An Analytical Explanation for the X-43A Flush Air Data Sensing System Pressure Mismatch between Flight and Theory

Following the successful Mach 7 flight test of the National Aeronautics and Space Administration X-43A hypersonic scramjet-powered research aircraft, unexpectedly low pressures were found to have been measured by the aft set of the onboard Flush Air Data Sensing System pressure ports. These in-flight aft port readings were significantly lower below a speed of Mach 3.5 than had been predicted by theory. Similar lower readings were also seen in the Mach 10 flight of the X -43A and in wind-tunnel data. The preflight predictions were developed based on two-dimensional wedge flow, which fails to predict some of the significant three-dimensional flow features in this geometry at lower Mach numbers. This three-dimensional flow primarily affects the aft pressure ports because of their location. Using Volterra’s solution to the wave equation as a starting point, a three-dimensional finite wedge approximation to flow over the forebody of the X-43A is presented. The surface pressures from this approximation compare favorably with the measured wind-tunnel and flight data at speeds of Mach 2.5 and Mach 3.