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22. Taking it to the limit!

27/06/2012

Yesterday we carried out the first stage of testing on the point where the flying wires meet the top wing – which is also where the strut fits.

This picture shows the way we set it up. There’s a short piece of spruce cut to the dimensions of the rear spar, with the fitting bolted to it. it’s actually upside down – on the aircraft, the spar would be horizontal and the other way up, and the pull on the wires would be downwards, but it’s easier in our case to have the pull horizontal (it’s being applied at the top left of the picture), and the fitting on top so that we can keep a close eye on things.

Calculations show that the weak point in the design is likely to be the bolt that goes through the middle of the spar – we would expect the wood fibres to be crushed and the bolt to start pulling to the left of the picture.

For modern light aircraft, we calculate the loads in straight and level flight, and call those 1g – that is, 1 times the force of gravity. we than apply a safety factor of four to allow for turbulence in the air, normal manoeuvring, and so on, and we say that the aircraft must be able to withstand that without any permanent damage. It can bend, but it mustn’t break. That’s called proof load.

But we also expect the aircraft to stand a 6g load before anything breaks – and this is called the ultimate load. The structure must be able to withstand this load for a mere 3 seconds.

The Bristol Scout was designed before they invented such limits, and will only be flown under carefully controlled conditions, so what’s acceptable will be up to the LAA – but clearly the better the figures we get, the easier their decision will be!

Well, we applied the load in a series of tests, increasing the load each time and checking for any movement or damage each time. Essentially, everything went fine until we reached 3.3g, when the bolt started to pull through the wood as expected.

We’d drawn pencil lines round the metal fitting where the bolt goes through the spar before the start of the test and you’ll see that it no longer lines up – the whole fitting has moved fractionally  to the left. You can see the same thing with the line drawn next to the bolt, which no longer lines up.

Essentially, this means that the hole in the timber has opened up just a fraction. allowing that movement to occur. It hasn’t quite met the 4g we’d expect of a modern aircraft, but it’s a whole lot better than the 1.8g predicted by our initial calculations!

Thereafter we carried on applying gradually increasing loads, peaking out at about 5.4g, by which time the bolt had pulled a considerable distance through the wood, but everything else was intact. Oh – apart from the test rig, a couple of bits of which had to be beefed up following exciting failures!

There’s a video of the day’s testing at http://www.youtube.com/watch?v=ZaTMasQcRd8.

We finished about lunchtime, and set to on the wings themselves, drilling the necessary holes in the spars, and completing an impressive set of cables, all tested and whipped. So tomorrow we’re all set to start some actual assembly!

 

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