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Replicating Fabric Wings in Depron

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Dave Hopkin16/05/2015 14:42:11
3672 forum posts
294 photos

As a convert to Depron, all my inadequate efforts so far have been on aircraft that in real life were metal skinned as that suits Depron construction perfectly - But I love the look of fabric covered wing and older aircraft, so I decided to have a little experiment to see what could be done......

The method was originally suggested by Tony Bennet as uses undersize ribs between the normal ribs, so that when the skin is glued to the ribs the undersize ones will pull the skin into a curved indentation to imitate the shape of fabric wings.....

So a small section of wing plate was cut out, a main spar, aileron spar and a aileron leading edge fitted (to mimic how I would build a proper wing) and rib positions fitted thus:

001-plate and spars.jpg

The solid lines are where normal ribs will go, the dotted lines for the undersize ribs

The full ribs in place and faired into the shape

002 - main ribs in.jpg

Undersize ribs added (undersized at the spar end merging to full size at the Aileron spar

003 - sub ribs added 002.jpg

Skin cold formed on the kitchen unit edge

004 - cold formed skin.jpg

Rolled till its a reasonable fit

005 - ready for glue.jpg

Now the messy bit - glue smeared and kin and frame mated together - the skin was massaged down to touch the undersized ribs, the back of a metal spoon proved a useful tool here!

The leading edge was simple "pinched" with a gentle pressure on the ribs and a massage with the heel of the palm....

006 - skinned.jpg

007 - skinned.jpg

So there you go.... I dont think there is a great deal of difference between the front and back of the spar and I am wondering if there is actually a need for the undersized rib at all

Once covered with some light glass tissue and the weave NOT fully filled in with PU Varnish leaving a woven surface effect.....

So what do we think chaps N chapesses does it look like a fabric wing to you?

Mowerman16/05/2015 17:03:22
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1536 forum posts
105 photos

Yes, you have got the effect nicely.

Simon Chaddock16/05/2015 17:17:37
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5416 forum posts
2831 photos

That's not bad at all but from a purely structural point of view 'depressing' the skin between ribs will significantly reduce the skins ability to resist compressive loads so the underlying structure would have to be made stronger/stiffer to compensate.

Glueing the skin to the lower intermediate rib is better but I suspect it would be a bit of nightmare to do on a typical 'fabric' wing that tends to have quite closely spaced ribs already.

Dave Hopkin16/05/2015 17:32:50
3672 forum posts
294 photos

Yes Simon, I noticed on the test piece, there is more longditudinal flex than I would of expected - so I suspect depressing the skin does weaken the whole.....

I dont think intermediate ribs are the way to go, perhaps a web between the ribs with a curved profile - so it forms another spar would be a better way to go - a Carbon strip could be introduced underneath to give additional strength?

Colin Leighfield16/05/2015 17:44:20
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5920 forum posts
2481 photos

image.jpgThat looks the part, although Simon's observation needs to be noted.

However, it seems to me that as long as you protect the Depron with a couple of coats of EzeKote or PolyC, you can actually dope tissue, nylon or silk onto it, as you do with balsa. Therefore if you build an open structure, as long as it is strong enough, it's possible to do this and accurately replicate a fabric covered surface. I've done it image.jpgon the Chipmunk rudder and elevators. The only extra action is to put 1/32" balsa cap strips on the ribs first.

Simon Chaddock16/05/2015 19:56:15
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5416 forum posts
2831 photos

The other aspect of depressing the skin is the resulting distortion to the wing section particularly that ahead of the spar.

It was common on many WW1 types to use short intermediate 'riblets' to reduce the 'sag' and so better maintain the the leading edge of the wing profile. By contrast the curvature aft of the spar tends to be more gentle so the sag is less noticeable indeed the low pressure can actually make the fabric aft of the spar bulge slightly when in flight.

Now that would be quite trick to do in Depron!. wink 2

Dave Hopkin16/05/2015 23:10:30
3672 forum posts
294 photos

Certainly 3mm riblets could be added in the leading edge that would be pretty easy and minimal weight increase

Bulging behind the spar in flight, short of fitting a concealed EDF blow air into the wing I think I'll pass on that!

Seriously though, how much impact on lift is that leading edge distortion going to make? will it be significant in a model (as opposed to the full size)??????

Colin Leighfield16/05/2015 23:42:49
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5920 forum posts
2481 photos

Dave, I think at the sizes we are building, you can ignore it for all the difference that it will make. It's just about how accurate you want the appearance to be. It just seems to me though that if you want to reproduce a fabric appearance, use fabric.

Dave Hopkin16/05/2015 23:54:38
3672 forum posts
294 photos

Thanks Colin

Partly its the challenge of making Depron do what I want to do with it! Partly its a concern about wing warping using shrinking dopes on a open structure Depron wing

My skills dont allow me to even consider approaching the true scale beasties, so just stand (well) off (and squint) scale will do for me......

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