Ground and Air Resonance Characteristics of a Soft In‐Plane Rigid‐Rotor System

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Abstract
A flight research program was conducted on a soft in‐plane matched‐stiffness rotor in which it was demonstrated that a soft in‐plane rigid rotor is subject to mechanical instability in flight as well as on the ground. The research included extending existing analysis techniques to include aerodynamic forces. Analytical results correlated well with measured data, and thus provided an analytical tool for evaluating air and ground resonance. Tests included the use of a unique whirl tower stand which was gimbaled to permit pitch and roll motion, and flight tests on an XH‐51A helicopter. It is concluded that positive control of body inertia, and especially of rotor speed limits, are required to insure safety of flight when using n soft in‐plane rigid rotor.

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