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119. Linen covering – a summary

06/07/2014

On Sunday we more or less had a day off to recharge the batteries, watch the Silverstone Grand Prix and the Wimbledon Men’s final, and eat a delicious Sunday lunch provided by Theo’s lady Fran.

It’s been a tough week, and here’s a list of what we’ve learned during it.

Before You Start

Check to make sure the structure is absolutely complete, that the paint, varnish, etch primer and any other coatings are complete.

Check that all fasteners are permanently secured; locknuts tightened down, screws fully home, split pins doubled back.

We applied Wax-oyl to the ends of the internal bracing wires to try and stop them going rusty.

Wrap the metal parts that the fabric comes into contact with. In our case this is the wing and tailplane leading and trailing edges, and the entire structure of the elevators and rudder. You should use tape wound round the metal in a spiral, with a small overlap. We found 1” wide was the best, and made our own from offcuts, splitting it using a craft knife pinched in a Workmate and drawing the fabric across it, using a pencil mark as a guide. Fix the first end with dope, then wind all the way down to the other end. Where it has to be fitted round a fitting, such as a rib end, try and make sure that any creases are on the inside, out of contact with the fabric. When you get to the end of the tape, fix it with another dab of dope, and then dope all along the wrap to secure it. I only bothered with the back of the wrap, since the front side would get plenty of dope from the fabric.

We also applied anti-chafing tape, 1/2in wide, wherever the fabric would be in contact with wood, or might come in contact. For clear doped fabric, this also helps to prevent any colour from the wood bleeding through. The fabric doesn’t follow the line of the ribs – in between the ribs it will tend to pull into a straight line. This means that in addition to the rib caps, you will need to consider many of the spars and stringers inside the wing. I started using Superseam cement, but found that dope was easier and quite good enough.

This process also makes you check for any lumps and bumps that might be in contact with the fabric. This might be screw heads, lumps of glue, etc.

Now check the internal structure all over again. It’s the last time you will be able to see all that beautiful work you’ve done, and it’s amazing how difficult it is to spot every last error of omission. It’s a really good idea to get someone else to look at it. Even if they know nothing about building an aeroplane, it’s likely their eye will light on something completely obvious you’ve overlooked.

Fixing the fabric

Traditionally the fabric was sewn into a ‘bag’, which was sewn by machine as far as possible, then closed with hand stitch, and some places where it’s glued in place with dope.

Machine stitches are much faster, but can only be used on straight seams. Hand stitching is slow but can manage curves.

Gluing with dope can only be used where there’s sufficient area of wood for the glue joint; you couldn’t use it on the leading or trailing edges, for example.

Machine Sewing

These are done with a balloon stitch, which requires the two edges to be folded inside each other, and lines of stitching run just inside each fold. You can get a twin needle machine that will do it all automatically, but doing it with a normal machine takes a bit more effort. It’s made a lot easier if you do a first tacking stitch to hold the two edges together, then iron the folds in place and pin them, before doing the two main stitches.

It’s generally a two-man job, particularly if you are doing a long seam; an extra pair of hands is necessary for handling the bulk of fabric and holding the folds in place while they are ironed.

For something like a wing, sewing the first seam (at the leading edge, for example) is relatively straightforward, but doing a second (such as the trailing edge) is a lot more complex, since it has to be in exactly on the trailing edge, and the fabric has to be exactly the right tension. After a very great deal of experimenting and head scratching we found the best solution is to make pencil marks (DON’T use biro – it bleeds through onto the finished surface!) on both bits of fabric where you want the middle of the seam to be. Then mark a line exactly 15mm from the pencil marks and cut the fabric. Now tack the edges together with one edge 10mm from the other. It’s extremely important which one stick out more, as it determines whether the tacking stitch is on the same side of the tube as the first seam!

Folding, ironing and pinning are done as before, but the seams may have to be sewn from each end, as the whole tube may not fit through the machine.

Hand Sewing

The American bible, AC43-13, says to use a baseball stitch, but we gathered that the standard British way was different, more complex and time-consuming. Since the whole thing is hidden under edge tape and the scout was made before any such manuals were written (and maybe the seamstresses in the factory in 1915 didn’t follow the instructions!) we decided to go with the baseball stitch. The edges are cut about 1/2in long and tucked inside, and you sew through the main fabric and the tuck under. The tricky bit is getting the fold in just the right place so that when the stitches are pulled together you have just the right tension in the fabric.

As I said in the previous post, it’s unbelievably time-consuming; about 18in (500mm) per hour, and you can’t watch television while you’re doing it or the stitches go wrong.

The baseball stitch being applied to the tailplane trailing edge

The baseball stitch being applied to the tailplane trailing edge

Gluing

Originally they would have used dope, with loads of little tacks to hold it in place while it dried. The tacks tend to go rusty and don’t do the wood any good, so we used modern glue – called Superseam cement – in order to comply with the airworthiness requirements.

It’s horrible stuff to use; glutinous and stringy, and the tin has a tiny lid which makes it very difficult to get at, and becomes completely glued up. You need at least 30mm of wood to stick to, and we found the best way was to make sure the wood surface was horizontal, then pour a small amount onto it – enough for around 4-6in of glue joint. Spread it with an old kitchen knife, then lay the fabric into it, adjusting the position to get the tension correct. Now use the knife as a squeegee to force the glue through the fabric. If necessary, dip the knife into the tin to get more glue on top. It starts to dry very, very quickly, which is why you only do a little bit at a time. We used glue joints on the sides of the centre section, and the curvature of the wing section means that the fabric won’t lie perfectly flat on the wood. With a bit of luck you can persuade the wrinkles to lie flat if the curvature isn’t too great. If you can’t do that, you’ll need to make slits in it, but it’s better not to leave this until you’re half way through gluing!

Fabric finishing

Modern polyester covering materials are shrunk using heat – often using an iron. It will shrink a huge amount, and the tension can be increased by using a hotter iron. This means that it’s not particularly important to have the fabric taut when it’s applied.

Linen is different, however. The amount of shrinkage is much, much smaller, so you will need to get it sitting almost perfectly before starting the shrinking and doping process.

Machine seams are the least easy to adjust for tension. Hand sewn seams are better, but glued seams are the best. On the wing, we used machine seams on the leading and trailing edges and on the straight part of the tip. The curve at the tip was hand sewn, and the inboard ends and the cutout for the aileron were glued.

Watering

The next step is to apply water to the fabric. You must use demineralised water and apply it with a spray. It’s a simple and very satisfactory process, and within a few minutes the creases have all disappeared, the fabric has stretched itself gently taut, and you’re likely to feel confident that once it’s fully dried, it’s going to be perfect. But you’d be wrong.

Doping

The next stage is to apply dope. There are two types. The original substance, developed in 1911, was cellulose nitrate and was the first to provide a permanent, waterproof finish that didn’t stretch over time. The downside is that it’s very flammable, and a chance spark from an engine backfire or any other source can ignite the whole airframe. In the 1950s, an alternative, butyrate dope, was developed. Both systems have the disadvantage that they will continue to shrink the fabric over its lifetime, and we were warned that it was very important not to get the fabric too tight to start with, or it would distort the frame underneath in only a few years.

Both systems can be bought as tautening and non-tautening (which contains a plasticiser), and it’s standard practice to apply a couple of coats of tautening dope to start with, followed by subsequent coats of non-tautening in order to limit the continuing fabric shrinkage.

Applying the dope, like spraying with water, is a real pleasure. You thin the first coats down about 70:30 with acetone thinner, then slap it onto the fabric. There’s no need to be careful with the brush strokes – it will soak into the fabric and be quite even. If you apply vast quantities it can form drips on the inside surface of the fabric, but it’s not very easy to do this.

What then happens then is very disappointing. Instead of tightening, the fabric actually relaxes, going really quite slack. It’s touch dry in about an hour, and we found that although it tightened up a bit, it was rarely perfectly tight, and there were wrinkles in some places. Applying a second coat of tautening dope sometimes helped, but not always enough.

The tailplane after water shrinking and in the early stages of applying dope. Note the lovely taut surface after water shrinking.

The tailplane after water shrinking and in the early stages of applying dope. Note the lovely taut surface after water shrinking.

The tailplane after the initial coat of dope was applied. Look how the fabric has sagged compared to the previous picture.

The tailplane after the initial coat of dope was applied. Look how the fabric has sagged compared to the previous picture.

We had used the modern butyrate system because of its fire resistance, but after considerable experimentation with smaller pieces (ailerons, rudder, elevator) and advice from other experts, we’ve decided for the wings to use nitrate tautening dope (which tautens the fabric more) followed by finishing coats of butyrate. This is a very commonly used system on aircraft, and should meet our needs.

As we saw in the previous post, one surprise bonus is that the butyrate non-tautening contains a small amount of tan dye in it to make it easier to see where it’s been applied which reasonably accurately recreates the look of the final varnish coat which would have been applied to clear doped aircraft of this period.

Rib Stitching

With the first two coats of dope applied, you have to ‘sew’ the fabric to the ribs, or it can pull away in flight, modifying the aerofoil section with potentially disastrous consequences.

Waxed linen cord is used for this with very long needle to post it from side to side. First of all, however, you have to dope a piece of 1/2in wide cotton tape along the line of the rib. Then mark out the positions of the holes – on the Scout they should be every 3in and carefully make the holes with the needle. The first loop is secured with a reef know on each side, and then the cord is run alongside the reinforcing tape to the next set of holes, where it’s looped round the rib and tied in a modified seine knot and so on. For the undercambered wing, of course, the fabric will need to be pulled in quite a bit until it contacts the rib.

Taping

Now dope tapes over all of the rib stitching. The Scout has tape 1.25in wide, and we had to trim these down from 3in tapes. It may be a good idea to add tapes wherever there’s a risk of the fabric contacting the internal structure or other parts of the airframe.

Finally, tape must be doped around the edges of the piece. In the case of the rudder, this involves four quite tight curves, and as we saw in the previous entry, what’s required is brute force. The tapes have sat down very satisfactorily round the edges, with no wrinkles. After that it’s a couple of coats of non-tautening until you get a satisfactory finish. For more modern aircraft, this means a deep, lustrous, mirror-like finish which takes any amount of care and attention, and would be way beyond our level of skill. Thankfully, the Scout won’t need anything like that, and more or less anything that’s waterproof will be good enough!

The rudder after completion of the initial doping, ribstitching and taping. There's a coat of the non-tautening butyrate dope on top which shows the sort of off-white colour we expect the majority of the airframe to have on completion. The rudder itself will be red, white and blue eventually, of course!

No apologising for the repeat of this photo. The coat of the non-tautening butyrate dope on top shows the sort of off-white colour we expect the majority of the airframe to have on completion. The rudder itself will be red, white and blue eventually, of course!

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