Wing strength q

[David perry 1]

Just pondering. Wing dihedral braces, joiners, call them what you will, impart the strength to prevent wings applauding our flying skills. Often we add centre section bandage with pva or fibreglass too 

 

But which gives more ?  I remember building a super 60 with standard plywood braces and then another with pva bandage and that one was unbreakable in flight.  Mind you, so was the first.  
 

Hhas anyone got any reasonable comparison to say whether more strength comes from wood or bandage or is it just a summation? Would you fly a wing without plywood braces and just bandage for example?

 

Actually as I type this a thought occurs…Maybe the plywood prevents flexing and the bandage separation. 

 

[John Lee]

3 hours ago, David perry 1 said:

Would you fly a wing without plywood braces and just bandage for example?

 

Tens of thousands of kit built Wot4s, AcroWots, Wots-Wots and Phase 6s have been flying for the last 40 years with this configuration! They are foam wings of course where plywood braces give localised stress points whilst bandages can spread the forces over the whole chord.

 

For built up wings I think plywood braces are in general a better engineering solution as the stresses can be transferred directly from the spars, but as ever there are multiple solutions depending on the design for the task in hand.

[leccyflyer]

Foam wings with fibreglass bandages and without ply braces are perfectly acceptable. As John says, a large number of models with foam wings use such a configuration without any problems whatsoever. In fact it doesn't even have to epoxy or polyester resin used for the job - PVA works perfectly well.

[Tom Gaskin 1]

Foam wings use the veneer as a stressed skin so use bandage to spread the load. An open weave surgical bandage can be used with PVA ( the flexible stuff) as Leccy says. A built up wing uses the ply braces to connect the main spars together, so it's 'horses for courses'.

 

On built up high wing models you don't need a too strong centre section if you use struts. My original plan built Senior Telemaster had balsa spars and 16swg struts from the bottom of the fuz, out to mid-span, and could be looped and flick rolled with impunity!

 

Tom

[Geoff S]

I remember the foam winged Precedent Funfly had a plywood spar that fitted into pre-cut slots in the wings and, IIRC, no bandage. I built 2 many years ago. I used the same method on my Percival Mew Gull which works well, too.  Obviously bandage works perfectly well but is harder to disguise completely.  Another Funfly feature I've adopted with foam wings is to fit a soft balsa block at the leading edge join to fit the retaining dowel - easier than drilling a 6mm hole right where the glue is.

[David perry 1]

Yes I have used PVA to great benefit for many years.  Ref the Acrowots etc, I'd forgotten those.    Butt jointed and centre section bandage...

 

Anyway, I have chickened out and opened up my wing to pop in a good ply brace prior to PVA bandage...  at least I shall sleep better.

 

Thanks chaps.

[Andy Stephenson]

The original Acrowot kit had some kind of woven book binding tape instead of fibreglass, it was thinner and made a much neater job.

[Jonathan M]

One advantage of tape around the centre-section is that it helps protect against bruising from the wind-saddle.

[Jonathan M]

I meant to type "helps protect against bruising from the WING-saddle."

[Martin Dance 1]

As a side note, many years ago I built a Ghost Rider 50 a David Boddington design. It had a very thick wing probably in excess of 20% of the chord. David advocated no wing brace or anything else. I flew it with Galloping Ghost and quickly discovered that my lack of skill flying GG an the inherent characteristics of GG did mean the wing needed some sort of wing brace 😀. Once repaired the model flew well until a 'proper' proportional radio was built and fitted to an O/D model.

 

I guess it depends on what you choose to believe when it comes to wing joining/reinforcement of centre sections.

[Andy Stephenson]

The GG50 also suffered from another problem where it could get into an unrecoverable spin. I think it should have had a larger fin/rudder.

[shepeiro]

depends on build weight. For my lightweight slow flier wing I found some glass fibre/CA seamed to work really well to help stiffen and protect the trailing edge and centre section from the rubber bands

[Simon Chaddock]

Its is personal point of view but why joins spars with plywood?

The biggest force on a spar is either tensile on the lower surface or compressive on the upper. The spar material is chosen to best resist these forces so surely they should be joined with a similar material. There is also smaller force trying to squeeze the spar(s) together.  

Plywood is a uniform material so for a given weight it is weaker in tension or compression than the spar material and over strength for the smaller squeezing force.

Of course ply being in sheet form makes it easy to cut to the required shape.

It does rather depend on how fanatical your are about structural strength to weight but spars should be joined taking into account the structural requirement particularly at the root where the bending moments and thus the forces are the highest. 

How often do you see a wing structurally fail other than at the root? This suggests the "conventional" root join design tends to be weaker than the rest of the wing.   

 

You can, like they do in full size, actually test a wing.

This foam sub 250g Super Cub is centre loaded to 3 times its flying weight and supported just by its wing tips.

The wing does bend but so supported it is generating a root bending load that is about 4 times that from flying so with the extra centre weight the root is withstanding roughly the equivalent of pulling a 12g manoeuvre which the plane has neither the speed or power to ever achieve.

Contact with the ground will break it but not by what I can do with the controls in the air. 😉

[Nigel R]

Bandages work. 

Braces work.

 

One or other might be easier for any given wing.

 

Horses for courses.