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21. Testing times

26/06/2012

We’re down in Dorset again this week for another week of wing-building. As we drove down, we were firmly convinced we were all ready to assemble the first wing, and by the end of the week we’d have all four of them slung from Theo’s roof.

That was yesterday.

By the end of today, we’d (a) realised that there were an awful lot of holes to drill, fittings to fettle and wires to splice, and (b) there’s a potential limitation on one of the fittings which is likely to mean some redesign…

On the plus side, Theo is now producing really beauuutiful wire splices, and we’ve tested the ones he’s done before, which have passed with flying colours.

Cable testing rig

Rick and I spent a couple of days making a massive testing rig, since each cable has to be pulled to 2/3 of their maximum load. In the picture, you can see the two big timber beams which has the cable in between. The left hand end is fixed to the timber; the right hand end is attached to the bottom right of the metal lever you can see sticking up. The lever is pivoted at the top right hand end, and then the carefully calculated weight – a bucket full of scrap metal – is popped on the far end. The bucket weighs 30lb, but the load on the cable is 1350lb, thanks to the mechanical advantage. This (a) proves the cable is strong enough, (b) tightens down the splice and neatens it up, and (c) takes out the initial stretch in the splice joint so that it should stay more or less the same length throughout its service life.

When they were done, I whipped – or served – the tested splices. Before you get too excited, whipping (or serving) a splice simply means wrapping it with special thin twine to make it neat. Alan Hazeldine, our welder, very kindly sent us a length of it, and it’s very satisfying. The cut ends of the wire tend to stick out and are pretty painful when you’re handling the cable. Well, the whipping wraps those ends tidily away and looks good into the bargain.

So now we were all ready to start assembling a wing. Well, no.

We soon discovered we were going to have drill a whole lot of holes in the wing spars before we could do any assembly, and the one that would cause us the most problems was the one fixing the brackets where the interplane struts fit. In 1915 they were called stanchions (not struts) – and in fact the wings were called planes!

But these fittings do a lot of work, and the LAA’s  Francis Donaldson had identified them as a potential problem area at his initial look at the drawings. We’d asked Paul Welsh to look at the problem and he looked at some calculations I’d done to try and justify the design. He corrected my mistakes, and it showed that there was a potential problem where a bolt goes through the middle of the spar. But he said the only way was to carry out a test.

Well, we decided it should be possible to modify the cable tester to do the required testing, so Rick got started on that job. It took all day, but we’ve uncovered some interesting facts, which the calculations hadn’t identified. They go to show how Francis Donaldson’s wisdom in questioning this part, and Paul Welsh’s wisdom in advising a load test. There’s some interesting video to go with it, but we’ll save that for tomorrow’s post…

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