There are various considerations which govern the use of materials in engineering construction. The use of many of them is founded on precedent, and in the case of the older branches of engineering the original reason for their use is sometimes obscured.

From time to time as new classes of materials are developed they aregradually incorporated, often breaking their way with difficulty through the wall of prejudice with which engineering caution so often surrounds its designs.

The problems connected with the material used in the construction of aircraft are very numerous, and the time available in one lecture is far too short to consider them in any kind of detail. It seems to be more appropriate to approach the matter broadly and to consider some of the basic problems with a little thoroughness than to examine a number of details in a superficial manner. The main objection that can be urged against this method is that the conclusions reached are by no means confined in their application to aircraft materials, but apply almost equally to most of the arts in which the same or similar materials are employed. After all, the aeronautical engineer is mostly concerned to obtain the ideal sample of any chosen material.

Although the static air-cooled radial engine is by no means an innovation irr the history of aero-engine development, its career has been somewhat chequered. Some ten years ago several practical comparatively low-powered air-cooled radials were produced and successfully flown, but they were ousted by the rotary engine on the score of power/weight ratio.

When in 1917 it was found that the limit of the air-cooled rotary had been reached, a demand arose for an engine of the same characteristics of manoeuvrability, compactness and light weight as this type, but of much higher power, and with a general all-round performance and fuel consumption comparable with the water-cooled engine of that date.