Racing airscrews differ essentially from Service or commercial types by reason of the extreme values of engine horse-power, revolutions and forward speed which enter into the specified requirements.

These three variables, associated with the condition that the thrust–horsepower shall be a maximum, determine the airscrew completely, but the exceptional values which they assume for racing aircraft have various important reactions on the efficiency obtainable.

Thus high power and revolutions, especially the latter, have a distinctly detrimental effect, whereas high forward speed contributes to a large extent to restore the position.

I propose to examine very briefly the relative importance of these factors and to indicate how they influenced the process of selection and design in the particular conditions of last year's race.

Let us first consider a typical power curve (Fig. I). As it is not permissible at this stage to publish actual figures in detail I have made use of relative scales which should be sufficient for our purpose.