Differential Throttles

[Frank Skilbeck]

Posted by Gary Manuel on 25/11/2015 09:10:05:

Frank,

Yes, the radio goes above 100. It actually goes to 125. It needs to to do 100 plus 20% when the mix is applied.

Well maybe not, what would happen if you were at fully throttle and applied rudder is that instead of speeding one up the other would slow down. So with full throttle applied and left rudder, you would get RH motor = 105%, LH motor =125%, no rudder applied and they are both at 125%. If the throttle stick was then at 100% you'd get RH = 120% and LH = 80%.

BTW many years ago a friend of mine gave me a circuit for a twin engined water plane, where if you put in full rudder and then advanced the throttle only the engine on one side would speed up. Made steering on water a lot easier.

[Gary Manuel]

Kevin,

That ESC sounds a bit dangerous to me. Throttle off should be throttle off. I can see that you fixed it, but I would have expected calibration to only take place at binding / special calibration mode. What type of ESC is it, so I can try to avoid them?

[Kevin Wilson]

Like I say, it certainly caught me by surprise Gary.
i wouldnt say I fixed it; more a fudge to prevent it happening again.

I have an idea of the make/model but will check before passing false info.

[Former Member]

[This posting has been removed]

[Gary Manuel]

Thanks Steve.

Just on with it now. My first problem is that each wing has a "Y" lead inside it. I need to bring each one out separately, which involves removing the retracts to get to the "Y" leads, then splitting the wiring before soldering and crimping new servo plugs on. I'll soon know whether it works as planned.

[Gary Manuel]

All done and tested on the bench.

It works exactly as as I hoped, including the fail-safe, which stops all 4 motors simultaneously

I am running the motors with strips of masking tape to load the motors, instead of propellers (a good safety tip I picked up from a youtube video). The wings are laid loose on foam padding at the moment, but it sounds fantastic already. There is a definite shift in engine tone from centre to left to right as the rudder stick is moved. It's like adjusting the balance control on a stereo.

One thing I had a bit of a problem with is that 3 of the 4 motors worked first time, but with the throttle off, they would randomly spin up every few seconds. This was sorted by adjusting the sub-trims of each one of these 3 channels - they are all now set to -2, and work perfectly. The other ESC / motor did not work initially, but the ESC beeped once the throttle was increased slightly, and when returned to throttle off, the motor continued to run slowly. This was sorted by adjusting the sub-trim to +4.

Next step is to build the model and put all this table-top work into practice.

I'm very pleased and quietly confident that this is going to work

[Gary Manuel]

Steve.

I'm glad you told me how to set up Preset Fail Safe mode, because I can't see instructions on how to do it in the AR8000 manual!

[Former Member]

[This posting has been removed]

[Gary Manuel]

It's a very handy feature to have and it works well.

All 4 ESC's sing their chorus in perfect harmony as soon as the receiver has bound. When tranny is switched off, all 4 motors stop together, then recommence together after it is switched back on. It's exactly what I was hoping for.

[Fredrum]

Posted by Biggles' Elder Brother - Moderator on 24/11/2015 23:24:19:

I've done this on my Twin Otter - using the Taranis the mix is quite easy to set up. But I would strongly agree with Chris - I made the mix switch outable in the air as the effect can be quite disturbing! Good on the ground though!

BEB

Hi BEB. I use a Taranis and will probably look into differential throttle in the future. I'd be very interested to hear how you set up the mix.

[Gary Manuel]

I hate to reopen such an old thread, but I think this is the appropriate place for what I'm about to add.

My Hobbyking Lancaster discussed earlier in this thread was removed from my JR transmitter last year, as I had decided to sell it. It didn't sell and I've since acquired a Horus 12s transmitter, so I've decided to set it up on the Horus. Again, it's as much about the technical challenge of setting up 4 channel throttle differential as it is about flying. This is a record of how I've set it up - for my own benefit and for anyone else who might be interested.

All settings can be seen from the OpenTX companion screens and I'll highlight anything of note:

This is the simulator screen, which does a decent job of impersonating the actual transmitter layout.

This is the top half of the Settings menu:

The only thing of note here is my Model naming system. Each model has a number physically attached which helped me to find it on my old transmitter. Not really needed on the Horus as models can be found by their avatar but I've retained it anyway and made sure that the models were added in numerical order so that they appear in numerical order when selecting the active model.

The other thing I've done is add "XXX" to the end of the name when I added the model to the transmitter, but is basically a copy of the generic template I've prepared and tested. This becomes "XX" once I have configured it to suit the actual model. This reduces to "X" once it's been bound and set up on the bench. The final "X" will be removed once it's been flown and I'm happy with it. I've used this system because I have 20-odd models added and I need something to help me keep track of where I am with each model.

This is the bottom half of the Settings menu:

The thing to note here is that the internal radio is switched off and the external radio is on. This makes use of a Jumper 4in1 module to allow my old DSM / DSM2 / DSMX receivers to be reused. Note that there is an extra tick box on the actual transmitter which allows you to FORCE the DSM2 22ms setting needed for non-digital servos. The radio would otherwise set itself to the highest spec possible for the receiver i.e. 11ms, which can damage the servos.

continued ....

[Gary Manuel]

This is the settings for Flight Mode 0.

No switches are stated, which makes this the default flight mode:

Flight mode 1 is entered by having switch "B" in the centre position.

and flight mode 2 for switch B in the downward position.

This is the Input screen:

The thing to note here is that I have got two ways of changing the rates of the three main controls: either separately using the 3 small switches A, D and E, or all together using flight mode switch B as previously set up. Note that the flight mode switch has priority over the individual switches and that the default low rate with no switch positions specified is at the bottom as a catch-all.

I have also assigned an input for "Gear" and "Tail" as will be explained later.

[Gary Manuel]

Now the important screen as far as differential throttle mixing is concerned - the Mixes Menu.

4 Separate channels (1, 7, 8 and 9) for the 4 motors, based upon the throttle input. These are mixed with rudder using the appropriate positive or negative percentage - 20% for the inbourd motors and 10% for the outboard motors. This can be adjusted later if I need more or less differential rudder control. Note that Flight mode 0 and Low Rudder Rates gives 0% as defined in the bottom entry of the I4:Rud Mix on the previous screen. I can therefore disable differential throttle easily. This explains why I assigned a separate input for controlling the tailwheel as I would not want this to be disabled under any conditions.

Another line of note is the Ch3:Elev, which has Rudder applied according to curve CV2:R-E. This is because I remember that the model had a tendancy to nose-down when differential rudder / throttle was asigned. This curve applies a small amount of up elevator when the rudder control is used, to compensate for this.

The final line of note is the Gear Mix which lowers or raises the gear according to curve CV1:Gea. This is just a simple mechanism for converting the 3 position switch into 2 state operation. I'm sure there are other ways of doing this, but this method works.

These are the two curves mentioned above.

This is the Output screen. Nothing of note other than to say that the channel order is based around the order that was needed by my JR transmitter in order to make everything work. I could easily have made this more logical using the Horus as it is completely flexible in that respect, but I chose to leave the channel allocation as it was to avoid rewiring.

For completeness, these are the Sticky Kill Switch latches I have used as a motor kill switch.

These are the special functions for overriding the four motor channels using Latch L02 along with generating sounds when any functioning switch is operated.

[Gary Manuel]

One thing I forgot to mention is that on the Output screen, I have set the downward moving elevator to move only about 50% of the upward moving one. This is to provide Aileron Differential in an attempt to reduce the tendency of these models to tip-stall.

That's it. I hope everything works.

[Frank Skilbeck]

Posted by Gary Manuel on 23/02/2020 16:54:28:

One thing I forgot to mention is that on the Output screen, I have set the downward moving elevator to move only about 50% of the upward moving one. This is to provide Aileron Differential in an attempt to reduce the tendency of these models to tip-stall.

That's it. I hope everything works.

Did you mean aileron?

Also I believe Open Tx does have the option to set aileron differential and that maybe a better option than just limiting the down travel, I've seen on glider forums where people have allocated a control to aileron differential which allows you to make adjustments on the fly, useful when you are fine tuning a model.

[Gary Manuel]

Posted by Frank Skilbeck on 23/02/2020 17:38:28:

Posted by Gary Manuel on 23/02/2020 16:54:28:

One thing I forgot to mention is that on the Output screen, I have set the downward moving elevator to move only about 50% of the upward moving one. This is to provide Aileron Differential in an attempt to reduce the tendency of these models to tip-stall.

That's it. I hope everything works.

Did you mean aileron?

Also I believe Open Tx does have the option to set aileron differential and that maybe a better option than just limiting the down travel, I've seen on glider forums where people have allocated a control to aileron differential which allows you to make adjustments on the fly, useful when you are fine tuning a model.

Yes I did mean Aileron. My mistake. Thanks Frank.

I'm also aware that OpenTX allows differential to be applied to ANY input or Mix, using the DIFF function. I raised a question HERE some time ago about the point of using "differential" rather than "travel adjustment" for applying aileron differential to any generic transmitter. Nobody convinced me that their was any point in using the differential option.

With respect to making adjustments during flight, this is not something that I would trust my poor ageing brain to do! If I did, I could just as easily assign a global variable to the percentage value of the downward moving aileron as I could to the value of Aileron DIFF. Either method could easily be achieved by assigning a switch or slider to give the value of the global variable.

Regarding gliders, the amount of reflex for example, could similarly be adjusted on the fly.

OpenTX is unlimited in it's ability, but needs to be used with care in my humble opinion.

 

Edited By Gary Manuel on 23/02/2020 22:18:04

[Toni Reynaud]

I spent hours today sorting out mixes on my DX7 to get differential throttle control on the Flair Beaufighter. Why? I got fed up with having to repair the motor mounts every time it landed in the long grass, so I fitted electric retracts. It would still have to be a hand launch because it has no rudder. It would add too much weight to fit one, so to get directional control on the ground for take-off, differential thrust is the answer. It'll be interesting to see if it works.

Tx is Spektrum Dx7

One ESC on THR, one ESC on AUX1

Mix throttle to AUX2 100%

Mix throttle to RUDD 100%

Check both motors work

Mix Rudder to Throttle

Mix Rudder to AUX 2

First, set the AUX2 switch towards you. Leave it there at all times, or things don’t work.

Connect port ESC to RUDDER, connect stbd ESC to AUX2.

Using PMIX5 and PMIX 6 means that they will respond to THROTTLE channel trim settings

Check all settings on the control monitor display as you make them. Only make and check one setting at the time.

[Toni Reynaud]

First mix on PMIX 6 [PROG. MIX6] THRO -> RUDD ON RATE +100% +100% SW:ON OFFSET: 0	This makes the port ESC work independently of the throttle channel. If this were not done, any rudder movement would tend to override the throttle output instead of being mixed.
Second mix on PMIX 5 [PROG. MIX5] THRO -> AUX2 ON RATE +100% +100% SW:ON OFFSET: -100	This makes the stbd ESC work from the throttle channel

[Toni Reynaud]

Third mix on PMIX4 [PROG. MIX4] RUDD-> RUDD ON RATE -50% 0% SW:ON OFFSET : 0	Because the rudder is now mixed to the port ESC, it will control port motor speed independently of the main throttle setting. Left rudder will decrease the port motor speed.
Fourth mix on PMIX 3 [PROG. MIX3] RUDD-> AUX2 ON Rates -50% 0% SW:ON OFFSET: 0	Because the rudder is now mixed to the stbd ESC, it will control port motor speed independently of the main throttle setting. Right rudder will decrease the stbd motor speed.