A quadcopter with monoblade propellers
By cutting away one blade on each propeller, about the same lift
force was retained but the flight time was increased by 30%. The
motors and the battery are cooler at the end of the flight.
The motors used on this 28 grams quadcopter derived from a H8 Mini
are meant to turn at high speed. When the blade of a propeller is
cut away, to get the same lift force the motor has to turn faster to
One blade can exert the exact same lift force as two... provided
that the propeller turns faster.
DC electric motors have a better yield when they turn at a higher
speed; closer to their maximum speed. They need to be fed less
electric power, because they will spill less of that power into
heat, while producing the same mechanical power. Details here: http://www.ericbrasseur.org/emamem.html
. To summarize, for a given electric force (in Volts):
- If the motor is blockaded it will produce no mechanical power
and it will draw maximum current (in Amperes). It will produce
the maximum amount of heat while producing no mechanical power.
- If the motor can turn free, with no load at all (no propeller)
then it will turn at a given high speed and still produce no
mechanical power since there is no load. But it will draw very
little current and produce almost no heat.
- To produce the maximum amount of mechanical power (make a
propeller blow the most air...) the motor needs to turn at half
its maximum speed. It will also produce a fair amount of useless
- To have the motor turn slower than half the maximum speed is
pointless since it will produce less mechanical power while
producing more useless heat. Lots of electric power consumed,
that will be converted in little useful mechanical power and
lots a useless and damaging heat.
- To have the motor turn faster than half the maximum speed can
be a sound engineering decision. The motor will sure produce
less mechanical power but it will also produce much less heat. A
greater share of the electric power will be converted in
mechanical power; the yield will be better.
To avoid unbalance and very strong vibrations that would render
flight impossible, the loss of the blade has to be compensated. This
is why 0.1 millimeter copper wire is wound around the remaining part
of each blade that was cut away:
The wound copper wire is drenched in superglue. A length of a few
decimeters is wound at a time, then drenched in glue. The excess
glue is absorbed using a paper towel. To check the balance the
propeller is placed on a needle. The last length of wire and its
drench of glue, will make the cut side too heavy. From that point
on, the needed length of wire will be unwound and cut away, a few
centimeters at a time, till the balance is attained.
The angular momentum of the propeller is retained yet its weight
A more usual approach would have been to use two-blade propellers
that have half the chord. That way too, the rotation speed needs to
be increased to achieve the same lift force. The problem is that
such narrow blades can be more fragile and... I didn't have such
propellers with narrower blades.
The next photograph shows both the usefulness and uselessness of
monoblades. The two normal H8 Mini propellers are mounted on
standard H8 motors. The two monoblade propellers are mounted on high
speed Chaoli 59000 RPM motors. The end result is about the same. The
quadcopter's microcontroller sees no difference between the two
motorization systems. For a same electric power injected in each
motor, the lift force will be about the same. At the end of the
flight, the motors will have heat up about the same.
The build technique for these monoblades was different. One of the
blades was shortened using scissors. Soldering wire was flattened
with a tang and applied on both sides of the shortened blade, then
fastened with many loops of polyester sewing thread. The whole was
drenched in superglue then cut and sandpapered till the propeller
was balanced again.
Are monoblade propellers the future of quacopters? Probably not. But
it's useful to know that when tuning the amount of blades of
propellers, less than two is also a possibility. Advantages are that
it makes the quadcopter easier to tidy up and it will be less likely
to stay hooked in a tree.
Eric Brasseur - April 23
till October 20 2018