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Torque rods vs wing servos

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Tim Donald04/12/2019 15:45:08
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I'm building an old slope soarer kit (SAS Apache) that originally used torque rods for the ailerons driven by a single servo. I was thinking about adapting it to use two servos so that I can alter the camber etc. but the design doesn't lend itself well to wing servos (low wing so servo covers and control horns would have to be on the top looking unsightly). I am now thinking just keep the torque rods but use one servo for each. I know torque rods are not in fashion now but are there really any benefits to using wing servos over this method?

Jon - Laser Engines04/12/2019 15:49:58
5557 forum posts
270 photos

Rigidity in the linkage is the main reason as far as i am concerned. Some would argue reliability but i dont consider that a factor as servo failure is very rare these days.

If you wanted you could still use the torque rods but fit two servos in the middle. You could then do any flap mixing you feel you might need.

Alan Gorham_04/12/2019 15:51:02
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1292 forum posts
145 photos

I might be old fashioned but I still prefer torque rods to servos in the wings for most smallish models.

The proviso is that the torque rods are made and installed so that they cannot develop slop over time. I have several models with torque rods and 2 servos (one per rod!).

Tim Donald04/12/2019 16:42:18
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57 forum posts
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Thanks Jon / Alan. That settles it then, torque rods and two servos it is.

Nigel R04/12/2019 16:54:52
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3981 forum posts
714 photos

I'm with Alan. I find torque rods require less time/effort in both covering and building stages. I believe they're quite common in full size too. Rigidity is just a case of fitting a big enough rod!

J D 804/12/2019 17:03:34
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1523 forum posts
84 photos

One small benefit of two central servos is keeping the mass in the middle of the aircraft but doubt it makes much difference. Have a couple of models with this system an they have worked well for years.

wingcoax04/12/2019 17:44:05
104 forum posts
2 photos

I have fitted torque rods in all my models since i started flying 40 years ago, with one exception when I went overboard and fitted two, one in each wing, although it was a rather thick wing. Never had a problem as I still have the wings from 30 years ago.Note to oneself`»"must build new fuselages"

eflightray04/12/2019 18:59:14
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624 forum posts
132 photos

Twin servos do allow for 'flaperons', should you want to try them.

It is probably years since I last used torque rods.

.

Simon Chaddock04/12/2019 21:18:14
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5733 forum posts
3034 photos

The only consideration I can offer for wing servos acting directly on aileron horns is there are fewer bearing surfaces so the mechanical efficiency is slightly better.

Of course if the servo arm can be fixed directly to aileron so the servo itself acts as the inner hinge then there is no linkage thus giving the best possible mechanical efficiency and with nothing external.

Stephen Jones04/12/2019 23:31:10
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2853 forum posts
1618 photos

Torque rods vs wing servos

The longer the Control surface, Aileron full length of the wing for example and If made from a foam for example .

Would be less effective the further from the control point.

So if you mount a servo in the middle of a foam wing to control the full length Aileron there would be less slop at the tips than there would if was at one end like a Torque rod would have to be.

Now Torque rods are fine on a balsa model so long as the Aileron is made from a hard balsa or if the Aileron is tapered so is thinner the further from the control point.

I still like and use Torque rods on my balsa models as it keeps the wing clean . No nastie control horns to see or disturb the airflow or for fuel grime to congregate.

So consideration is needed when choosing the type of control Material to control , looks , Will control arms be caught up in grass , fast model, ease of installation, how wide is the control surface how flexible.

And as has already been mentioned above the torque needs to be up to the job of if the rod is long or you will get slop.

My 2p worth. take it with a pinch of salt.

Steve.

 

Edited By Stephen Jones on 04/12/2019 23:35:47

Nigel R05/12/2019 09:46:37
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3981 forum posts
714 photos

Rigidity of the aileron itself, is a good point.

I find solid medium balsa surfaces work for me.

Tim Donald05/12/2019 11:26:40
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57 forum posts
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Thanks again everyone. I'm going with torque rods and twin servos. The ailerons are fairly still so I don't think flexing would be an issue as it might be with thinner ones or foam. Good point to remember for future though. The supplied torque rods seem pretty strong so hopefully the finished mechanism will not have too much flex or slop. It's funny that I only ever read about how torque rods are so out dated but actually lots of people seem to still use (and prefer) them.

Stephen Jones05/12/2019 15:45:35
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2853 forum posts
1618 photos

Hi ,

I could be wrong but i think we saw the introduction of servos in the wing being used in ARTF`s of which most were foam, Balsa ARTF`s came later. My Guess it was easier to manufacture that way.

So most will of come to learn on those models and that way would of been the norm.

Steve

Doc Marten05/12/2019 16:33:49
626 forum posts
7 photos

2 servo torque rods is what I used them on my last build:

img_20190813_210734.jpg

Nigel R05/12/2019 16:36:24
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3981 forum posts
714 photos

I don't know about out dated. Unfashionable perhaps. They do require a touch of care when fitting, to get nice free movement (and enough of it).

Wing mounted servos definitely win for ARTF manufacturers, as Stephen says, the manual labour involved in neatly fitting torque rods to the wing root can't work for them. When considered against a laser cut liteply servo box which would just need supergluing in place.

Don Fry05/12/2019 16:56:37
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4557 forum posts
54 photos

An observation about how stiff we need the assembly. Ailerons are not potential big loads like rudders, or elevators. They are a pair, and roll the machine round their centre of gravity. And also, as a pair, if there is a bit of induced twist, it's symmetrical.

High G aerobatic, precision aerobatics not included in this observation.

Nigel R05/12/2019 17:22:17
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3981 forum posts
714 photos

I think flutter might be the only real gotcha with a weak aileron held in place at one end by the torque rod.

As for forces, ailerons and even the elevators are a light load. There used to be an online calculator somewhere, into which I plugged a bunch of numbers, and came to the conclusion that two ailerons together was approximately the same load as the elevator. Rudders, yes, much bigger, about double the load of the other surfaces.

The calculator I refer to was removed when it turned out people had been using it to calculate forces involved with full size microlights as the target application.

From notes, long story short, etc - 60 size model, travelling at 100mph, ailerons/elevator need ~30 oz in (bog standard S148 / JR507 / etc) and rudder needs ~60 oz in (average standard digital). And most don't travel at 100mph. For a 40 size model, forces were about half - a bog standard standard servo covered everything.

Edited By Nigel R on 05/12/2019 17:22:40

kc05/12/2019 18:03:34
6575 forum posts
173 photos

Don't you think that a single servo has some advantage in that the loads on the servo gear are somewhat counterbalanced? Not very significant perhaps but worth considering if substituting 2 small servos for one standard servo. Surely the twin servos need to be each as good as the single one they replace and not just half as powerful as a first guess might suggest.

A slight disadvantage to torque rods is that you need to remember to make a handed pair and not end up with 2 left or 2 right hand rods! Also you probably need to place the bearing parts on before bending. and ensure any differential is bent in correctly.

Another disadvantage with torque rods ( and wire elevator joiners) is that over time the rods can loosen in the balsa and give less movement which is not easily spotted.

Martin McIntosh05/12/2019 18:10:03
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3471 forum posts
1216 photos

Torque rods and kwik links yuk! Two servos and ball links for me other than on drag brakes. Horns under a wing rarely get ripped off on landing.

Doc Marten05/12/2019 18:19:11
626 forum posts
7 photos
Posted by Martin McIntosh on 05/12/2019 18:10:03:

Torque rods and kwik links yuk! Two servos and ball links for me other than on drag brakes. Horns under a wing rarely get ripped off on landing.

I'll change them immediately.

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