Reversed Motor/Prop Rotation in a Quadcopter

This article explains some of the benefits of reversing your motor and propeller rotation for quadcopter and mini quad flying.

Aerodynamic benefits

The most significant benefit of inverting your motor/prop rotation is Aerodynamics. This video is quite good at explaining it and here is my personal understanding.

Center of thrust

When propeller spins at a stationary spot, centre of thrust is right in the hub. But when the quadcopter begins to fly forward, force from air resistance is applied on the the propeller and thus shifting the centre of the thrust to the advancing blade (the red circles).

This means the advancing blade will tend to generate more thrust and pull more air than the retrieving blade. Advancing blade, stronger. Retrieving blade, weaker.

Front propellers, centre of thrust is outward, and that of back props are inward. The faster the copter is flying, the more significant it becomes. Which means back propellers are working less efficient than the front propellers.

Air Disturbance

Furthermore, the air turbulence from the front propellers can make the back propellers work even less efficiently. (That’s one reason why “stretch X” frame was designed to combat this issue to make front motors further away from the rear motors)

By reversing the prop rotation, the centre of thrust of the back propellers are now outward when flying forward, and it compensates the penalty we get with the dirty air disturbance from the front props.

Yaw turns

Back motors are less likely to go into “stall” at tight yaw turns with inverted rotations.

However

These benefits would not make much a difference during a hover or 3D aerobatics flying.

Other benefits

One of these reasons might be enough to make you adapt to this idea.

Since your front props are not spinning toward the FPV camera but rather outward, your camera lens won’t collect dirt from the props such as grass or water.

Another positive thing in changing prop rotation is that most of us often get into situations where we have no CW props left but many CCW, or the other way round. That happens because we are left or right handed and tend to make more corners to one specific direction instinctively. Now we can just change rotation and keep flying.

How to reverse motor rotation in a quadcopter

Here is how to apply the custom mix for a pure X quad in boards using Baseflight / Cleanflight or Betaflight

  • Remove all propellers
  • Change motor direction with ESC, BLHeliSuite
  • Go to BetaFlight CLI, type
set yaw_motor_direction = -1
save

And now install your props in the reversed order, CW instead of CCW and vice versa, not flipping it upside down. :)

And make sure you check it every time you flash/update firmware in case that setting gets reset, since this is not the traditional way of doing it.

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