Matching propeller to engine
For aerodynamic efficiency large slowly rotating propellers are preferable, but unfortunately, small piston engines develop their best power-to-weight ratio at relatively high rotational speeds. For light aircraft, therefore, the added cost, complexity, weight and mechanical losses of gearing sometimes make it preferable to use direct drive, and accept a slight degradation in engine efficiency due to running at low rotational speed. When small automotive engines are adapted for home-built aircraft, some form of gearing is often used. In the case of turbo-prop propulsion, the rotational speed of the primary engine shaft is so high that gearing is almost essential.
Once gearing is accepted, then the propeller diameter is limited only by practical considerations such as ground clearance, so highly efficient propellers can be used. The propellers of the Lockheed Super Hercules shown in Fig. 6.6 are driven by geared gas-turbines.