Seaspeed was established as a wholly owned subsidiary of British Railways to operate high-speed non-conventional marine craft, concentrating initially on the SRN4 hovercraft. As a first preliminary it was decided to establish an SRN6 route across the Solent to gain operating experience. Since those early days, with two SRN6s, two HM2s and two SRNs the company has ferried 1 026 595 fare-paying passengers a total approaching ten million passenger miles.

The Solent was chosen for a number of reasons. It was the birthplace of the hovercraft and manufacturers were all readily at hand. The weather conditions in relation to the size of the N6 were judged to be representative of the weather conditions facing the N4 on the Channel. Further, as we saw that successful hovercraft would have to operate in crowded terminal conditions, operating in and out of Cowes would give us valuable experience that we could transfer to N4 operating. Our first N6 route was, therefore, selected as Southampton/Cowes.

Over the past twenty years a large part of the activity of Turbomeca has been directed first to the creation and next to the subsequent development of propulsion units for helicopters.

The use of the gas turbine brought with it a fundamental advantage—low specific weight which led to the exceptional expansion of this specialised branch of the aircraft industry, and without wishing to minimise the constant improvement of rotor systems, we believe that above all else the qualities of the engine have played a significant part in the success of this system of transport and in the more specific success of this or that particular helicopter.

The sound of a rotorcraft is very complex in nature since it is a combination of the sound produced by several individual sources, which in the main generate acoustic energy by more than one mechanism. Externally, the important sources are rotor noise and engine noise; while internally transmission system (gear) noise is an additional source. The frequency ranges associated with the various sources on a rotorcraft are summarised on the narrowband spectrum shown in Fig. 1. Main rotor noise is treated as two separate entities, rotational and broadband noise, while the tail rotor noise (if applicable) is dominated by its rotational components.

The following discussion is limited to gas-turbine-engined rotorcraft as, except for some of the small light-weight helicopters, the majority are powered by this type of engine. It is worth noting that on a piston-engined helicopter, the engine itself is the main noise generator, with the primary source being the exhaust noise emanating from the periodic expulsion of the hot gases of combustion. This produces harmonics at the engine firing frequency as indicated on Fig. 1, and in addition to being of high amplitude is very annoying since it occurs in the 200-1000 Hz range.

My connections with,the helicopter industry covered a period of some ten years, working principally on the jet reaction rotorcraft Fairey Rotodyne, and my only regret about this exceptional aircraft was that it never entered airline service. VTOL/STOL projects currently being bandied around today for civil use appear to'have made very slow progress in noise reduction since that time.

It was on this aircraft that I really got to grips with the noise problem, and with this experience behind me, my present task with Rolls-Royce became much more invigorating and refreshing. If for no other reason than, if you generate the noise, you are the principal agency which is required to set about removing it.

My present paper is really a factual presentation of the Ansty Noise Facility and its relationship with the general lowering of sound levels of the next generation of gas turbine engines. It is therefore slanted more to the fixed wing CTOL type of aircraft powerplant, although VTOL/STOL applications are now being studied at scale size.

The need to understand the mechanism of generation and to be able to predict the noise of aircraft has increased enormously in the present decade. The questions which the aircraft designer now poses to the noise engineer are much more difficult to answer than was the case in 1964 when rotorcraft noise was discussed by this Section. For example in the case of conventional transport aircraft at the beginning of this decade it was only necessary to show that on take off the peak noise did not exceed 110 PNdB at a predetermined monitoring point at certain airfields. With the advent of noise certification it is now necessary to know the complete noise time history of the aircraft for points under the take off path, at the side and on the approach before the aircraft flies or the engine is made. The increased emphasis on noise research and the resulting developments in the theory and practice is not surprising.

Practical problems often arise in which the aerodynamic loadings on wings and fins, due to nearby free vortices, are required. For example, aerodynamic loads on helicopter blades are affected by the tip vortices which are continuously being shed and which lie beneath the rotor plane. And vortices shed from the nose of an aircraft fuselage can induce significant side loads on a fin.

The estimation of the aerodynamic load distribution on a moving wing in the neighbourhood of a free vortex (whose axis is primarily in the free stream direction) is an interesting problem which, as far as the author is aware, has not been discussed previously in the literature. Standard techniques for conventional linearised wing theory cannot be applied directly to this problem because the trailing vorticity shed from the wing trailing edge is affected by the flow field of the free vortex and so the mathematical application of the Kutta trailing edge condition is more complex.

The problem of vibrations of a circular ring on an elastic foundation finds application in the study of vibrations of gears mounted on elastic shafts and launching vehicles having a ring-stiffened shell structure.

The inplane flexural vibrations of a circular ring were investigated by many authors in connection with the study of vibrations of gears and frames of electrical machines. The transverse vibrations of a thin ring of circular cross-section were first investigated in 1890 by Michell. There appears to be less amount of work in the literature which deals with the effects of rotatory inertia and shearing deformation on the transverse vibrations of rings. In the present investigation, flexural vibrations of a circular ring on an elastic foundation involving both displacements at right angles to the plane of the ring and twist are studied. The foundation is considered to provide restraints against the transverse and torsional motions of the cross-section of the ring linearly.

In 1969, the writer, as a member of the panel on studies of the economics of route facilities of the International Civil Aviation Organisation (ICAO), submitted a paper about costs allocation of 13 international Canadian airports including six alternates to the North Atlantic Service. The data in the paper might be used as a numerical illustration for costs allocation at international airports to the domestic operation and of the alternate airports as well.