Build your first flying scale model - Chapter 8 - Covering the flying surfaces
Not sure why, but the tops of the wings seem to be the most difficult parts of a model aircraft to cover neatly
- especially if as in this case the leading edge is right at the bottom of the section. For this reason I tend to
cover the top first, to get it over with.
So, we need to cut a piece of tissue to cover between the centre and outermost ribs. To minimise the sag between ribs, make sure the
tissue grain runs spanwise.
Line the tissue up wih the frame and apply a spot of dope thinners to the gusset where the top spar joins the centre rib.
Now slosh some thinners through the tissue on the leading and training edges, and easing the tissue outwards with your thumbs,
work your way down the wing, pushing the tissue onto the wood.
As you move towards the tips, apply more thinners as you go. Remember we are only sticking the tissue to the centre rib, tip rib and leading and trailing edges - NOT any of the other ribs.
When you get to the last rib, check that the tissue is sitting nicely on the outermost rib. You sometimes find you have excess loose tissue,
so to avoid a wrinkle, ease the tissue out more towards the trailing edge, and try to chase the excess out along the trailing edge - working back towards the
centre of the wing again. You may need to apply more thinners through the tissue so it will still slide over the doped balsa.
When dry, the excess tissue at the leading edge can be trimmed with a scalpel, then any overlap at the end rib, as shown here. The tissue at the trailing edge
can be trimmed using emery paper or a scalpel.
A bead of full strength dope is applied carefully along the top of the outmost rib and allowed to dry.
The separate tissue tip piece is added in the usual way by flooding dope thinners through the tissue.
And here it is trimmed.
The underside is much simpler as it is flat and can be covered in just one piece of tissue.
Trim the excess tissue using your favoured method, then repeat the whole process for the other wing.
I put this photo in to show the less than drum tight finish you would expect to have at this stage, So if your tissue looks
a bit like this, don't panic!
We are going to have to hold the wing flat while the tissue is water shrunk, and I use a pair
of balsa offcuts, 1/8" thick to hold the wing off the work surface. This allows air to get under
the wing, and helps the underside to dry out.
Shrink one wing at a time - here the panel has been sprayed with water on both sides.
You may have noticed a small piece of wood in the photo before last - this is what it is used for. It is inserted at the trailing edge of
the wing tip prior to adding the weights. The purpose is to lock in a small amount of washout at the wing tip
(less incidence, or angle of attack, at the wing tip than at the wing root). On some models the addition of this washout can make the difference
between a having a laterally stable or unstable model. It is very probable that a naturally stable design
like the Swift with ample dihedral would work perfectly well without any washout at all, but I do tend to build it in as
standard on all my models "just in case". On the Swift I am adding 1/16" washout to each wingtip.
We could get into a more complicated discussion here about using differential washout on both tips to help control the flight pattern, but I think that will have to wait for another article.
Very useful things, spice jars.
Here is the result after both wings have been shrunk. You may be surprised just how strong and rigid the wing has now become.
This photo makes an interesting comparison to the pre-shrinking shot. Although not completely perfect, at least the major leading edge wrinkle has been avoided and I am sure the extra gussets helped.
Turning our attention to the tailplane - this is very simple to cover with one piece of tissue each side.
The edges being trimmed with emery paper
And the finished article covered on both sides.
The bare balsa edges do stand out a bit when using dark coloured tissue, which is not an issue if you are going to paint the model,
but will be if you are relying on a coloured tissue finish. In this case, trim the upper piece with a sharp knife leaving a bit of an overlap.
Run a wet finger round the edge,
and stick down by flooding dope thinners through the tissue. If you want to go a less smelly route, stick the edge down with a glue stick.
The above applies equally to the wings, of course.
At this point, to avoid being sued by people who aquire bad scalds, I ought to say "DON'T TRY THIS AT HOME"
Instead I will just say, "be careful". It might be sensible to wear gloves while you do this (I didn't, which I suppose makes me wildly irresponsible)
Why am I doing such a thing? Well, a full water soak on this part, which is considerably more fragile than the wing
(especially with the light wood I used) is going to make it end up looking like a potato crisp (or chip to our American friends).
It will just not cope well with the shrinkage. Now you could use alcohol to shrink the tissue, which is gentler than water, or you could waft
it over a boiling kettle like I did. The tissue can be seen starting to go limp - when this happens, place on a flat surface and weight it down while the tissue dries.
I said spice jars were useful. If there are still areas that look a bit wavy after it dries, you can steam again, or if the problem is very localised, just lick your
finger and apply to the problem area. Actually, this trick can be used on any part if you have a wrinkle after the first water shrink.
There seems to be something in saliva which helps to give that little extra shrink you sometimes need to get rid of an obstinate wrinkle. Faintly disgusting, but there you are.
Proof of flatness photo! This is what your tailplane should look like after the tissue has been shrunk. If it comes out warped, bite the bullet, strip the tissue off, and try again.
You are bound to have problems later on flying a model with a warped tailplane.
Time for a question - should the fin be covered or not? If you are going for a coloured tissue finish, the answer is obviously yes. If a painted finish, you don't have to, but it will
give a more even surface appearance to the model, and hide the wood grain of the part. You will also need less paint to give the
required match to the rest of the airframe. I would think it adds a bit of strength too.
Very easy to do - just attach an oversized piece of tissue to both sides. The fin has previously had a coat of sanding sealer all over, followed
by a coat of full strength dope. The tissue is just cut from the sheet and applied dry, by flooding dope thinners through it.
If you cover both sides at the same time there is less chance of the part warping. There is no need to water-shrink the tissue.
Here is the trimmed item - I used emery paper to remove the excess tissue. If you find you have some loose tissue edges, run a damp finger around the perimeter,
and then brush dope thinners around the edge. The damp tissue will be more happy to sit down neatly at the edges than dry tissue.
All that remains is to dope all the parts we have covered and shrunk in the last two chapters. I am a believer in the principle that all the necessary shrinking
should have already been done by the time we come to dope the model. If you have wrinkles at this point, you are not going to get rid of them using shrinking dope.
I use non-shrink dope exclusively - the stuff in the old preserve jar here is thinned 50/50 with dope thinners, and the whole model will receive just one coat.
I use an old flat brush to apply the dope and try to get a nice even coat. It is easy to see where you have applied the dope while it is wet, because the tissue
goes almost transparent. Thus you should not miss a bit. This process is extremely smelly, so take it outside if you can, or at least to a well ventilated spot.
Even though the dope is sold as non-shrinking dope, I see no point in taking chances. Thus, the wings and tailplane get weighted down for a second time for a couple of hours while the
dope dries completely. Let it flash off before doing this, otherwise they could stick rather messily to the work surface. No need to do this with the fuselage or fin though.
Looking at this photo, I am sure there are times when my long suffering wife wishes I had a different, less smelly hobby, which took place rather further from the kitchen.
Time for another weight check. Here are all the parts made to date, covered and doped and ready for assembly.
I am very happy with 13 grams so far - it will be interesting to see how much the paint and details will add.
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