Aerodynamic active control for fin-buffet load alleviation

The efficiency of an active auxiliary rudder system in alleviating fin buffeting of modern fighter aircraft is investigated. Low-speed wind-tunnel tests are performed on a detailed 1/15-scale model of canard-delta-wing type. A digitally controlled auxiliary rudder is installed on a specific fin model providing harmonic oscillations at varying frequency and deflection angle. The vertical tail is instrumented to measure unsteady surface pressures, fin-tip accelerations, and auxiliary rudder moments. At open-loop tests the fin unsteady pressure field is fed with energy at the frequencies of the auxiliary rudder motions. The corresponding rms values increase with increasing frequency and deflection angle at all angles of attack tested. The motion-induced rms pressures reach values well above the levels of the nonoscillating case. Thus, a potential to reduce rms buffet loads by approximately 18% for closed-loop operations exists. There is no decrease in the rudder moment with increasing angle of attack substantiating the effectiveness of the auxiliary rudder concept also for high angles of attack. The active control system uses single-input single-output control laws to alleviate buffeting in the fin first bending and torsion mode, respectively. With active control the spectral density peaks of fin-tip accelerations at the frequencies of the considered eigenmodes can be reduced by as much as 60% at angles of attack up to 31 deg.