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Should we reverse prop rotation direction on our quads?

We recently noticed a video by Kabab FPV detailing a number of improvements in switching the rotation of props on a quad. Here’s an explanation of the advantages of reversing the prop rotation directions on your quad.

Firstly, it means when you fly low to the ground, all the gunk that your props pick up will flick it away from your camera. So you can still see where you’re going!

However there are a number of advantages that are not quite as obvious as the one listed above. The first one is that in the classic prop rotation direction, the most effective part of the rear prop rotates inwards, this means it receives all the disturbed air that has travelled from over the front of the quad. This reduces the amount of thrust it can make. Although when we compare the velocity of the props to the velocity of the quad, the difference due to the inward prop moving forward into faster air is likely to be negligible.

The most important difference we think is that when a quad turns, we need to spin up 2 opposite motors in order to “yaw” the quad around the corner. In addition one side of the craft needs to “dip” in order to complete the turn. This means one of the rear props needs to spin down significantly to yaw and to drop its side of the quad to glide into the turn. As a result, the prop can stall and takes much longer to spin up to regular speed, making the quad unstable. On the other hand, the other rear prop needs to speed up in order to yaw AND in order to lift its corner of the frame. This quite often leads to the motor being pinned at max throttle when turning, even if you are not at max throttle. When you reverse the prop rotation directions, one of the props spins up to turn it, and the other spins up to raise the back side, instead of one prop having to do both tasks. This means no prop has to spin up to max, and no prop has to spin so slow that it stalls either.

It’s quite hard to explain how this all works in writing, so here’s a video by Kabab FPV detailing how it works.

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