DISCRETE-TIME, CLOSED-LOOP AEROMECHANICAL STABILITY ANALYSIS OF HELICOPTERS WITH HIGHER HARMONIC CONTROL

This paper presents an aeromechanical closed loop stability and response analysis of a hingeless rotor helicopter with a Higher Harmonic Control (HHC) system for vibration reduction. The analysis includes the rigid body dynamics of the helicopter and blade flexibility. The gain matrix is assumed to be fixed and computed off-line. The discrete elements of the HHC control loop are rigorously modeled, including the presence of two different time scales in the loop. By also formulating the coupled rotor-fuselage dynamics in discrete form, the entire coupled helicopter-HHC system could be rigorously modeled as a discrete sys-tem. The effect of the periodicity of the equations of motion is rigorously taken into account by converting the system into an equivalent system with constant coefficients and identical stability properties using a time lifting technique. The most important conclu-sion of the present study is that the discrete elements in the HHC loop must be modeled in any HHC anal-ysis. Not doing so is unconservative. For the heli-copter configuration and HHC structure used in this study, an approximate continuous modeling of th