Fuselage strengthening advice please

[David P Williams]

Hi all - I'm (slowly) building a Flair 1/4 scale Stearman, intermittently documented in another thread. The fuselage is entirely made up of formers and stringers or longerons as here -

 

 

 

I've never built a fuselage this big the doesn't have some sheeting somewhere and although it seems quite stiff and resistant to bending it seems worryingly floppy in torsion. It is built exactly as the plan and instructions, but I have this mental image of the tail shaking itself to pieces.

 

Do you think the intended Solartex covering will impart enough strength to resist this?

 

If not, how should I go about bracing to improve the torsional stiffness?

 

What is the experience of other Flair Stearman owner/builders?

 

All opinions and advice welcomed

 

[John Stainforth]

I would be tempted to put strips of sheeting half way down the fuselage sides, between the middle pairs of longerons; or, if that is going to spoil the scale look too much, a zig-zag of diagonal struts along the fuselage sides, between the formers on the inside of the longerons.

[GrumpyGnome]

Would braces across the formers help? Totally invisible from outside..... top right to bottom left on front of former, top left to bottom right on rear.  Probably quite a challenge!

 

GG

[Nigel R]

I think you'd need braces between the formers GG, I don't think across the formers will stop the torsional twist.

 

3 hours ago, David P Williams said:

Do you think the intended Solartex covering will impart enough strength to resist this?

 

If not, how should I go about bracing to improve the torsional stiffness?

 

Yes, I think it most likely will provide enough strength (unless it gets baked and slackens off)

 

and

 

if you want to brace with wood it has to be diagonal between formers, although it looks a bit late to get easy access to the inside now.

 

Another option would be to do doped nylon or silk or similar traditional covering, as that will not slack off.

[Don Fry]

I don’t think I would worry too much. Flair kits were sorted. Adding weight won’t be a benefit. The solartex will stiffen it fine.

[Piers Bowlan]

I think once the model is covered the model should be torsionally stiff. Solartex, if you can get it, or Oratex is preferable or as Nigel suggests, a doped traditional fabric. I bought some very lightweight Poly-Fibre fabric for a project from the LAA Sywell show a couple or years ago but havn't used it yet. I think it is polyester fabric and 'Aircraft Coverings' can supply adhesive as well as filling dope and cleaner etc. Could be a bit pricy for a one off project but would make for a very tough/stiff covering. Once glued on the fabric is shrunk with an iron then the weave filled with dope. The lightweight fabric is not approved for full sized aircraft although it is used on some microlights.

Edited March 22, 2021 by Piers Bowlan

[David P Williams]

Thanks everyone. I found an old thread on RCUniverse where someone had infilled between formers and stringers in four strips with 1/64 ply, but looks like a huge amount of faff and others hadn't bothered and seemed fine.

I bagged two 10m rolls of Solartex when they closed down, so that's what I'lll be using.

I think I'm just worrying too much ?

[Simon Chaddock]

David P

What will have the greatest impact on the fuselage torsional rigidity will be its skin.

This type of construction relies on the skin being taught and reasonably stiff. It was likely doped heavy weight tissue or possibly doped silk. Once the dope has shrunk the fuselage will have no discernible torsional 'give'. The only limitation is too much shrinkage will cause the stringers to sag between the formers and give the fuselage a 'starved horse' appearance. 

If you intend to use a modern heat shrink film it may be slightly more flexible than a doped skin but remember a slight torsional movement does not alter the incidence between the wing & tail plane.

[David P Williams]

Thanks Simon, I don't have any worries about incidence changes as the fuselage is very stiff in bending even with no covering. The Flair instructions recommend Solartex which is what I'm intending to use, so hopefully all will be OK.

I was intending to make the 'baggage compartment' hatch behind the rear cockpit functional to hide switches etc so will have to do some local reinforcement anyway.