A presentation of the solution of linear equations with complex coefficients is given, in which current checks can be introduced to obviate inaccuracies and thus engender confidence in use. The method is based on the elementary concept of elimination of each unknown in turn, and details only of its application are given since the theory demands lengthy exposition. Evaluation of determinants is also illustrated.

“Weight” is the key to everything that matters in aircraft construction. It is of paramount importance to performance, comfort and safety. Incidentally, it is also many a designer's excuse for any shortcoming on his machine.

Analysing the weight figures for an aircraft, one can classify them into three groups:—

1. 1. Weights over which the designer has no influence.—Namely, engines and accessories, airscrews, instruments, wheels and brakes, etc. This group represents about 35 per cent. of the empty weight of a normal commercial machine.

2. 2. Weights over which the designer has a limited influence.—Namely, fuel tanks and pipes, cowlings, engine controls, cabin equipment, upholstery and windows, heating, etc., which represents some 20 per cent. of the empty weight.

3. 3. Weights within the designer's influence.—Namely, wings, fuselage structure, tail unit, chassis, flying controls, etc., which represent about 45 per cent. of the empty weight.

The most powerful theoretical tool in the solution of the aerodynamic problems of aircraft is the theory of small perturbations, which states that if a wing is thin (or a body slender), and if the incidence is small, then in inviscid flow the fluid velocity at any point can be treated as a small perturbation from the stream velocity. The backbone of our knowledge of the aerodynamics of aircraft is provided by this theory, to which the effects of thick wings and large incidences, and the effect of viscosity, introducing as it does the concept of boundary layers, can be added as additional or correction effects. It is known that at subsonic and again at supersonic speeds, the theory of small perturbations is a linear theory; that is, the assumptions implicit in it lead to a linear partial differential equation for the velocity potential, with linear boundary conditions.

An investigation into the preparation of Data Sheets based on the work of Kuhn and Griffith at the N.A.C.A., revealed that it might be possible to present non-dimensional curves for the ultimate shear strength of aluminium alloy webs in incomplete diagonal tension, in a form similar to that of Structures Data Sheet 02.03.13, which are independent of material specifications. The values of “basic allowable shear stress” are founded on flat plate details from early experimental work and on estimated values for fully developed diagonal tension. Later work by Ross Levin provided further test results extending the range of diagonal tension developed, and the application of these to thick webs has been confirmed.