Ground Resonance Analysis for an Eight-Degrees-of-Freedom Rotorcraft with Double-Stage Oleo-Pneumatic Shock Absorbers

摘要

In rotorcrafts, the ground resonance may arise from interaction among lead-lag motion of the rotor blades,nfuselage inertia, and stiffness and damping characteristics of the shock absorbers in landing gears. Approximatenprediction of the ground resonance is often conducted by linear modeling and analysis of characteristic roots of thensystem, although the effect of nonlinear quadratic damping was discovered recently by applying harmonic balancenmethod. Double-stage oleo-pneumatic shock absorbers, employed for softer landing and more comfortable taxiing,nshowvery strong nonlinear stiffness characteristics depending on dynamic amplitude,which is not known a priori. Innthis study, how to deal with this nonlinear pneumatic stiffness problem by the harmonic balance method in anniterativemanner is presented for an eight-degree of freedomrotorcraft systemand simulation results are discussed inntwo aspects.One is the necessity of the nonlinear treatment of the dual-stage pneumatic stiffness relative to the linearnstiffness approximation.The other is howto choose the ratio of pneumatic pressure in two chambers or location of thenbreakover point relative to the static equilibrium point.