Pitch-Flap-Phase Coupling. A Preliminary Analysis of an Evolving Concept of Helicopter Autostabilization

摘要

A six degree of freedom model of helicopter dynamics is presented based on Newtonian mechanics, with the acceptance of quasi-steady rotor forces and moments referred to the flight state invariant shaft axis reference system. Use of the classical small disturbance theory allows linearization, while partial differentiation furnishes all rotor forces, moments and state derivatives. The modification of rotor state derivatives with introduction of pitch-flap-phase coupling is presented. Modal analysis is effected primarily longitudinally, based on a weakly coupled systems hypothesis necessitated by low directional stability attributable to tail rotor omission. Overall asymptotic longitudinal stability of the system is achieved, subject to a future rotor stability analysis, a precondition being the reversal of the natural rotor derivatives by use of sufficient negative pitch-flap coupling. Phase lag/lead of 170-190 deg and -0.5 coupling gain infers maximum positive damping, typically 20 sec to half amplitude in the most weakly damped mode, ensuring at the same time the maintenance of the control derivatives at the desired unity level.