Introducing Captain FAY, MR FITZWILLIAMS said it would be generally agreed, that very few people, if any, in the world were better able to discuss the subject The lecturer held the rank of Captain in British European Airways, and had served with the Helicopter Unit of the B E A since its inception in 1947, when it was based on Yeovil, and when the Chairman had had the pleasure of meeting him and his colleagues The activities of that Unit had covered a very wide range of helicopter operation, including an intensive pioneering effort in helicopter blind flying

As a Fleet Air Arm pilot he had learned to fly in 1940, and had served for two years in H M S “Victorious” Afterwards he had joined the Naval Service Trials Unit, and was converted to helicopters at the first helicopter training course in this country, at Hanworth, in 1944

The helicopter as we know it is a good example of a potential vibration producing machine It has in it the means for providing vibratory frequencies similar to those which we meet in fixed-wing aircraft and, in addition, it can throw up periodic disturbances having much lower frequencies With this in mind it will be easy to appreciate that the vibrations produced in a helicopter can be a major source of worry to the operator unless they are carefully kept under control

To the helicopter operator, vibratory disturbances mean possible structural failure from fatigue, and probable difficulty in the protection of equipment from the lower frequency disturbances In addition, vibratory sensations have to be contended with in respect of both passengers and crew Many of these sensations come within the aural frequency band, they are all within the frequency range which can be felt, and the lower frequency displacements originating from the main rotor may be seen and followed visually These sensations, if excessive, will result in physical fatigue and discomfort, further they are not conducive to safety

Rotor Unbalance Rotor unbalance is a once per rev impulse This is a case of clear separation of impulse and response Unbalance is a rotating force Its origin can be mass unbalance of blades or geometric dissymmetry in their distribution which can be caused by (a) unequal pitch, (b) errors in the geometry of articulation, (c) unequal damper settings Unequal pitch can occur in spite of equal pitch settmg if the elastic twists are unequal due to unequal pitching moments In practice these are mainly aerodynamic moments which can be corrected with tabs or avoided by superprecision in blade making

Response depends on rotor mountings Rotor r p m must be removed from the natural frequency of the hub on its mountings Subcntical and supercritical mountings are practical Supercritical mountings due to the isolation of the rotor system are mostly introduced for other reasons (discussed below)