Parametric Study of Dynamic Inflow for Coaxial Rotor System

Abstract

The paper presents a comprehensive study of the sensitivity of aerodynamic response predictions of coaxial rotor systems to a wide range of variations in the embedded dynamic inflow model at various flight regimes. The dynamic inflow model is founded on the classical first order system hypothesis and is extracted from a generic high fidelity aerodynamic scheme using the single frequency analysis methodology. The methodology is carrying out by a systematic series of wake excitations and is primarily based on time simulation via a time marching free wake analysis. A free wake model is activated by a sequential periodic rotor loads that excite various periodic responses of the wake. The corresponding time histories of the induced velocity enable the determination of the mass and gain matrices associated with the dynamic inflow model. Unlike existing estimations, the present methodology enables a detailed determination of all elements of the above matrices and shows regions where the first order system behavior, which is a fundamental assumption of the dynamic inflow formulation, is valid. The parametric study presented in this paper deals with both single and coaxial rotor systems operated in hover and forward flight. The paper provides analysis by free wake model and comprehensive aeromechanics numerical solutions in addition to various analytic insights into the associated phenomena and interactions. In particular, the issue of the liftoffset effect on the dynamic inflow characteristics is analytically reviewed along with analytic modelling of the crosscoupling between the rotors in coaxial rotor system that is induced by the dynamic inflow. In addition, the paper addresses the importance of the rotors flapping stiffness and rotors clearance. Wherever meaningful, comparisons with similar prediction of single rotor systems are also presented.