Aircraft engines are classed as air-cooled or water-cooled, accordingly as they are cooled directly by contact with air or indirectly by air through the intermediary of the water which surrounds the cylinders. The method of cooling used has been a cause of wide divergence in design and arrangement between the two types. The air-cooled engine, except for the smaller powers, appears in the radial form in which exposure of each cylinder to the air stream is simplified at the expense, however, of a large frontal area while, with obviously greater latitude, the water-cooled engine can take a form having a smaller frontal surface.

In January of this year Mr. R. McKinnon Wood read a paper before the Society on the new American. wind tunnels, and in his paper he emphasised the importance of making tests at values of the Reynolds number which are actually obtained in flight. These modern developments of wind tunnel design comprise firstly the compressed air tunnel in which high values of the Reynolds number are obtained by testing models of the usual dimensions in a stream of air compressed to 20 atmospheres, and secondly the large wind tunnel with a free jet of 20 to 25 ft. diameter in which it is possible to test the airscrew and fuselage of an aeroplane of moderate size. Mr. Wood also emphasised the importance of a high speed wind tunnel for certain special classes of experiment.

Confusion has arisen regarding just what is meant by adjustable variablepitch and controllable-pitch propellers, and the matter should Be more clearly defined in the following way:—Fixed-pitch propellers to apply to both (a) the ordinary fixed-pitch propeller, made up with the hub and blades integral, and the material the same throughout, and (b) the detachable-blade or ground-adjustable type that will usually have the hub and blades of different materials, and in which the blades will be clamped in the hubs on the ground to an angle that is assumed, usually by experiment, to give the best general or some specially desired performance results.