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61. Chassis

02/08/2013

This is the start of two solid weeks dedicated to the Bristol Scout airframe. Early on Monday morning, I set off south for Theo’s, arriving about 1100.

But Theo had beaten me to it, and already had the fuselage turned upside down in order to work on the chassis (now more commonly known as the undercarriage).

We’ve got three major tasks requiring two of us to get completed. The undercarriage is the first; the cabane (a strange word that apparently means ‘cabin’ in French, and usually means the structure that fixes the top wing to the fuselage), and then attaching the wings themselves.

All of these need struts – external bits of timber that are shaped to an aerofoil section to reduce drag, and they are being made by Rupert Wasey at Hercules Propellers using his CADCAM system. Their length is clearly critical; if one chassis leg is too long, the aircraft will sit with one wing low, or maybe steer round in a circle. If all we had to do was set them up in the saw and cut them off square to an exact length, this wouldn’t be a problem, but for the chassis in particular, nothing could be further from the truth.

There are two rear struts, the bottom of which is fiendishly complicated in order to fit into the metal fitting.

You can see the problem here:

Lower ends of the undercarriage legs, with all the complex cuts to be made.

Lower ends of the undercarriage legs, with all the complex cuts to be made. The front leg is on the left, with the rear leg on the left. The sides aren’t parallel, the end isn’t square (and in fact is sloping too), and the top is tapered. then you’ve got to make that 1.5in hole in it in EXACTLY the right place, and that’s drilled on the slope too! The front strut doesn’t have a hole in it but the sloping cut on the right hand side must match the left side of the rear strut…

You can’t afford to have a single slip up in this area, because it’s all very heavily loaded every time you come into land, and any slackness in there will very quickly result in unacceptable play building up.

And as you can see, they fitted!

And as you can see, they fitted!

But it’s more complicated still, because all those angles are obviously affected by the position of the axle with respect to the airframe. if its too far back, it could tip over on its nose on landing, and if it’s too far aft, it will tend to bounce. And if it’s too low, the propeller could hit the ground…. oh, you get the picture. According to the drawings the axle needs to be 550mm aft of the front engine mount and 1300mm vertically below the top longerons. Of course, you could set up a fancy jig to hold the axle in position while you make the legs to fit the allocated positions, but we do have some drawings with the dimensions on, so we did some calculations to see if they seemed about right.

We made up a pair of dummy legs (one front, one rear) using scrap timber. We cut them in half and concentrated on getting the ends to fit the metal fittings first, then joined them together using ply straps so that we could adjust the length. In fact the drawing dimensions seemed pretty much spot on, so we then transferred the angles onto dummy legs in scrap wood that Rupert made up for us to play with for the other side. 

As you can see, the result looked pretty mush like an aeroplane (albeit upside down) and the measurements – the 550mm and 1300mm were spot on as well. Normally this kind of skilled fitting is left to Rick, but he can’t be here this week, and we were pretty chuffed that we’d managed to do it so quickly and accurately ourselves!

The completed trial assembly of the wheels, using dummy legs. I'm sure you realise the fuselage is upside down!

The completed trial assembly of the wheels, using dummy legs. I’m sure you realise the fuselage is upside down!

So now, when Rupert produces the finished struts, we can be much more confident off being able to make them fit correctly…

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