It was between flying seasons, meaning the Minnesota winter was here and had already put nearly 4' of snow on my favorite fly location. Yep, time to put together a new plane for the upcoming summer. After looking at what was available, I opted for a PNP cargo transport model from Horizon Hobby, the EC-1500 Twin. With a 60" wingspan and some really good reviews in the RC forums, it looked like it would be a really fun aircraft to fly.
The EC-1500 has a pair of 650Kv brushless motors controlled by 40 amp ESCs. As delivered from Horizon Hobby, the EC-1500 ESC signal wires are connected to the receiver's throttle channel via a "Y" cable. This means both ESCs are controlled by a single throttle channel on the receiver. I've had a couple of twin motor planes in the past and I flew them with a differential thrust configuration. That being said, I couldn't think of a reason why I would not want to try this with the EC-1500. I also decided that being able to enable/disable the diff thrust with the flip of a switch would be a good option.
Horizon Hobby labeled all of the servo connectors that plug into the receiver. I identified the existing throttle channel wire and removed the "Y" cable connecting it to the two ESCs. One ESC signal wire was then plugged into the existing throttle channel on the receiver and the second ESC signal wire was plugged into a spare channel that would be configured for controlling the second motor.
In reading the forum comments, I found a pilot that mentioned the EC-1500's ground control could be better when the aircraft was on a grass strip. No surprise there....simply put, grass is not asphalt. Grass height, ant mounds, etc. all are issues at a grass only flying field. Even though the nose gear is linked to the rudder to enhance ground steering, the handling for a plane this size could possibly be improved. Because of this, I decided to set up my differential thrust programming to support a "Ground Mode" as well as the normal "Flying Mode". The "Ground Mode" would provide a mix which increases the level of differential thrust and causes the aircraft to maneuver more easily on the ground. "Flying Mode" would reduce the rudder to throttle mix and provide a more appropriate level of differential control when the plane is in the air. These two custom modes, along with the ability to disable all differential thrust, would be controlled via a 3 position switch on my transmitter.
Towards the end of last year's flying season, I bought a new transmitter, the FrSky Horus X10S Express. It took me an evening before I upgraded the radio to OpenTX firmware. I'd been flying my Taranis X9D+, also using OpenTX, for 5 years and wanted another transmitter. The plan was to let the Taranis become my backup and also fill the role when buddy flying. I should mention now that this differential programming is applicable to most any FrSky transmitter, not just the X10S Express. I'll be using the X10S Express, which is running on OpenTX 2.3.5, for the remainder of this article when showing screen shots.
I'm flying an FrSky
S8R 3-axis stabilization receiver in the EC-1500 Twin. This
is a 16 channel receiver with 8 conventional channel outputs assigned as follows:
Channel 1 - Ailerons
Channel 2 - Elevator
Channel 3 - Throttle Left
Channel 4 - Rudder
Channel 5 - not used
Channel 6 - Throttle Right
Channel 7 - Door (loading ramp)
Channel 8 - Flaps
I will be using two global variables, as shown in the above screenshot, in my throttle mixers. Global Variable #1 (GV1) will be used for the "Flying Mode" mix. Global Variable #2 (GV2) will be used for the "Ground Mode" mix. I've set the value for GV1 to 5% and the value for GV2 to 20%. I'll mention now that these two values were initially selected based on my personal flying experience for GV1 and some steering testing for GV2. Changing the value of a variable will either increase or decrease the differential thrust you experience for that mode.
I will be using Switch C (SC), a 3 position switch, to control select a mode
or disable differential thrust altogether.
SC (up) = Flying Mode
SC (mid) = Differential Off
SC (down( = Ground Mode
This screenshot shows the mix for the left throttle. The first line below CH3 is the replace line for the throttle cut mix and has nothing to do with differential thrust but does help with pilot safety. The next line is additive for "Flying Mode" only. The weight for the rudder to throttle mix is the value of GV1, which was previously set at 5%. This line is active when SC is up. The next line is also additive but for "Ground Mode" only. The weight for the rudder to throttle mix is the value of GV2, which is 20%. This line is only active when SC is down.
This screenshot shows the mix for the right throttle mix. The first line below CH6 is the throttle cut mix and has nothing to do with differential thrust but does help with pilot safety. The next line is additive for "Flying Mode" only. The weight for the rudder to throttle mix is the value of GV1, which is -5% for this channel. This line is active when SC is up. The next line is also additive but for "Ground Mode" only. The weight for the rudder to throttle mix is the value of GV2, which is -20%. This line is only active when SC is down.
So how does it work? Let's walk through it.
1. When ready to fly, the pilot would first disable the throttle cut. This will allow the motors to spin once throttle is increased.
2. When SC is placed in the middle position, differential thrust is disabled and the plane flies just like it came from the factory.
3. When the throttle is increased enough to move the plane (and the rudder is neutral), the plane will move straight forward because both motors run at the same speed and the rudder has no effect.
4. When SC is placed in the down position (Ground Mode), and the rudder is still neutral, the plane will continue to move straight forward.
5. When some right rudder is applied, it causes the right motor speed to decrease and left motor speed to increase....and the rudder will likewise help turn the plane to the right.
6. When some left rudder is applied, it causes the left motor speed to decrease and the right motor speed to increase....and the rudder will likewise help turn the plane to the left.
When steering right, the reason the left motor speeds up is because the CH3 mix adds the positive rudder input (+GV2) to the throttle value which increases the throttle signal on Channel 3. The right motor slows down because the CH6 mix adds the negative rudder input (-GV2) to the throttle value which decreased the throttle signal on Channel 6.
The same functionality occurs when SC is up (Flying Mode) except that the rudder to throttle mix change is less pronounced because the value in GV1 is much smaller than the GV2 value used in Ground Mode mixing. Both of these global variable values can be adjusted to the pilot's personal preference. At no time should either of these variable be entered as negative numbers.
So that wraps up the crash course on OpenTX differential thrust mixing. This is one of resaons I enjoy using a transmitter that supports OpenTX. OpenTX allows you to make these kid of mixes once you understand the basics of using it. As a retired software developer, I enjoy the freedom it provides me for setting up my transmitter to do things my way.
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