Publication

Abstract : Flight test data of helicopters indicate that vibratory levels in the fuselage exhibit a wide spectrum of frequencies including the dominant blade passage frequency and its integer multiples. The present work attempts to understand the reason for the existence of several frequencies in the response of the fuselage and possible cause for this observed phenomenon by formulating a computational aeroelastic model. In this theoretical study, a systematic approach has been undertaken to identify the effects of inflow modeling and sectional aerodynamic load evaluation, on helicopter trim, rotor blade response, and hub loads. Five different combinations of aerodynamic models of increasing complexity, representing rotor inflow and sectional aerodynamic loads, have been proposed. The differential equations of motion are solved in time domain in a sequential manner to obtain the response of all the blades in the rotor system, the inflow variables, and the sectional loads at every time step. The results of the present study show that the aerodynamic model incorporating dynamic wake and dynamic stall effects introduces a wide spectrum of harmonics in the hub loads including blade passage frequency and its integer multiples. The influence of aerodynamic modeling on the variation of trim parameters with forward speed has also been brought out. It is observed that the aerodynamic model incorporating dynamic wake and dynamic stall effects predicts the trim parameters whose variation with forward speed resemble qualitatively similar to those obtained in flight test. A comparison of the variation of blade sectional lift for various aerodynamic models indicates that in the advancing side of the rotor, a dynamic stall model introduces a shift in the azimuth angle at which the minimum lift occurs. The effect of structural flap—lag coupling due to blade pretwist on trim and rotor loads has been studied, and these results are compared with those pertaining to a straight blade configuration.