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Old Sep 03, 2003, 10:43 AM
Dragonfly IIV is offline
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Waterloo, ON, Canada
Joined Aug 2003
79 Posts

The physics of propulsion are quite complicated even for something as simple as a small plastic propeller/motor combination. One difference between a car transmission and a propeller is the efficiency of converting power into forward velocity. A car transmission is highly efficient at this. Propellers will have varying degrees of efficiency depending on many factors (rpm, forward velocity, shape of the blades, etc.)

When a full-sized fixed-pitch propeller is designed, its performance is "mapped" before it is put into use on an aircraft. This map shows how much power is required to drive the propeller at different rpm and forward speeds. This is then compared to the aircraft requirements. The power required to pull or push an aircraft in straight and level flight is proportional to the cube of its forward velocity (V^3). This defines a curve on the propeller map. The motor is the final variable; it sets power available for a given propeller rpm.
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