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 study, dynamic stall and dynamic wake effects are incorporated in a coupled aeroelastic analysis to investigate blade sectional loads, hub loads and trim condition of the helicopter. 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 influence of aerodynamic modeling on the trim condition and aeroelastic response of the rotor blade in forward flight has been brought out. It is found that the aerodynamic model incorporating dynamic wake and dynamic stall effects predict the trim parameters whose variation with forward speed resemble qualitatively similar to those obtained in flight test. A comparison of variation of blade sectional lift for various aerodynamic models indicates that in the advancing side of the rotor, dynamic stall effects introduce a shift in the azimuth angle at which the minimum lift occurs. It is also shown that the structural coupling due to blade pretwist significantly influences the rotor blade response and loads compared to an untwisted rotor blade. Copyright 2008 by the American Helicopter Society International, Inc. All rights reserved.