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
compensate. One blade can exert the exact same lift force as two...
provided that the propeller turns faster. The thing is that 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 avoid unbalance and very strong vibrations that would render any
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.
Are monoblade propellers the future of quacopters? Probably not. But
it's useful to know that when tuning the amount of blades of your
propellers, less than two is also a possibility. And it makes the
quadcopter easier to tidy up.
Eric Brasseur - April 23 2018