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

On behalf of our Association, I have pleasure in extending the warmest welcome to our guests, and in particular to our guest of honour who is to lecture to us on the Flight Testing of Helicopters.

Mr. Frank O’Hara is an M.A. of both Cambridge and Edinburgh, and is a Principal Scientific Officer of the Ministry of Supply stationed at the Airborne Forces Experimental Establishment at Beaulieu.

He joined A.F.E.E. in 1942 on leaving his previous appointment at the Marine Aircraft Experimental Establishment at Helensburgh. During the last 8 years the A.F.E.E. have increasingly looked to him for the solution of their research problems and as a fertile source of inspiration in overcoming the many difficult practical problems which the Establishment has found.

In the course of its work the A.F.E.E. has been responsible for developing the technique, and the necessary background of theoretical and practical knowledge, required for the flight testing of helicopters for the Ministry of Supply.

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

Prototype helicopters at the Bristol Aeroplane Company have been produced and servicing schedules have been made up detailing the maintenance work and inspection that has to be carried out after the following periods : (a) between flights, (b) daily, and (c) after 30, 60, 120 and 240 hours flying.

At the time these schedules were made it was not fully realised that the work would not be carried out exactly in this way.

We now learn that the airline operators are dividing their 30, 60, 120 and 240 hour inspection and maintenance schedules into small parts. One of these parts is added to the routine daily inspection, and so, by the time 240 hours of flying have been completed the whole of the interim schedules have also been completed, without withdrawing the helicopter from service. This system can only be operated providing that all work in the schedules can be split into tasks of less than, say, three hours each; there must, of course, be exceptions as obviously the overhauling of an engine takes longer than three hours.

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)

In preparing this paper it was obvious to the writer, from a perusal of the titles of the other papers arranged for this meeting, that some of the subjects discussed will have been dealt with in some measure by previous speakers, as this paper also deals with the Westland-Sikorsky S-51 helicopter. Where such repetition occurs the writer tenders his apologies.

The Westland Sikorsky helicopter is well known in this country through its commercial activities over the past three years, and more recently its application to the Navy and R.A.F., whilst its American opposite, the Sikorsky S-51, has accumulated many thousands of flying hours to its credit, proving that the design has dependability and adaptability for the many roles of operation it has to meet. A close liaison between the American and British makers ensures the best possible service to all its operators.

The Westland-Sikorsky S-51 inherits to a large degree the same qualities as the American model, and in discussing its maintenance many of the practices used are common to both models.

In the field of commercial aviation, the Helicopter is a comparatively new vehicle and, to my mind, one requiring considerable development in respect of servicing, maintenance and overhaul.

As we are all aware, serviceability has an important influence on an aircraft’s revenue flying time. Therefore, I think that it is essential for us to grapple with maintenance problems at an early stage and try to place them in their true perspective.

From my point of view, as an engineer, it is most important to use the aircraft at a high “Utilisation Rate,” without impairing its safety.

It will be appreciated that, in this short paper, only brief reference can be made to various aspects of importance to the design and performance of rotary wing aircraft. Nevertheless, it is suggested that these references will serve as a basis for discussion, as well as to emphasise the necessity for more investigational work.

Because of the rapid development in the “science” of rotary wing aircraft, the subject of materials of construction will become increasingly important and most probably difficult as the designers’ demands and desires increase. Today, therefore, any discussion on materials can only be based on current design knowledge and requirements, with indications to the materials which may be considered most suitable for future projects.

Bearing in mind such limitations in scope, together with the fact that a certain amount of choice of material is left to the designer, it is proposed to refer to various aspects of materials and their selection which, in the opinion of the author, warrant the attention of those concerned with the responsibility of design development and research.