Thin blade propellers
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
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
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
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:
- Easy to build.
- The fast turning speed makes them less sensitive to the travel
speed of the plane than slow propellers.
- Less visible.
- They brake less the plane when they stop rotating.
- Fragile. Should thin blade propellers be mass produced,
probably they will have to be replaced after each flight. The
landing of a conventional RC electric glider can do nothing but
destroy a thin propeller.
- Dangerous. Beware your fingers.
- It can only be used for little planes. For real planes the
outer part of the blades would go beyond the speed of sound.
Eric Brasseur - August 1 2001
till October 30 2018