As in most things, progress in aviation has proceeded in distinct steps. A technical breakthrough resulting in a significant performance increment would be immediately adopted by all designers in the international competition for orders and recognition. One might recall the cantilever wing, the retractable landing gear, the jet engine, the swept wing, the delta wing and the supersonic wing. Performance and economics counted and if there were a few new handling or operational problems introduced— well, they would be attended to in good time. Reduced margins of stability were often accepted by test pilots; partly in their employer's interest, and partly because there is no easy answer to “what's good enough?” Handling qualities did have a substantial impact on economics, of course, but cause and effect were not yet clearly related.

In December 1968, the Boeing Company defined a two-phase programme having the objective of demonstrating for the US Army those characteristics which experience with operation of the CH-47 Chinook had indicated would be required in the next generation of US Army transport helicopters. The programme, which was named the Model 347 Programme, would encompass simulation, wind tunnel, whirl tower, and bench testing and culminate with flight test development and demonstration of a modified CH-47A airframe. The first phase of the programme was aimed at establishing new standards for the flying qualities (handling, vibration and noise) of transport helicopters in a conventional pure helicopter arrangement. The second phase was to allow assessment of the value of providing a high manoeuvre capability for a transport helicopter as could be provided through the use of a fixed wing.

Planning for the development of a future air traffic control system, or for the improvement of an existing system, requires a substantiated appreciation of the capacity and performance of the present system and of the projected future system.

With the growing volume of air traffic and the advancement of modern technology, ATC planners are concerned with making decisions relating to the organisation, development and control of an increasingly complex and sophisticated dynamic system. In many instances it is no longer possible to decide solely on the traditional basis of experience, intuition and “rule of thumb”, especially when the decision involves the selection of the best of several alternatives, involves the employment of very large sums in capital investment and affects the safety and security of large numbers of human lives. Under these circumstances it has become essential to turn to the Operational Research techniques which have been developed and which are now readily available to derive data on which sound decisions may be based.

In addition to direct solar radiation, this paper includes the effect of direct earth radiation, its albedo and shadow on the attitude dynamics of gravity orientated systems, thus extending the analysis of Part I down to the earth's effective atmosphere. The importance of these radiations was indicated by a preliminary force analysis, which revealed that they can exert a force comparable to that of direct solar radiation for close earth satellites. Thus, the need for a rigorous investigation including the effect of earth radiations on satellite attitude dynamics is evident.

Earth radiation forces were obtained in integral form by Clancy and Mitchell using elementary radiation principles. However, in a test on an IBM-7044 digital computer, it was found that the time required to evaluate these integrals renders a study of satellite attitude dynamics virtually impossible.

The von Mises yield criterion for sheet material under conditions of plane stress is given by

where σ1, σ2 are the principal stresses in the plane of the sheet and σy is the yield stress under uniaxial tension. If equation (1) is expressed in terms of elastic strains by virtue of the relations:

where v=Poisson's ratio and E=Young's modulus, it is found that when

the term in ∊1∊2 vanishes and the criterion for the onset of yield simplifies to

Now v* is very close to that of most materials of structural interest.

The stress analysis of an axisymmetrical body under the influence of a field of centrifugal body force is of considerable importance to the design of rotating machinery. The classical problem of the rotating disc of uniform thickness was considered by Maxwell as long ago as 1850. For this, the simplifying assumption that the stresses are functions of the distance from the centre only, with no change across the thickness, was made. Since then, the same basic analysis has been extended to discs of conical and hyperbolical cross-section profile, with or without a central hole, references to which may be found in a number of text books. The analysis of a disc with an elliptical cross-section was published by Chree in 1895. The solution for a disc of constant thickness has been refined to allow stress variation across the thickness. Although these solutions, in common with many analytical solutions in the theory of elasticity, do not satisfy all the boundary conditions exactly, they serve as very good approximations in practice.

In trying to assess the effect of a new material on design there is little past experience on which to draw: the only time when a really new material was introduced was when light alloy began to replace wood as a major structural material. Nevertheless, a few general conclusions can be drawn from this period. Although in the long term considerable changes in structural form may be anticipated, there is unlikely to be any sudden dramatic change in design thinking, rather, such changes will be introduced as a result of continuing development soundly based on previous experience. Improvements in design and analytical techniques will play their part, but the dominant influence will be an increasing awareness and appreciation of the material characteristics; the best ways to use the properties of the material and the manufacturing techniques to which it lends itself.

After an initial evaluation of the carbon fibres and matrix resins available at the time (1968) Hawker Siddeley Aviation embarked, under the sponsorship of the then Ministry of Technology, on the design and development of a number of representative aircraft structural components in order to gain experience of the overall problems involved and to give a lead in determining the direction of the necessary materials and process development programmes. An essential feature of this work was that the components produced should be cleared for extended flight trials so that not only would a good deal of useful service experience be gained but also many principles of quality and process control would be established and a platform for discussions with the various official bodies would be provided. In addition, by using the normal shop facilities for the manufacture of the components, it was hoped to introduce as many company personnel as possible to the new material.

The structural use of fibre composites in helicopters, both for primary and secondary structures, has been recognised for some time. In 1964, Westlands developed a glass fibre composite tail boom and tailplane for the Wasp helicopter (Fig. 1) and the results were encouraging. The non-corrodable structure of the glass composite is considered a great advantage, especially for Naval helicopters. Strength testing of the tail boom was successfully completed with a weight of the component close to the existing metal tail boom, but resonance testing showed that a small additional weight was necessary to meet the stiffness requirements. The tailplane in glass fibre composite showed an appreciable saving in weight and after satisfactorily completing static and fatigue testing, one is now flying on a Service aircraft.