The importance of the flying boat was first generally recognised in England, and no other country can look back on so long continuous and systematic development of building flying boats. For this reason I feel it a special distinction that the Royal Aeronautical Society has asked me to give a lecture here, and I meet their wishes with pleasure.

The chief aim of the vibration engineer is to prevent the occurrence of structural failure due to excessive vibration. A secondary aim of only slightly less importance is to ensure comfort for personnel by the elimination of physiologically unpleasant vibrations; this consideration is a vital factor in the effective design of operational aircraft. The technique of vibration research is largely a matter of detecting and avoiding resonant conditions, and any method of predicting natural frequencies by calculation is welcome if only because it offers the possibility of reducing the practical testing programme.

At some stage in the design of every aeroplane it is necessary to estimate or to measure the resonance modes of vibration. This has not always been the case, but the problems of flutter, control reversal and dynamic loads have increased in importance as speeds have risen. Nowadays, it is an airworthiness requirement that these effects be considered and the aircraft made safe for all conditions of flight. A knowledge of the normal modes of vibration is essential for all accurate estimates of these aeroelastic effects.

Taking flutter as an example, the technique of flutter investigations consists of first determining which combinations of the various possible degrees of freedom are liable to excite dangerous oscillations. Typical degrees of freedom for a wing are bending and twist in each normal mode, aileron deflection and tab deflection; for a tailplane and elevator we might consider tailplane bending or twist, elevator deflection, tab deflection, fuselage bending and twist, and pitching of the whole aeroplane.

The helicopter, because of its complicated nature, provides the mathematician with a delightful set of problems on which to exercise his ingenuity, but the purpose of this paper is to interpret the functioning of the helicopter in terms of its physical aspects.

The paper is divided into three main parts: —

1. 1. General Performance

2. 2. Control and Stability

3. 3. Rotor Vibration Problems.

The present trend of fighter design is towards high speeds at greater heights.

For some time it has been apparent that two types of fighter are necessary. Firstly, the small, highly manœuvrable and very fast single-seater fighter with high rate of climb and comparatively short range, and secondly, a larger type for escort duties which is necessarily somewhat slower and less manœuvrable. As in all engineering, and particularly in aircraft engineering, the conclusions which are reached to arrive at a final design never produce an ideal. If this were possible it would only be necessary to consider one type of fighter, for the small single-seater would have the range of the escort fighter, or alternatively the escort fighter would have the speed, manœuvrability and climb of the small single-seater.

The association of gas turbines with aircraft designed to make effective use of the special characteristics of jet propulsion has brought the trend of development into a state of transition and uneasy experiment. Already speeds equivalent to a low level Mach number of 0.80 have been recorded under observed conditions.