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Thin blade propellers

Translation in Polish by Alice Slaba:

A way to augment the thrust of propellers for model planes.

Let's assume that an electric motor is used and that it will be used at the same yield throughout this text (see the text Data and formules about little electric motors for details).

The electric motor produces a given amount of mechanical power, that is transferred to the propeller, producing a stream of air.

A little propeller will produce a narrow stream of air while a large propeller will produce a broad stream of air. Yet the power transferred to the stream of air is the same in both cases. Hence the narrow stream of air will have a higher speed than the broad stream of air. The narrow but fast stream will carry the same power as the large but slow stream.

While both propellers transfer the same power to their respective streams of air, the lift force that they produce will not be the same. The large propeller will produce more lift force!

A propeller with a threefold diameter will produce a twofold lift force.

That's why the Flyer of the Wright brothers had such huge propellers. That's too why little gear or belt speed reductors are being sold for electric model plane motors, together with big propellers. The bigger the propeller (and the slower it turns, in return), the more air mass it moves and the higher the thrust; using the same motor turning at the same speed and consuming the same electric power.

Yet a speed reductor is not mandatory. A thin blade propeller can have the same diameter as a big propeller, yet with much less blade surface. It will need to turn at a much higher speed to transfer the same power to the air. Hence it can be used without speed reductor; it can be mounted directly on the motor. It sweeps over the same surface as a normal big propeller, catches the same quantity of air and blows it away at the same speed. Yet it turns much faster.

My first thin blade propeller was made out of needles and two little blades cut out of a steel beer can. The chord of the blade is a little curved, like the wings of early planes. The angle of the blades is about 5°.

The two blades were soldered to the needles. Three needles were used, making the propeller have a diameter of about 20 cm.

The surface of the blades was about a third of the normal surface of a propeller designed for the motor. But the surface the blades swept over was at least twice that swept over by a normal propeller. This allowed the motor to lift itself up into the air, which was not possible with a normal propeller. (This was before the advent of modern quadcopter motors.)

My next attempt will be with flat blades of constant chord and an angle of about 7°, resembling the blades of an helicopter.

Thin blade propellers have advantages:

And disadvantages:

Eric Brasseur  -  August 1 2001  till  October 30 2018