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20. Getting together

12/05/2012

This last week the three of us involved in the project – myself, my brother Rick, and our long-term friend Theo Willford – got together at Theo’s place in Dorset to try to assemble a wing.

Did we do it? Well, no. And here are our excuses.

People often find it hard to understand why a relatively simple piece of equipment – like a light aircraft – should cost so very much more than a family car, for example, when the car is so very much more sophisticated. The answer, as I’m sure you know, is that it’s all a question of manhours, and if the light aircraft manufacture could be automated as for a mass-produced car, the cost would come rocketing down.

In just the same way, we looked at the materials we had available for making the wing, and reckoned all you had to do was bolt them together. Wrong. To give you an example, here’s a very simple bracket that holds the ends of the wire bracing inside the wing. By getting the CAD drawing done, and transferring it to the waterjet cutter, we’d saved ourselves the business of marking each one out individually and cutting it out with a hacksaw.

But while your eye can now imagine the part as a finished piece, the reality is very different. First of all, each one (there are eight all told) has to be carefully bent in exactly the right place to exactly the right angle. You mustn’t make the bend to sharp, or that will weaken it, so you’re going to need a steel former of the right radius to carefully form it round. The waterjet cutting process leaves all the cut edges too rough for use on the finished article. any tiny imperfections in the surface can act as stress raisers and the starting point for fatigue cracks to appear, so they need to be cleaned up with a medium and then a fine emery.

The same applies to the holes, so they are cut to a smaller diameter than finally required. Then you drill to just under the final size, and finally hand ream them before cleaning up the corners with a rose bit. Then we need to reinforce one end with washers (these help to spread the load where the wire cable and thimble go through them) and so the washers need to be brazed in place.

If you haven’t tried brazing, it’s very satisfying when it’s done right. All the contact surfaces have to be covered in an acid flux, which you make by mixing a powder with just enough water (or spit, if you don’t have water to hand) to make a paste. Clamp the joint together and warm with an oxy-acetylene torch played gently over the whole surface until the steel turns a dull cherry red. If you’ve done it right, you touch the filler rod for the briefest second, and it immediately fills the gap, making a really strong joint. Very satisfying.

So are we done yet? Hell, no.

The heated part is discoloured, and the edges of the reinforcement need to be dressed off, using a grinding wheel, then linished using the old combination of medium and fine belts on a Powerfile. Then there’s the hole in the middle that needs drilling and reaming. And the other end also needs reinforcing using a stainless steel pop rivet.

For this you need to drill the hole as described above. Then insert and pull the rivet. Then grind the surplus off the formed end, punch out the remains of the mandrel, peen the end down with a ball peen hammer, drill a clearance hole and chamfer with a rose bit, and finally linish again to remove any marks left in the surface by the other operations.

There. It’s ready for painting now – etch primer followed by a top coat. That’s one fitting down; there are seven more like that, and another two sets of eight to go.

But don’t think that was all we did. We’re an experienced team who work exceptionally well together, and each of us knows his own comfort area. So while I concentrated on the metal fittings, Rick – the master joiner – concentrated on fitting the metal caps to the inboard ends of the wing spars. You can imagine that this is an extremely important bit of the structure, and if he made a single slip up, we might have to start again with a new piece of timber; and perfectly straight pieces of spruce like that don’t grow on trees, you know!

If you look at the picture, you’ll see that he had to make a cutout in the wood to fit the curved metal fitting. It had to be perfect in every dimension, with the spar fitting a tight sliding fit over it. What you probably can’t see is that the fitting isn’t exactly in line with the spar, That’s because the wings have dihedral – that is, they slop upwards towards the tips – but of course the metal fitting has to sit flat against the vertical side of the fuselage, and so a tiny bit of angle has to be incorporated into the measuring. Rick decided to make a jig and use a pattern-following  router so that he could do all eight ribs (two per wing) exactly the same. In the photograph we’ve left the fitting slightly loose so that it’s easier to get off without damaging it, but believe me, it’s a simply superb fit. And so are the other seven!

And Theo? Well, Theo is our most experienced cable splicer, so that’s what he did. Back in chapter 11 Consulting the Experts, I explained how the ends of cables were spliced rather than joined using a ferrule. It’s an art which is accompanied by a lot of mystique, and there’s an explanation of how it’s done here, and I have to say ours don’t look quite as good as that yet. But they aren’t too bad, and I hope they’ll improve as we go along!

The end result doesn’t look too bad , but Theo’s a perfectionist and is hoping to be able to match the experts shortly!

We also learned a lot by doing some trial assemblies. We’ve been looking at the drawings for years and years, but there’s nothing to beat actually assembling the ribs onto the spar, and trying to work out exactly what goes where, and how it will need to be finally assembled. Needless to say, we had a couple of goes at it – here,  and here. We’ve come to a number of decisions about how to assemble stuff, and when we do – finally – get to the stage of doing final assemblies on wings, we’ll have done the preparatory stuff on all foru, so they should go together much more easily.

We hope!

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