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64. Erection

05/08/2013

Calm down, now. It’s exciting, but not that exciting.

What we’ve been doing today is to start the critical job of fitting the wings to the fuselage. On this depends how – indeed whether – the aircraft is fit to fly. Unless it’s set up absolutely perfectly it could end up having a permanent turn, or really unsafe stall characteristics.

In fact, some of the final tuning can be done by adjusting the turnbuckles in the various rigging wires, but there are some bits where this really isn’t practical – and in any case, it would be great to be able to have it fly straight off the drawing board, so to speak!

As you can see, the first problem was to decide where and how to set the whole thing up to work on it. There’s no need to have both wings set up at this stage – once we know the lengths of the various rigging wires, we simply make a second set for the other pair. Theo’s idea as shown below was great – the front of the fuselage and the lower wing were all set onto the workshop floor, which is perfectly flat and firm.

How to fit the plane in the workshop to rig one set of wings!

How to fit the plane in the workshop to rig one set of wings!

This didn’t take too long to do, with the top of the fuselage perfectly level in both directions, and we soon got the wing spar ends inserted in the fuselage fittings and clamped to the longerons, with supports at the outer end.

The next thing is to check that it’s perfectly at right angles to the fuselage. Theo had a piece of 4” x 2” aluminium box section about 3m long, and this was the perfect tool for the job. We rigged it up across the two fuselage longerons at the front of the cockpit, perfectly at right angles, and supported at the end near the wingtip. We could use a plumbline to check that the wing was exactly at right angles too. (Later biplanes often have the wings set backwards a little – called sweepback but the Scout predates that kind of sophistication). It wasn’t perfectly aligned, and both spar ends have to be absolutely firmly butted up against the fuselage, so the way to adjust them is by altering the tensions in the internal bracing wires inside the wing. It’s surprising how few turns of the turnbuckle it takes to move the wingtip backwards or forwards by 50mm.

The aluminium beam was absolutely perfect for checking the wing alignment.

The aluminium beam was absolutely perfect for checking the wing alignment.

Next we had to set the dihedral. This is the amount the wings rise towards the tips, and critically affects the handling. On the earlier types it was set at 3%, and once again our aluminium beam came in extremely handy. You’d think it would be very difficult to judge exactly when the beam was exactly level with the fuselage top, but if you gently raise and lower it you can feel extremely precisely the point at which it changes from one longeron to the other – to within 1mm, I’d say.

With this set up we could measure the drop at two points and we adjusted it until the height difference was 3% of the horizontal distance between them.

Finally, we needed to make sure there was no twist in the wing. Most modern aircraft incorporate some twist (called ‘washout’) so that the outboard end is not pointing upwards quite as much as the inboard end, but the Scout doesn’t have any, so we used a digital spirit level to check this. With the lower wing perfectly placed, we could measure the lengths of the landing wires (these are the ones that go from the cabane to the lower wings, and these take the weight of the wings when the aircraft isn’t flying – hence the ‘landing’ wires.) With this done, Theo set too to splice them, while I made a start on the wing struts in preparation for installing the top wing.

We had two dummy ones ready – one hand-carved by Rick, the other roughed out by Rupert at Hercules Propellers – but the lengths and the angles at the ends have to be very carefully cut so that they are a snug fit in their sockets, and the ends bear evenly on the end of the socket. The overall lengths are given in the American drawings, but as we saw yesterday, they aren’t always accurate, so I’d done calculations as well. But you can’t afford any error here, since they determine the dihedral and the twist of the upper wing. We cut them to about the right angle, and about 20mm too long to start with.

When the landing wires were completed and installed, we popped the struts in place and lowered the top wing in place. We clamped it onto the cabane, with a sling from the roof to hold it roughly in place and checked the sweepback and dihedral by reference to the lower wing. I’m delighted to say that with the lengths set in exact accordance with my calculations, the top wing was exactly parallel to the lower in both directions. We measured up the various cables – the flying wires which go from the bottom of the fuselage to the top wings (and are called ‘flying’ wires, logically enough) and the diagonal wires between the struts. It was a very satisfactory day’s work and the only downside for Theo is that he’s got a LOT more splicing to do – particularly since the Scout was the first aircraft to be fitted with doubled up flying wires for additional safety.

Starboard wings fitted and aligned. Just got to splice all those cables now!

Starboard wings fitted and aligned. Just got to splice all those cables now!

Ah well, if he didn’t have that to do he’d only be sat in front of the tele…

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From → Building

One Comment
  1. What incredible dedication. it is going to take me several hits to take all this in. Put it on Kindle! Dave Thomas.

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