Matching propellers to the power of the engine and to a particular aeroplane is mainly a matter of experience and experiment Engine tests carried out on behalf of model aircraft magazines and published therein usually state a range of propeller sizes and rotational speeds achieved, which form a very useful guide. The test reports also usually include a graph of power against r. p.m. (Power is rated in Watts and Kilowatts in SI units and Horsepower in Imperial measure).

An equation which is of some help in choosing a propeller diameter for a particular engine and application is the following:

where propeller diameter, D, is in centimetres and in Imperial units:

Although based on work done for full-sized aircraft this gives results which seem to be applicable to model engines and propellers, although not necessarily quite accurate in every case. A scientific pocket calculator with a ‘fourth root’ function is required. With a rough idea of the speed of flight to be flown the pitch of the propeller follows approximately from Figure 14.12.