
Primary users DARPA
NASA
Number built 2

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The X-31 aircraft returns from a test flight for VECTOR.

A diagram of the Herbst maneuver. (NASA)
The X-31 showing its three thrust vectoring paddles.
The X-31 in Oberschleißheim
The three thrust vectoring paddles.

The collaborative U.S.-German Rockwell-MBB X-31 Enhanced Fighter Maneuverability program was designed to test fighter thrust vectoring technology. Thrust vectoring allows the X-31 to fly in a direction other than where the nose is pointing, resulting in significantly more maneuverability than most conventional fighters. An advanced flight control system provides controlled flight at high angles of attack where conventional aircraft would stall.

 

X-31 History

Two X-31s were built, and over 500 test flights were carried out between 1990 and 1995. The X-31 featured fixed strakes along the aft fuselage, as well as a pair of movable computer-controlled canards to increase stability and maneuverability. There are no horizontal tail surfaces, only the vertical fin with rudder. Pitch and yaw are controlled by the three paddles directing the exhaust (thrust vectoring). Eventually, simulation tests on one of the X-31s showed that flight would have been stable had the plane been designed without the vertical fin, because the thrust-vectoring nozzle provided sufficient yaw and pitch control.

During flight testing, the X-31 aircraft established several milestones. On November 6, 1992, the X-31 achieved controlled flight at a 70-degree angle of attack. On April 29, 1993, the second X-31 successfully executed a rapid minimum-radius, 180-degree turn using a post-stall maneuver, flying well beyond the aerodynamic limits of any conventional aircraft. This revolutionary maneuver has been called the "Herbst maneuver" after Dr. Wolfgang Herbst, an MBB employee and proponent of using post-stall flight in air-to-air combat.[1] Herbst was the designer of the Rockwell SNAKE, which formed the basis for the X-31.[2]

In the mid-1990s, the program began to revitalize and a $53 million VECTOR program was initiated capitalizing on this previous investment. VECTOR is a joint venture that includes the US Navy, Germany’s defense procurement agency BWB, Boeing's Phantom Works, and the European Aeronautic, Defense and Space Company in Ottobrunn, Germany. As the site for the flight testing, Naval Air Station Patuxent River in Maryland was chosen. From 2002 to 2003, the X-31 flew extremely short takeoff and landing approaches first on a virtual runway at 5,000 feet in the sky, to ensure that the Inertial Navigation System/Global Positioning System accurately guides the aircraft with the centimeter accuracy required for on the ground landings. The program then culminated in the first ever autonomous landing of a manned aircraft with high angle of attack (24 degree) and short landing. The technologies involved a differential GPS System based on pseudolite technology from Integrinautics, California, and a miniaturized flush air data system from Nordmicro, Germany.

Serial numbers

 

X-31 Specifications

General characteristics

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Performance

 

References

  1. Smith, R. E.; Dike, B. A.; Ravichandran, B.; El-Fallah, A.; Mehra, R. K. (2001). "Discovering Novel Fighter Combat Maneuvers in Simulation: Simulating Test Pilot Creativity" (PDF). United States Air Force. Retrieved on 2007-01-16.
  2.  "Partners in Freedom: Rockwell-MBB X-31." Langevin, G. S.; Overbey, P. NASA Langley Research Center. October 17, 2003.

Wikipedia

 

 

X-31 Enhanced Fighter Maneuverability Demonstrater

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.

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 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.

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 carbon-carbon -- an advanced carbon fiber reinforced composite -- the paddles could sustain temperatures of up to 1,500 degrees celsius for extended periods. 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.

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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 high-performance 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.

 

X-31 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 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.

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 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.

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 X-31 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 Daimler-Benz (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 X-31 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 AoA-limit 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.

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A diagram of the Herbst maneuver. (NASA)

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.

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 post-stall 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.

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 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 X-31 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.   More Information down the page

 

The 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.   More Information down the page

 

Program Management

An international test organization of about 110 people, managed by the Defense Advanced Research Projects Agency (DARPA—re-designated 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

 

X-31 3-view drawing

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.

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.

References

Bosworth, John T. and P. C. Stoliker, "The X-31A Quasi-Tailless Flight Test Results" (Edwards, CA: NASA TP-3624, 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 CP-10143), 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," X-Press, May 21, 1993

 

 

Flap Splitting & Setting Of The X-31 Wing

The Problem, its Importance, and its Challenges.

A common requirement for fighter planes is the ability to reach high roll angle accelerations, as this parameter is one of most important determinants of the plane's maneuverability.

X31 Wing Pressure coefficient distribution
Fig.1 - Pressure coefficient distribution for a sample wing design

If a plane has two edge flaps on the wings, deflecting the two independently will cause a rolling moment to ensue, which in turn induces a roll acceleration. Of course, key factors in determining the roll acceleration are the authorities of the inboard and outboard flaps, and the position of the flap split.

Unfortunately, these parameters cannot be chosen at will, because during the maneuver aerodynamic and mass loads are imposed on the structure, creating stress that the structure must be able to withstand. To guarantee the structural integrity is then necessary to add material, increasing the wing weight and reducing the range.

Accordingly, the solution must be a good compromise between good roll performance and low structural mass.

 

Mode Frontier's Contribution To Its Solution

To address this problem, DASA used modeFRONTIER to optimise the flap splitting (one discrete variable) and the flap settings (two continuous variables) on the X31 wing model.

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Fig.2 - The X31 experimental aircraft

Two conflicting objectives were pursued:

DASA's HISSS-D subsonic and supersonic solver was used for the aeroelastic design evaluation, while the structural weight for a given configuration was minimized, while maintaining structural integrity, using LAGRANGE, another proprietary code. The software run on an SGI Origin2000 parallel computer.

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Objective space for the designs by modeFRONTIER
Fig.3 - Objective space for the designs found by mode FRONTIER.

Optimization results using 16 individuals and 8 generations - using the MOGA algorithm - were sufficient to precisely describe the Pareto Frontier, thus singling out the set of optimal designs for different trade-offs.

For a given weight it became then possible to pick the design providing the highest acceleration possible. Conversely, given a desired roll acceleration it was possible to immediately find the design with the lowest weight.

 

 

References
[1] Stettner, M. and Haase, W., DASA (Germany)
Multiobjective Aeroelastic Optimisation
NATO meeting on Aerodynamic Design and Optimisation of Flight Vehicles in a Concurrent Multidisciplinary Environment
Ottawa (Canada), 21-22 October 1999

 

 

The X-31 Enhanced Fighter Maneuverability Demonstrator

The X-31 Enhanced Fighter Maneuverability (EFM) demonstrator, flown at NASA's Dryden Flight Research Center, Edwards, Calif., provided information which is invaluable for proceeding with the designs of the next generation highly maneuverable fighters. The X-31 program showed the value of using thrust vectoring (directing engine exhaust flow) coupled with advanced flight control systems, to provide controlled flight to very high angles of attack. The result is a significant advantage over conventional fighters in a close-in-combat situation.

Click on Picture to enlarge

An international test organization, managed by the Advanced Research Projects Agency (ARPA), is conducting the flight tests. In addition to ARPA and NASA, the International Test Organization (ITO) includes the U.S. Navy, the U.S. Air Force, Rockwell Aerospace, the Federal Republic of Germany and Deutsche Aerospace. About 110 people from the ITO agencies are assigned to the program. NASA is responsible for flight test operations, and aircraft maintenance. Research engineering is an ITO team effort. The X-31 is the first international experimental aircraft development program administered by a US government agency. It is one, if not the most, successful effort initiated by the NATO Cooperative Research and Development Program.

"Angle-of-attack" (alpha) is an engineering term to describe the angle of an aircraft's body and wings relative to its actual flight path. During maneuvers, pilots would like to fly at extreme angles of attack to facilitate rapid turning and pointing against an adversary. With older aircraft designs, entering this flight regime often led to loss of control, resulting in loss of the aircraft, pilot or both.

Three thrust vectoring paddles made of graphite epoxy and mounted on the X-31's aft fuselage are directed into the engine exhaust plume to provide control in pitch (up and down) and yaw (right and left) to improve maneuverability. The paddles can sustain temperatures of up to 1,500 degrees centigrade for extended periods of time. In addition, the X-31s is configured with movable forward canards, wing control surfaces, and fixed aft strakes. The canards are small wing-like structures located just aft of the nose, set on a line parallel to the wing between the nose and the leading edge of the wing. Normally "weathervaned" with the prevailing airflow, these devices are programmed to be used for aerodynamic recovery from high angles of attack in event of thrust vectoring system failure. The strakes are set along the same line between the trailing edge of the wing and the engine exhaust. The strakes supply additional nose down pitch control authority from very high angles of attack. Small fixed nose strakes are also employed to help control sideslip.

The X-31 flight demonstration program was focused on agile flight within the post-stall regime, producing technical data to give aircraft designers a better understanding of aerodynamics, effectiveness of flight controls and thrust vectoring, and airflow phenomena at high angles of attack. This is expected to lead to design methods providing better maneuverability in future high performance aircraft and make them safer to fly.

Phase One

Phase I was the conceptual design phase. During this phase the payoff expected from the application of EFM concepts in future air battles was outlined and the technical requirements for a demonstrator aircraft were defined.

 Phase Two

Phase II carried out the preliminary design of the demonstrator and defined the manufacturing approach to be taken. Three major government design reviews were held during the phase to thoroughly examine the proposed design. Technical experts from the U.S. Navy, Federal Ministry of Defense and NASA all contributed to the careful examination of all aspects of the design.

Phase Three

Phase III initiated and completed the detailed design fabrication and assembly of two aircraft. This phase required that both aircraft fly a limited test flight program. The first aircraft rolled out on March 1, 1990, followed by a first flight at Air Force Plant 42, Palmdale, Calif., on Oct. 11, 1990. The aircraft was piloted by Rockwell chief test pilot Ken Dyson, and reached a speed of 340 mph and an altitude of 1 0,000 feet during the 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.

 

Flight Summary

During the program's initial phase of flight test operations at the Rockwell Aerospace facility in Palmdale, Calif., the two aircraft were flown on 108 test missions, achieving thrust vectoring in flight and expanding the post-stall envelope to 40 degrees angle of attack. Operations were then moved to Dryden in February 1992 at the request of the Advanced Research Projects Agency (ARPA). At Dryden, the International Test Organization (ITO) expanded the aircraft's flight envelope, including military utility evaluations that pitted the X-31 against similarly equipped aircraft to evaluate the maneuverability of the X-31 in simulated combat. The ITO, managed by the Advanced Research Projects Agency (ARPA), includes NASA, U.S. Navy, the U.S. Air Force, Rockwell Aerospace, the Federal Republic of Germany, and Deutsche Aerospace (formerly Messerschmitt-Bolkow-Blohm).

The first NASA flight under the ITO took place in April 1992. By July 1992, the X-31 program was continuing the initial stage of post stall envelope expansion. The X-31 achieved controlled flight at 70 degrees angle of attack at Dryden on Nov. 6, 1992. On the same day, a controlled roll around the aircraft's velocity vector was accomplished at 70 degrees angle of attack. On April 29, 1993, the No. 2 X-31 successfully executed a rapid 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 term "J Turn" is also used to describe this type of maneuver, when flown to an arbitrary heading change.

The first tactical maneuver with a cooperative F/A-18 as adversary was accomplished in June 1993. In August 1993, the X-31 demonstrated full capability in flying Basic Fighter Maneuvers. In October 1993 the program logged its 300th flight. The final tactical evaluation phase, consisting of Close-In-Combat (CIC) tests with un-choreographed flights against the F/A-18 adversary, began in November 1993. During November and December 1993 the X-31 also reached supersonic speed (Mach 1.28). A total of 160 flights were completed by the X-31 program in 1993 setting a new annual experimental aircraft record. One of the two X-31s flew 103 of those flights. The program also set a new monthly record of 21 research flights in August 1993.

The evaluation of the X-31's unique capabilities in close combat (CIC) was completed on March 1, 1994. Evaluation of the X-31 as a fighter maneuverability demonstrator by the ITO concluded in early 1995.

The No. 1 X-31 ship was lost in an accident Jan. 19, 1995. The pilot, Karl Lang, ejected safely at 18,000 feet before the aircraft crashed into an unpopulated region of the desert just north of Edwards Air Force Base. There was no private property damage.

 

Quasi-Tailles Flight Demonstration

 

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Quasi-Tailless....

In 1994, software was installed in the X-31 to demonstrate the feasibility of stabilizing a tailless aircraft at supersonic speed, using thrust vectoring. This software allows destabilization through the control laws of the aircraft in incremental steps to the goal of simulation 100 percent tail-off. Quasi-tailless tests began in 1994. The first phase started with supersonic evaluations at Mach 1.2. Later subsonic evaluations were performed. During the flights the aircraft was destabilized with the rudder to stability levels that would be encountered if the aircraft had a reduced size vertical tail.

The quasi-tailless testing provided data to industry on the benefits of drag reduction, radar cross section, and weight reduction that could be used for future commercial and military designs and modifications. Simulated tailless X-31 flight tests conducted for the Joint Strike Fighter program successfully provided an initial demonstration that thrust vectoring could provide yaw control and, thus, reduce or eliminate the need for an aircraft vertical tail.

 

Helmet Mounted Visual / Audio Display

 

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Installation of a Helmet Mounted Visual/Audio Display (HMVAD) was completed on the X-31 (aircraft No. 2) in October 1993. The purpose of the HMVAD is to provid

e out-of-the-cockpit situation awareness and a simulated helmet-mounted sight to the pilot during high angle of attack combat maneuvering.

The system consists of a GEC Viper helmet with symbology projected on its visor by a monocular CRT. Also included is a Polhemus head tracker and an angle-of-attack audio cueing device. Both of these features have been demonstrated on the X-31, during post-stall close-in-combat, a first for any aircraft. This equipment will be the baseline for a follow-on virtual adversary program to demonstrate the feasibility of combat training against onboard and up-linked targets displayed by the helmet.

The Vectoring Extremely Short Take-Off and Landing Control Tailless Operation Research (VECTOR) program is a low-cost, highly leveraged approach to developing and demonstrating thrust vectoring and supporting technologies to enable complete flight control/engine/thrust vectoring integration for ESTOL and tailless flight. The Navy is particularly interested in thrust vectoring benefits in its unique take-off and landing environment. Germany is interested in the integrated FCS design and a major supporting technology, an Advanced Air Data System (AADS), which provides accurate air data information throughout the AOA range. Current systems develop inaccuracies at high AOA.

In February 1998, the participating contractors started VECTOR Risk Reduction and Requirements Definition. Efforts included defining detailed program and design requirements, identifying technology risks and performance goals, as well as agreeing on work and cost share in the Technology Demonstration Program. An enabling milestone was accomplished in April 1999 with the signature of the Memorandum of Agreement (MoA) between the German and U.S. Governments to conduct the VECTOR program.

In previous testing, the X-31 provided data for air combat maneuvering. In the VECTOR program, however, the aircraft will be exploring thrust vectoring technology in the take off and landing environment. In that flight regime, thrust-vectoring technologies may have a potentially significant pay-off in a number of critical areas, including operational capability, performance, safety, vehicle complexity, maintenance, and total cost of ownership.

The Boeing Company is responsible to the Naval Air Systems Command (NAVAIR) for VECTOR program integration and the flight control system hardware effort. DaimlerChrysler Aerospace of Germany is responsible to the German Federal Office for Defense Technology and Procurement (BWB) for the flight control law software, Advanced Air Data System development, simulation build-up, and aircraft wings and thrust vectoring vanes. In partnership with Dasa, Boeing will lead X-31 aircraft re-activation, modification, maintenance, repair, and flight test technology. Boeing is also responsible for the Extremely Short Take-Off and Landing activities. This effort includes development and integration of highly accurate navigation equipment. Reduced Tail studies are pursued in a joint effort of Boeing and Dasa. Major subcontractors of the VECTOR program include IntegriNautics, Honeywell, RJK Technologies, Moog, and Nord-Micro. Government participants include the Naval Air Systems Command, the German federal test center for military aircraft (WTD 61), and the German aerospace research center (DLR). Flight testing will be led by NAVAIR at the Patuxent River facility.

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An international test organization, managed by the Advanced Research Projects Agency (ARPA), is conducting the flight tests. In addition to ARPA and NASA, the International Test Organization (ITO) includes the U.S. Navy, the U.S. Air Force, Rockwell Aerospace, the Federal Republic of Germany and Deutsche Aerospace. About 110 people from the ITO agencies are assigned to the program. NASA is responsible for flight test operations, and aircraft maintenance. Research engineering is an ITO team effort.
The X-31 is the first international experimental aircraft development program administered by a U.S. government agency. It is one, if not the most, successful effort initiated by the NATO Cooperative Research and Development Program.

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The X-31's first fully automated, thrust vectored landing was completed on April 22, 2003.

Flight testing of the VECTOR X-31 ended April 29, 2003 at NAS Patuxtent River, where it had first arrived in April 2000. This followed a week of successful demonstrations of the world's first fully automated, thrust vectored landings at up to 24 degrees angle of attack. For three years, the VECTOR test team had been working to demonstrate the viability of thrust vectoring for extremely short takeoff and landing, using the unique X-31 as a test bed for the concept. During the last flight, the thrust-vectored jet completed an automated ESTOL landing at 24 degrees angle of attack and 121 knots, a 31 percent reduction from the aircraft's normal landing speed of 175 knots.

The X-31 typically requires 8,000 feet to stop after a conventional landing, but following the ESTOL touchdown, just 1,700 feet were needed to slow the X-31 down enough to turn around in the middle of the runway and taxi in a complete circle.

The program was then set to move into a data analysis and reporting stage, creating would essentially amount to a how-to manual for thrust-vectored ESTOL and the technology demonstrated on the X-31. , Young has been saying that the product of the VECTOR program would be knowledge. "Our intention was to get the data to aid government and industry in transitioning this technology to production aircraft," she said.

The X-31 is the first international experimental aircraft development program administered by a U.S. government agency. It is one, if not the most, successful effort initiated by the NATO Cooperative Research and Development Program.

The X-31 program logged an X-Plane record total of 524 flights in 52 months with 14 pilots from NASA, U.S. Navy, U.S. Marine Corps, U.S. Air Force, German Air Force, DASA, Rockwell International, and Deutsche Aerospace, flying the aircraft.

 

Specifications

Designed and constructed as a demonstrator aircraft by Rockwell Aerospace, North American Aircraft and Deutsche Aerospace.

The X-31 is a single seat aircraft with a wing span of 23.83 feet (7.3 m).

The fuselage length is 43.33 feet (1 2.8 m).

The X-31 is powered by a single General Electric P404-GE-400 turbofan engine, producing 16,000 pounds (71,168 N) of thrust in afterburner.

Typical takeoff weight of the X-31 is 16,100 pounds (7,303 kg).

The X-31's normal flight envelope includes speeds up to Mach 0.9 with an altitude capability of 40,000 feet (12,192 m). For specific tests to determine thrust vector effectiveness at supersonic speeds the aircraft was flown to Mach 1.28 at 35,000 feet.

 

 

 

The X-31 Configuration Evoluion

 

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X-31 with F-16 canopy during tests in the Langley 14- by 22-Foot Low-Speed Tunnel.

Free-flight tests of the X-31 in the Langley Full-Scale Tunnel.

The Rockwell and MBB X-31 design team merged their configuration candidates into a canard fighter powered by a single General Electric F404 engine with a single vertical tail. The initial design included an F-16 canopy for cost-saving purposes. Extensive tests of the initial X-31 configuration were carried out at Langley during 1987. These tests included static wind-tunnel tests and configuration component evaluations in the Langley 14- by 22-Foot High-Speed Tunnel, rotary-balance tests in the Langley 20-Foot Vertical Spin Tunnel to determine aerodynamic characteristics during spins, and dynamic force tests in the Langley Full-Scale Tunnel. Unfortunately, in 1988 the X-31 configuration was revised, and an F-18 canopy was incorporated. This change was regarded as significant, and a major portion of the previous wind-tunnel tests had to be repeated for the revised configuration.

Rotary-balance tests of the revised configuration were conducted in 1988, and spin tests and static and dynamic tests were completed in 1989 for the updated configuration. In 1989, a 0.19-scale model of the X-31 underwent extensive aerodynamic and free-flight tests in the Langley Full-Scale Tunnel. Results from these ground-based studies indicated that the X-31 might have marginal nose-down control at high angles of attack and that the configuration might exhibit severe, unstable lateral oscillations (wing rock) that would result in a violent, disorienting roll departure and an unrecoverable inverted stall condition. Fortunately, the results also indicated that a simple control law concept could prevent the aircraft from entering a spin. The awareness that such phenomena might exist for the full-scale aircraft enabled the X-31 design team to configure the flight control system for maximum effectiveness.

An exhaustive test, which included 498 paddle and nozzle configurations of the multiaxis thrust-vectoring system, was conducted by Langley researcher Francis J. Capone in the Jet Exit Test Facility during 1988. These data were used to select the final paddle and nozzle multi-axis thrust-vectoring configuration. These data were also critical to the design of the X-31 flight control system, since vectored thrust imposes large forces and moments in addition to the normal aerodynamic parameters.

A 0.27-scale drop model was used by Langley to evaluate the post-stall and out-of-control recovery characteristics of the configuration. The model, which weighed about 540 lb and included extensive instrumentation, was flown without an engine to assess the capabilities and characteristics of the basic airframe. The objective was to demonstrate that the X-31 would be agile and have satisfactory characteristics without the additional augmentation provided by thrust vectoring. The drop-model test identifies characteristics and large amplitude flight motions that cannot be assessed in conventional wind or spin tunnels. In the X-31 Program, the technique proved to be invaluable as an early indicator of the highly unconventional behavior of the configuration. In particular, the violent roll departure indicated by tests of the free-flight model was encountered during the drop-model tests. Several control schemes were evaluated to eliminate this problem. In addition, the drop-model test technique provided solutions to barrier problems during the full-scale flight-test program.

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One of the X-31 aircraft in flight.

Langley researcher Mark Croom (l) discusses the X-31 drop-model program with an X-31 program manager.

Mark Croom points to the aft-fuselage strakes defined by
Langley tests and subsequently incorporated on the X-31 aircraft.

The first flight of the first X-31 aircraft occurred at Palmdale, California, on October 11, 1990, and the second aircraft made its first flight on January 19, 1991. During the initial phase of flight-test operations at the Rockwell facility at Palmdale, the two aircraft were flown on 108 test missions. On the test missions, the aircraft achieved thrust vectoring in flight and expanded the post-stall envelope to 40-deg angle of attack. Operations were then moved to Dryden in February 1992, at the request of DARPA.

At Dryden, the International Test Organization (ITO) expanded the flight envelope of the aircraft, including military utility evaluations that compared the X-31 to similarly equipped aircraft for maneuverability in simulated combat. The ITO, managed by DARPA, included NASA, the U.S. Navy, the U.S. Air Force, Rockwell Aerospace, the Federal Republic of Germany, and Deutsche Aerospace (formerly Messerschmitt-Bolkow-Blohm). The first NASA flight under the ITO took place in April 1992. As the X-31 full-scale aircraft flight tests began at Dryden, the Langley staff maintained a close support role for consultation and ground testing capability.

Two problems surfaced during the X-31 flight-test program, and both were considered significant enough to curtail flight tests until solutions were found. The first problem was encountered in the flight-test program when it became apparent that the pitch control effectiveness of the aircraft at post-stall conditions (particularly at aft center of gravity conditions) was marginal. Pilots reported that their ability to obtain positive, crisp, nose-down aircraft response was unsatisfactory and that increased control effectiveness was required if the X-31 was to be considered tactically responsive at high angles of attack. As part of the X-31 Team, Langley was requested to conduct wind-tunnel tests to explore options to provide the increased control at high angles of attack. Mark Croom and his team quickly responded and evaluated 16 configuration modifications to improve nose-down recovery capability in the Full-Scale Tunnel. Results of the investigation recommended that a pair of 6- by 65-in. strakes be mounted along the fuselage after-body to promote nose-down recovery. The Langley recommendations, which were given within a week of the test request, provided a timely solution to the problem. The aft-fuselage strakes were incorporated in the X-31, and the pilots reported that the nose-down control was significantly improved.

The second problem that occurred in the X-31 full-scale flight test was caused by large out of trim asymmetric yawing moments at high angles of attack. Shortly into the high-angle-of-attack, elevated-g phase of the envelope expansion, a departure from controlled flight occurred as the pilot was performing a maneuver at 60-deg angle of attack. Data analysis by the X-31 team indicated that a large asymmetric yawing moment, in excess of the available control power, had triggered the departure. In response to an urgent request for solutions, Croom and the Langley team conducted tests in the Langley Full-Scale Tunnel to design nose strakes that would minimize the problem. Once again, Langley responded rapidly with a strake configuration that permitted the flights to continue.

The X-31 Program logged an X-plane record of 524 flights in 52 months with 14 pilots from NASA, the U.S. Navy, the U.S. Marine Corps, the U.S. Air Force, the German Air Force, Rockwell International, and Deutsche Aerospace.

Evaluation of the X-31 as an enhanced fighter maneuverability demonstrator by the ITO concluded in early 1995.

 

 

The Role Of The X-31 In High Angle Of Attack Technology

Langley Flight Test Center NASA

 

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The accompanying photograph shows three thrust-vectoring aircraft, each capable of flying at extreme angles of attack, cruising over the California desert in March 1994.
The F-18 HARV (top), the X-31 (middle), and the F-16 MATV (bottom) in flight.

Highlights of Research by Langley for the X-31

  1. Langley contributed to exploratory studies of a fighter configuration designed to exploit high angles of attack in a precursor to the X-31 Program with Rockwell.
  2. Working with Rockwell, Langley identified unacceptable characteristics of the initial design and successfully revised the configuration.
  3. At the request of the Defense Advanced Research Projects Agency, Langley participated in the International X-31 Program and provided information on stability and control, control system effects, configuration effects, thrust-vectoring system, spin recovery, and recovery from out of control conditions.
  4. During flight tests of the two X-31 research aircraft at NASA Dryden Flight Research Center, Langley provided facility support and technical consultation and analysis.
  5. On two occasions, Langley provided timely solutions to critical X-31 deficiencies that had stopped the flight program.

The Rockwell (now Boeing) and Messerschmitt-Bolkow-Blohm (MBB) X-31 Enhanced Fighter Maneuverability (EFM) Demonstrator was designed to demonstrate the effectiveness of controlled maneuvers at extreme angles of attack during certain close-in air combat scenarios. The first International (U.S. and Federal Republic of (West) Germany) X-Plane Program showed the value of using thrust vectoring (redirecting engine exhaust flow) with advanced flight control systems to provide unprecedented levels of controlled flight to very high angles of attack. Whereas many previous fighters experienced loss of control in this regime, the X-31 was able to maneuver without fear of loss of control or inadvertent spins, which provided the pilot with new tactical options. The X-31, along with the NASA F-18 High Alpha Research Vehicle, was used in extensive flight tests at NASA Dryden Flight Research Center in the 1990’s to provide the technologies and tactical evaluations to remove the high-angle-of-attack “barrier.”

Langley became involved in the X-31 Program in 1984 in a cooperative research program with Rockwell to develop a fighter configuration capable of highly agile flight at extreme angles of attack. Free-flight model tests at Langley led to a major redesign of the Rockwell candidate configuration. When Rockwell, the Defense Advanced Research Projects Agency (DARPA), and the West Germans formed the X-31 Program, the staff at Langley was requested to participate in the configuration development. Langley researchers conducted extensive studies of the stability, control, and thrust-vectoring system of the vehicle. Langley remained active in the program as Dryden became the responsible test organization during the flight tests of two X-31 demonstrator aircraft. Flight tests began at Dryden in February 1992 and concluded in 1995.

During the flight evaluation tests at Dryden, Langley provided technical support and on two occasions provided rapid solutions to critical stability and control problems that had stopped the flight tests.

Langley support of the X-31 included tests in the 30- by 60-Foot (Full-Scale) Tunnel, the 20-Foot Vertical Spin Tunnel, the 12-Foot Low-Speed Tunnel, the 14- by 22-Foot Tunnel, the 16-Foot Transonic Tunnel, the Jet Exit Test Facility, a radio-controlled drop model, and piloted simulators.

 

 

The Langley Contribution To The X-31

Langley Flight Test Center NASA

Background

Langley participated in the X-31 Enhanced Fighter Maneuverability (EFM) Program during four separate activities. From 1973 to 1984, Langley was active in the planning, testing, and analysis of the remotely piloted Highly Maneuverable Aircraft Technology (HiMAT) research vehicle. From 1984 to 1985, Langley cooperated in a program with Rockwell International to develop a representative fighter configuration that could demonstrate the advantages of exploiting high-angle-of-attack maneuvers during close-in air combat. From 1986 to 1991, Langley participated in the analysis and configuration development of the International (United States and Federal Republic of (West) Germany) X-31 Program. From 1991 to 1995, Langley supported the flight-test program, which was conducted at NASA Dryden Flight Research Center by the International Test Organization.

 

The HiMAT Program

Following the Vietnam conflict and renewed emphasis on close-in air-to-air combat, the U.S. military became interested in aircraft maneuverability. As a result, the requirement for high speeds, long considered the key factor in successful air combat, became a secondary objective. NASA initiated a joint program with the Air Force known as the Highly Maneuverable Aircraft Technology (HiMAT) Program. The staffs of the Langley, Ames, and Dryden Research Centers all participated in planning the HiMAT Program, with William P. Henderson serving as the technical lead and coordinator for Langley. The focus of the HiMAT Program was flight research and maneuverability demonstrations of a representative advanced configuration in the form of a remotely piloted subscale vehicle at Dryden. The goals of HiMAT included a 100-percent increase in aerodynamic efficiency over 1973 technology, and maneuverability that would allow a sustained 8-g turn at a Mach number of 0.9 and an altitude of 25,000 ft. The program ultimately achieved all goals.

 

 

The X-31 Aircraft At The Paris Air Show

 

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In another aviation first, the unique maneuvering capabilities of the X-31 high-performance experimental fighter aircraft were demonstrated Saturday to the international aerospace community in a performance at the 1995 Paris Air Show. The X-31's performance is the first international air show flight demonstration by an X-plane.

The X-31's demonstration included a series of unique maneuvers in which the aircraft dramatically exceeded the aerodynamic stall angle, a condition in which ordinary aircraft lose control. The X-31 is able to exploit this high angle-of-attack "post stall" capability to turn and maneuver more quickly and over shorter distances than can conventional aircraft.

The specific maneuver set demonstrated during the air show included a post-stall loop after takeoff, followed immediately by a rapid, so-called "helicopter turn" in the opposite direction; a low-altitude, horizontal, post-stall break turn termed the "mongoose"; a slow-speed, high angle-of-attack turn in the opposite direction called the "Herbst turn"; and, finally, a climbing, high-speed entry into a post-stall loop, followed by rapid, sequential re-pointing of the aircraft in opposite directions.

In preparation for the low-altitude air show demonstration, the X-31 had conducted 34 flights in less than a month. This represents a record for X-aircraft, bettering the previous achievement of 22 flights during one month; the previous record was also held by the X-31.

Two X-31 experimental aircraft were built and flew during a four-year exploration and test program to demonstrate the feasibility of thrust vectoring control in the post-stall flight regime. The X-31 used maneuvers similar to those in its air show repertoire in mock, close-in air combat engagements against a variety of front-line fighter aircraft, dramatically dominating many of these "adversaries."

The X-31's maneuvering achievements have also been complemented by another significant aviation first when the aircraft demonstrated that flight without a tail is possible at supersonic as well as subsonic speeds. Designing aircraft without tails offers the potential for reduced weight and increased performance, efficiency and stealth. The X-31 demonstrated flight without a tail through a novel supersonic in-flight experiment in which the flight control system was fooled into reacting as though the aircraft had no tail. The thrust vectoring capability was then used to provide necessary aircraft stability, trim and control.

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One aircraft crashed during a test flight in January 1995, after a departure from controlled flight not attributed to any of the aircraft's unique systems or maneuvering capabilities. The remaining X-31 aircraft was brought back to flight status in April.

The X-31 aircraft was developed jointly by Rockwell International's North American Aircraft Division and Daimler-Benz Aerospace (formerly Messerschmitt-Bolkow-Blohm), under sponsorship by the U.S. Department of Defense and the German Federal Ministry of Defense. The program has been operating under the auspices of the X-31 International Test Organization (ITO) from the NASA-Dryden Flight Research Center, Edwards, Calif. The ITO is comprised of participants from the DoD's Advanced Research Projects Agency, NASA, the U.S. Navy, U.S. Air Force, the German Government, German Air Force, and the two prime contractors, Rockwell International and Daimler-Benz. Note: Video footage of the X-31 performing maneuvers similar to those performed at the air show is available from Ken Carter, Room 2E765, Pentagon, at (703) 697-6161.

DoD

 

 

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Last Updated

02/10/2014

 

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