FROM the aerodynamic standpoint, propellers for engines of several times present powers will operate just as efficiently as those for smaller powers. For long-range aircraft operating at altitudes of 20,000 to 30,000 ft. three-blade propellers increased somewhat in diameter over what is current practice are definitely indicated although, for medium-range types, the diameters of present practice scaled up by the square root of the take-off power ratio appear satisfactory.
As alternatives to the increased diameters for the long-range high-altitude aircraft, the three-blade propeller of about 15 per cent smaller diameter with a two-speed reduction gear will give equal range but a poorer take-off, or the four-blade propeller of the same diameter as the smaller three-blade design will give very close to equal range without a two-speed reduction gear and slightly poorer take-off.
The importance of pitch distribution designed for operating speeds is indicated, as is the necessity for keeping blade-shank sections faired when exposed to the airstream particularly in liquid-cooled installations. Propellers which can be feathered in case of powerplant failures are considered essential to long-range aircraft from the standpoint of safety and increased performance on the remaining engines.