[WWI] vac wings

Shane Weier bristolf2b at hotmail.com
Fri Feb 9 04:12:26 EST 2007


iban says

>>Speaking about undersurfaces, look at the most modern Roden or Eduard 
>>kits:
>>the ribs are recessed on them! Fabric, attached to the ribs, was sagging
>>between them
>
>interesting.  are there any photos to back this up?

Thousands.

Over  10 years ago I pointed out on this list that the only correct 
representation I'd ever seen (though exagerated) was on the DML Fokker Dr.1 
which was maligned in model clubs and magazines everywhere.

Since then model manufacturers have actually *looked* at fabric wings and 
the undersides of the better ones now show the slight depression along the 
ribs on the concave part of the undersides.

Incidentally the fabric is NOT NOT NOT sagging on either the top or bottom 
surface. It should be tight as a drum.

Think about what is happening. The fabric is tightened between the ribs, and 
pulls taut, but is also tring to pull flat in a chordwise direction between 
the leading and trailing edges.

Draw a curve and see what a straigthened line between LE and TE does - it 
drops below the curve.  It does the same on BOTH surfaces, so is lower than 
the ribs on the top, and also lower on the bottom.

Now think about the shape of the wing. More curved above, less below. That 
means that the fabric pulls further into the curve above, and less below it 
beneath because the underside starts out flatter.

There's two other factors. One - The underside isn't entirely concave, but 
usually convex about the first third, so there the ribs stick out, and 
further back the fabric sticks out

And two - the degree to which this happens depends on the amount of 
undercamber, the pitch of the ribs, the width of the wing, whether there's a 
ply covering on the LE and whether the TE is solid or wired.

I measured this on 6 different types of period aircraft 10 years ago. 
Sometimes the depth underneath is so small that only a straightedge reveals 
it, but it *is* there.

Shane
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