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A quadcopter with monoblade propellers









H8 Mini 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:



copper wire to balance a monoblade propeller           copper wire to
          balance a monoblade propeller



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.



balancing a monoblade propeller on a needle



The angular momentum of the propeller is retained yet its weight increases.

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
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