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74. Onwards and upwards

18/10/2013

This week I’m down with Theo again, consolidating the work we did in the summer. It’s not going to be quite so spectacular, since we will end up we are sort of filling in and finishing off the bits we put together for the big assembly in August, but it’s satisfying nonetheless.

Since then, we’ve had the pukka struts made by Rupert Wasey, and these have to be fitted in place of the temporary ones we used in August. It should, in theory, be simple to make the final ones just the same as the dummy ones, but we can’t afford any mistakes, and in some cases there are minor errors that we’d like to correct.

Rupert’s machine cuts the wood in layers, leaving a sort of striated finish, and it takes quite a while to smooth these down to a perfect finish.

Struts before and after finishing. there's quite a lot of elbow grease to get from one to the other!

Also, it’s necessary to finish the ends and – for the undercarriage at least – this is horribly complicated, since although the front of the struts are straight, nothing else is, and there’s not a single right angle anywhere.

I've marked up all the angles and dimensions on here in case we ever have to make another one.

I’ve marked up all the angles and dimensions on here in case we ever have to make another one.

You can see all the different cuts we had to make in this picture. We had to make a jig to hold the strut at fixed angles in two planes in the bench saw in order to get the necessary precision, and even so, getting everything exactly right is really hard. It’s also very important to mark on each end of each leg where it’s going to go on the aircraft – bottom right inside, top left outsiide, etc., and to draw on it roughly where the cuts have to go. If you don’t do this, it’s amazingly easy to cut two left legs, or cut them so that the top and bottom ends are for opposite sides!

One of the odd things is that the front and back legs enter the fitting at the bottom at very slightly different angles. When you do a trial assembly, the top of the front leg naturally wants to sit about 100mm too far in, and initially we thought we could sort this out by just forcing it. But when you do that, other things spring slightly out of line, and so we had to make that tiny adjustment in the metal fitting at the bottom.

We’ve also decided to make a small change from the drawings. On the original, there’s a wooden cross-member that goes behind the axle, and because of its location, the big cables that cross over and hold the undercarriage straight must pass through it. Apparently the original had holes drilled at an oblique angle for the cable to pass through, but it quickly became apparent that we’d have to either make a pretty enormous hole to fit the end of the cable through, or make the cables up and splice the ends, with them already threaded through the crossmember. We didn’t fancy either of those solutions (particularly since we’d already made up the cables!), so we’ve made a slot in each end of the crossmember. The material we’ve taken out doesn’t contribute to the strength at all, and it will barely be visible since the slot is mostly covered up by the metal brackets at each end, so we don’t feel particularly guilty about this small modification!

By the end of Tuesday, we had the undercarriage rear legs all ready to fit, and the top legs only needed to be finally trimmed to length at the top, so we slapped on a coat of paint / Danish oil to the bits that will get bolted together so that they will be ready for final assembly on Wednesday.

While those were drying, Theo made up a couple more cables (these will stop the axle sliding sideways) and tested those and some flying wires he’d made up previously, and we cut and fitted the cabane struts.

I drilled all the final holes to fix the tailplane in place.

So we’re making steady, if undramatic progress. We should be on track to have another complete assembly later on in the week – hopefully on Satruday when our inspector comes to have another look.

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