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121 Provisional Relief

10/07/2014

This process has been used to cover tens of thousands of aircraft. From 1911 to 1940, it was how the vast majority of aircraft from the smallest sport plane to large airliners, from the slowest glider to the fastest fighter plane were covered. It was applied on an industrial scale by manufacturers large and small worldwide, by workers who were not highly trained experts. It must therefore have been studied and documented by any number of governments, manufacturers and universities and been well understood and repeatable.

So how does it work? How does the doping process shrink the fabric to provide a taut, flat surface? Nobody knows. The experts we’ve spoken to don’t know; I can’t find any explanation on the internet.

How much does doping shrink the fabric?

My test piece, using an unrestrained 500mm square with a single coat of nitrate tautening dope showed no shrinkage at all. After 24 hours it still measured exactly 500mm square. One expert said that it will only tauten the fabric if it’s already in tension, and yet the initial application of the dope will always make it relax, so that wouldn’t seem to be the explanation.

How much tension should be applied to the fabric to start with? Based on the above test, the answer would seem to be that the fabric must be stretched as much as you will need on the finished aircraft, and yet experts say that it should be straight, but not in tension, otherwise the structure can be crushed.

Should you apply water to the fabric before doping? Two experts say you should apply as little as possible. Although it will tighten the fabric beautifully, it will cause it to sag somehow when the dope is applied. The third expert says you should apply a good even spray coat of water to get a nice even tension. Our experience is that no matter what you do, initial application of the dope will cause the fabric to sag alarmingly. As the dope dries it will shrink back again but generally not back to its original state.

All agree that it should be demineralised water applied with a spray. As far as I can tell, this is the only thing on which they agree.

What sort of dope should be applied? The manufacturer firmly states that butyrate dope should be applied. All the experts prefer nitrate, particularly for the initial, tautening coats.

Should it be thinned with solvent before application? Two experts say it should be thinned considerably for the initial coats; one says it should be applied unthinned and warmed.

All the experts and the manufacturer agree that there should be sufficient time between coats, but no-one agrees how much time that should be.

How many coats should be applied? Some experts say no more than three; others say up to eight, provided they are thinned right down.

If there are wrinkles after application, how can they be removed? The manufacturer clearly says you should apply more coats of shrinking dope. Some experts advise the use of boiling water, others recommend re-activating the dope with thinners, and others recommend the use of many coats of well-thinned dope.

Everybody warns that dope will continue to shrink throughout its life to the extent that it can crush the structure underneath. Our own experience with wrinkly fabric after being left a week is that it’s no different.

So far, we have covered eight small pieces of the aircraft. Of these the first, the rudder, has come up successfully. It was the first to be sewn and doped, and was therefore the least expertly done. All the others, no matter what initial tension has been applied, or how much water sprayed on, or what sort of dope was used, or how much the dope was thinned, are wrinkly to a greater or lesser extent.

We have had limited success in removing wrinkles, but it can take an hour or more to have any effect on an area of a square foot, and with a total fabric area of 400sq ft, we could be there for some considerable time, and in any case, subsequent coats of dope will cause the fabric to relax, undoing all the good work you did…

So today, we applied water to the last aileron and, as always, it came up perfectly flat and taut. We let it dry – it’s been a roasting hot day – ideal conditions and we were keen to make the most of it.

Then we applied a coat of dope 50:50 with thinners. When it was dry, miracle of miracles – no wrinkles! On that basis we decided to do the same to the first wing, and lo and behold, it too dried to the same perfectly flat surface as when it had been water shrunk.

The wing looking pristine!

The wing looking pristine!

Theo had been applying dope and thinners alternately to the other wrinkly surfaces and putting them outside in the sun, and the improvements in the centre section while not dramatic, were sufficient to suggest it was probably okay, so I got on and ribstitched it and applied the rib and edge tapes.

Rib stitching holds the fabric onto the ribs and is done with waxed linen cord. This is the centre section

Rib stitching holds the fabric onto the ribs and is done with waxed linen cord. This is the centre section

The result is certainly satisfactory. Meanwhile Theo was busy doing the last of the sewing on the last wing.

Then in the late afternoon we applied a second coat of dope, thinned 70:30, to the aileron, and all the wrinkles appeared. By now we were committed to doping the first wing, so we decided to apply a second coat, but still thinned 50:50.

And the result? Wrinkles. We will have to wait now to see whether they sort themselves out, but by last thing at night we felt that they were looking pretty much okay, and felt a sense of provisional relief.

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