The Vibration Characteristics of a Coupled Helicopter Rotor-Fuselage by a Finite Element Analysis.

The dynamic coupling between the rotor system and the fuselage of a simplified helicopter model in hover is analytically investigated. Mass, aerodynamic damping, and elastic and centrifugal stiffness matrices are presented for the analytical model; the model is based on a beam finite element, with polynomial mass and stiffness distributions for both the rotor and fuselage representations. For this analytical model, only symmetric fuselage and collective blade degrees of freedom are treated. Real and complex eigen-analyses are carried out to obtain coupled rotor-fuselage natural modes and frequencies as a function of rotor speed. Vibration response results are obtained are the coupled system subjected to a radially uniform, harmonic blade loading. The coupled response results are compared with response results from an uncoupled analysis in which hub loads for an isolated rotor system subjected to the same sinusoidal blade loading as the coupled system are applied to a free-free fuselage. It is shown that although the fuselage response as a function of forcing frequency and rotor speed is similar in the two cases, the responses resulting from the approximate, uncoupled analysis are significantly greater than those resulting from the coupled analysis.