Large Model Association Forum

I would be gratefull if someone can point me in the right direction with regard to the stress requirement of a wing
I have built a 8-ft wing with 15" chord plus 3" for flaps and ailerons from 4mm Correx. The ribs are 2 times 4mm correx there are four half inch square spars the full length of the wing front and back of ribs, both spars have been webbed the full length of the wing in 8th light ply. The correx is of course in one rap round piece when it is finished. So I put this wing on two tressles at each tip. What would be an acceptable weight to put in the centre to check whether the wing will not fail under normal conditions. The whole model is being built as a large trainer type model with this material as two 8x4 sheets only cost £13.00 and the rest of the model I am using anything left over in the hanger (garage).
I await your replies.
Cheers
John Evans 1915

Hi John. Stressing a wing as you describe ,if it is a cantilever wing, would be a complicated analytical maths problem. Also, supporting a wing at the tips is not how a wing supports the plane in flight. It would be better to support the model at the centre of lift, and in the absence of a plan form I would suggest supporting the model half way between root and tips.Add weight to the fus to allow for g forces and a safety factor, about 3 or 4 times fus weight.
Another good check is to measure the deflection at the tips under load, a good indication of stiffness, not too much deflection. Thats my two pence worth, some other informed member my be able to help further.
Hope to see you soon, Binham or Wintn.and mind that tree, you can't stess it for hitting that.LOL
Cheers, Barry.

John- the main requirement would be a wing joiner/spar spanning the centre 600mm (or more!) of the wing. You do not say what your spars are, but even with spruce you will need to strengthen the centre section. 8-10 mm ply (birch, no liteply!) would be ok. Don't forget that trainers can be subject to 'unusual' stresses- one reason why I stopped using my Greenley as a trainer! Smaller models are much more suitable to the vagaries of first time fliers! Make sure the brace is well glued to the spars and webs. watch for the tree....