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9. Cutting metal and spoiling wood

20/01/2012

All three of us – myself, my brother Rick, and our very good friend Theo, have built aircraft before, and we knew that wood was a lot easier to work than metal.

Although the metal parts were relatively simple – they could be made from tube and sheet material that was easily available off the shelf – each little bracket is a complicated shape with all sorts of weird-shaped lightening holes, and making each of them by hand was going to be a loooong job. So we took the decision straight away to make use of modern metal-cutting facilities and have them transferred onto CAD, so that they could be cut by machine.

Until recently, that would have meant a plasma cutting machine – a tiny stream of incredibly hot ‘gas’ that melts the metal and cuts a path through it. It’s a very good system, but there is one little snag. Next to where it’s cut the slot, the metal warms up and expands, and so the cut that you make doesn’t go quite exactly where you want it, and it’s not really possible to predict how it’s going to expand, or to compensate for this movement.

But technology has come up with an even more extraordinary solution – a water jet. It may seem incredible that you could actually cut metal with a stream of water, but this jet is very thin and contains fine abrasive powder, and it’s at a pressure you can’t really imagine. It can cut a very, very narrow slot -1mm or less – to levels of accuracy that leave one’s imagination reeling. Because it doesn’t heat the metal up, there’s no distortion, and it leaves a perfectly clean cut. It will cut metal up to an inch thick (though we don’t need anything like that of course).

Initially we gave the task of putting the metal part drawings onto CAD to Derek Walton of Defra. We threw him in at the deep end with the undercarriage (i.e. the bits where the wheels are attached), because they were by far the most complicated part, and he did a superlative job. These days, it’s pretty old hat to see a ‘drawing’ in three dimensions, and to be able to turn it round and look at it from every angle, but it was still pretty cool to us!

But shortly afterwards we were approached by Dave Graham, who’d just completed building – from scratch – a very, very fancy biplane with a radial engine called a Great Lakes Biplane. His enthusiasm won us over, and he took on the very tedious and time-consuming task of transferring the details of every bracket from the original 1915 drawings to the computer.

That was pretty much my job, and meanwhile Theo had cleared his workshop of his other aircraft, and made a start on the Scout. Theo had inherited a large baulk of aircraft quality timber with another project. ‘Aircraft quality’ is a very high grade of timber. It’s normally spruce, though Douglas Fir is commonly used too, and you have to be very, very picky about which bits you use.

The first thing is to get it kiln-dried (this means cooking it in a very low oven for ages and ages until the moisture content all the way through is within tightly controlled limits. If you don’t, the uneven moisture within the baulk may twist and split the timber when you slice it up and expose new bits to the air.

Next, you can only use timber from the right part of the log. Too near the middle, and the grain is circular, not straight, and the relative lack of sap can make it weak or even rotten. But the outside’s not ideal either – it can have faults in it that have worked their way in from the outside, and there’s too much sap flowing, making it less dense.

And for the bits (like the main wing spars)  that take the most strain it’s critical that the grain is going in the right direction to give the wood strength in the direction you want it.

Oh, and the grain must be straight down your cut piece and the correct number of grains to the inch too, or its strength will be hopelessly compromised.

And finally, of course, it must have no faults in it; no cracks, splits, knots, or shakes.

Well, the trouble is that although you have some control over the first of these requirements, much of the rest is a matter of pot luck. You take a piece of wood that looks perfect on the outside; you plan carefully how to cut it up with minimum waste, and making sure that the grain direction is correctly oriented with the strain on the finished piece, and then you hold your breath and pop it on the bandsaw.

Then you take up the cut piece and examine it to see what it looks like, because until you’ve cut it to size, you will have little idea about the straightness of grain, the grain density, or the defects. And it’s not uncommon to have to cut two or three pieces before you get one that’s good enough to go on an aircraft, and you end up with a lot of scrap wood which – if you’re lucky – can be cut down for smaller parts. It may be possible to use it for less critical areas. Or it may end up on the fire…

At any rate, I know what started off as a very large baulk of timber that more or less took two of us to lift it became a very small pile of usable bits.

The wing ribs are made from plywood, and there are lots and lots of them. Thankfully, the wings are pretty much rectangular, so most of the ribs are the same. Theo made a master rib out of thicker material first, and used it as a pattern to cut out all the others using a following router bit. They have to have a ‘capstrip’ – a little strip of ordinary wood – glued onto the edges, and that was done using a jig – a solid wooden baseboard with blocks on it to hold the rib in place, and lots of little wooden cams that rotate to hold the capstrips firmly against the rib until the glue dries.

The spars – the main timbers that run the full length of the wings and give it most of its strength – are the largest and most critical bits of wood in the aircraft. There are eight of them – two in each wing – and we had to wait for months until suitable material came through from our timber guru, Dudley Pattisson, known, slightly off-puttingly, as ‘Duds’. In fact there was a bit of a scare with these rare and precious bits of timber when they were machined to size. A couple of resin inclusions came to light, and we wondered if they might have to be declared ‘Duds’, but thankfully they are considered okay!

The end result is very impressive; take a look.

Wing ribs and spars assembled in Theo’s workshop

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From → Building, Technical

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