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.
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 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%.
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!