The power plant of an aeroplane is usually designed by a different person situated in a different works from the aeroplane designer, yet it is essential that both should understand something of each other's work, if the best result is to be attained. It is of the first importance that the engine designer should have a clear idea of the value to the aeroplane designer attaching to low weight per b.h.p. and low fuel consumption per b.h.p., when considering a new design or comparing the merits of existing ones.