Abstract :
Autorotation is an abnormal mode of operation for a helicopter because no powerplant torque is applied to the main rotor. Thus, descending flight requires an upward flow of air through the rotor to sustain rotation and, therefore, lift. There is a negligible literature on the flight mechanics of helicopter autorotation, although the aerodynamic phenomenon is well understood. The objective of this paper is, therefore, to examine helicopter flight mechanics across the autorotation flight envelope, while addressing the modeling requirements for simulation in autorotation. A nonlinear individual blade\blade element model of a conventional single main and tail rotor helicopter is used to generate a variety of data, including trim states, time, and frequency responses. It is concluded that contemporary mathematical modeling can mimic the general performance characteristics of helicopters in autorotation with no special development. Although rotorspeed can vary signicantly, even during maneuvers that embody only small perturbations in the body states, linearized models can remain an appropriate basis for analysis. Finally, distinctive aspects of helicopter flight mechanics in autorotation, dissimilar to level flight, are readily explained, and it is suggested that they are benign in nature.
