Need For Orme's Law
The main purpose was to demonstrate to the modeler that the electric motor was not the source of power like an internal combustion engine. With an IC engine, a particular sized engine will have a particular power output. Typically a model airplane manufacturer will recommend a 2-cycle glow plug motor by its size (typically .40-.60 cubic inch for the average sized plane) Since electric motors operate over a much wider range, there was no good way to recommend an electric motor for a plane.
Orme's Law makes it clear that the battery pack is the power source, and it determines the available power, not the electric motor. Once the power requirements of any particular airplane model have been determined, an electric motor can be chosen to match the power source. i.e., a 10 cell NiMh battery pack produces approximately 1 volt per cell under load. At 30 amps, that translates to .4 HP.
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