Stability and Control of Helicopters in Steep Approaches. Volume 1. Summary rept.

Abstract
Aerodynamics and control characteristics of helicopters in level flight and steep descents at low speeds are analyzed. Single-rotor, tandem- rotor, and compound helicopters are considered and are specifically represented by the S-58, AH-56A, and YHC-1A, for each of which derivatives and transfer functions are presented. New analytic methods are used to describe the flow about descending rotors. Using these methods, predictions are made of the boundaries of the vortex-ring state and of tip loss factors. Good agreement with experiment is obtained. A new modular stability derivative program, MOSTAB, is described and is used to calculate derivatives for the S-58 and AH-56A, including lateral-longitudinal cross-coupling derivatives. It is shown that, for these helicopters, control of flight path by collective pitch becomes difficult for low-speed steep descents even with stability augmentation, due to a nonminimum phase transfer function and the associated wrong-way step response characteristic. The effect is identified as an important factor in the observed degradation of flying qualities in steep approaches.

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