Differential Motor Thrust

As designed, the GWS P-38 has fixed, non-functioning rudders. For most RC flying this isn’t a real big deal, as having rudder control is in the “nice to have” rather than the “need to have” category. However, I generally do like having rudder control available, but in this case I decided not to go to the trouble of cutting and hinging the rudders, adding another servo, and working out the control linkages. Instead, since this is a twin motor airplane, I’m going to try getting the same effect by using differential motor thrust to give me yaw control. This can be done by programming the radio to mix the throttle and rudder functions such that at least one motor’s rpm is varied in response to input from the rudder stick. 
PwrWiring_withDiffThrustReceiver assignments for differential thrust

The first two photos show the wiring set-up I’m using to allow this to happen, first on the bench to test the idea, then installed in the airframe. The left motor’s ESC is plugged into the throttle slot on the receiver as normal, the ESC for the right side motor is plugged into the rudder position. The hot wire (black)  from that ESC is removed from the connector and taped out of the way, because the receiver only expects to be getting power from one source. It’s easy to unplug the wire, so don’t be tempted to just cut it or you’ll have more work if you ever want to use that ESC on another plane later. A Y-harness is used to connect both ESCs to a single battery. If the rudder were going to be operational with a servo, then the second ESC would have to be plugged into the AUX channel and the mixing would be different than what I show below. This set-up is a little simpler.

I’m using a Spektrum DX6i radio. It turns out that this radio has limited ability to mix throttle to other channels, compared to the DX7 and higher radios. This means that I’m not able to set it up to have both motors vary their RPM as a function of rudder control input, but only one motor, in this case the right side motor. I think this will work fine in practice though. The second pair of photos show the two mixes I came up with to accomplish this.

Mix 1 settings

Mix 1 causes the rudder stick to act like a throttle lever for the right side motor, with full right rudder stick being off (even though R is set at 100%, just trust me, it works!) and full left giving full throttle plus another 25%.

Mix 2 settingsMix 2 makes the rudder channel follow the throttle stick, and modulates it so that full rudder stick deflection will give +/- 25% of the current throttle setting.

That all sounds confusing, I know. Here’s what happens in practice. When the throttle is advanced with neutral rudder, both motors ramp up together. At full throttle, both motors will be at their nominal 100% rpm. Adding full left rudder causes the right motor to speed up an additional 25%, yawing the airplane to the left. Full right rudder slows that same motor to 75%, yawing the airplane right because the left motor is still at 100%. At less than full throttle the same thing will happen, with the right motor’s rpm varying by up to +/- 25% of the current throttle setting depending on the rudder control input. That 25% is a starting point and may be adjusted after flight testing.

That’s the theory, and the set-up works fine on the bench. I can’t wait to try it out in the air!



This entry was posted in P-38 Build, Projects. Bookmark the permalink.

6 Responses to Differential Motor Thrust

  1. Tony Watson-Paul says:

    Hi Ron I have a Guanli Canadair 415 When I first hooked it up 1 motor would start the other would not come on until about 1/4 throttle which was hopeless as I couldn’t steer it I read your article and followed your advice and it works a treat My question is I would like to fit an undercarriage and I would like a working rudder connected to the nosewheel Is this mix possible with the Spektrum DX6I Thanks Tony

    • Ron says:

      Hi Tony,
      Thank you for having a look at my website; I’m glad you found it useful. I think nosewheel steering is usually accomplished either with mechanical linkage from the rudder servo to the nosewheel, or with a separate servo just for the nose wheel. In the latter case that servo would share the rudder port on the receiver with the rudder servo via a Y-harness. You would just have to mount the servos such that the throws of both are in the correct direction. Neither of these methods would require any mixing at the radio.

      If you have further questions feel free to ask, otherwise I hope you’ll let me know what you come up with and how it works out.


  2. Francisco Soublette says:

    Hi Ron we apereciate wery much for your response.

    I commited a mistake wiring ueach motor with an individual batt. After wiring a single batt for both motor the reported problem was reduced only to an abnormal interference with ailerons cannel to rudder (right motor).

    In fact moving ailerons cause only a single pick current to rudder (right motor).

    While using both mix, (1 and 2) what other mix may exist? (I am a beginner)


    Francisco Soublette

    • Ron says:

      Hi Francisco,
      I’m glad that you have found the problem. You are right that there are only two mixes available on the DX6i radio. I only mentioned the other possible mixes because I didn’t know exactly what your set-up was and to offer some possibilities to help you work through it.

  3. Francisco Soublette says:

    Hi Ron
    I apreciate very much your page for diferenctial motor thrust.
    I followed the instructions, and as you said, it worked.
    But the right motor, linked to rudder channel ,moves for few seconds, when we act with alrerons, flaps and elevator.
    Whats wrong?

    • Ron says:

      Hello Francisco,

      I’m glad you have found the information on my site useful, but I’m sorry it is not working quite correctly for you. I’m not sure why the motor would run as you describe.

      When my DX6i is set-up as shown, the right motor will run when the rudder stick is moved to the right, even when the throttle is off. This is correct. However, the motor should not run when either the ailerons or elevator only are moved. Perhaps you have some mixing of the ailerons and rudder, and of elevator to ailerons? If so this might cause the motor to run when those controls are moved. I hope you can get it sorted out, and I will be happy to help as much as possible.


Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>