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        Discussion Parallel Servos

#1 Chophop Jan 20, 2013 08:31 AM

Parallel Servos
 
I am looking at the 27% GP Edge 540 and they parallel the rudder servo. Might this cause as fight between servos if they are not performing exactly ?

The two servos are linked together by two control rods, one on each horn arm, then to the rudder. No photo available but is in the assembly manual.

http://manuals.hobbico.com/gpm/gpma1414-manual.pdf

#2 davidmc36 Jan 20, 2013 08:38 AM

Yes it sure can cause them to fight.
What are you using for a radio. Many radios now have a built in function to allow you to equalize their travel and even do it with a curve so it is equal all the way through the travel.

#3 Chophop Jan 20, 2013 10:49 AM

I can mix and limt the travel. Looks like a minute difference to trim. I thought it may be a bad idea if the servos were mismatched. If I get the plane I'll look for a single servo to do the job, but I suppose two standard servos would be a lot cheaper, the whole idea.

#4 davidmc36 Jan 20, 2013 11:58 AM

Just mixing and limiting travel may not be quite enough if there are differences throughout the travel of the rudder. May be just fine if you are very careful installing the horns so the travels on both sides of the rudder are equal.

If they are not exactly the same servo one may want to travel faster than the other. If you have a radio with the ability, that problem can be equalized out with the servo matching function.

#5 62pilot Jan 20, 2013 01:18 PM

Hey chop, if needed you can always put each servo on it's own channle and run two pushrods, one to each ele half. Once again, just make sure travle is equal on each side or you will induce a roll.

#6 Chophop Jan 20, 2013 02:59 PM

I don't have servo match. No problem, I'll get around it.
62, it's for the rudder. I guess the rudder on a 27% Edge (78 inch) is about as big as wing on some small planes. I want to find out the need for two rudder servos. I do hear some are using a single high torque no problems, but now maybe next summer it will be "my rudder servo failed, pece of junk".

#7 Chophop Jan 20, 2013 03:47 PM

1 Attachment(s)
I found the photo. I can't see why a single high torque servo couldn't do the job. Maybe high stress knife edges ?

#8 davidmc36 Jan 20, 2013 05:06 PM

I don't see why one strong servo would not be fine for that setup. I saw recently a bit different setup that used two servos but it was set up with a lost motion device so you would have partial control if one servo failed. It was set up so that the failed servo would not have to be driven by the servo that was still working. Personally I don't see the point of the set up in that photo. Maybe somebody can enlighten us but I would go with one high torque servo and you have the pull-pull cable there so you don't have the problem of a push-pull rod bending under load.

#9 Alien_Tech Jan 20, 2013 10:01 PM

You can always use one of these: http://www.swbmfg.com/categories/2X-...g-Servo-Trays/
Minute or even large differences in travel make no difference. Heck, you could even use two totally different types of servos. :cool:

#10 Chophop Jan 20, 2013 10:25 PM

Since so many people are running this setup there must be a reason. Two standard size servos won't break me. I can run 'em in the excersizer and see what happens.

I can think of one good reason for it. Wanna see my dual servo rrrrrruder setup honey ? Check this out ! See how well they move ? Waaaaannnna give it try ? :D

#11 Magne Jan 30, 2013 07:19 AM

Quote:

Originally Posted by Alien_Tech (Post 23877025)
You can always use one of these: http://www.swbmfg.com/categories/2X-...g-Servo-Trays/
Minute or even large differences in travel make no difference. Heck, you could even use two totally different types of servos. :cool:

I would not have put one of these things into a 27% model.
It weighs 10 os (280 gram) and cost $140. Two cheap servos and this thing costs more than a high torque servo, which weighs 2.3 oz.
I use one servo for the rudder (Futaba BLS 157 HV) in models up to 2600mm/102" wingspan) and one Hitec 7990 servo in a 3,2 meter (126") wingspan IMAC model.
That works fine for me, flying IMAC Advanced.

(Maybe not as cool, but functional.)

Magne

#12 Chophop Jan 30, 2013 01:02 PM

I appreciate your honesty Magne. $140 IS too steep for a servo tray. Those, like the $50 hamburger won't go far. I'll look those servos up, i am Hitec confident and will need to try some Futaba's.

#13 ChillPhatCat Jan 30, 2013 01:41 PM

I don't see how it would really matter if you run a pull pull cable with one cable on each servo... there's a little give in the system, probably enough that it will equalize out as long as they're pretty much equal, which two servos of the same model with the arms in the same place probably can do (provided they're on separate channels).

That said, I have a hitec 805BB servo for about $40 however... it puts out over 300oz-in of torque @6v. It's a little heavy but man does it pull.

#14 davidmc36 Jan 30, 2013 01:53 PM

Quote:

Originally Posted by ChillPhatCat (Post 23981043)
I don't see how it would really matter if you run a pull pull cable with one cable on each servo....

Interesting idea, never thought of that one before. Different than the picture linked above with the two servos hard-linked together. But I guess it would only be the same as using one servo, since only one at a time would be doing the work depending on which way you were pulling the rudder.

#15 Vince Herman Jan 30, 2013 02:13 PM

Quote:

I can't see why a single high torque servo couldn't do the job. Maybe high stress knife edges ?
Quote:

Since so many people are running this setup there must be a reason.
I understand that the reasoning behind ganging multiple servos is to increase the torque while keeping the speed.


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