Some thoughts on the production of smaller gliders...

[Barry_Cole]

No noticeable difference.



BC

[chris williams]

...rather than a continuous length between the spars?

No continuous length here, Peter, each webbing plate is separate

[Peter Balcombe]

Sorry Chris, I forgot that bit.
So what is the difference - structurally - between having the beam in the centre, or on front or rear edge of the spars?
I am sure that the centre position is better, but is it enough to be worth the effort of putting the extra bits in?
Peter

[chris williams]

Having the spar webbing centrally between two separate spars seems to be crucial to the extra strength that accrues... Imagine this: one 50cm length of spar, just the spar, no ribs, consisting of two 6 x 3mm spruce strips tied together on one side with a length 0.8mm ply. If you were to put this structure over your knee and proceed to bend it what would happen? I'm guessing the spruce would twist away from the ply, separating the glue joint before breaking.
With two smaller spruce strips, and the ply in the middle, (once again this is guesswork), the spar might well break cleanly once the wood's tensile strength had been breached. Either way, the case has been proven with my last four or five models, which have significantly lighter wings than with my old methods.

Also, what extra work? 2 minutes to cut the extra spar slots; 5-10 minutes to apply the PVA (the nozzle's always blocked!) and clamp the spar in place.
This is up-to-date-info, because I've just this minute done this on one of the Flamingo wings!

[Jolly Roger]

Chris,

I like! Apart from the benefits of symmetry you mention, there's another practical benefit.

On the 2 occasions I've had spars fail in flight, I've had a good post mortem gander at the structure. Both times the spar had snapped where the grain had an irregularlity in it - a small knot or a section where the grain was diagonal rather than running along the spar.

Irregularities are hard to avoid when working with an organic material, but at least if a spar is made from two pieces of wood then you're highly unlikely to have a fault run through both at exactly the same point.

Another step to optimise the spar structure is to cut the ply shear webs at 45 deg. In theory it improves their performance although I've no practical proof of this and it's probably just a quick way to waste time and precious ply. But I'm a perfectionist so I had to type all that.

Thanks for a really useful set of posts.

Rog

[chris williams]

Thanks Rog, hadn't thought of the double spars in terms of redundancy...as to the shear webs, it would take some considerable twisting force to split them, by which time the pilot's underpants would surely have failed first?

[Jus Gordon]

You should write a book Chris

[chris williams]

Nice idea, Jus, but think of the downside...all those book signings, celebrity status (having to be interviewed by Piers Morgan) and the responsibility of disposing of vast sums of money. I think I'll just stick to building gliders

[mick a]

i was looking at your method chris
and too late now for me , but what if i used 4 pieces of 3mm sq and laminated as i put them in the top spar with the webbing in the middle , a bit more work , but do you think it would be stronger ?

[chris williams]

If you've already built the inner panel, Mick, I fear it's too late (I'll have some thoughts on the Minimoa wings a bit later on in this thread)