Design Sensitivity Analysis for Helicopter Flight Dynamic and Aeromechanic Stability

Abstract

An efficient technique is described to calculate the sensitivities of bandwidth and phase delay (defined according to the ADS-33 specification) and poles of a helicopter with respect to the blade torsion stiffness GJ. Thetechnique is based on the derivation of expressions for the sensitivities using chain rule differentiation of appropriate portions of the equations of motion. Two configurations are studied, similar to the Eurocopter BO-105 and the Sikorsky UH-60. The study shows that the semi-analytical sensitivities are in excellent agreement with the corresponding finite difference-based sensitivities and that they are far less sensitive to step size. For the BO-105 configuration, phase bandwidth, gain bandwidth, phase delay, and poles are only weakly nonlinear functions of the torsion stiffness GJ, and therefore, linear approximations are accurate for broad variations of GJ. The same is mostly true for the UH-60 configuration, except that move limits are recommended for linear approximations to the phase delay at low GJ. The new technique is computationally very efficient. The additional cost of calculating the sensitivities is 4% of that of a complete linearized analysis, regardless of rotor configuration and flight speed. The figure is expected to apply also to sensitivities with respect to flap and lag stiffness.