To Brace Or Not To Brace That Is The Question

[Dai Fledermaus]

Are ply centre wing braces a remnant in aeromodeling left over from the days before we had Epoxy and Glass Cloth? Are they really necessary to produce a strong accurate dihedral joint?

[Bob Cotsford]

In a built up wing the stresses go through the spars so you need the ply brace to tie them together, I wouldn't bother with additional glasscloth reinforcement. In a foam wing the veneer acts as a stressed skin so you use glasscloth to tie the skins together.

Horses for courses.

[Geoff S]

Back in the spring I built a Skyshark Fantasy from a kit I bought on eBay a few years ago. The wing is a built up structure and the instructions are to join the wings with the supplied glass cloth bandage. I've used that method with foam wings but never with built-up ones. The biggest bind with that method is trying to hide the bandage when you cover. That's unavoidable with a foam wing (though IIRC Precedent/SLEC used a couple of spars embedded in slots to join their Funfly wings). I deviated from the instructions and fitted a plywood wing brace which I think is far more elegant solution.

I must be right because Peter Miller does the same.

Geoff

[IanN]

Posted by Dai Fledermaus on 03/01/2016 11:30:03:

Are ply centre wing braces a remnant in aeromodeling left over from the days before we had Epoxy and Glass Cloth? Are they really necessary to produce a strong accurate dihedral joint?

I'd go the other way and question whether epoxy/cloth bandages are necessary for a built up wing?

A ply brace to tie the wing halves together via the spars is imo a must

[Biggles' Elder Brother - Moderator]

I agree Ian. In a built-up wing structurally the bit that looks most obvious to us (ie the ribs) contributes very little to the strength. Its the spars, allied to any leading edge sheeting and shear webs to form a D-box that provides the strength. If the wing is built is two halves (as is common) they you must have a way of allowing the two sets of spars to act as single unit through which the loads can be distributed. The ply brace is the neatest, lightest and most effective way that has been developed so far to do this.

An alternative - beloved of the ARTF manufacturers - is to provide a "wing joining tube" - the holes for this tube being fixed to the spars. This has the advantage of course of allowing the wings to be taken apart into two units. But the principle - ie connecting the spars top and bottom, left and right, is still valid.

Foam veneered wings - as has been pointed out - are a completely different type of structure, a stressed skin and so different rules apply. Now you must connect the skins - hence why a F/G bandage works.

BEB

[Erfolg]

George Stringwell's book "Thermal Gliding" indicates and discusses many ways of joining wing panels.

Perhaps Georges strongest suite is that his approach builds on practical experiences and then relates this to theory. Interestingly, bandage type jointing has little place in the world of large glider type models.

Although the joining of butt jointed wing ribs seems a poor way of joining wing panels, in the real world, this does work, with some advantages. As with most things though it needs to be done well.

[Robin Colbourne]

The problem with ply wing joiners is that at least 1/3 of the wood is going in the wrong direction. I once saw the wings on a Precedent Hiboy Mk1 'clap hands' as it snapped its ply joiners pulling out the bottom of a steep dive. Fortunately as the wings came off they knocked the tail off as well, so the fuselage came down from 200ft or so, falling horizontally rather than lawn dart fashion, as engine torque was spinning in opposition to the propeller. It landed in a bush and was flying again the following weekend, this time sporting a fibreglass bandage.

If the dihedral angle isn't great, joiners of hard balsa or spruce are equally good, provided they taper away to nothing to avoid a stress point where they end. Ply is just easier to use. Make sure you sand off the release agent from ply manufacture though.

[Simon Chaddock]

A "mitre" joint is the about the lightest possible way to join two spars that are at an angle.

To work well not only does it requires some fancy joinery but the also wing has to be built in one piece!

[PatMc]

If two pegs & two screws are used for wing fixing there's a lot less stress on the wing joiner.

I have an old Protech Unlimited that uses two soft 2mm ali sheet joiners, the front one is 12mm deep, rear is 9mm. They go into ply sided boxes formed between main & rear spar respectively, main joiner is 22mm long, rear 14mm.

If it wasn't for the pegs & screws the joiners would not have been able to take the loads & would have bent after the first sharp elevator input. As it is the model can be thrown around no problem, wing loading is about 15oz/sq ft.

[Henri Squier]

Dear Fledermaus ,

I agree with the facts it depends on many factors : size, use (quiet / aerobatic), plane or glider, wing structure (sheeted or not / foam venered), aesthetic ...

But finally in these specific situations you don't really have much choice.

However, up to my experience for planes < 4 pounds both works well. Up to you to choose what suits best for the project.

In the way to build Tony Nijhuis 46 Spit I was quite worried by the glass cloth as only joining system : I always built planes with ply joiners and thought it was the only way to do.

So I made - sideway - an other plane with this kind of joining :

Before releasing by fear of destruction, the wing (110cm / 43 inches - for a 2 pounds plane) could handle 3 pounds at the tip (pressure on the middle of the wing) and 5.5 pounds midway.

As someone made comment at this time "plane are built to fly not to overcome a crash", I think we usually built to strong or at least much more stronger than really needed.

[Erfolg]

The version of the ply brace that Simon shows is presented by George, where he indicates  a better engineered solution.

In Georges version, the brace has a open birds mouth at each end of the brace, this puts the load bearing material on the outer surfaces preferentially, with a shear web effectively reducing in strength.

In all examples, the best principle is always to get the load carrying fibres at or close to the outer face of the load carrying beam.

Studying large model gliders, typically +100" does show how to effectively control stresses and bending loads, in addition to torsional forces in wings. So many have solved the problems and learnt the lessons in the past, it does seem that we owe it to our selves to learn from the experiences of others.

Edited By Erfolg on 03/01/2016 17:49:54

[Peter Miller]

I have mentioned this in other threads but it bears repeating.

My Dancing Queen which is very similar in size to Oodalally and Ballerina uses my standard constrcution witha 1/8" ply dihedral brace. No glass skinning and the wing with a very accurate butt joint. IT was powered by an OS Wankle engine.

I fitted an Eagle Tree G meter. Then I did a long, full power dive and hit full up elevator.

The G meter recorded 24.8 G which meant that the model weighed 95 lbs at pull out.

NO DAMAGE AT ALL

Does that answer your question?

[Dai Fledermaus]

Posted by Peter Miller on 03/01/2016 18:36:17:

I have mentioned this in other threads but it bears repeating.

My Dancing Queen which is very similar in size to Oodalally and Ballerina uses my standard constrcution witha 1/8" ply dihedral brace. No glass skinning and the wing with a very accurate butt joint. IT was powered by an OS Wankle engine.

I fitted an Eagle Tree G meter. Then I did a long, full power dive and hit full up elevator.

The G meter recorded 24.8 G which meant that the model weighed 95 lbs at pull out.

NO DAMAGE AT ALL

Does that answer your question?

Thanks Peter I guess that's the definitive answer. In any case, I found it a pain, when joining the wings on my Uno Wot, recently to get a decent finish with epoxy and glass cloth particularly at the edges. I wouldn't use it on a built up wing if I didn't need to. I was playing Devil's Advocate in posing the question.

[Martin Harris - Moderator]

Posted by Peter Miller on 03/01/2016 18:36:17:

I fitted an Eagle Tree G meter. Then I did a long, full power dive and hit full up elevator.

The G meter recorded 24.8 G which meant that the model weighed 95 lbs at pull out.

NO DAMAGE AT ALL

Does that answer your question?

Don't try this at home with a 16 ton 91 lb weight...remember that the effective weight of the wings under G loads opposes the bending moment - still proved the point, though!

[Martin Harris - Moderator]

...and whatever you do, don't copy the maker of a small Extra that a friend bought from a well known retailer at a show - they used lite-ply for the brace which lasted until the first application of G in a normal loop entry. Rather disappointingly, they didn't want to know my friend when he complained - and neither he nor I have ever been back in their tent or considered an on-line purchase!