AE/Flutter Simulation and Flight Test Correlation of the F/A-18 Active Aeroelastic Wing (AAW)

Introduction The AAW 1 technology program involves mutidisciplinary, aero-structural-controls interactions in an optimized fashion that maximizes the performance of a flight vehicle. This is achieved by the use of a more flexible, lighter wing that provides desirable wing twist response at higher speeds and deforms aeroelastically into favorable shapes for optimum performance; in this process the wing produces the control forces. The goal of this program is to generate control laws that will maximize the vehicle performance taking into account the flexibility of the wing. This is a joint flight research program involving NASA, Air Force, Navy and the industry. The AAW program uses a F/A-18 aircraft with suitable modifications of the wing that reduces the torsional stiffness characteristics. Thus the aft upper and lower wing covers were replaced by more flexible ones, made of thinner aluminum and composite skins, as appropriate. Apart from structural changes modifications were effected in the leading edge flap drive system (LEFDS), the flight control computer (FCC) and the baseline flight control laws. This paper presents the results of a set of aeroelastic and aeroservoelastic analyses of this modified aircraft along with flight test data in support of an ongoing research effort in the area of aeroelastic(AE) model validation and test data analysis . Nomenclature AE = aeroelasticity ASE = aeroservoelasticity CFD = computational fluid dynamics Ae(ki) = aerodynamic matrix, calculated for given Mach number M∞ and ki values K, M, C = elastic stiffness, inertia and damping matrices ki = reduced frequency ωb/V, ω and b being the natural frequency and wing semichord length, respectively fa(t) = generalized aerodynamic load vector ∗ Associate Fellow, AIAA AIAA 2005-0233 43rd AIAA Aerospace Sciences Meeting and Exhibit 10 13 January 2005, Reno, Nevada AIAA 2005-233 This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. 2 fI(t) = generalized impulse force vector u = displacement vector q = generalized displacement vector ρ = free stream density Linear Aeroelastic (AE) and Aeroservoelastic (ASE) Analysis In this procedure the unsteady aerodynamic forces are computed by the standard panel methods and the analysis follows the same pattern as the earlier X-29 project 4 , using the STARS program. Thus the equation of free vibration of a flight vehicle discretized by the finite element method may be written as M u +Ku = 0 (1) in which M , K are respectively the inertia and the stiffness matrices, u being the displacement vector. An effective eigensolution of Equation (1) yields the natural frequencies ω and the associated vectors Φ . A coordinate transformation of the form u = Φq (2) when applied to Equation (1) yields the generalized equation of motion ˆ M q + ˆ C q + ˆ K q + q ˆ A e(ki)q = ˆ P (t) (3) in which ˆ M = ΦMΦ and similarly ˆ K , ˆ C and ˆ A e ; q is the dynamic pressure and ki is the reduced frequency; ˆ A e is the aerodynamic influence coefficient matrix, calculated for a given Mach number and a set of k values, ˆ P (=ΦP) being the external loading function; the generalized coordinate q= qR qe qc [ ] and the modal matrix Φ = ΦR Φe Φc [ ] incorporating rigid body, elastic and control surface motions, respectively. The aerodynamic forces matrix may next be expressed as standard PADE polynomials in i*k and the generalized equation of motion may be written in state-space form, which for the plant dynamics is given as below x = Ax + Bu (4) after transformation of the equations from Earth-fixed (inertial) to body-fixed coordinate system. In these equations A = plant dynamic matrix, B = control surface influence matrix, x = generalized coordinates and u is the control surface motion input into plant. In the presence of sensors, the required sensor output relationship signifying motion at the sensors due to body motion (C) and control surface motion (D) is given below y = Cx + Du (5)