High-Fidelity Linear Time-Invariant Models for Higher Harmonic Closed-Loop Rotor Control Studies

摘要

Linearized time-periodic models are extracted from a high-fidelity comprehensive nonlinear helicopter rotor model at a trim flight condition. Subsequently, a Fourier expansion-based model reduction method is used to obtain linearized time-invariantmodels fromthe time-periodic models. These linearized models predict the effects of on-blade control systems such as active flaps on the rotor hub loads and hence are an important component required for vibration control and flight control interaction studies. The linearized models are verified against the nonlinear model at low-speed descending and cruise flight conditions. Inclusion of aerodynamic states corresponding to the unsteady aerodynamic model reduces the error in the peak-to-peak hubload amplitude predictions from over 80% to less than 10%. The time-periodic and time-invariant models are in excellent agreement. The linear time-invariant (LTI) models are also verified for closed-loop vibration reduction performance fidelity using active flaps activated through the higher harmonic controller. The flap deflection histories and the vibratory loads predicted using the LTI model and nonlinear model agree very well when the flap deflection is limited to 2 degrees. The errors between the two models increase with the flap deflection amplitude.