[WWI] vac wings

iban ibancorp at TDS.net
Fri Feb 9 06:05:11 EST 2007

thank you shane (and paul for forwarding shane's message).

now that you've pointed it out and explained it, i have to admit it 
makes complete sense.  i was leery of any fabric sagging anywhere, 
because i know what that causes in terms of flapping, vibration, and 
drag, not to mention damage to the canvas (just leave one corner of a 
truck tarp loose and take a long day's drive to see what i mean).  but 
as you've well explained, it's nothing to do with anything sagging at all.

thanks for the insight.


Shane Weier wrote:
> 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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