Over the 40-year evolution of the helicopter, man's understanding of vibration control techniques has greatly increased. However, with the performance of helicopters also increasing, the extent of their vibration problems has not diminished. Crew vibration therefore remains a significant factor in the design of all current helicopters. With more complex and critical demands being placed on aircrews it becomes more important that vibration should not impair their performance. Thus, as man's greater understanding of vibration control techniques provides the ability to constrain the vibration the definition of appropriate vibration limits becomes both more necessary and more useful.

Nowadays we enjoy a high level of safety in air transport. One generation ago, in the late 1940s, it was not so good. Two generations ago, when air transport began in the 1920s, safety was by today’s standards quite deplorable. For example, in UK passenger services, fatal accidents in the early 1920s occurred at a rate of one per half million aircraft-miles. In the first half of the present decade, the corresponding rate is around one per 500 million aircraft-miles. This represents an astonishing improvement of the order 1000-fold. To what extent this improvement came about to satisfy public demand, or as a result of pressure from the powers-that-be, or from a professional desire to do an increasingly good job, is hard to say. Probably all these motivating factors played a part.

Reference 1 describes a numerical procedure for calculating the roll-up of the wake behind a wing of finite span, when the wake vorticity is assumed to occur within a layer of finite thickness with thin elliptic cross-section normal to the stream. The vorticity is assumed to be a function of spanwise position only (corresponding to a linear transverse velocity profile when the wake is thin), the process is treated as a two-dimensional unsteady one, as in the work of Westwater, Moore and others, and viscous and turbulence effects are ignored.

In Ref. 1 the technique was applied to the classical elliptic loading, to calculate the very early stages of the roll-up. In the present paper these calculations are extended to larger values of the dimensionless time t* and the results are seen to tally fairly well with those of Moore, which were obtained by using a point-vortex model. Similar calculations have also been carried out for a particular non-elliptic loading which was dealt with by Clements and Maull by a two-dimensional, point-vortex method. They gave this loading as an example of a distribution which produces two roll-up cores on each half span, with an induced drag only 10% greater than that of the elliptic loading having the same Ch. The results of the new calculations are presented here, and are seen to be similar to those of Ref. 4 in a general way, both qualitatively and quantitatively.

The theory of non-stationary motion around aerofoils has many important applications in the field of aeronautics and turbomachines. The most significant problems are flutter, fluctuating loads experienced by blades of a typical turbomachine which move through a non-uniform field of flow, disturbed by the induced effects of other stationary and moving blade rows. It is important to predict the fluctuating lift force on a blade or aerofoil due to a flow disturbance, because both the blade vibration and noise generation are related to fluctuating lift and moment.

In World War II when Spitfires and Mustangs and Messerschmitt 109s and other aeroplanes of that category were quite common, we were all aware of the troubles that these particular aeroplanes got into when starting to dive. When getting up to around 80 per cent of the speed of sound, the aeroplane starts shaking, buffeting we call it; some of them lost ailerons and tails and it proved to be fatal to some of the pilots. It was quite obvious to all people who have an engineering mind that something just was not quite right, especially in the speed range they were flying in those days.

I think I need say nothing more today about why we have a Mandatory Occurrence Reporting system; we have it and it has been working for well over a year. It has had the inevitable teething troubles, and we still have a long way to go before we get the level of reporting right and get the right balance in the frequency and quantity of information we re-publish. Nevertheless we can see many of the aims of the system being achieved or within sight of achievement. What has not changed significantly from the previous voluntary reporting system is the proportion—the very small proportion—of reports which reveal human error, especially reports by people who have themselves made errors. Human nature being what it is, there is a little less reluctance to report other people's errors!

One of my US friends in the forecasting fraternity wrote a paper some years ago which he called ‘The Perils of Prediction'. He concluded, on the evidence of successes and failures, that all air traffic forecasters needed to have their heads examined for staying in the business. So why am I here today to talk about air transport demand in the next quarter of a century? Only because I am totally persuaded that the industry cannot do without forecasts —and, moreover, very long term forecasts—of the future volume and nature of demand. I shall examine this proposition in detail shortly. But, in spite of this conviction, you will note that I have added the words ‘through a glass darkly’ to my title. Those of you familiar with St. Paul's Epistle to the Corinthians will remember the words:

'When I was a child, I spoke as a child, I understood as a child, I thought as a child, but when I became a man I put away childish things for now we see through a glass darkly, but then face to face'.

Considerable public debate has arisen recently over the Warsaw Pact's increasing military capability both in quantity and quality of land, sea and air weapons systems. The Soviet military potential has moved from mainly defensive to increased offensive power and a considerable proportion of Soviet resources is going into military research and development. These developments and trends are of increasing concern to the Western democracies particularly in the light of Soviet communism's constant aims and ambitions.

Professional military correspondents and commentators have been warning us on these trends in Soviet military power for a decade or more but a more public and general concern has arisen only in the last year or two, inspired in part perhaps by the lack of progress in SALT and MBFR negotiations as well as the disappointments arising out of the Helsinki agreements.

I was both honoured and pleased to be invited to give this Mitchell Memorial Lecture. Aside from the personal gratification there were, and still are, two other substantial reasons for my satisfaction:

• (i) Mitchell, and his distinguished contemporary, Camm, are without doubt still the best-known names in British aircraft design. Both were heroes of my boyhood—I had not yet taken my eleven plus at the time of R. J. Mitchell’s death. Later, I worked for 16 years for and with Sir Sydney.

• This lecture therefore provides me with an opportunity formally to express the lifelong respect I have always had, in absentia, for R. J. Mitchell and his achievements.

• (ii) The Southampton Branch Committee were tolerably firm in their invitation: ‘Sea Harrier’, they said. Since at the time Sea Harrier was occupying —and still fills—over 75% of my professional lot, I was specially pleased with the choice of subjects offered.

Throughout the aviation industry the term ‘third level operation’ has many different definitions, ranging from ‘an operator whose aircraft have 12 seats or less’, the definition used in the Edwards Report, to the more general interpretation used in the United States, where the concept is normally that of either a small regional air carrier, operator of feeder liner services, or commuter services.

Current design studies for tomorrow’s aircraft are increasingly utilising composite materials, especially fibre reinforced plastics, in a primary load-bearing role. By suitably orienting and aggregating (stacking) uni-directional pre-pregs (laminae) of these materials, very stiff and very light laminates or sheets may be produced. These two attributes suggest that such laminates are well suited to the role of face sheets (skins) in sandwich panels, which are commonly used in a primary load-bearing context in aircraft construction.

No one, least of all professional pilots as a body, would wish to condone evasion of the issue where a pilot has wilfully or recklessly hazarded his machine and the lives of those on board. Equally there is common ground throughout the industry that air safety demands the rigorous selection of pilots for ability, together with extensive training to create and maintain necessary pilot skills.

Bearing in mind the initial licensing requirements that the State lays down and the regular re-examination that a pilot submits to in competency checks there is, on the face of it, every opportunity for rejecting basically unsuitable applicants and for correcting later shortcomings in the performance of an established pilot.

This seems to imply that we have to look further for the possible origins of whatever comes to be called ‘poor airmanship'.

I am engaged in operating the B 737 mainly in Europe and my employer has utilised another operator's 737 flight simulator with three axis motion for some eight years for the purpose of training. Currently we are using a six-axis motion moving base 737 flight simulator with computer generated image visual attachment.

During this period the ratio of simulator to aircraft training for type conversion has changed progressively from 6 hours simulator and 12 hours aircraft initially, to approximately 32 hours simulator and 3 hours aircraft currently. Similarly the checking pattern has changed from using the simulator for the I/R and one competency check annually to the current practice of completing all refresher training, regulatory authority check requirements and some aspects of command training on the simulator.

Although this paper is intended to contribute to the symposium a view on the subject from within a major equipment supplier to the aerospace industry, the views expressed, whilst based on experience gained from working within the Aviation Division of Smiths Industries at Cheltenham are my own, and do not necessarily represent those of the Company.

'Avionics’ is used in the title for brevity, although if one accepts the definition of avionics as being the application of electronics to airborne systems and equipment, it does not strictly cover the whole range of products with which Smiths Industries Aviation Division at Cheltenham is concerned. However, since electronics form the major part of Head Up Displays and Flight Control Systems, and an increasing proportion of Flight Deck Instruments and Components, it is the most convenient generic term.