Transonic Flutter Suppression Control Law Design, Analysis and Wind Tunnel Results

The benchmark active controls technology and wind tunnel test program at NASA Langley Research Center was started with the objective to investigate the nonlinear, unsteady aerodynamics and active flutter suppression of wings in transonic flow. The paper will present the flutter suppression control law design process, numerical nonlinear simulation and wind tunnel test results for the NACA 0012 benchmark active control wing model. The flutter suppression control law design processes using classical, and minimax techniques are described. A unified general formulation and solution for the minimax approach, based on the steady state differential game theory is presented. Design considerations for improving the control law robustness and digital implementation are outlined. It was shown that simple control laws when properly designed based on physical principles, can suppress flutter with limited control power even in the presence of transonic shocks and flow separation. In wind tunnel tests in air and heavy gas medium, the closed-loop flutter dynamic pressure was increased to the tunnel upper limit of 200 psf. The control law robustness and performance predictions were verified in highly nonlinear flow conditions, gain and phase perturbations, and spoiler deployment. A non-design plunge instability condition was also successfully suppressed. Introduction The benchmark active controls technology (BACT) and wind tunnel test program at NASA Langley Research Center was started with the objective to investigate the nonlinear, unsteady aerodynamics and active flutter suppression of wings in transonic flow. Under the initial wind tunnel test program, a NACA 0012 airfoil rectangular wing, equipped with pressure transducers, active trailing edge control surface, and two spoilers were constructed for active flutter suppression tests. The model was mounted on a pitch and plunge apparatus in the NASA Transonic Dynamics Tunnel in order to test flutter suppression control laws and measure unsteady pressure distributions in nonlinear flows with oscillating shocks and boundary layer separation. It was necessary to develop a flutter suppression system that would be stable under these flow uncertainties. This paper describes flutter suppression control law design processes using classical and unified linearquadratic Gaussian minimax techniques. A unified general formulation for the linear quadratic Gaussian and minimax methods based on the steady state differential game theory is presented. Lessons learned in evaluating and improving the singular value based multi-input multioutput system robustness are described. Design considerations for digital implementation are outlined. Numerical simulation of the control law performance, and wind-tunnel test results for flutter suppression, are also presented.