Pressure Testing Success Stories:
X-31 Dynamic Pressure Wind Tunnel Test

Wind tunnel tests were conducted on the X-31 to understand the aerodynamic characteristics of the canard configuration. This aircraft has been used extensively for experimental high angle of attack flight testing. During those tests dramatic rolling moment flight characteristics have been seen. These occurred at high angles of attack while deflecting the canard. Force and moment as well as pressure data were measured for this model in both static and dynamic conditions. Analysis was then performed to understand how canard deflection modified the aerodynamic characteristics.

For these tests pressure ports were installed on all of the model surfaces. Each port was connected to a Pressure Systems Inc. (PSI) 32 port ESP module with a range of +/- 10” of H2O. A total of 8 modules were used for 256 ports. All pressures were measured and processed by the data acquisition computer. The pressures values were nondimensionalized by the tunnel dynamic pressure to produce a pressure coefficient. These data were stored for later analysis. For data acquisition, the test conditions were set and all ports were scanned at a rate of 10 khz. The scan was repeated every 100 ms to produce a time history of pressure values. For static conditions the time history values for each port were averaged for a single value. With dynamic testing the time histories were kept intact. The surface pressure coefficients at any point in the dynamic motion could then be plotted for analysis using different colors to represent levels of pressure.

The force data was measured first and used to determine the test conditions for the pressure tests. This helped reduce the test time and cost while optimizing the quantity of pressure data. Rotary balance force test results at 35∞ angle of attack indicated that canard deflection produced a large change from damped to propelling rolling moment with rotation about the velocity vector (Wb/2V). Corresponding pressure data was then measured for the same test conditions.

The following are some examples of the force/pressure results at 40∞ angle of attack. In the first case, the measured aerodynamic rolling at a body axis roll rate +0.06 indicates well damped rolling moment value with the canard set at zero defection. Deflecting the canard 40∞ trailing edge up produces very propelling rolling moment for the same roll rate. The 60∞ amp or 360∞ roll are just an indication of the test motion. In both cases the rolling moment was measured at the same rate, angle of attack and sideslip. As can be seen the results of the two motions are very similar at that rate. The important point is that the canard deflection produces very propelling rolling moments.

The pressure results for the same dynamic motions and conditions are shown in the following pressure plot. It shows that the cause of the propelling rolling moment is a large change in the wing surface pressure values. The two plots on the left are for neutral canard. Their negative values of rolling moment at +0.06 pb/2V are produced by the large negative pressure value on the top surface of the starboard wing. The two plots on the right are the pressure results with the canard deflected 40∞ trailing edge up. The pressure change on the wing is significant. Now the port has much more negative pressure than the starboard wing. This differential makes the airplane want to roll to the right and produces the positive values of rolling moment seen in the force data. Also of interest on these two pressure plots is the reduction of pressure towards the trailing edge of the canard that was not present with neutral controls.

The following is a second of correlating force and pressure data. In this case the X-31 is still at 40∞ angle of attack, but at 20∞ of sideslip. This produces a negative rolling moment bias for all values. The following plot shows the force results for body axis roll rate. At comparisons in this case are at +/-0.06 pb/2V. The rolling moment values at +0.06 are the same with and without canard deflection. At –0.06 pb/2V, the neutral canard reduces the negative rolling moment while 40∞ trailing edge up deflection produces a more negative value.

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