Aerodynamic Considerations Affecting Propellers for Large Engines 380153
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.