How To: Mixing differential thrust for easy handling.

DIFFERENTIAL THROTTLE MIXING Many twins can benefit from differential thrust to improve handling on the ground, or on the water with seaplanes and flying boats. The trick though, is to be able to activate or cancel the differential thrust at will, rather than to leave it on all the time. An unwanted input of differential thrust in the air at the wrong moment can spell disaster.

Below are most of the programming screens from my Futaba 10C that show exactly how I set up differential thrust with the Kondor Model Products Canadair CL-215 water bomber, reviewed in the September 2009 issue of Fly RC magazine.

Because this model is a dedicated flying boat, and has no landing gear, I was able to use switch E, traditionally used for retracts, to control the differential throttle mixing. I plugged the ESC controlling the right motor into channel 3 on the receiver as the master throttle. The left motor’s ESC is plugged into channel 5 and programmed as the slave.

I fly with my index fingers between the switches on the top of the transmitter, and so program my normal flight parameters so that the forward switches, labeled E and G on the 10C, are toward me, and the rear switches, F and H, are toggled away. This provides the most comfortable positions for my fingers and reinforces that the switches are set properly for my normal flight condition.

When I program a deviation like differential thrust or aerobatic heli modes that I want to know are on, I set the switches to tighten the spaces between them when the non-standard modes are active. Tighter spacing for my fingers when I pick up a transmitter indicates a complex mix is active, and provides another level of protection against accidents.


The 10C’s main screen shows that I have the proper model selected.

Press the Mode button for at least 1 second to access the Basic Acro Menu.



These are the two screens of the Basic Acro Menu. The servo menu on page 2 can be used to set up the programming before powering up the model.


Pressing the Mode button again will toggle to the Advance Acro Menu. The Programmable Mix submenu is show highlighted, and is accessed by pressing down on the rotary dial.


I set program mix #1 to mix Throttle into Landing Gear, since this model does not have retracts and is dedicated to flying from the water, and I used the Futaba R617FS 7-channel receiver. Ganged flaps and separate ailerons filled the remaining channels.


Pressing the dial accesses the parameters for that mix, and I set 100-percent mix to give full throttle range on the slave motor. Note that the Mix is set to On. The POSI>NULL setting ensures that the Master-Slave mixing is always active.


Mix #5 is set to slave the Throttle to the Rudder inputs. This links the right motor to the rudder when active.


As you can see, neutral rudder does not bias the throttle at all. Adding right rudder decreases the power to the right motor, while adding left increases the throttle. The curve looks exactly opposite of this because the throttle channel of Futaba radios need to be reversed with most electronic speed controls.


Recalling that the left motor is plugged into the gear channel, we want to slave that channel to the rudder as well.


Note that this mixing curve is exactly opposite that of the one I set up for the right motor. I guessed at the mixing amount I wanted and set mixing to give +/- 50-percent throttle with full rudder deflections. That has proven to be a great setting for this model.

Both mixing curve screens are flagged 1 of 2. The second screens allow you to assign the mix to any switch and position. I set them both to be active when the gear switch H is pushed away from me, presenting a tightened gap for my fingers. Part of my pretakeoff check list is to make those index fingers comfortable, deactivating the mix after lining up into the wind.


The SERVO screen shows you the channel outputs without powering up your model. Here we see both throttle channels (3&5) are at low power, with the flaps up on channel 6. All other channels are at neutral.


A left rudder command is shown on this screen by the Channel 4 bar below the center line. With outputs 3&5 identical, you can see that the differential thrust mixing is not active and both throttles are still low.


Here we can see that the mixing is active, as evidenced by the different throttle settings influenced by the rudder input. The right motor would now be turning, based on the –50 output on channel 3. The left motor is still not turning.


Setting the throttle stick at mid throttle and deflecting the rudder to the left (down on #4) slows the throttle on the left motor (#5) from half power, and increases the right throttle (#3). This differential thrust turns the model to the left.


The differing prop blurs clearly show that the motor on the left wing (right when looking from the front) is turning much faster than the motor on the right.


Flaring for a smooth touchdown, the Canadair CL-215 kicks up a dramatic wake as it drags its rudder.

Note that I used channel 5 for my second throttle. If you have retracts, you can plug your second speed control or throttle servo into any other aux channel you have available. I used Futaba’s 7-channel R617FS receiver and had no other ports available. Programming differential thrust on a model with retracts would require at least an 8 channel receiver, or using a y-harness to gang the ailerons to one channel.