Improved sine-trim2 model for the simultaneous elimination of aerodynamic lift, roll, and pitch moment vibrations of rigid rotor helicopters

Abstract

An improved sine-trim method called sine-trim2 is proposed from the original sine-trim method, based on the phasor superposition principle, to simultaneously eliminate the aerodynamic lift, roll, and pitch moment vibrations of rigid rotor helicopters. The advancing blade concept rotor aerodynamic analysis model is modified and applied on the basis of the free-wake method associated with the inner-trim and outer-trim modules. A hybrid individual blade control system with the blade pitch and linear twist is applied to simultaneously adjust the blade lift and lift arm at each azimuth angle. In the sine-trim2 method, the number of rotor blades should be greater than or equal to four to successfully apply the phasor superposition principle for all kinds of aerodynamic vibration phasors of each blade. Two kinds of lift arm distributions (constant lift arm and sinusoidal lift arm) in one revolution that can both meet the requirements of vibration elimination are considered. Calculation results show that although the sine-trim2 method can eliminate the aerodynamic lift, roll, and pitch moment vibrations of the rigid rotor with either lift arm distribution. A proper sinusoidal lift arm as the control target will have lower control difficulty and efficiency loss than the constant lift arm. The optimal individual blade pitch and twist control distributions, which are both comprehensive superpositions of many higher harmonic inputs, are obtained in each azimuth in one rotation revolution.