The past several years have seen a steady advance in the techniques employed in flutter and vibration analysis. At the same time aircraft performance has been improved on every front and margins have had to be reduced: it is only by making full use of these modern methods that the accepted margins of safety have been preserved.

The paper reviews the processes which are currently used in the prediction and prevention of aircraft flutter in contrast to those of a few years back. The more recent types of flutter which have had to be considered are discussed briefly in relation to the methods which have been developed to deal with them. Ground and Flight testing methods are reviewed together with the use to which the results are put.

Finally there is a discussion of the increasing need to predict sub-critical response. The methods employed are discussed in relation to the more important problems.

I felt very honoured when asked to give this lecture in memory of one of the leading figures of the period which saw the greatest British achievements in aeronautics. The contrast between the technical situation of those times, and the uncertainties of today, immediately sprang to mind, especially in regard to British achievement (or lack of it) in space.

So also, very naturally, did those occasions in the 1940's when I used to visit Mr. Chadwick to discuss the propeller problems of various Avro aircraft, both real and projected. During the course of these sessions, we often discussed future propulsion trends, including the relative merits of piston and turbine engines, propellers and jet propulsion, and so forth. I cannot honestly recall that the question of space flight ever came up, although it probably did, because propulsion is even more of a fundamental problem for flight in space than it is within the atmosphere, and even in those days I was what the Americans call “a space cadet.”

It is an honour, a privilege and a pleasure to deliver the Inaugural Lecture of the Test Pilots’ Group. It is encouraging to see such strong support for this function and on looking round it is only too apparent that there are many more able and deserving pilots present to deliver this lecture.

It is perhaps appropriate, first, to say a few words about the Group itself.

On the 28th November 1963 the Council unanimously approved the formation of a Test Pilots’ Group within the Society. The aim of the Group is The general advancement of aeronautical knowledge in the test flying field. It is hoped that through the Group the test pilot will be able to make a worthwhile contribution to the objects and purposes of the Society, at the same time advancing his own professional knowledge and standing.

In projecting future turboshaft engines for helicopter appUcations the engine designer is faced with the problem of selecting the size of engine to meet the likely size, configuration, performance and economic requirements of future helicopters. It is also desirable to meet as many different helicopter applications as possible with the same basic engine design. The helicopter designer is, likewise, faced with the problem of which engines could be available for the future and how they could best be tailored to his requirements. This is the perennial problem of the “chicken and the egg.”

This paper summarises the results of a preliminary study of the problem, based on some assumed airworthiness requirements and simple engine assumptions and, while it makes no claim to be a method in itself of assessing the problem, it does show the sort of trend which could be expected.

The development of the P.531 family of helicopters covering the period of approximately 5 years is presented. Evolution from the first basic aircraft towards a land-based general purpose helicopter, the Scout, and towards a shipborne helicopter operating from small ships, the Wasp, is presented. The basic design features of the Scout are described briefly as broadly representative of the family. Development experiences are reviewed under the following main headings:—

• (i) Ground resonance investigations.

• (ii) Development of satisfactory flight handling characteristics including engine/rotor control system

• (iii) Substantiation of structural integrity,

• (iv) Development of systems and operational equipments.

• (v) Environmental and climatic testing.

Development of miniaturised auto-stabilisation/autopilot system, for small helicopters, considered to be an important programme, is briefly reviewed. More detailed consideration is given to operations from small ships. The possible design solutions to ensure satisfactory operation from small ships are briefly reviewed, and the reasons for the solution chosen for the Wasp, i.e. four wheel undercarriage are given. Development experiences with the preferred solution are briefly described. It is shown that for operation from small ships in the Naval application a fair degree of complexity is unavoidable independent of the size of a helicopter.

Some statistical data on the development efforts expended are given and some tentative conclusions drawn, aiming at determination of the necessary conditions which would promote a relatively speedy development of a new helicopter.

In some recent impact tests in which thin walled square tubes were loaded axially to produce large scale buckling of the walls, a difference occurred between the behaviour at high speeds and that at low speeds. The tubes, standing on an anvil, were struck by a falling steel tup which was guided to strike the tubes approximately axially. At the higher speeds damage was confined to the impact end of the tube, with a short wavelength buckle adjacent to the end and longer waves farther from the end. At lower speeds damage occurred at either end, or, jn many cases, in the central region of the tube. The wavelengths in these low speed tests were longer than those occurring at high speeds. These two types of behaviour are illustrated on Fig. 1.

In air-tunnel studies it is often necessary to measure the absolute magnitudes of pressure and velocity or to conduct tests at a given Reynolds number. For this purpose it is important to determine precisely the density of the air in the tunnel as a function of temperature, humidity, and barometric pressure. Because of the laborious evaluation involved, the influence of the latter two quantities is usually neglected, despite the fact that it is of the same order of magnitude as the influence of temperature. In the following a nomogram is developed which permits determination of the air-density without time-consuming calculation. The effect of density changes upon pressure and velocity measurements is illustrated by a factor C for the instrument most commonly employed in air-tunnel studies: a Pitot tube as the sensing element and a differential manometer as the recording device. An analogous factor could be derived for other measuring techniques.

A recent paper on the potential incompressible flow through two-dimensional contracting ducts raises some interesting points that are relevant to the hodograph technique for solving this problem.

In this paper, Lau obtains a contraction shape by placing a source in a uniform flow that is contained in a doubly infinite strip. The resulting flow pattern is sketched in Fig. 1. The flow is from a region of unit velocity to one of velocity R, where R is the contraction ratio. A streamline, intermediate in position between ψ = 0 and ψ=1.0, is adopted as the contraction profile.