“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.

Since the Armistice the gradual development of the various types of aero engines, together with the abandonment of windmill driven accessories, has necessitated an increase in the number of auxiliaries incorporated in the engine structure, and the engine designers are to be complimented for the ingenuity and compact arrangement of the auxiliary drives and housings on present-day aero engines.

The auxiliary drives for valve operation, dual magnetos, oil pressure and scavenge pumps, water and petrol pumps, supercharger, turning, starting and gun gears, are usually grouped in a housing or housings at the rear of the engine, except that on the radial air-cooled engine the valve gear is arranged at the front and the water pump is unnecessary.

Most engineers have a sound working knowledge of the handling of materials of different kinds, and of their properties, and make use of these wellknown properties. If, however, the engineer attempts to explain any of these properties he frequently finds that his training has not equipped him with an acceptable explanation.

In recent years much work has been done towards explaining these properties, but this work is dealt with in the specialist works of chemists, physicists and metallurgists, and is usually not easily available for the engineer, involving as it does a great deal of reading and investigation.