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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.

The potential sources and paths by which the propeller produces structural responses resulting in vibration and noise in the cabin of a transport aircraft are discussed. New low-cost, convenient experimental and analytical techniques are described for evaluating the excitations-propeller airborne pressures on the fuselage shells, slip-stream-induced forces on the wing and tail, and oscillatory forces on the propeller. The techniques described make use of ground-determined structural signatures to relate forces with vibrations or noise, and of propeller signatures from flight tests which define the vibroacoustic contributions of individual propellers. Knowing these, the propeller-produced excitation forces can be deduced by the relations shown; design approaches to control the fatigue and vibroacoustic environment can then be enunciated logically.

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