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The X-29

Two X-29 aircraft, featuring one of the most unusual designs in aviation history, were flown at the NASA Ames-Dryden Flight Research Facility (soon to be renamed the Dryden Flight Research Center), Edwards, Calif., as technology demonstrators to investigate advanced concepts and technologies. The multi-phased program was conducted from 1984 to 1992 and provided an engineering data base that is available in the design and development of future aircraft.

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The X-29

The X-29 almost looked like it was flying backward. Its forward swept wings were mounted well back on the fuselage, while its canards ­ horizontal stabilizers to control pitch ­ were in front of the wings instead of on the tail. The complex geometries of the wings and canards combined to provide exceptional maneuverability, supersonic performance, and a light structure. Air moving over the forward-swept wings tended to flow inward toward the root of the wing instead of outward toward the wing tip as occurs on an aft swept wing. This reverse air flow did not allow the wing tips and their ailerons to stall (lose lift) at high angles of attack (direction of the fuselage relative to the air flow).

The concepts and technologies the fighter-size X-29 explored were the use of advanced composites in aircraft construction; variable camber wing surfaces; the unique forward-swept wing and its thin supercritical airfoil; strake flaps; close-coupled canards; and a computerized fly-by-wire flight control system to maintain control of the otherwise unstable aircraft.

Research results showed that the configuration of forward swept wings, coupled with movable canards, gave pilots excellent control response at up to 45 degrees angle of attack. During its flight history, the X-29's were flown on 422 research missions 242 by aircraft No. 1 in the Phase 1 portion of the program; 120 flights by aircraft No. 2 in Phase 2; and 60 flights in a follow-on "vortex control" phase. An additional 12 non-research flights with X29 No. 1 and 2 non-research flights with X-29 No. 2 raised the total number of flights with the two aircraft to 436.

Reverse airflow-forward-swept wing vs aft swept wing. On the forward-swept wing, ailerons remained unstalled at high angles of attack because the air over the forward swept wing tended to flow inward toward the root of the wing rather than outward toward the wing tip as on an aft-swept wing. This provided better airflow over the ailerons and prevented stalling (loss of lift) at high angles of attack.

 

 

The X-29 Program History

Before World War II, there were some gliders with forward-swept wings, and the NACA Langley Memorial Aeronautical Laboratory did some wind-tunnel work on the concept in 1931. Germany developed a motor-driven aircraft with forward-swept wings during the war known as the Ju-287. The concept, however, was not successful because the technology and materials did not exist then to construct the wing rigid enough to overcome bending and twisting forces without making the aircraft too heavy.

The introduction of composite materials in the 1970's opened a new field of aircraft construction, making it possible to design rugged airframes and structures stronger than those made of conventional materials, yet lightweight and able to withstand tremendous aerodynamic forces.

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Construction of the X-29's thin supercritical wing was made possible because of its composite construction. State-of-the-art composites permit aero-elastic tailoring, which allows the wing some bending but limits twisting and eliminates structural divergence within the flight envelope (i.e., deformation of the wing or breaking off in flight).

In 1977, the Defense Advanced Research Projects Agency (DARPA) and the Air Force Flight Dynamics Laboratory (now the Wright Laboratory), Wright-Patterson AFB, Ohio, issued proposals for a research aircraft designed to explore the forward swept wing concept. The aircraft was also intended to validate studies that said it should provide better control and lift qualities in extreme maneuvers, and possibly reduce aerodynamic drag as well as fly more efficiently at cruise speeds.

From several proposals, Grumman Aircraft Corporation was chosen in December 1981 to receive an $87 million contract to build two X-29 aircraft. They were to become the first new X-series aircraft in more than a decade. First flight of the No. 1 X-29 was Dec. 14, 1984, while the No. 2 aircraft first flew on May 23, 1989. Both first flights were from the NASA Ames-Dryden Flight Research Facility soon to be renamed the Dryden Flight Research Center.

 

Flight Control System

 

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The lower illustration shows how the canards on forward swept-wing X-29 will share the lifting load and reduce drag as compared to a conventional aircraft in the upper diagram.

The flight control surfaces on the X-29 were the forward-mounted canards, which shared the lifting load with the wings and provided primary pitch control; the wing flaperons (combination flaps and ailerons), used to change wing camber and function as ailerons for roll control when used asymmetrically; and the strake flaps on each side of the rudder that augmented the canards with pitch control. The control surfaces were linked electronically to a triple-redundant digital fly-by-wire flight control system (with analog back up) that provided an artificial stability.

The particular forward swept wing, close-coupled canard design used on the X-29 was unstable. The X-29's flight control system compensated for this instability by sensing flight conditions such as attitude and speed, and through computer processing, continually adjusted the control surfaces with up to 40 commands each second. This arrangement was made to reduce drag. Conventionally configured aircraft achieved stability by balancing lift loads on the wing with opposing downward loads on the tail at the cost of drag. The X-29 avoided this drag penalty through its relaxed static stability.

Each of the three digital flight control computers had an analog backup. If one of the digital computers failed, the remaining two took over. If two of the digital computers failed, the flight control system switched to the analog mode. If one of the analog computers failed, the two remaining analog computers took over. The risk of total systems failure was equivalent in the X-29 to the risk of mechanical failure in a conventional system.

X-29 - designed with relaxed static stability to achieve less drag, more maneuverability, increased fuel efficiency. Arrows in upper illustration indicate drag-producing opposing downward forces on rear stabilizers to achieve stability. X-29 canards share lifting loads, reducing drag.

 

Phase 1 Flights

The No. 1 aircraft demonstrated in 242 research flights that, because the air moving over the forward-swept wing flowed inward, rather than outward as it does on a rearward-swept wing, the wing tips remained unstalled at the moderate angles of attack flown by X-29 No. 1. Phase 1 flights also demonstrated that the aero-elastic tailored wing did, in fact, prevent structural divergence of the wing within the flight envelope, and that the control laws and control surface effectiveness were adequate to provide artificial stability for this otherwise extremely unstable aircraft and provided good handling qualities for the pilots.

The aircraft's supercritical airfoil also enhanced maneuvering and cruise capabilities in the transonic regime. Developed by NASA and originally tested on an F-8 at Dryden in the 1970s, supercritical airfoils flatter on the upper wing surface than conventional airfoils delayed and softened the onset of shock waves on the upper wing surface, reducing drag. The phase 1 flights also demonstrated that the aircraft could fly safely and reliably, even in tight turns.

 

Phase 2 Flights

The No. 2 X-29 investigated the aircraft's high angle of attack characteristics and the military utility of its forward-swept wing/canard configuration during 120 research flights. In Phase 2, flying at up to 67 degrees angle of attack (also called high alpha), the aircraft demonstrated much better control and maneuvering qualities than computational methods and simulation models had predicted. The No. 1 X-29 was limited to 21 degrees angle of attack maneuvering.

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High angle of attack smoke test

During Phase 2 flights, NASA, Air Force, and Grumman project pilots reported the X-29 aircraft had excellent control response to 45 degrees angle of attack and still had limited controllability at 67 degrees angle of attack. This controllability at high angles of attack can be attributed to the aircraft's unique forward-swept wing- canard design. The NASA/Air Force-designed high-gain flight control laws also contributed to the good flying qualities.

Flight control law concepts used in the program were developed from radio-controlled flight tests of a 22-percent X-29 drop model at NASA's Langley Research Center, Hampton, Va. The detail design was performed by engineers at Dryden and the Air Force Flight Test Center at Edwards. The X-29 achieved its high alpha controllability without leading edge flaps on the wings for additional lift, and without moveable vanes on the engine's exhaust nozzle to change or "vector" the direction of thrust, such as those used on the X-31 and the F-18 High Angle-of-Attack Research Vehicle. Researchers documented the aerodynamic characteristics of the aircraft at high angles of attack during this phase using a combination of pressure measurements and flow visualization. Flight test data from the high-angle-of-attack/military-utility phase of the X-29 program satisfied the primary objective of the X-29 program to evaluate the ability of X-29 technologies to improve future fighter aircraft mission performance.

 

Vortex Flow Control

In 1992 the U.S. Air Force initiated a program to study the use of vortex flow control as a means of providing increased aircraft control at high angles of attack when the normal flight control systems are ineffective.

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The No. 2 X-29 was modified with the installation of two high-pressure nitrogen tanks and control valves with two small nozzle jets located on the forward upper portion of the nose. The purpose of the modifications was to inject air into the vortices that flow off the nose of the aircraft at high angles of attack.

Wind tunnel tests at the Air Force's Wright Laboratory and at the Grumman Corporation showed that injection of air into the vortices would change the direction of vortex flow and create corresponding forces on the nose of the aircraft to change or control the nose heading.

From May to August 1992, 60 flights successfully demonstrated vortex flow control (VFC). VFC was more effective than expected in generating yaw (left-to-right) forces, especially at higher angles of attack where the rudder loses effectiveness. VFC was less successful in providing control when sideslip (relative wind pushing on the side of the aircraft) was present, and it did little to decrease rocking oscillation of the aircraft.

 

Vortex flow control involves pneumatic manipulation of forebody vortices as shown in the diagram. Exhausting air through the nozzles at the top of the airplane's forebody results in alteration or movement of the forebody vortices. As the diagram shows, air exhaused through the right nozzle accelerates the flow of the right vortex and pulls it closer to the forebody. As this occurs, the left vortex is pushed further away from the body. This results in lower pressure on the side of the blowing right nozzle, resulting in a right yawing movement of the aircraft as shown.

 

Summary

Overall, VFC, like the forward-swept wings, showed promise for the future of aircraft design. The X-29 did not demonstrate the overall reduction in aerodynamic drag that earlier studies had suggested, but this discovery should not be interpreted to mean that a more optimized design with forward-swept wings could not yield a reduction in drag. Overall, the X-29 program demonstrated several new technologies as well as new uses of proven technologies. These included: aero-elastic tailoring to control structural divergence; use of a relatively large, close-coupled canard for longitudinal control; control of an aircraft with extreme instability while still providing good handling qualities; use of three-surface longitudinal control; use of a double-hinged trailing-edge flaperon at supersonic speeds; control effectiveness at high angle of attack; vortex control; and military utility of the overall design.

 

The Aircraft

The X-29 is a single-engine aircraft 48.1 feet long. Its forward-swept wing has a span of 27.2 feet. Each X-29 was powered by a General Electric F404-GE-400 engine producing 16,000 pounds of thrust. Empty weight was 13,600 pounds, while takeoff weight was 17,600 pounds.

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The aircraft had a maximum operating altitude of 50,000 feet, a maximum speed of Mach 1.6, and a flight endurance time of approximately one hour. The only significant difference between the two aircraft was an emergency spin chute deployment system mounted at the base of the rudder on aircraft No. 2. External wing structure is primarily composite materials incorporated into precise patterns to develop strength and avoid structural divergence. The wing substructure and the basic airframe itself is aluminum and titanium. Wing trailing edge actuators controlling camber are mounted externally in streamlined fairings because of the thinness of the supercritical airfoil.

 

Program Management

The X-29 program was funded initially by the Department of Defense Advanced Research Projects Agency. The program was managed by the Air Force's Wright Laboratory, Aeronautical Systems Division, Air Force Systems Command, Wright-Patterson AFB, Ohio.

The flight research program was conducted by the Dryden Flight Research Center, and included the Air Force Flight Test Center and the Grumman Corporation as participating organizations.

FAS

 

Langley Research Center NASA

 

1. Langley, Grumman, Defense Advanced Research Projects Agency (DARPA), and U.S. Air Force cooperated to validate analytical methods for aeroelastic divergence of forward-swept wings.
2. Tests in the Langley 16-Foot Transonic Dynamics Tunnel of the X-29 resulted in advances in design methods for aero-elastically tailored forward-swept wing configurations.
3. Langley studied the high-angle-of-attack behavior of the X-29 and identified potential problems such as wing rock, divergent rolling motions at post-stall angles of attack, and longitudinal tumbling motions, which permitted timely design of a robust flight control system.
4. Critical information on engine inlet performance at high angles of attack was obtained during tests in the Langley 14- by 22-Foot Subsonic Tunnel.

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The Grumman (now Northrop Grumman) X-29 demonstrated the feasibility of several advanced technologies, including the aero-elastically tailored forward-swept wing, and the ability to routinely operate with extremely high levels of inherent longitudinal instability (relaxed static stability). Under the Defense Advanced Research Projects Agency (DARPA) sponsorship, Grumman designed two X-29 aircraft that underwent joint DARPA, Grumman, NASA, and U.S. Air Force flight tests at NASA Dryden Flight Research Center from 1984 to 1992. The exhaustive flight-test program covered aspects such as structural and aerodynamic performance, as well as high-angle-of-attack maneuverability. The X-29 aircraft flew 422 research missions. The joint X-29 Program obtained a vast amount of detailed data and analysis methods that will be applied to future high-performance aircraft.

Langley cooperated with DARPA and Grumman in the areas of flight dynamics and engine inlet performance at high angles of attack and aero-elastic divergence of forward-swept wings. Highlights of these tests included early and accurate projections of aerodynamic, stability, and control characteristics that allowed for resolution of problems before flight tests; rapid acceleration and validation of design methods for the avoidance of wing divergence for forward-swept wing configurations; and risk reduction for engine operations at high angles of attack.

Langley facilities used to support the X-29 program included the 30- by 60-Foot Tunnel, the 20-Foot Vertical Spin Tunnel, the National Transonic Facility, the 16-Foot Transonic Dynamics Tunnel, the 14- by 22-Foot Subsonic Tunnel, radio-controlled drop models, and the Differential Maneuvering Simulator.

Evolution of Forward-Swept Wing Configurations

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Research on the swept wing as a drag-reducing mechanism for high subsonic and transonic speeds during the late 1930’s and early 1940’s resulted in some of the first conventional aft-swept wing aircraft during World War II. At that time, it was also recognized that forward-swept wings (FSW) could produce the same beneficial effect for performance. Furthermore, the FSW also promised improved low-speed controllability. Stalls were expected to start at the wing root rather than the tip (in contrast to aft-swept wings), thereby maintaining the effectiveness of outboard ailerons and their contributions to roll control at low speeds. The onset of shock waves at high speeds was also expected to begin at the wing root, which again maintains aileron effectiveness at high speeds. With lateral control effectiveness assured across the operational envelope, there would be no need for drag-producing leading-edge high-lift devices. Finally, the general layout of the FSW resulted in a more aft location of the wing spar carry-through structure in the fuselage, which results in more fuselage internal volume.

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During 1942, the German Junkers Design Bureau initiated studies of an FSW bomber designated the Ju-287. First flown in 1944, the Ju-287 exhibited several problems, the most serious of which was a tendency to increase g-loading during a turn without control inputs from the pilot. The analysis of the problem by Junkers revealed that the cause was wing structural deformation from the aerodynamic loads on the forward-facing wingtip panels. At high speeds, the deformation was predicted to become very severe, exceed structural limits, and result in wing failure. This potentially catastrophic phenomenon was referred to as aero-elastic divergence. Further analysis indicated that the structural modifications required to avoid the divergence problem for the aluminum wing of the Ju-287 would result in excessive weight and unacceptable performance penalties. Other interest in FSW configurations during World War II came from the American Cornelius Aircraft Company, which worked on several configurations, including the XFG-1, a piloted towed glider used to transport fuel.

Researchers at Langley also investigated FSW as part of a program to develop variable-sweep wings. In one of these investigations, an existing wind-tunnel model of the Bell X-1 was equipped with a variable-sweep wing, and tests were conducted for a FSW version of the aircraft in the Langley 300-MPH 7- by 10-Foot Tunnel.

Following World War II, the only significant FSW aircraft built was the German Hansajet business jet, which was designed by the same chief engineer who designed the Ju-287. The aircraft never enjoyed a large market.

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The German Junkers Ju-287 forward-swept-wing bomber.

The problem of aero-elastic divergence stood squarely in the progress of FSW options for relatively high-speed aircraft, and the challenge of providing sufficient rigidity versus weight turned many designers away from the concept.

In the 1970’s, two activities coupled to stimulate interest in FSW configurations. First, Grumman became interested in conducting aerodynamic research to determine methods to improve its Highly Maneuverable Aircraft Technology (HiMAT) configuration, (which had lost in the design competition to Rockwell) including revolutionary wing configurations. The second activity was the remarkable advocacy and influence of Dr. Norris J. Krone, Jr., a retired Air Force pilot who had written his doctoral thesis on eliminating aero-elastic divergence of FSW configurations by using advanced tailored composites for structural rigidity. Krone subsequently became a manager at the Defense Advanced Research Projects Agency (DARPA), and Krone’s discussions with the Grumman managers led to a resurgence of interest in an examination of the FSW concept. During 1977, DARPA released a request for proposals (RFP) for a highly advanced technology demonstrator that would integrate advanced aerodynamics (with emphasis on the FSW) and advanced flight controls. Responses were received from Grumman, Rockwell, and General Dynamics.

On December 22, 1981, DARPA announced that Grumman had been selected to develop the new technology demonstrator, to be known as the X-29.

Initial information exchanges between DARPA and Langley on independent high-angle-of-attack evaluations of the competing FSW configurations occurred in early 1980, when Krone visited Joseph R. Chambers and his staff at the 30- by 60-Foot (Full-Scale) Tunnel. Langley agreed to provide support in this area as requested for all three competing industry teams. General Dynamics proposed an FSW version of the F-16 as a candidate design for the DARPA competition. Exploratory static wind-tunnel data had already been generated by cooperative tests led by Langley researcher Sue B. Grafton in the Langley 12-Foot Low-Speed Tunnel in 1978. Final tests of the General Dynamics design occurred in the Full-Scale Tunnel in April 1980. Rockwell’s FSW configuration underwent preliminary static and dynamic tests in the Full-Scale Tunnel in March 1981. Grumman’s FSW configuration was tested in the same tunnel during three entries beginning in November 1980. As a result of these tests, DARPA was provided with a timely, independent assessment of the high-angle-of-attack characteristics of all three competing designs. Langley gained considerable experience with the unique aerodynamic, stability, and control characteristics of FSW configurations at high angles of attack.

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Following the award of DARPA’s X-29 contract to Grumman, Langley’s support in the area of high-angle-of-attack technology expanded to include additional dynamic force tests and free-flight model tests in the Full-Scale Tunnel, spin and spin recovery tests in the Langley 20-Foot Vertical Spin Tunnel, control system development studies and piloted assessments of high-angle-of-attack behavior in the Langley Differential Maneuvering Simulator (DMS), and assessments of spin-entry and post-stall motions using a radio-controlled drop model.

One of the most important, unexpected results of the X-29 high-angle-of-attack study came during preliminary static and dynamic tests of a 0.16-scale free-flight model in the Full-Scale Tunnel. The X-29 government and industry team (and the entire technical community) had expected the X-29 to be heavily damped in roll at high angles of attack as a result of the tendency of the FSW to maintain attached airflow at the wingtips during stall. However, when Sue Grafton conducted the first dynamic force tests to measure aerodynamic damping in roll, the results indicated that the X-29 configuration would exhibit very unstable values of roll damping at angles of attack above about 25 deg. This result came as a complete surprise, and additional tests were quickly planned to confirm the suspected impact of the unstable damping. Grafton conducted a special “free-to-roll” test, in which the X-29 model was mounted to a sting assembly that contained a roll bearing which provided a 360-deg roll capability. The test technique evaluated the tendency of the model to display unsatisfactory roll characteristics at high angles of attack. At low angles of attack, the model was very stable, with no tendency to oscillate or diverge (in agreement with the results of the dynamic force test). When the angle of attack of the model was increased to about 25 deg, however, the model suddenly exhibited large amplitude wing rocking motions of a periodic nature. The wing rock was a nonlinear phenomenon, in that the model motions were self-initiating and built up to a limited amplitude independent of the magnitude of the initial disturbance.

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Sue Grafton with the F-16 forward-swept-wing model.

The early identification of the wing rock led to more tests, wherein it was determined that the wings of the X-29 were indeed working as predicted. That is, the airflow remained attached at the wingtips. However, vortical flow shed from the long, pointed fore body of the X-29 was found to be interacting on the upper fuselage and inner wing and causing the unstable damping, which was so large that it overwhelmed the stabilizing influence of the attached flow at the wingtips. Interestingly, the X-29 incorporated the forward fuselage of the Northrop F-5A, which is also known to exhibit wing rock at low speeds and high angles of attack because of the same phenomenon. With the cause of the instability identified, the X-29 flight control system could be modified to increase the level of the artificial roll damping provided by feedback to the flaperons. Fortunately, the flaperons of the X-29 retained their effectiveness because of the favorable flow patterns of the FSW at high angles of attack. Estimates indicated that sufficient damping could be provided by the flight control system via the roll damper, which utilized the flaperons.

Free-flight tests of the X-29 model were first conducted in the Full-Scale Tunnel in January 1982. A special challenge faced the Langley team, since the X-29 airframe was designed for a very high level of aerodynamic instability in pitch (relaxed static stability) with a highly responsive, redundant flight control system that provided stability augmentation. The X-29 would be un-flyable without the stabilizing inputs of the stability augmentation system. No other aircraft (and no other free-flight model) had ever flown with such a high level of relaxed stability. The X-29 incorporated a level of relaxed longitudinal stability (-32 percent at low speeds) that was an order of magnitude more unstable than the F-16. Under the leadership of Luat T. Nguyen, the staff programmed the X-29 control laws into the Langley computer that was used to replicate full-scale controls for the model flight tests. With a vane on the model nose boom providing information on angle of attack, the control system of the model performed flawlessly during the entire test program. The X-29 model became the first flying vehicle with such a level of relaxed stability.

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X-29 free-flight model undergoing tests at high angles of attack in the Langley Full-Scale Tunnel.

During the free-flight tests led by Daniel G. Murri and Sue Grafton, the model exhibited large amplitude wing rock near an angle of attack of 25 deg when the roll damper component of the flight control system was turned off, as had been observed in the free-to-roll tests. The wing rock became more severe with increasing angle of attack, and flights usually resulted in loss of control of the model near an angle of attack of 30 deg. The amplitude and frequency of the motions were in close agreement with the preliminary tests. When the roll damper was engaged, the motions quickly damped out and the model displayed satisfactory characteristics.

The timely identification of the unstable roll damping of the X-29 configuration was a major contribution of Langley for the aircraft. If the full-scale aircraft flight program had begun without the provision for adequate control system gains and an awareness of a possible roll-damping problem, the high-angle-of-attack characteristics of the X-29 could have been unacceptable. Ultimately, the X-29 did display the wing rock tendency in flight (although not to the degree of severity indicated by the model tests). However, the controls of the full-scale aircraft were even more effective than those of the free-flight model, and the control system had been designed with provision to increase the gain of the roll damper. The two X-29 aircraft subsequently exhibited entirely satisfactory characteristics in flight, and the technical community learned a lesson regarding the integrated aerodynamic contributions of FSW configurations with long, pointed fuselages.

In 1983, Luat Nguyen led a piloted assessment of the X-29 at high angles of attack in the DMS. The objectives were to study the high-angle-of-attack flying characteristics of the aircraft and its susceptibility to departure under maneuvering conditions, identify and develop control law concepts to provide good flying qualities and a high level of departure-spin resistance, assess the effectiveness of the Grumman flight control laws at high angles of attack and provide recommendations for modifications, and provide support for flight-test planning and coordination with NASA Dryden Flight Research Center.

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A priority issue in the DMS studies was the effect of Reynolds number on aerodynamic characteristics. Results of high Reynolds number tests in the NASA Ames 12-Foot Pressure Tunnel indicated that significant effects of Reynolds number on pitching moments and yawing moments existed for the X-29 configuration. Working with flight control team members from Grumman and Dryden, Nguyen completed an in-depth study of X-29 handling qualities at high angles of attack and provided numerous recommendations for the specific design of the flight control system for high-angle-of-attack conditions. Control law concepts were identified for control surface interconnects for optimum roll coordination, wing rock suppression, and automatic departure and spin prevention.

Tests of the X-29 in the Langley 20-Foot Vertical Spin Tunnel, which began in 1981 under Raymond D. Whipple, concentrated on three areas. First, the developed spin and spin recovery characteristics of the X-29 were determined for various aircraft loadings and erect and inverted spins. As previously mentioned, large Reynolds number effects had been predicted for the X-29 configuration. These effects were very noticeable at very high angles of attack (near 90 deg), where the model might exhibit critical spins. To correct for these effects (which were caused by the unique forebody shape that was incorporated from the F-5A) at the low speeds involved in the Spin Tunnel conditions, the Langley staff employed auxiliary strakes on the nose of the X-29 spin model. The X-29 model exhibited two types of spin. One type was flat, with an angle of attack of about 85 deg, with marginal to unsatisfactory spin recovery. The second spin was very oscillatory, with satisfactory to excellent recovery characteristics.

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The second area of interest was determination of the size of the emergency spin recovery parachute required for the number 2 aircraft, which would be used in the high-angle-of-attack flight-test program. Working with Grumman and Dryden, Whipple and the X-29 team arrived at a recommended parachute size, truss structure, and deployment mechanism. As a result of the outstanding maneuverability and spin resistance exhibited by the X-29, the emergency parachute system was never utilized in flight tests to terminate post-stall maneuvers.

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The spin parachute and truss assembly on the X-29 during high-angle-of-attack tests at Dryden.

X-29 model mounted on single-degree-of-freedom pitch-tumble test apparatus in the Langley Spin Tunnel.

The third area of interest in the Spin Tunnel test program involved concern over the possibility of a longitudinal tumbling phenomenon for the X-29. This concern had risen because of the high level of inherent longitudinal instability designed into the X-29. Specifically, Langley researchers expressed concern over whether the combination of very low airspeed and very high angle of attack (such as during recovery from a “zoom climb” to zero airspeed) might result in the aircraft pitching over into an end-over-end tumbling that would result in incapacitating g-levels for the pilot. The Spin Tunnel staff first addressed this issue in free-spinning tests wherein the X-29 model was launched tail first, without rotation, into the vertically rising airstream. The results of these exploratory tests showed that, without inputs from the control system, the model would indeed pitch over and develop a continuous tumbling motion about its center of gravity. The wild gyrations quickly caused the spin tunnel model to impact the walls of the tunnel, so an additional test technique was developed to study the issue under more controlled conditions. In these tests, the model was mounted on a special single-degree-of-freedom test apparatus that permitted 360-deg free-pitching motions. The additional studies of tumbling on this apparatus provided insight to a possible solution to the problem. The X-29 flight control system included aft-fuselage strake flaps, which were intended to be used only as trimming devices. However, during the tumbling tests it was found that the strake flaps were extremely efficient in promoting recovery from the tumble motions. In view of this result, the control system was modified to permit use of the strake flaps as control devices. This approach prevented the X-29 from entering the uncontrollable tumbling motions.

 

Drop-Model Tests

In 1987, Langley upgraded its drop-model test technique and conducted studies of the high-angle-of-attack and post-stall characteristics of a 0.22-scale model of the X-29. The high degree of relaxed stability of the X-29 made high-fidelity simulation of the control system a mandatory feature of the drop model. Langley had never flown an unstable model before the X-29 program. The Langley staff, led by Mark A. Croom, upgraded nearly every element of the drop-model operation, such as the model control actuators, transmitters and receivers, data encoders and decoders, and operational displays including a new cockpit display and operations monitor. The advanced X-29 control laws developed in the DMS study were programmed into a ground-based computer for proper simulation of control effects. This activity was by far the most challenging drop-model project ever conducted by Langley to that time.

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X-29 drop model mounted on launch apparatus on side of helicopter prior to release for post-stall tests.

The results of the drop-model program further confirmed the need for special control system concepts for high-angle-of-attack conditions. Without artificial stability augmentation in roll, the drop model exhibited the same type of large amplitude wing rock motions previously displayed by the wind-tunnel free-flight model. In the case of the drop model, when the angle of attack was increased to 30 deg and beyond, the oscillations became so divergent that the model exhibited uncontrollable 360-deg rolls that evolved into a roll departure and post-stall gyrations. When the wing rock suppression system was engaged, the motions were damped and the model was controllable to very high angles of attack.

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X-29 powered-inlet model tests in the Langley 14- by 22-Foot Subsonic Tunnel.

During some flights at extreme angles of attack, asymmetric yawing moments were encountered that caused the model to yaw and generate relatively high rotational rates. When the pilot intentionally applied prospin control inputs during these conditions, the model sometimes entered the unrecoverable flat spin mode exhibited by the spin tunnel model. With the departure and spin prevention system engaged, the model was highly resistant to intentional spins and was extremely maneuverable at high angles of attack.

Another valuable contribution of the X-29 drop-model program was a parameter identification effort conducted by David L. Raney and James G. Batterson of Langley, in which they analyzed the wing rock motions of the drop model and extracted values of the critical aerodynamic parameters that caused the motions. This complex analysis required the identification of rapidly changing aerodynamic derivatives over a range of angle of attack. The results of the study clearly identified unstable values of aerodynamic damping in roll to be the cause of the wing rock motions.

Grumman requested support from Langley in assessing the static and dynamic pressure conditions that might be experienced by the engine inlets during the high-angle-of-attack flight tests of the X-29. The data were required to ensure satisfactory engine operation and avoid engine stalls that could result in loss of hydraulic power during the spin maneuvers. Langley responded with tests of a powered X-29 model in the 14- by 22-Foot Subsonic Tunnel. The model was equipped with a propulsion simulator, flow-through inlets, and extensive instrumentation. These tests occurred in 1991 under the leadership of John W. Paulson, Jr.

 

Aero-elastic Divergence

The phenomenon of aero-elastic divergence had dramatically constrained international interest in the application of conventional metal FSW concepts. Franklin W. Diederich and Bernard Budiansky of Langley studied and summarized the major challenges of the divergence phenomenon in a NACA Technical Note in 1948 (ref.1). However, the emergence of composite materials and the aggressive advocacy of Norris Krone to utilize aero-elastic tailoring to reduce divergence led to cooperative studies of FSW technology by the staff of the Langley 16-Foot Transonic Dynamics Tunnel (TDT). Rodney H. Ricketts, Robert V. Doggett, and Wilmer H. Reed, Jr. planned and participated in numerous studies with DARPA, the Air Force, and industry to develop and verify analytical predictions of the divergence phenomenon. The studies investigated systematic generic wings and specific configurations, including the Rockwell and Grumman FSW designs.

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Rodney Ricketts with the Grumman FSW model during aeroelastic divergence tests in the Langley 16-Foot Transonic Dynamics Tunnel.

The analyses and tests of generic wing models with variations in aspect ratio, airfoils, and sweep provided invaluable data and methods that significantly expanded the database, especially in the transonic regime. Six sub-critical response test techniques were formulated and evaluated at transonic speeds for accuracy in predicting static divergence, and two divergence stoppers were developed and evaluated for use in preventing structural damage of wind-tunnel models during divergence tests.

Ricketts led cooperative tests of the proposed Grumman and Rockwell FSW configurations in the TDT in 1979. NASA, DARPA, the U.S. Air Force, and the industry teams participated in the cooperative tests. The Rockwell model consisted of a semi-span wing mounted to a splitter plate on the tunnel sidewall. The Grumman model included a representative fuselage shape. Conclusions from the divergence tests included dramatic demonstrations that aero-elastic tailoring was extremely effective in suppressing divergence for FSW configurations and that new nonlinear aerodynamic theories were required for complete analysis of the phenomenon. After the DARPA X-29 contract was awarded to Grumman, additional tests were conducted in the TDT in 1983 which demonstrated the potential coupling of the wing structural modes with the rigid body pitch mode to create an instability called body-freedom flutter. Techniques were developed to analyze these wing-body interactions, and the wing divergence prediction methods developed at Langley were used in the flight-test program of the X-29 at Dryden.

 

Advanced Engine Nozzles

 

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In the early 1970’s, an advanced thrust-vectoring nozzle for V/STOL aircraft was designed by the General Electric Company under a contract funded by the Navy. The nozzle was designed with a deflectable internal hood to permit large pitch thrust-vector angles for V/STOL operations, while deflection of the single expansion ramp could be used for smaller thrust-vector angles during air-to-air combat. This nozzle, known as the augmented deflector exhaust nozzle (ADEN), received considerable test and developmental support in the technical community during the 1970’s and early 1980’s. Much of this activity was advocated by the Department of Defense (DOD), NASA, the U.S. Air Force, and the U.S. Navy ad hoc interagency non-axis-symmetric nozzle working group that included Langley researcher Bobby L. Berrier as the NASA representative. The Propulsion Aerodynamics Branch at Langley conducted much of the development work on the ADEN under the direction of William P. Henderson and Bobby Berrier. Numerous cooperative ADEN research programs with DOD and industry were conducted to optimize nozzle performance and define suitable propulsion-airframe integration methodologies on a series of generic and specific (e.g., F-18) fighter configurations. The first full-scale non-axis-symmetric nozzle test was conduced at the Glenn Research Center in 1976 by running an ADEN nozzle attached to an augmented General Electric YJ101 engine in an altitude test cell. Thus, by the time of the DARPA X-29 award to Grumman in 1981, a mature thrust-vectoring nozzle design was available.

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Simple ADEN-like nozzle configuration (pitch and yaw vectoring) flown on X-29 free-flight model in the Full-Scale Tunnel.

During the early phases of the X-29 program, NASA Headquarters expressed interest in the application of the ADEN to the canard configured X-29 as an exploratory flight-test bed, since the canard of the X-29 could counterbalance the effects of the deflected ADEN nozzle. The NASA interest in the X-29 ADEN was to promote technical progress in vectorable nozzles and ensure that a sufficient number of candidate nozzles were considered for further development.

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X-29 ADEN model flying at an angle of attack of 80 deg in the Langley Full-Scale Tunnel.

NASA Headquarters requested Langley’s assessment of the X-29 ADEN configuration for further advocacy discussions with Grumman and DARPA. Although no induced lift or aerodynamic wing lift augmentation would occur for this configuration, the addition of vectoring was believed by Langley to improve high-angle-of-attack controllability. In response to this request from Headquarters, Joseph L. Johnson, Jr. and his staff at the Full-Scale Tunnel quickly modified their X-29 free-flight model and conducted exploratory evaluations of the handling qualities of the X-29 ADEN at high angles of attack in 1984. During these tests, a yaw-vectoring side-door capability (a concept defined and tested by the Propulsion Aerodynamics Branch in the Langley Jet Exit Test Facility) was added to a simplified ADEN-type nozzle to provide both pitch and yaw vectoring, and the resulting flight tests demonstrated a dramatic improvement in controllability and agility of the X-29 at extreme angles of attack. The model could be flown with precise and effective control to angles of attack as high as 85 deg. These positive results were typical of those that had been obtained with several different aircraft configurations equipped with other thrust-vectoring nozzles. The national progress in non-axis-symmetric, thrust-vectoring nozzles led to interest in other nozzle configurations and the eventual successful F-15 short takeoff and landing and maneuver technology demonstrator (STOL/MTD), F-18 High Alpha Research Vehicle (HARV), and X-31 flight-test programs.

Langley Research Center NASA

 

 

The Forward-Swept Wing

 

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This illustration shows the reverse airflow on forward swept wings vs. the airflow on the swept-back wings. On the forward swept wing, the air tended to flow inward toward the root of the wing rather than outward toward the wing tip as on the swept-back wing.

Three views of the X-29 forward-swept wing aircraft.

Two X-29 aircraft, featuring forward-swept wings, were flown at NASA Dryden Flight Research Center in a program conducted from 1984 to 1992. The plane looked almost like it was flying backward.

The X-29 Ship No. 2 technology demonstrator was flown by NASA Dryden Flight Research Center in a joint NASA-Air Force program to investigate the unique design's high angle of attack characteristics and its military utility.

The lower illustration shows how the canards on forward swept-wing X-29 will share the lifting load and reduce drag as compared to a conventional aircraft in the upper diagram.

The Grumman X-29 aircraft were flown from December 1984 to 1988.

Sometimes technological developments to overcome aeronautical challenges succeed but provide such a small improvement in performance or operations that they are never widely adopted. Such is the case for forward-swept wings, a design concept that has never really caught on for any kind of aircraft.

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In 1936, a German aerodynamicist first postulated developing an airplane with its wings swept forward, but nobody built any actual models at the time. During World War II, the Germans finally conducted tests of such an aircraft. The Messerschmitt company built the tailless Me 163B to explore the design. The German firm Junkers produced the jet-powered Ju 287 light bomber with forward-swept wings. They did this not because the design had any inherent aerodynamic advantages but rather to enable the wings to be mounted behind the bomb bay. In 1944, the obscure airplane manufacturer American Cornelius built one of the oddest aircraft ever to fly, the XFG-1 fuel transport glider, which was an unpowered fuel tanker with forward-swept wings. Only two of the ugly-looking craft were built.

After the war, the National Advisory Committee for Aeronautics (NACA) in the United States conducted wind tunnel tests of forward swept wings. NACA engineers even mounted a model of the Bell X-1 with forward-swept wings in a wind tunnel. But they found little inherent aerodynamic advantages to such a design. Even the Russians conducted full-scale model flights of forward-swept-wing gliders but abandoned the concept.

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In 1964, the German airplane manufacturer Hamburger Flugzeugbau built the HFB-320 business jet with forward-swept wings. This design allowed the wings to be mounted behind the passenger cabin along the sides of the fuselage. Only 50 of the aircraft were manufactured and it remains the only aircraft with forward-swept wings to enter actual production. For years the primary purpose for developing forward-swept wings was structural—to allow the wings to be mounted farther back on the fuselage so that their connecting structure did not interfere with anything inside the fuselage (like bombs or people). Wind tunnel tests made it clear that there were many problems with forward-swept wings and few aerodynamic advantages. One major problem was that the wingtips tended to bend upwards and cause the plane to stall—inevitable for metal wings. But in the mid-1970s, a U.S. Air Force officer noted that new composite materials then becoming available for aviation could be incorporated into the wings of a modern jet and eliminate the tendency of the wingtips to bend upward and cause the plane to stall. At the same time, several U.S. aviation companies were exploring ways to make planes that were highly maneuverable at transonic speeds (i.e., near the speed of sound).

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Aircraft with forward-swept wings are highly maneuverable at transonic speeds because air flows over a forward-swept wing and toward the fuselage, rather than away from it. By the late 1970s, the Defense Advanced Research Projects Agency (DARPA) sponsored a competition to build an experimental forward-swept-wing airplane. Rockwell International proposed the Sabrebat fighter and General Dynamics proposed modifying an F-16 Falcon jet fighter. But in 1981, DARPA finally selected Grumman, which had proposed using parts from several different aircraft to develop an experimental lightweight airplane soon designated the X-29. The X-29 used the fuselage from the Northrop F-5A, the main undercarriage and other equipment from the F-16, and an engine from the F/A-18. Its wings were made of advanced composites and it was equipped with small wings called canards mounted on the forward fuselage rather than on the tail where horizontal stabilizers are usually located. These helped increase the plane's maneuverability. The reverse airflow inward from the wing tip toward the root of the wing did not allow the wing tips and their ailerons to stall at high angles of attack.

The Grumman X-29 first flew in 1984. It had a strange appearance, with the wings mounted well back on the fuselage, and almost looked like it was flying backward. The aircraft could only be flown with the help of an advanced computer control system. In numerous tests over the next several years, the X-29 demonstrated that the forward-swept wing design produced a 15 percent better ratio of lift to drag in the transonic speed region. But Department of Defense officials were not significantly impressed by this performance improvement to approve any further experimental aircraft and the two X-29 aircraft were soon retired to museums.

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Forward-swept wings remained dead as a concept until the surprising appearance in 1997 of the Russian Sukhoi S-37 Berkut ("Golden Eagle") with its forward-swept wings and canards. The S-37 uses the front fuselage of the popular Su-37K fighter, but is otherwise an entirely new aircraft. It is significantly larger and heavier than the X-29 and when it first appeared, Western experts speculated that it was a prototype heavyweight naval fighter. But after several years of laboriously slow flight tests, Sukhoi did not appear ready to begin producing large numbers of forward-swept wing naval fighters and the S-37 remains a one-of-a-kind aircraft. Whether this is because the Russians have been unimpressed with their forward-swept wing airplane's performance, or simply because of lack of money is unknown. But it is clear that the forward-swept wing remains a novel solution to a problem that nobody feels the need to solve.

--Dwayne A. Day

Sources and further reading:

Braybrook, Ray. "Forward Sweep Is Back." Air International, February 1998, 119-123.

Wallace, Lane. Flights of Discovery: 50 Years at the NASA Dryden Flight Research Center. National Aeronautics and Space Administration, NASA SP-4309, 1996.

Miller, Jay. The X-Planes: X-1 to X-45. Hinckley, England: Midland Publishing, 2001.

X-29 Fact Sheet. National Aeronautics and Space Administration. Dryden Flight Research Center, April 1998. http://trc.dfrc.nasa.gov/PAO/PAIS/HTML/FS-008-DFRC.html

 

 

The X-29 Photo Gallery

 

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