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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.

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: . To summarize, for a given electric force (in Volts):
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:

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.

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.

FPV quacopter with monoblade and biblade propellers

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