Recent Dynamic Measurements and Considerations for Aerodynamic Modeling of Fighter Airplane Configurations

As airplane designs have trended toward the expansion of flight envelopes into the high angle of attack and high angular rate regimes, concerns regarding modeling the complex unsteady aerodynamics for simulation have arisen. Most current modeling methods still rely on traditional body axis damping coefficients that are measured using techniques which were intended for relatively benign flight conditions. This paper presents recent wind tunnel results obtained during largeamplitude pitch, roll and yaw testing of several fighter airplane configurations. A review of the similitude requirements for applying sub-scale test results to fullscale conditions is presented. Data is then shown to be a strong function of Strouhal number both the traditional damping terms, but also the associated static stability terms. Additionally, large effects of sideslip are seen in the damping parameter that should be included in simulation math models. Finally, an example of the inclusion of frequency effects on the data in a simulation is shown. Nomenclature b wing span c mean aerodynamic chord Cl rolling moment coefficient lβ static lateral stability derivative lp body axis roll damping derivative lr rolling moment due to yaw rate derivative lΩ stability axis roll damping derivative *Aerospace Engineer, Senior Member AIAA Copyright © 1998 by the American Institute of Aeronautics and Astronautics, Inc. No copyright is asserted in the United States under Title 17, U.S. Code. The Government has a royalty-free license to exercise all rights under the copyright claimed herein for Government purposes. All other rights are reserved by the copyright owner. Cm pitching moment coefficient m0 pitching moment at α = 0 mα static pitching moment derivative mq pitch damping derivative Cn yawing moment coefficient k Strouhal number (reduced frequency) ωl/2V l characteristic length (b/2 or c/2) p body axis roll rate p̂ nondimensional roll rate, pb/2V q body axis pitch rate q̂ nondimensional pitch rate, qc/2V V free stream velocity α angle of attack α0 mean angle of oscillation β angle of sideslip φ roll angle ω oscillation frequency Ω stability axis rotation rate Introduction At high angles of attack, unsteady aerodynamic effects may have a major impact on the maneuverability and controllability of an airplane. Currently, some modern fighter airplanes are capable of performing transient maneuvers involving high pitch rates to extreme angles of attack; this has been graphically demonstrated in the so-called "Cobra maneuver" flown by Soviet aircraft at recent airshows. The advent of innovative high-α control effectors such as thrust vectoring and forebody controls will enable even greater capability to effectively exploit a substantially enlarged envelope for air combat. The impact of unsteady aerodynamic effects on airplane flight dynamics (including stability and control effects during rapid high angle-of-attack maneuvers) and a practical means of utilizing these unsteady effects need to be addressed. Because current generation developmental aircraft programs are relying more heavily on simulation based predictions of airplane dynamics to reduce risk and cost of flight test programs, accurate prediction of aircraft