Correlation between static rolling moment slope and unsteady-response time constants

Abstract

The time scales over which aerodynamic responses to aircraft maneuvers act are typically assumed to be much smaller than the time scale of the maneuver itself. However, unsteady rolling moment data taken using 65” delta wings undergoing body-axis rolling motions have demonstrated that this assumption is not always valid. In analy%ng these data, a correspondence was observed involving the slopes of the experimentallyand numerically-determined static rolling moments and the time constant of the unsteady rolling moment response. This relationship appeared to be rooted in the presence of leading-edge vortex burst. This comparison may be of use in determining whether vortexdominated flows containing critical states elicit unsteady-response time scales which are significantly greater than the convective time scale. Introduction In most approaches to aircraft design, the time required for the aerodynamic response to reach its equilibrium value is assumed to be much smaller than the maneuver’s time scale.‘Y2 This premise is a fundamental assumption in the development of linearcoefficient-based stability and control models. As a result, these models cannot account for aerodynamic responses which are time-lagged, or “slow,” in comparison to the conduct of typical maneuvers. However, slow aerodynamic responses are sometimes found when the flow field involved includes separatedand/or vortex-dominated flows. With the envelope expansion of new military aircraft and missiles into higher angle of attack regimes, the limitations of these stability and control models are becoming apparent. Fig. 1 presents data which demonstrate slow temporal response characteristics. These data were acquired during ramp-and-hold body-axis-roll (I$) maneuvers of a 65” delta wing conducted at a body-axis angle of attack ((r) of 30” and freestream Mach number of 0.3. The unsteady rolling moment data clearly did not track the quasi-static data, instead requiring over 30 convective time units (croot/2U) to reach the final equilibrium value.3 * Aerospace Engineer; Senior Member, AIAA This paper is declared a work of the U.S. Government and is not subject to copyright protection in the United States. I Pi 0.03, I8