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Help! Twisted wing

Advice please on how to proceed

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Phil B11/11/2019 10:31:23
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212 forum posts
196 photos

I have acquired a 48 inch DB Cirrus Moth.

Nicely made and finished with sprayed polyester wing coverings.

The wing with the ailerons is true, but the upper wing with no ailerons has a warped left wing half. The twist is tracing edge high, which I think is wash out, which is excessive, about 10 degrees. If the right half was the same maybe this would be OK, but the right half is true, no washout. I can hold the wing and twist it true, wrinkling the covering severely. If I could fix down the right half flat and twist the left flat whilst heating, would this get the twist out or will it just return. With the dihedral, it is not easy to pin down the whole wing. Any ideas? Might it be better to uncover the left side and somehow untwist the structure then recover?

Peter Christy11/11/2019 10:42:16
1831 forum posts

Is the polyester covering heat-shrink? If so (and maybe even if not), simply twisting that wing BEYOND true and warming it with a hot-air gun should get it back in to shape.

It will need a bit of trial and error. When I've had this problem in the past, I sit on a stool, clamp the centre section between my knees, and twist the warped wing section by a roughly equal and opposite amount with one hand, whilst wafting the hot-air gun over it with the other. Let it cool while holding it in place, and when released it will spring back a bit (hence the over-correction). After a couple of goes, it should come true.

You may have to repeat the procedure periodically, as over time, it may return to its original twist.

Once you've done it a couple of times, it becomes second nature, and is a lot quicker than recovering and repainting!

--

Pete

will -011/11/2019 10:50:25
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587 forum posts
19 photos

Hello,

I had this with my SE5A

the easiest solution was to make the rigging functional. I could get rid of the twist and iron out the wrinkles, but as the stress was still there in the structure it just warped back over time.

My solution is to leave the thing rigged and add a cross brace between the struts to pull it square.

Made a huge difference to the flying qualities of the aircraft.,

Geoff S11/11/2019 12:25:49
3701 forum posts
29 photos

I assume the smaller Cirrus Moth is like its bigger 58" version and has leading edge sheeting (neither the 58" DB Tiger Moth nor the 60" Sopwith Pup do) and I would guess is almost impossible to correct without serious surgery - eg removing the the sheeting, pinning everything flat and refitting/recovering.

The top wing is fairly straight forward and it would be easy to make a new one. Perhaps the best solution for the sake of a few bits of balsa. Do you have the drawing?

You could see if DB could supply a wing only kit or, failing that, the drawing. Eddie Stocker supplied me with a complete set of drawings for the 58" Tiger Moth I'd been given for £10 plus postage about 8/10 years ago.

Geoff

will -011/11/2019 12:37:00
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587 forum posts
19 photos

Incidentally this is one reason I prefer to build wings "in the hand" rather than pinned down. I find the pinning down technique can lead to warps appearing as the pins are released as there are stresses built in which are masked by the pins.

Simon Chaddock11/11/2019 12:54:30
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5736 forum posts
3034 photos

Phil

As Geoff and others have pointed out the severity of a warp depends on the nature of the underlying structure.

If the warp is simply due to the covering deliver unequal tension then within reason the wing can be set true and the covering re shrunk.

If the warp is derived from the actual structure than re shrinking the skin will not last long. As soon as covering goes slack for any reason (cold and damp) the warp will return.

Removing a structural warp will depend on the nature of the structure itself. If without any covering the wing is pretty flexible in torsion a long term solution would be to simply recover the wing, if however the wing was structurally stiff and warped then it would have to be rebuilt, at least in part, and any warped components replaced.

A 10 degree warp in a structure that is intended by design to hold a high degree of torsional accuracy suggest something is seriously wrong internally.

Phil B12/11/2019 10:22:01
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212 forum posts
196 photos

Thanks for all the helpful advice.

Yes, the wing has l. e. sheeting top only.

I thought there's nothing to lose in heat treatment first.

I measured and there is 5 deg washin on one tip and 5 deg wash out on the other. So I held the washin tip with a twist to match the 5 deg washout and held it in front of my hot air gun set at 150 deg c. Until all the diagonal wrinkles pulled out to smooth. Now both tips have 5 deg wash out. I will see how long this holds.

I also need to check what happens when connected to the flat lower wing.

Geoff S12/11/2019 10:37:18
3701 forum posts
29 photos
Posted by will -0 on 11/11/2019 12:37:00:

Incidentally this is one reason I prefer to build wings "in the hand" rather than pinned down. I find the pinning down technique can lead to warps appearing as the pins are released as there are stresses built in which are masked by the pins.

I couldn't disagree more.

If the structure is pinned or held absolutely flat by another method (I now use a magnetic building board) and the components are fitted properly then it will not have in built stresses and will remain flat when lifted off the board. I've just built 2 pairs of wings for my Sopwith Pup with them firmly held on the board both for building and joining and they are both warp free as are numerous wings I've built the same way.

You only have to read Peter Miller's design/build threads to see that pinning a wing down on an absolutely true surface is the best (perhaps only) way have getting warp free wing.

Geoff

will -012/11/2019 10:47:41
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587 forum posts
19 photos

Hi Geoff

So long as it works for you, carry on. I've done it both ways and found the free method to be much more reliable.

If you use pins to pull things into shape you haven't eliminated the twist you've merely forced it into a particular shape. Once the pins are removed it pings back (unless you've added a component to resist the twist like D-box sheeting or similar). Of course at this stage you've put a lot of effort into the wing so fixing the problem is annoying.

If you do it free, if there's a twist as you're building you can see it immediately and do something about it as you go. The resultant wing should be stress free and stay untwisted.

Engine Doctor12/11/2019 10:47:51
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2536 forum posts
39 photos

Hello Phil . just a thought but when you shrunk out the diagonal wrinkles did yo do both side of the wing ? the reason I ask is that I told a club member how to do this some time ago and they said it did'n't last long so I said come round to my place and we will sort it . I did the top of the wing while he held it then said keep the twist on and turn it over so that I can shrink the other side and he had a light bulb moment enlightened.

Phil B12/11/2019 11:30:50
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212 forum posts
196 photos

Yes both sides done thanks.

Phil B12/11/2019 12:39:49
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212 forum posts
196 photos

This is the upper twisted wing before heating. The ruler is on the tip rib at the one end of the wing and the black rod is on the tip rib at the other end. The difference in incidence is the angle of twist. The far wing tip has wash in and the near tip wash out.

img_20191112_075723.jpg

I've now changed the wash in tip to be the se as the wash out tip.

Maybe both neutral would be better and to do that I'll have to heat both wing tips. I'll see how it settles first.

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