The computer can be used in many ways to assist in the project and design stages of aircraft development. As would be expected it was used first to assist in the more unwieldy calculations of the design stage. Massive calculation sequences which previously involved much tedious labour, or were effectively beyond the capacity of a design office to perform, suddenly became a few hours work for one or two technicians. Project analysis, being concerned with an earlier stage of development where calculations are simpler and more approximate, tended to remain largely unaffected. However, it became clear that the very simplicity of the preliminary project calculations would, at least in principle, allow the entire project analysis phase to be computerised, so allowing many parametric variations to be done in a short time with minimum effort. The natural follow-on to this was the optimising project analysis program of which examples now exist. With such programs one is able to predict an optimum configuration to meet a given requirement, given only the basic data to define the scope of the calculation.

The problems involving vibration which affect helicopters can be divided into two major categories:

• (a)Forced vibration effects

• (b)Aeroelastic and aeromechanical instabilities.

Forced vibration effects are subdivided into two categories.

• (i)Effects due primarily to aerodynamic forcing; and

• (ii)Effects due primarily to mechanical forcing.

Forced vibration effects have important repercussions in the area of:

• (i)Crew comfort and efficiency

• (ii)Fatigue strength of dynamic components and the airframe

• (iii)Reliability of electronic equipment.

The most important forcing effect is aerodynamic in origin, and this will be considered first.

Modern technology places great demands on structural material performance. Until the 1950s, structures were invariably designed according to safe-life principles, for which a crack-free service life was required and assumed. However, the tendency to much longer utilisation of aircraft, and latterly the introduction of high strength materials, have focused attention on damage tolerant design in which allowance must be made for possible cracking and its certain detection before catastrophic failure. Especially for high strength materials is this change in philosophy necessary, because the improved static strengths (and hence potentially increased structural efficiency) rarely result in comparable improvements in fatigue properties, and are usually accompanied by a decline in fracture toughness and in greater susceptibility to environment-induced cracking.

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.

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.