Feature: The slow homebuild Currie Super Wot
PUBLISHED: 12:47 19 July 2018 | UPDATED: 12:57 19 July 2018
Nick Bloom gets his second homebuild Currie Super Wot project off to a slow start - just the way he likes it | Words and photos by Nick Bloom
When it comes to building an aeroplane, I’m in the slow lane. I already have a Currie Super Wot, so building another one is a labour of love, and the longer it takes the better. Which is probably just as well, because Wot building can’t be rushed.
Actually, I suppose it can, if you don’t mind how much you spend. The six Wots built in 1967 by Slingsby to look like SE5a aircraft for the film Darling Lili were certainly produced in record time. For homebuilts, the rules only insist on the owner doing at least 51 per cent of the work, so a great deal can be farmed out, but if you pay others to do the specialist stuff you will end up with an aeroplane worth a fraction of what it cost to build.
I intend to do as much as possible myself, and that includes making all the metal fittings, the covering in fabric and painting, but even I will still be farming out a huge amount. I could − but won’t − make my own turnbuckles, for instance.
Or instruments. Or − taking it to extremes − turn my own bolts, weave my own cloth and make my own dope, paint and glue. And I could, but I won’t, make my own propeller. It’s all relative.
There are no kits for the Wot, but the LAA will sell you a set of plans for eighty quid. The design started life in the 1930s as a scaled-down de Havilland DH60 Moth with simplified construction and a single cockpit. Over the decades the plans have been copied, updated and altered.
For instance, Dr John Urmston came up with a list of modifications to allow aerobatics, and these were included in the package sent to me by the LAA.
I decided to begin the construction of my Super Wot (a clipped-wing variant) with the tail surfaces, and drew up a cutting list from the plans. I sent this to SkySport who accepted the order and delivered the wood cut to size around six months later, with a bill for £750, which sounded rather steep, until I stopped to consider what exactly I was paying for.
And that, in a word, is labour. It takes a long time to saw and plane wood to size and my first tip is don’t pay someone else to do it, do it yourself. I did later on, with the wing rib stock, and it isn’t difficult. In fact, it’s quite satisfying. (Perhaps making my own turnbuckles isn’t such a crazy idea.)
The stripwood and more bulky items, such as the tailplane spars all have to conform to fairly strict rules. The grain must run straight and have a reasonable density of grain lines. Obviously you can’t have any knots or wood discolouration. SkySport’s wood was perfect in every way, possibly rather more so than strictly necessary.
I began by making the tailplane ribs. These are built up from strip wood in a lattice shape, with plywood ‘biscuits’ glued over each junction. The Wot’s tailplane has a fairly thick, but flat cross-section and the curved leading edge is covered with thin plywood. The ribs are directly under this plywood, supporting it.
My first job was to devise a means of steaming the rib ‘booms’−they run continuously from trailing edge to trailing edge with a sharp curve in the middle at the tailplane leading edge. I drilled a hole in each side of an electric kettle to make the steamer. The next task was to devise some kind of former to produce the bend.
Using my bandsaw I cut a suitable curve in a piece of thick plywood, splitting it in two. Clamping the two halves together with the steamed rib boom in between them ought to produce the desired shape. I tried first with scrap wood. I gave it two minutes in the steamer, then placed it between the two halves of the former, clamped in a vice, and carefully forced the two halves together. The scrap wood snapped long before it had anything like enough curve.
I experimented with different steaming durations and style, and with more closely grained wood (which helped), but couldn’t get anything like the required bend. Could I have mis-read the plans? No, they did call for one continuous rib boom.
Both sides of the rib had plywood glued on at the nose, so that must be the main source of strength, not the rib boom. It was tempting to cheat by substituting a block spacer, but I decided to persevere. In the end, the solution was to steam a modest bend in one former and allow it to set; then after more steaming, a tighter bend in a second former; finally the tight bend called for in the plans. Three sets of steaming, allowing an overnight set between each.
Fortunately I had over-ordered the rib stock because, despite this cautious approach and the excellent quality of the wood, two booms snapped before the third in the series of formers could be clamped shut.
So now I had a set of shaped rib booms. The next job was to make a jig to form the ribs. I stapled greaseproof paper on to a sheet of thick plywood, then drew on it an outline of the latticework−dimensions are in the plans. Next, I screwed on a dozen pairs of plywood blocks, one pair for each component of the lattice (i.e. length of stripwood).
This ensures that each rib exactly matches the others. The idea is to cut lengths of stripwood to make the lattice so that each strip abuts neatly with its neighbours and the rib boom, ready to be glued in place. Finally, I cut a lot of small squares and triangles from thin plywood to make the biscuits and ‘D’ shapes for both sides of the nose of each rib. You have to lightly sand the side of the biscuit that is to be glued on, or the glue won’t take−aircraft plywood has a thin protective coating.
Eventually I was ready to assemble my first tailplane rib. The glue I am choosing to use on my Wot is the kind that isn’t coloured; Aerolite 306. (The other kind is red and works in two equal parts like epoxy resin. It used to be called Aerodux and is now called Prefere 4050 Resin and 5750 Hardener. It isn’t acidic, so I’ll be using it later in the project, near metal fittings).
Aerolite comes as a powder in a tin and acid in a bottle. You mix the powder with water and can either apply the resulting paste to one side of the joint and some acid to the other side, or mix acid and paste together. If you mix them, you have very little time in which to apply the glue and then clamp both sides of the joint together, because once the glue starts to harden it loses its strength. I learned to use it mixed on the previous Wot project and others, and prefer it used in this way for making lots of small joints.
Preparing the small batch of Aerolite required for something like a tailplane rib is quite an art. I use a small wooden spatula and the base of a disposable plastic cup. Delicately dribble in a small quantity of water, then, using a plastic spoon, add about a quarter of a teaspoon of powder. When mixed it should be about the consistency of pouring cream − and you have to make allowances for adding more thin liquid with the acid.
The quantity of acid, ideally, is a single drop, which gives a relatively slow ‘set’. Two drops won’t weaken the glue, but does give less time for applying it before it starts to go off. I make periodic tests on scrap plywood to ensure that my gluing techniques really are working; the test is (after an overnight set), when pulled apart, it’s the wood that fails, not the glue. I made the ribs in winter, which gives a slower set, but still needed on average two successive glue batches per rib. This wasn’t me running out of glue, it was the batch showing signs of setting before I had finished.
So, having mixed a batch, I’d spatula glue on to each end of each piece of lattice, spread glue under each plywood biscuit and then assemble everything. Next, before the glue has a chance to set, it all has to be stapled together. I use a hand stapler from B&Q and it’s quite an art (again) to get each staple through the biscuit and squarely into the stripwood under it. This bit requires both speed and accuracy. Tip: paint an arrow on the head of the stapler to show exactly where the staple emerges.
Once secured with staples, you can lift the rib out of the jig. The glue will have set, but not set hard, so it isn’t difficult. After leaving the rib overnight for the glue to gain full strength, the next task is to remove the staples. I do this with a sharpened spatula eased under one end of a staple, then long nose pliers to lever out the rest.
Now biscuits can be glued on the other side of the rib. I use butterfly clamps rather than staples for this. Staples seem rather brutal, but they leave a delicate hole which doesn’t appear to weaken the rib stock. Incidentally, the plans seem to suggest leaving the staples in place which perhaps made sense in the 1930s when less reliable organic glues were the norm. My LAA inspector says synthetic glues are now so good that staples should be removed, not least because they can start to rust (and they do add infinitesimally to weight).
A rib made this way seems enormously strong in relation to its weight. It is a small work of art, and I particularly like the glued-on biscuit method of reinforcing a butt joint. Similar joints−in the fuselage at least−of my Stampe were held together by complicated fittings made from welded sheet steel with lugs for bracing wires.
What a relief it must have been to aircraft constructors when plywood and reliable glues came along. Plywood webs are used to reinforce the joints at the ends of the drag and anti-drag wooden struts inside both the tailplane and the wings in the Wot. Other designs (such as the Stampe, Flitzer and Pitts Special) use bracing wires with bolted-on metal fittings at each end. (My inspector says the Wot method is also used in wooden glider wings.)
After making the ribs, I assembled them and the other components of the tailplane on my work bench. The Wot’s tailplane tips are constructed like the wing tips on many aircraft, from multiple laminations, glued together over a former to make a curved shape.
Once attached to the tailplane (all joints reinforced with plywood webs) the tips have to be planed and sanded from a square cross-section into one shaped like a ‘D’. This all went more or less to plan, although the manual plane I used was rather laborious. Later I bought an electric planer, which is not only faster, but makes a lot less mess, because the shavings are caught in a bag attached to the machine.
The final stage in constructing the tailplane was to glue on the plywood leading edge, but, before that, the whole thing had to be given the okay by my LAA inspector−essential, because he wouldn’t be able to see inside once it was covered in. The thin plywood used on the Wot didn’t need steaming; dampening it with boiling water was sufficient for it to take up the required shape. I put towelling, then thick polythene over the tailplane and used that as a former. Left overnight, in the morning the plywood was curved and fully dry, ready to be glued on.
First, it had to be cut to make an exact fit. I attached it securely at one end, stretched elastic round it at each rib station, then marked the other end with a pencil−nervous stuff this, and even more so when cutting the far end and the edges with a Stanley knife.
Inevitably I didn’t get it quite right. Make the plywood a touch oversize and it won’t be glued to the ribs properly. I made it a smidgen undersize, which my inspector said was acceptable when he saw it a few weeks later. It meant a very slight reduction in the glued area where the plywood attached to the front spar but, like many things on the Wot, there is a margin for error.
The elevators, fin and rudder came next. They are much simpler to make, since they have a triangular cross-section. Clamp the leading edge−which has a flat face−to the table, clamp two side stringers to that, and clamp the upper ends of the side stringers to the trailing edge. Once you’ve done that twice (two stringers at each end) you’ve got a stable three-dimensional structure to which all the other stringers can be glued and clamped.
The work I have described so far was not done to a daily schedule, but in fits and starts in spare time between other activities, one of which, of course, was flying. The many interruptions included clearing thorn bushes which were catching in the fabric of my Super Wot when coming in to land, a Glis-glis invasion in the loft, fitting a new hot water tank to replace the old one (the water was becoming suspiciously rusty), other tasks around the house and the eight acres that came with it, plus, of course, social life and spending time with wife and family.
And, of course writing features for Pilot.
I needed to take some welding from the Super Wot for my LAA inspector to do, so I thought why not make the metal fittings for the tail surfaces and get him to weld them at the same time? I was bored with looking (for several months now) at the tailplane and elevators as separate objects and thought how much better they would look hinged together. The varnished woodwork looked almost naked without some black-painted fittings to adorn it.
The control hinges on the Wot are fairly mind-boggling, far from the flimsy bought-off-the-shelf things you find on some other aeroplanes. These are solid, massive and complicated to make, but (and I like this part) because they’re home-made they are cheap. Also, the area where one bit rubs against another is so vast that it’s never going to wear and develop play.
Basically, the Wot’s hinge on one side is an aeroplane bolt with its head sawn off and a right angle bend in it. On the other side it’s an eye bolt (the most expensive bit at around six quid). The tricky part is positioning the bend in the bolt in the right place, which is why some of my hinges have several washers under the nyloc nut.
Another complication is that the eye bolts currently obtainable are different from the ones sold in the 1930s and have a thick collar. This is superfluous because the Wot’s hinge already has a metal bracket under the eye bolt. If the collar is left on, it makes the gap between the two surfaces being hinged look silly (much too big) so, with my inspector’s acceptance, I grind the collars off.
There are two brackets to make per hinge, one with a right angle bend in it. Six hinges to make in all, and of course they all have to line up. Fortunately mine did.
While there’s nothing in the hinges that needs welding, the same can’t be said for the control horns or the component that joins both sides of the elevator−they need the attention of an aircraft-approved welder which, happily, my inspector is, and a very good one too. There are two horns (upper and lower) on each side of the Wot’s elevators, making four in total, plus two for the rudder.
You make a horn first by drawing out the shape on sheet steel, then cutting it out with an angle grinder−at least, that’s how I do it, though I believe real engineers use a nibbler powered by compressed air to do the cutting. When I first cut steel plate with the grinder, I failed to realise the combustible quality of the sparks gushing out backwards and set light to my apron. That was neatly solved by sewing a leather patch over the burned-out area, and so far the leather hasn’t caught fire.
Each side of the horn has to be given a right angle bend with a suitably mild radius. When mated together this gives a ‘T’ shape for the lower horn or an upside-down ‘T’ for the upper one. The upper and lower horns are clamped to each other by four bolts running through the elevator.
I drilled those four bolt holes in each horn and also the single hole at its tip, where a shackle pin will one day−many years from now−link it to a control cable. I then tightened B&Q nuts and bolts (to be discarded later) through all the holes, thus securely clamping everything together, ready to be welded. The last stage is to tidy up all the edges with the bench grinder.
All the welder has to do is run a seam along all the edges, joining both sides. A clean up, etch primer and some black paint to finish and then the big question: did I order bolts to the right length, and will they go through all the holes, including the ones I made in the tailplane leading edge? (This time, ‘yes’.)
So now the tailplane and elevators are a single unit complete with control horns and they hang on the workshop wall exactly like a piece of sculpture. The Japanese would appreciate all the small imperfections, I feel sure; they would see it as ‘art’, much like the cups in their tea ceremony.
The tailplane has been up there for so many months now though that I have ceased to admire it; it’s become invisible, part of the workshop. What I’ve been doing for probably over a year now, is making wing ribs.
I made up a cutting list for the wings several years ago and obtained a quote from an aircraft wood supplier, but thought it only polite to check with my LAA inspector first−he’s the chap who has to sign for everything. It turned out that he had wood in stock and when he offered to supply it at well below the quoted figure, I said yes, despite knowing that it would mean a delay.
My inspector has over sixty aircraft on his books and supports a number of homebuilders. He’s highly respected and I’ve been going to him for over half my life; you could say we’ve grown up together. If he has a shortcoming, it’s that he can’t be hurried. But then, I’m in no hurry myself. As long as it takes is fine by me−as I said earlier, I’m in the slow lane.
After a very long interval and some prompting I had a message to ring him. “The rib stock, is it really 5/32 inches? That’s awfully thin,” he said. I checked the plans and told him that was the measurement. “Hmm,” he said. “My planer won’t go down to that.” Between us we worked out a modus operandi.
He had lots of offcuts in stock of aircraft quality wood that I could saw into strips and plane down to size, and B&Q sold me an electric planer for around 35 quid, that looked as if it would do the job.
Some of the wood he gave me came off a Hurricane project. Other pieces were blocks of wood that had been rejected because of a knot on one side. All were thoroughly seasoned and had the required straight grain and grain density.
I marked out an oversize plank off the longest block−long enough to make rib booms. Then, very carefully, I ran it down the band saw. Despite my care, the cut was somewhat wavy, but not too ‘off dimension’. I then took the plank and marked out more lines on its longest side, six of them, again well over the required dimension.
This made six of what Dr John Urmston described in his book about building a Wot, Birds and Fools, as ‘pea sticks’, only oversize and of somewhat uncertain cross-section. I laid the six flat on the work bench and ran the electric planer down them, then turned them over and did the same again.
After a few such runs, the pea sticks had acquired the correct uniform dimension and were straight on two sides. Turning them through ninety degrees and more planing eventually resulted in six perfect rib booms. It was amazing how many of these one discarded block of wood could produce. And I felt so much more involved.
Dr John Urmston’s description of making wing ribs is one thing everyone who reads his book remembers − the sheer scale of production required. Thirty-six nose ribs. Twenty-six full-length ribs. Even producing one a day, as he did, that’s months of labour.
He was a busy GP, so whipped up a rib in the early morning, glued and stapled it and left it to dry, then glued biscuits on the other side in the evening, after work.
But, as I think I’ve said, I’m in no hurry. Far from producing one rib a day, there were periods when I averaged one a week. Last time my LAA inspector came to check out my (flying) Super Wot for its Annual, I had eight nose ribs and ten full size ones.
Half of them were a year old and with winter coming, I had varnished those, but not the others. (And not the areas that will be glued to spars, trailing and leading edge.) He gave them a quick look. “These seem okay,” he said, twisting and flexing. “You should see the state of some of the factory-produced ones.”
Wing rib production has now reached the halfway mark and I’m getting up a little more speed. You can picture me, classical music on the radio, doing my daily hour or two, drawing out endless rows of biscuits on thin plywood, cutting out a strip of them with the Stanley knife.
Then donning ear protection and slicing up the strip with the bandsaw. (John Urmston reportedly cut them up with scissors, which doesn’t work for me.) Steaming rib booms and clamping them to leave overnight so that in the morning they’re nicely curved and bone dry.
Fitting the booms into the rib jig, making repeated passes of both ends over bench sander and band saw until they fit perfectly. Cutting a ‘pea stick’ bit by bit into sections, sawing and sanding each section until it just fits, butting nicely against the booms to make the required lattice work.
Once a rib and its biscuits are ready for gluing, everything speeds up. The first batch of glue only takes me a third along the rib before it starts to get gummy, then I whip up a second batch. Each glue joint is followed by a pre-sanded biscuit and each biscuit requires up to four carefully positioned staples to clamp it down.
Usually I get it all done after whipping up a third batch of glue and then prise out the rib. So far the ribs have always come out in one piece. A quick clean of the jig removes any glue that got left behind. When I return to the rib the next day, everything is calm again. First, prise out the staples. Second, have a good look at all the joints to make sure they’re sound. Finally, glue biscuits on the other side of the rib, clamping them with butterfly clamps. And then on to the next rib.
Soon I’ll be getting on to my inspector, chivvying him to make up the rest of the wing cutting list. He will get around to it eventually. And there’s no shortage of other things I can be doing in the meantime. I will order some more sheet steel from LAS Aerospace, and get back to manufacturing multitudinous metal fittings.
I suppose a day will come, perhaps a decade from now (by which time I’ll be in my mid-seventies) when the tempo will quicken. I might even briefly move into the centre lane. There will be big sums to spend on engine and instruments (emphatically not new−everything second-hand, or costs will overrun), an endless series of orders and deliveries for the thousands of nuts, bolts, washers and other bits.
A fuel tank to make, undercarriage, interplane struts, goodness me, another engine cowling (the last one took fully three months). And then gluing on fabric, stitching, rib tapes, priming, painting, snipping control cables and bracing cables to length, adjusting turnbuckles, and a thousand-and-one details. Not to mention LAA paperwork and test-flying.
I’ll cope with it when it comes. For now, it’s one rib at a time. Life in the slow lane.