Significance of Unsteady Aerodynamic Effects in F/A-18C/D Pitching Moment

Abstract

Fighter aircraft have historically been challenged by low restoring nose-down pitching moment from high angles of attack. The consequence of weak nose-down pitching moment can be catastrophic – resulting in a deep stall condition, particularly at aft centers of gravity. The assessment of pitching moment at high angles of attack has mainly centered on largescale static wind tunnel testing and forced oscillation techniques for dynamic effects. Flight tests have recently been flown with F/A-18C and D model aircraft to directly measure the pitching moment at high angles of attack, offering an extensive flight-based data source. These tests revealed extremely large pitching moment disparities with respect to a model based largely on wind tunnel best-practice methods. The reasons for the disparities are identified in this paper: hysteresis effects of the strong vortex formed by the leading edge extension (LEX); delay in tail stall due to the rapid tail rotation during the test maneuvers; and delay in reestablishing normal flow on the tail post-stall. These effects are well known in the industry literature and have been typically analyzed and discussed in piecemeal. The results of this flight testing show that these unsteady aerodynamic effects – typically not modeled - are of great significance in actual practice particularly for aircraft with strong vortices, and need to be considered when assessing the pitching moment at high angles of attack. This paper will also discuss the techniques used to extract the pitching moment in flight test, and offer other ramifications of these unsteady effects such as divergent oscillation tendencies in high angle of attack maneuvers that would be otherwise predicted to be well damped.