Aeroelastic testing for freeplay induced limit cycles of flexible wing-aileron system

Control surface flutter or limit cycle oscillations (LCO) of civil aircrafts is significant aeroelastic instability, which may occur at a speed well below the flutter clearance envelope. Freeplay has been a source of control surface vibration and structural damages, which could result in reduced aeroelastic stability margin. This paper presents a low speed wind tunnel testing of a three-dimensional rectangular wing model. The half span model is constructed with linear wing and aileron surfaces. Adjustable nonlinear freeplay is introduced into the aileron deflection leaf spring. The experimental aeroelastic model is tested in wind tunnel up to the LCO or critical flutter wind speed. Data acquisitions are arranged for acceleration signals of wing surfaces and aileron deflection angles. The oscillation amplitude, frequency and damping of the flutter modes are further delivered from the frequency spectrum analysis. Limit cycles jumping and hysteresis induced by freeplay have been observed with wind speed increments. A combination of three wing-aileron deflection stiffness and four freeplay variations on flutter characteristics are evaluated. Through the testing, we obtain a good observation of flutter and limit cycle phenomena on the control surface. The oscillation amplitude of the control surface increases with the wind tunnel speed at each deflection freeplay. The correlations of freeplay with critical oscillation speed, besides oscillation amplitude are explored. Linear regression analysis of the above variables is illustrated, based on which the control surface divergence speeds for other freeplay are predicted within speed envelope. Verification practices for freeplay induced limit cycle oscillation are recommended for civil aircraft certification.