The purpose of this paper is to give a fairly detailed description of where carbon fibre composites are most likely to be used in the structures of future military aircraft together with an indication of the structural configurations.

Criteria for the design of helicopter rotor blades has been long established but the introduction of composite materials opens up new means of achieving these.

As an introduction to the subject I would like to look briefly at the design considerations under the headings:

1. 1. Aerodynamics

2. 2. Dynamics

3. 3. Strength

Structural integrity is a key ingredient of aircraft design. It takes its place alongside flight performance, operational effectiveness, reliability, maintainability and economy in determining, or contributing in very large measure to, the success of a project in service.

What is expected from the contractor for a new project in respect of these qualities is set down by the customer in a specification which includes reference to a set of mandatory design requirements. Thus, flight performance parameters such as speed, range and payload, are defined in the specification : there too is to be found a description of the sorties to be flown, numbers and types of manoeuvres and their severity (conventionally including the levels of normal acceleration) and the regions in which the aircraft is required to operate. Sufficient detail is given to establish, as the process of design evolves, the loading and environmental conditions to which the structure will be subjected.

The high values of specific strength and stiffness of carbon fibre composite materials have already been discussed by Mr. T. Sharpies in his paper and require no further comment. Therefore, I would like to consider the benefits to direct operating costs (DOC) which can be obtained by the application of Carbon Fibre Composites (CFC) to a civil transport aircraft.

Figure 1 is taken from a paper presented by Stewart and Campion at the Atlantic Aeronautical Conference last year. In this figure are shown the benefits in DOC which arise due to the application of the following advanced technologies:

Design studies have shown that the successful application of carbon fibre composites to suitable components in the RB 211 engine nacelle could achieve initially a weight saving of 200 to 250 kg per power plant.

In the future the employment of the long mixed flow nacelle, and possibly higher by-pass ratio cycles, aimed at optimising engine design for maximum fuel and operating economy, will emphasise even further the need for lightweight nacelle structures.

The interesting article by Archer, Sapuppo and Betteridge leads to the problem of determining the optimum lift distribution along the wing during flapping motion. Adopting the assumptions made in Ref. 1, the problem may be reduced to one of minimising the induced drag for a specified, periodically varying, bending moment at the wing root. The wing-root bending moment should be of primary importance to a bird or, for that matter, to a mechanical flapping-machine, since it is at this point that most of the flapping energy is introduced.