Why Reverse Motor/Prop Rotation in a Quadcopter? Props out vs. Porps in

This article explains some of the benefits of reversing your motor and propeller rotation for quadcopter and mini quad flying. This is known as “props out”.

Aerodynamic Benefits

One of the most significant benefit of inverting motor/prop rotation would be aerodynamics. This video explains this very well and here is my personal understanding of it.

Center of Thrust

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

This means the advancing blade will tend to generate more thrust and pull more air than the retrieving blade.

On the front two propellers, the COT is located outward, and that of the back props are inward. The faster the copter is flying, the further the COT is away from the hub. That implies the back propellers would be working less efficient than the front propellers due to principle of moment.

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 by moving front motors further away from the rear motors)

By reversing the prop rotation, the centre of thrust of the front propellers are now inward while that of the rear props are outward, and it compensates the penalty we get with the dirty air disturbance from the front props.

Better Yaw Turns

With inverted rotations, the back motors are less likely to go into “stall” at tight yaw turns. The benefit and effect is especially noticeable for smaller quadcopters, like tiny whoops and other brushed micro quads with weaker motors.

However…

These benefits would not make much a difference during a hover, or for 3D aerobatics flying where the props could be spinning both directions.

Other Benefits

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

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. However your FC stack will :)

But when you use turtle mode, the props runs the opposite direction when you are up side down, and you are more likely to get your lens dirty. So there is a contradiction with props out.

With reversed rotation you are also less likely to get caught by gates or branches, as it would just bounce off obstacles in collisions.

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

Here is how to do “props out” on a quadcopter in Betaflight.

Alternatively you can do this in BetaFlight CLI, by entering

set yaw_motor_direction = -1 (command name changed in newer Betaflight version)
set yaw_motors_reversed = on
save

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

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

Do I Recommend Props Out?

Yes and no.

I personally just run standard rotation (props in) on my 5″ quads (actually, 3″ or larger), mostly because of laziness. I don’t have to remember to re-configure every quad after updating the firmware.

But I always run props out (reversed rotation) in tiny quads – anything smaller than 2″. Because the flight performance improvement is a lot more noticeable.

Edit History

  • Jun 2017 – Article Crated
  • Aug 2017 – Added one more benefit to reversed rotation
  • May 2019 – Updated guide with Betaflight Configurator screenshot

15 thoughts on “Why Reverse Motor/Prop Rotation in a Quadcopter? Props out vs. Porps in

  1. Rodnet

    Ok so you need to reverse motors in Blheli to physically get them to change rotation direction and then in betaflight (either by change rotation in config or via command line) so betaflight sends correct signal commands to the motors?
    Also swap motors around to have the correct threading for the direction of prop rotation.

    Reply
  2. Jamie

    I’m in the process of putting together my 1st quad and was looking into what way the propellers should spin and why before I got started… Do you know why the convention of having the front props turn towards the chassis was established in the first place?
    Thanks, Jamie

    Reply
  3. Nigel

    I don’t know if this is another Betaflight change (now on 3.2.1) or if it is specific to my quad, (an Awesone Q95), but I also got the Invalid name error when trying to input the yaw_motors_reversed = ON command. Using the yaw_motor_direction = -1 worked for me.

    Reply
  4. Jase H.

    I’ve run all my setups this way since I discovered tiny whoops two years ago… Betaflight 3.2.0,
    The CLI command is yaw_motors_reversed = ON
    I tried using yaw_motor_reversed, and got an invalid name error. I did a dump check, and discovered it’s “motors”, not “motor” in the CLI command.
    Oscar, could this be a typo in your tutorial? Or maybe Betaflight changed it for 3.2.0? As I remember, 3.1.7 was still “yaw_motor_direction”. Anyhow, cheers Oscar! I love what you do!

    Reply
  5. Jay

    I installed my motors ESC’s and they’re spinning reversed. I can’t access BLHeli32 to change direction(or even get dshot600 to work). I know that I can swap two motor cables to change direction, but I’m pretty happy with the wiring as is. I googled reversed props and found this great page. Will right and left hand threads on the prop posts have any negative effect here? I have directional threads and I think I might have to swap all of the motors now to run reverse prop direction without the props and nuts flying off. Is thread direction an important detail to pay attention too here? I’m new to the hobby. Thanks.

    Reply
    1. Oscar Post author

      It’s best to install them correctly according to motor spin direction. It ensures your props won’t fly away during flight. But if you check that pre-flight everytime then it’s not really that big of an issue :)

      Reply

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