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Designing Electric Model Hovercraft

Fans

To run efficiently, the motor and propeller or fan must be correctly matched to each other. A small motor driving a fan from a hair dryer or vacuum cleaner will seem to produce a dramatic airflow, but will do so for a very short time.

Electric motors of the type which we will consider using run at very high speeds. A five pole motor connected to six cells will run at around 10,000 RPM when off load, while a three pole motor will run at around twice this speed. The faster we can get our motor to run, the more efficient it will be.

Hair dryer and vacuum cleaner fans run at 5-10,000 RPM, and are driven by motors rated at from 300 to 800 Watts. A three pole model motor will absorb only 100 Watts when heavily loaded, so if it is driving such a fan it will not be capable of reaching its operating speed unless it is highly geared. Coarse pitch propellers, which are designed to operate on model aircraft with high forward speeds, require very high powers to drive them when the model is slow or stationary. Model hovercraft travel at considerably lower speeds than model aircraft, so it is unlikely that a coarse pitch propeller will be operating at its optimum. It is apparent that a large propeller of any given pitch will require more power to drive it than a small one.

Having at first sight ruled out commercially available fans and large and coarse propellers, we are left with the small diameter fine pitch propeller. These perform very well with electric motors, especially when mounted in a close-fitting duct. Some of these propellers are available as three blade types, and are particularly suitable for model hovercraft.