Torque rods vs wing servos

[Tim Donald]

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 H]

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_]

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 Donald]

Thanks Jon / Alan. That settles it then, torque rods and two servos it is.

[Nigel R]

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 8]

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.

[wingcoax]

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"

[eflightray]

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

It is probably years since I last used torque rods.

.

[Simon Chaddock]

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 Jones]

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 R]

Rigidity of the aileron itself, is a good point.

I find solid medium balsa surfaces work for me.

[Tim Donald]

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 Jones]

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

[Former Member]

[This posting has been removed]

[Nigel R]

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 Fry]

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 R]

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

[kc]

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 McIntosh]

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.

[Former Member]

[This posting has been removed]

[John Minchell]

The blue foam wing on the Apache is deep enough to take two std size servos side by side and still using the torque rods be a successful set up - I am doing exactly the same thing with my Apache 20 years after buying it.

John M

[Tim Donald]

Thanks John, that is what I am going to do. I have built the wings now but not cut the servo holes yet as I might use smaller servos but I've spaced the torque rods to allow for standard size.

One problem that came flooding back to me was that if you mount torque rods high enough to top hinge it makes covering the tube or mountings difficult. If the torque rods are lower then they pull on the hinges. Never really worked out a good solution to that, it always feels a bit bodged. Any solutions to that would be good to know.

[Capt Kremen]

I'm in the process of renovating a 1990s (successful!) electric model, the 'Todays Hobbies Inc. " SkyVolt". This was a kitted version of the design from the pioneering US electric flight enthusiast and 'Model Aviation' columnist Bob Kopski. Although these days I would always use separate aileron servo, one in each wing panel, this design features a single centre(center!) servo.

The 'built up' ailerons are top hinged. I let-in a small off cut plastic tubing to the top surface and covered this with thin ply to strengthen and support the buried aileron wire. The attached pics I hope show this sufficiently clearly. Covering is no problem, at least the Solarfilm Fibafilm covering is still strongly attached some 30 years on from first application!

[John Minchell]

Tim

You can centre hinge the aileron, then the torque rod pivot would be in the cente of the thickness of the section and because you won't need any differential on such a short aerobatic wing there is no issue. You won't be using mega angles of down flap as it'll tip stall even with the washout in the pansls as standard, so a neater all round solution. And just a few degrees of down on both ailerons for a bit of thermalling assist. Well that's what I'm doing anyway.

John M

[Tim Donald]

Thanks Capt, that is effectively what I have done. It's ok but took a lot of faffing. I'm 50/50 if it was the best choice but its done now.

Thanks Jon, yes I did consider a centre hinge and looking back I think that is what I used to do. I didn't want there to be a gap on this one spoiling the top surface but it probably wouldn't have made a lot of difference.