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Old Jan 28, 2009, 09:27 PM
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Differential throttle-steering on an elec. twin

Building a GP PBY Catalina. I want to counter rotate the props and mix differential steering, however i don't know how to do this... I'm using a JR 8103 PCM that seems to do a lot of stuff... Any help would be appreciated THANKS.
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Old Jan 28, 2009, 10:27 PM
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Should be easy ...

Greggk,
Best treated as two different issues:
For counter-rotation - not a radio function, just swap two of the three wires on the preferred motor to reverse its direction.
For differential throttle - I have a 3810 (same radio, different numbering convention in Australia, I think) and would go about it something like this (I've mixed throttle and aileron to control a model with no control surfaces, but not rudder to throttle yet for differential thrust):
Plug one motor into the throttle outlet on the Rx, and the other into a spare channel (let's say AUX2).
Then mix throttle into AUX2 at +100% with no offset - you should now have both motors running in synch off the throttle channel.
Now mix Rudder into throttle at a convenient percentage (+50%?) - and Rudder into AUX2 at same amount, but opposite direction (-50%?).
Now, when you wiggle the rudder, you should get one engine accelerating (or starting up) and the other slowing down (or staying stopped).
If the yaw direction is wrong, you could swap the leads at the Rx or reverse the directions of mix for the same result.
Mind you, I haven't done it yet, but a Puddle Twin is somewhere in my future, I hope!
Cheers, and let us know how you go,
Pete
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Old Jan 29, 2009, 01:56 AM
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Hello,

I have used diff. throttle on a HVPīs Hu-16 and did that the way Pete mentions, althought on a completely diferent radio, Futabaīs FC-28.

It works really well on this model without water rudder...

Just one suggestion: program a switch to ON/OFF the rudder-differential power mix as you (if like me) will disable it after take off.

Good luck !

João
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Old Jan 29, 2009, 02:46 AM
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Good thinking!

I agree with Joao - if you choose the right mixers on the JR radio you can assign an on/off switch for the mix - perhaps to the Mix switch or the Land Flap setting. Some mixers also allow for trim to be mixed (5 & 6, I think) if you want throttle to be trimmable. I can never remember which is which, but a quick fiddle will tell you, and the Monitor screen will show what is happening before you turn on a Rx.
For info, I have had no problems with the motors I used if the programming sends them beyond 'full throttle' or below 'stop' - they were only brushed motors, but just stayed with the normal full-throw limits if the channel commanded them below zero or beyond full. Might need to play and check with brushless.
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Old Jan 29, 2009, 05:37 AM
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Great information guys.
I'm converting a 73" wingspan glow powered PBY, (scratch built by previous owner), to electric. I've been considering using Throttle differential instead of a water rudder, so your post is spot on for me.

I have two questions.
1-Do you think counter rotating props is necessary?
I've read that with a plane like the PBY and with the motors so close, counter rotating isn't necessary. However if it is done as suggested by simply changing the motor leads, does the prop need to be reversed???

2-I like the idea of using a switch to turn off the Throttle/Rudder mix BUT is this really necessary? How would the mix affect airborne fight?

Thanks for your comments,
Don
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Old Jan 29, 2009, 07:15 AM
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Here's how I set up throttle diff on my Twinstar.

http://www.rcgroups.com/forums/showthread.php?t=489099

I have the function switched, but I leave it "on" all the time. It works great! I even have the rudder/aileron mix enabled which also then activates the thottle diff even on aileron turns. I only use about 20% mix and it works very well for ground taxi and airborne stall turns. In the air it just makes the rudder feel more effective. I haven't found any downside yet.

We have just set up a friends glow powered Twinstar with this same mix and it works just fine there also. For glow you have to be careful not to allow the mix to cause the slow engine to go below idle and then stop. For glow we only mixed the rudder in one direction to speed up the one motor and put 0% mix in the other direction to keep it from slowing down the motor. We did the same for the other motor and the opposite rudder deflection.

Mike McD
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Old Jan 29, 2009, 07:35 AM
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Hey Mike,
You are fantastic!!!
Your article was so full of information my eyes are still spinning, (with the proper amount of differential, naturally).
Thanks for writing that review, it answered so many of my questions.
AWESOME,
Don
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Old Jan 29, 2009, 03:27 PM
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Caution! Caution!

*Danger*

Hi Guys/Girls,

Just a wee word about the throttle mix/rudder set up, from personal experience. I don't want to teach any of you to suck eggs, however.
1. Some radio systems have mixers that have an accumulative effect, add on.

2. With my radios, Graupner/JR, I mix in only a negetive value for the differential, just slow down the into turn engine(s). More than enough for water taxying.

3. I read my programming manual and it suppies caution, as you can go from 100% range of servo movement to 200%, the worse you do to a servo is load it, strip the gear teeth, break your push rods or have a high current draw, then they melt.

4. The dangerous set up is when your ESC is set up to read the 100% normal range of travel on the throttle channel output, then you increase it by another unknown amount with the mixer switched in, you might get away with a real sharp yaw in flight, or like me, you may get the model jump sideways across the bench, with one motor about to have a containment failure.

5. Just go easy and reduce, Not increase, read the manual for your specific radio, and keep those wee piggies out the of prop, or where the prop might end up, in the blink of an eye.

Cheers, regards to all, Al
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Old Jan 29, 2009, 06:48 PM
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1-Do you think counter rotating props is necessary?

Other than for very special cases - no.

I have two twins (one of them a PBY) and a tri-motor, none contra rotate. Think about it - you are just spreading the load over mutiple engines. Total power/torque is the same as the equivalent single.
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Old Jan 29, 2009, 07:28 PM
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Glad to help Don. Twinstar is still flying and I plan to have it back at SEFF this year.

Counter Rotating props have a lot of value in glow powered models. You NEED the torque to help when one of the glow engines die - and glow engines always die at the worst times! For electrics, the benifit is no torque effect on takeoffs or in loops and stall turns. My Twinstar will stall turn right or left exactly the same with the counter rotating screws. So I guess the answer is "no" you don't NEED them for electrics, but they sure are nice once you set them up. My two cents.

McD
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Old Jan 30, 2009, 01:21 AM
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Thanks for all the great informative info guys.


JOAO,
I have been trying to find a way to get the HVP Hu-16 Albatross in the US. Any ideas or if anyone will shio to the US from the EU? Thanks
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Old Jan 30, 2009, 02:52 AM
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Hello,

I got back in 2001 both a Hu-16 and a Catalina from PAF . No problems (within Europe) from Germany to Portugal.
It looks like they still have it:

http://shop.strato.de/epages/6131678...Shops/61316787

a) Got both birds without contra rotating props and they fly very well indeed

b) I have the same set-up as Al, that is, just slowing one motor for water taxing

c1) The "problem" with waterplanes larger than parkflyers is that you want to fly on sometimes windy conditions as opposite to very light models.
In windy conditions the models tend to weather vane strongly and without power diff. you canīt get them to turn at all !

c2) This problem is evem more critical in flying boats than in floatplanes (with 2 floats) because, in a turn, the "outside " float drags in the water, causing an effect similar to a down aileron drag, oposing the "will" to turn. Two float floatplanes have a more "symmetrical" water drag behaviour when taxing.

d) In windy days and inland water surfaces, expect sudden wind direction changes during take off imediatly apparent on the model, resulting sometimes in aborted take offs as you ara unable to reach the necessay speed and also the ocasional "water loop".

my 0.000000000000001 Euros

(A shot of my Hu-16 on a take-off in windy conditions)

Regards to all, Joao
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Old Feb 02, 2009, 06:49 AM
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I (FX-18) prefer the X-MIX:

Motor1 Ch3, Motor2 Ch7

VMX1 (switchable):
ACT, Master 4, Slave 7, 100%, Offset 50%

XMIX:
ACT, Master 3, Slave 7, Rate Master 100, Slave -30%

3 normal, 4 normal, 7 reverse

XMIX puts Throttle to both Ch3 and Ch7, Function3 controls the throttle, Function7 controls the difference, Ch4(Rudder) is mixed to Function7 (switchable)

The switch for VMX1 controls differential steering on/off. The amount of differential can be controlled by VMX1 Rate or XMIX Slave.

2 free mixers are not used.

RK
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Old Feb 02, 2009, 07:00 AM
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One issue I have found when I configured differential throttles is that the second channel - the one you mix to the throttle channel - does not have a failsafe like the throttle channel, at least on my Spektrum DX7 system. So if for whatever reason, you loose radio control, the slaved motor will continue to run while the throttle channel will shut down. Creates an interesting decent!!!

The JR X9303 with a AR9000 receiver does resolve the issue as it has a better failsafe system.

I suppose not a big deal unless you lose radio control for an instant as I believe I did.

Joe
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Old Feb 02, 2009, 08:12 AM
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Quote:
Originally Posted by jpgilbert
One issue I have found when I configured differential throttles is that the second channel - the one you mix to the throttle channel - does not have a failsafe like the throttle channel, at least on my Spektrum DX7 system. So if for whatever reason, you loose radio control, the slaved motor will continue to run while the throttle channel will shut down. Creates an interesting decent!!!
Lowtech solution: use an external X-mixer on the plane.

RK
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