The overall objective of design against fatigue is to achieve adequate safety and reliability for minimum structural weight and, having this end in view, the fatigue performance of an aircraft structure is a major consideration in the design, testing and operation of an aircraft. This has not always been the case, indeed until the early 1950s the design of aircraft structures was based almost entirely on strength considerations with little regard to fatigue. However, in the decade following the Second World War a series of accidents both in the military and civil field, culminating in the Comet disasters, focused attention on the need to consider the problem of fatigue in design and provided the climate of opinion to support requirements for much more extensive and expensive full scale fatigue testing than had been contemplated hitherto. Since that time, advances in safety and reliability have been achieved in the face of customer requirements for major increases in overall fatigue life and for higher standards of day-to-day reliability of the structure. For example, in the military field a required fatigue life exceeding 6000 hours would not be exceptional now for a combat aircraft (Fig. 1) as compared with a life in service of about 1500 hours for such an aircraft in the years immediately following the Second World War. On the civil side, whereas lives of about 20 000 hours would have been considered adequate in the early 1950s, operators now require lives in excess of 60 000 hours. The European Airbus shown in Fig. 2 typifies such aircraft currently in service. Both military and civil aircraft are now expected to be in service for 20 or more years.

The emergence of active control technology (ACT) as a design feature of future combat aircraft raises three fundamental requirements (a) the identification of those areas where active control is likely to make for more efficient designs (b) the development of a capability to produce system hardware and software satisfying the stringent integrity requirements which the envisaged ACT applications demand, and (c) the development of a capability to implement successfully the ACT benefits, emerging from (a), by means of appropriate control laws.

This paper is concerned primarily with Item (c). Subjects considered include the design of control laws for fast target acquisition and precise tracking, the design of ride-smoothing systems, control laws for manoeuvre limiting at high angles of attack, and flying qualities problems associated with aerodynamically-unstable air-craft.

The design of control laws takes place through a series of steps including initial computer-aided studies followed by ground-based simulation with the pilbt-in-the-loop and ultimate validation (probably with re-tuning of control-loop parameters) through flight test.

No eighteenth century designer of flying machines surpassed Melchior Bauer in imagination and aerodynamic insight. Born on 19th October 1733 at Lehnitzsch, a village near Altenburg, he was the son of a gardener, Hans Bauer'1'. Little is known of his life, but in 1763 he went to London, where he sought patronage from the recently crowned King George III for the construction of a man-powered aircraft. According to his own account, he got no further than the official scribe who refused to copy out the submission, saying that even if he were offered £500 he would never agree to send such folly to the Court. ‘If you could do all that you claim,’ said the scribe, ‘you would certainly be the foremost and greatest man on earth.’ [1 r](2).

An experimental study was undertaken to measure the instantaneous lift and propulsive forces generated by a rigid, sinusoidally flapping wing at Reynolds numbers ranging from 72 000 to 120 000. The wing was flapped about a fulcrum at the root at frequencies of 2.55 Hz and 3.30 Hz. Instrumentation and a data reduction system were developed to provide quick and reliable assessment of the effects of changes in flapping amplitude, flapping frequency, flapping plane angle, wing incidence, and windspeed. In this way it was attempted to model, however crudely, the forward flapping flight of birds.

The instantaneous values of lift coefficient and thrust coefficient were plotted against position in the cycle while the average CL and CT were plotted against advance ratio (ratio of the windspeed to the maximum tip speed velocity). The experimental results were compared to the theories of Walker(e) and Hoist ' Kuchemann(4) both modified to correspond more closely to the experimental conditions.