Applications of Computational Methods for Dynamic Stability and Control Derivatives

Initial steps in the application of a low-order panel method computational fluid dynamics (CFD) code to the calculation of aircraft dynamic stability and control (S&C) derivatives are documented. Several capabilities, unique to CFD but not unique to this particular demonstration, are identified and demonstrated in this paper. These unique capabilities complement conventional S&C techniques and they include the ability to: 1) perform maneuvers without the flow/kinematic restrictions and support interference commonly associated with experimental S&C facilities, 2) easily simulate advanced S&C testing techniques, 3) compute exact S&C derivatives with uncertainty propagation bounds, and 4) alter the flow physics associated with a particular testing technique from those observed in a wind or water tunnel test in order to isolate effects. Also presented are discussions about some computational issues associated with the simulation of S&C tests and selected results from numerous surface grid resolution studies performed during the course of the study. INTRODUCTION Aircraft stability and control (S&C) derivatives quantify the aerodynamic forces and moments used to model aircraft dynamics. S&C derivatives are used to predict, for example, the longitudinal short period, lateral pure roll, lateral Dutch roll, spin behaviors, and handling qualities sensed by pilots for a given configuration. Historically, aircraft static and dynamic S&C derivatives have been determined through wind tunnel tests, water tunnel tests, or with empirical formulations based on prior tests. However, most of the wind tunnels and all of the water tunnels used for S&C tests operate at low free stream velocities and Reynolds numbers. These limitations restrict the range of flight conditions that can be adequately simulated. Many S&C testing facilities provide unique dynamic capabilities that are not typically available in the wind tunnels used for aircraft performance tests. ---------------------------------------------------------------This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Because of their dynamic capabilities, these S&C facilities may be expensive to build, operate, and/or maintain. The range of motions that can be performed in such facilities may be limited by the wind tunnel walls or by the test rig’s kinematic and vibrational restrictions. In addition, many conventional dynamic test facilities do not provide the capability to determine damping and cross derivatives individually; instead, these facilities can only provide measurements of combined derivatives. The measurement of these quantities in pairs is not the preferred situation; it is simply a kinematic reality of the facilities available. A companion paper details several computational techniques to separate such paired dynamic derivatives. Some facilities are not equipped with a “slip ring” capability and thus, cannot perform continuous rotary motions; instead they rely on oscillatory motions to provide brief periods of steady rotational motion. The difficulty of many experimental S&C facilities to adequately model the flight characteristics of today's aircraft over the entire flight envelope, the cost of developing better S&C testing facilities, and the recent advent of novel morphing aircraft configurations all contribute to the difficulty of predicting S&C derivatives. Prior research efforts using CFD to predict S&C derivatives have made some progress toward bridging the capability gap, but these efforts are very time consuming and have failed to make a significant impact in the day to day business of S&C prediction. Because both CFD and experimental studies 31 have their unique disadvantages, aerospace companies spend millions of dollars fixing S&C problems discovered during certification flight tests or production use. The current situation for S&C prediction is similar to the situation seen before CFD methods were widely used to accurately predict aerodynamic performance. CFD offers capabilities and techniques that complement experimental S&C testing techniques for obtaining S&C derivatives. With CFD, support and ---------------------------------------------------------------* Senior Research Scientist, Multidisciplinary Optimization Branch, M/S 159, Senior Member AIAA ** Engineering Co-op Student, Multidisciplinary Optimization Branch, M/S 159, Student Member AIAA