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Thin blade propellers

Translation in Polish by Alice Slaba:

A way to augment the thrust of propellers for model planes. The text asumes the use of an electric motor, yet it can be transposed to any sort of fast rotating motor.

The first problem when designing a propeller for a given motor is the yield. Whatever the energy of the current you feed into that motor, you want as much as possible of that energy be transmitted to the air and become a strong air flow. If the propeller is too little, the motor will turn close to its maximum speed, consume few current and thus transmit few energy to the air. If the propeller is too big and braking the motor will turn slowly, consume a maximum amount of current that will be transformed into heat and transmit just a little bit of energy to the air. The right propeller is in between. In order to make the motor transmit the maximum possible quantity of energy to the propeller and hence to the air, the propeller has to brake it down to half its maximum speed (the maximum speed is the speed it turns when turning free, without a propeller) (using the same battery tension) (if you cannot measure the rotation speed then consider at half the maximum speed the motor consumes half the current it consumes when it is blockaded). At half the maximum speed the motor transforms 50% of the electric energy into heat and 50% into mechanical energy for the propeller. If you want a higher yield, say 70% of mechanical energy and 30% of heat, then you must use a more little propeller, which allows the motor to turn at a slightly higher speed. But then you must accept the motor produces less mechanical energy; you get a higher yield and less thrust. (A way round, to get both the same quantity of mechanical energy and a higher yield, is to use a bigger and thus heavier motor.) See the text Data and formules about little electric motors for details.

The second problem is the thrust produced by the propeller. Indeed two different propellers, both correctly designed and transmitting the same amount of energy to the air, can produce very different amounts of thrust. A fast turning little propeller will blow a small and fast vein of air; it will propel a little mass of air yet at a high speed. A slow turning big propeller will blow a huge volume of air yet slowly; it will propel a big mass of air yet at a low speed. Both masses of air received the same energy. But the laws of mechanics also instruct us the reaction force exerted in return by the air on the propeller will be mainly proportional to the mass of the air. So the big propeller, catching and blowing a bigger mass of air, will produce more thrust. 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 current.

Thin blade propellers are an alternative to big and slow propellers mounted on a speed reductor. A thin blade propeller has the same diameter as a big propeller, yet with a very little blade surface, even less than the surface of a normal propeller. This allows it to turn at a high speed, without braking too much the motor. So it can be mounted directly on the motor, without a speed reductor. 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.

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  -  1 August 2001