Thin blade propellers
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
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:
- 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.
1 August 2001