Challenging Technology: The X-29 Program

Meetings between Defense Advanced Research Projects Agency (DARPA) and NASA Langley personnel in early 1980 initiated planning for sup­port of an advanced forward-swept wing (FSW) research aircraft project with numerous objectives, including assessments and demonstration of superior high-angle-of-attack maneuverability and departure resistance resulting from the aerodynamic behavior of the FSW at high angles of attack. Langley was a major participant in the subsequent program and conducted high-angle-of-attack wind tunnel tests of models of the competing designs by General Dynamics, Rockwell, and Grumman dur­ing 1980 and 1981. When Grumman was selected to develop the X-29

research aircraft in December 1981, NASA was a major partner with DARPA and initiated several high-angle-of-attack/stall/spin/departure studies of the X-29, including dynamic force-testing and free-flight model tests in the Full-Scale Tunnel, spinning tests in the Spin Tunnel, initial high-angle-of-attack control system concept development and assess­ment in the DMS, and assessments of spin entry and poststall motions using a radio-controlled drop model.[1302]

Early in the test program, Langley researchers encountered an unan­ticipated aerodynamic phenomenon for the X-29 at high angles of attack. It had been expected that the FSW configuration would maintain satis­factory airflow on the outer wing panels at high angle of attack; however, dynamic wind tunnel testing to measure the aerodynamic roll damping
of an X-29 model in the Full-Scale Tunnel indicated that the configura­tion would exhibit unstable roll damping and a tendency for oscillatory large-amplitude wing-rocking motions for angles of attack above about 25 degrees. After additional testing and analysis, it was determined that the FSW of the aircraft worked as well as expected, but aerodynamic interactions between the vortical flow shed by the fuselage forebody with the wing were the cause of the undesirable wing rock. When the free-flight model was subsequently flown, the wing rock was encoun­tered as predicted by the earlier force test, resulting in large roll fluctu­ations at high angles of attack. However, the control effectiveness of the wing trailing-edge flapperon used for artificial damping on the full-scale X-29 was extremely powerful, and the model motions quickly damped out when the system was replicated and engaged for the model.

Obtaining reliable aerodynamic data for high-angle-of-attack tests of subscale models at Langley included high Reynolds number tests in the NASA Ames 12-Foot Pressure Tunnel, where it was found that sig­nificant aerodynamic differences could exist for certain configurations between model and full-scale airplane test conditions. Wherever possi­ble, artificial devices such as nose strakes were used on the models to more accurately replicate full-scale aerodynamic phenomena. In lieu of approaches to correct Reynolds number effects for all test models, a conservative approach was used in the design of the flight control sys­tem to accommodate variability in system gains and logic to mitigate problems demonstrated by the subscale testing.[1303]

In the area of spin and recovery, the Langley spin tunnel staff mem­bers conducted tests to identify the spin modes that might be exhibited by the X-29 and the size of emergency spin recovery parachute recom­mended for the flight-test vehicles. They also investigated a growing concern within the airplane development program that the inherently unstable configuration might exhibit longitudinal tumbling during maneuvers involving low speeds and extreme angles of attack (such as during recovery from a "zoom climb” to zero airspeed). This concern was of the general category of ensuring that aircraft motions might over­power the relative ineffectiveness of aerodynamic controls for configu­rations with relaxed stability at low-speed conditions.

Using a unique, single-degree-of-freedom test apparatus, the research team demonstrated that tumbling might be encountered but that the aft-fuselage strake flaps—intended to be only trimming devices— could be used to prevent uncontrollable tumbling.[1304] As a result of these tests, the airplane’s control system was modified to use the flaps as active control devices, and with this modification, subsequent flight tests of the X-29 demonstrated a high degree of resistance to tumbling.

In 1987, Langley conducted high-angle-of-attack and poststall assess­ments of the X-29 using the Langley helicopter drop-model technique that had been applied to numerous configurations since the early 1960s. However, the inherent aerodynamic longitudinal instability and sophis­ticated flight control architecture of the X-29 required an extensive upgrade to Langley’s test technique. The test program was considered the most challenging drop-model project ever conducted by Langley to that time. Among several highlights of the study was a demonstra­tion that the large-amplitude wing rock exhibited earlier by the unaug­mented wind tunnel free-flight model also existed for the drop model. In fact, when the angle of attack was increased beyond 30 degrees, the roll oscillations became divergent, and the model exhibited uncontrollable 360 degrees rolls that resulted in severe poststall gyrations. When the active wing-rock roll control system of the airplane was simulated, the roll motions were damped and controllable to extreme angles of attack.[1305]

Two X-29 research aircraft conducted joint DARPA-NASA-Grumman flight tests at NASA Dryden from 1984 to 1992.[1306] The first aircraft was used to verify the benefits of advanced technologies and expand the enve­lope to an angle of attack of about 23 degrees and to a Mach number of about 1.5. The second X-29 was equipped with hardware and software modifications for low-speed flight conditions for angles of attack up to about 70 degrees. The test program for X-29 No. 2 was planned and accomplished using collated results from wind tunnel tests, drop-model tests, simulator results, and results obtained from X-29 No. 1 for lower
angles of attack. Dryden and the Air Force Flight Test Center designed flight control system modifications, and Grumman made modifications. The high-angle-of-attack flight program included 120 flights between 1989 and 1991. Dryden researchers conducted a series of aerodynamic investigations in mid-1991 to assess the symmetry of flow from the fuselage forebody, the flow separation patterns on the wing as angle of attack was increased, and the flow quality at the vertical tail location.[1307] In 1992, the Air Force conducted an additional 60 flights to evaluate the effectiveness of forebody vortex flow control using blowing.

The results of the high-angle-of-attack X-29 program were extremely impressive. Using only aerodynamic controls and no thrust vector­ing, X-29 No. 2 demonstrated positive and precise pitch-pointing capability to angles of attack as high as 70 degrees, and all-axis maneu­verability for 1 g flight up to an angle of attack of 45 degrees with lateral – directional control maintained. The wing-rock characteristic predicted by the Langley model tests was observed for angles of attack greater than about 35 degrees, but the motions were much milder than those exhib­ited by the models. It was concluded that the Reynolds number effects observed between model testing and full-scale flight tests were respon­sible for the discrepancy, as flight-test values were an order of magni­tude greater than those of subscale tests.