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15. Odds and Sods

24/02/2012

In previous blogs, I said that the Bristol Scout was astonishingly similar to its competitors, given that it was designed only six years after the first aeroplane flew in Europe.

And while that’s certainly true of the general shape and construction methods, there are some strange corners that we’ve been wrestling with, and it would be interesting to know why designer Frank Barnwell, or his draughtsmen, came up with the solutions they did.

Most of these oddities relate to the metal fittings. Take a look at the Scout with its clothes off and you are immediately aware of the wood. Look a little closer, and it’s obvious there’s lots of wire too.

Leo Opdyke's Bristol Scout, Yeovilton

Leo Opdyke’s Bristol Scout, Yeovilton

But if you’re setting out to make something like this, you quickly realise that wood and wire can be sorted very easily; the bits that take all the time are the little metal brackets that join them all together.

I haven’t counted up, but there are probably hundreds of the little beggars; occasionally there are ‘standard’ fittings where you need more than one off, but not many.

Metal fittings on the wings

The ones pictured here are complicated ones that connect the wings together, and are actually made up of six individual components, each of which has to be cut out, filed, drilled, bent and some of them welded. There are ten of these fittings on the wings, which are relatively simple compared to the fuselage!

Bristol Scout wing strut fittings

Getting back to the designer’s oddities, the first of these occurs on these same wing fittings. You can see where the rigging wires are attached to the fitting, and the metal fitting there needs to be 3mm thick in order to be strong enough. But on the original drawings, it’s made up of two thicknesses, each 1.5mm thick, and welded round the edges. We’ve consulted with the experts on this, and have failed to come up with a good reason for it; welding (particularly in those days) introduced all sorts of stresses and strains in a piece of metal, and water could get in between the two thicknesses and cause it to rust from the inside. The only reason I can think of is that perhaps they simply couldn’t get hold of metal in the right thickness, so had to make do with what was available.

Here’s another oddity.

Normal Tailplane hinge

If you make the tail surfaces of steel tube, you’ll need to make a hinge so that the control surface can move easily. The way it’s been done since time immemorial is as on the right – three pieces of tube are welded in line; the two outside ones to the front, and the middle one to the back. You then push a pin through and Bob’s your uncle.

But for some reason this wasn’t how it was done at Bristol’s.

Tailplane hinge

What they specified was a piece of thin sheet metal wrapped around the hinge pin, in a sort of keyhole shape, posted through a slot in the tube, and welded in place. The first problem is making a nice even keyhole shape. It ain’t easy – we’ve tried it! In the end, we had to wrap the strip of metal round the hinge pin (or a bolt the same size), weld the ends temporarily together, and then press it down a slot just wide enough for the two thicknesses of metal. It’s very difficult to get it to sit tightly all the way round the hinge pin, and absolutely straight so that the whole thing is perfectly symmetrical.

Next you have to cut the slot in the tube. This is not easy either; we used a drill at each end of each slot, and joining up with a tiny cutting disc. We haven’t got as far as the welding yet, but one thing we know is that welding will distort the workpiece, so it’s likely that the hinge pin won’t fit after we’ve done it. Hmmph! We’ll keep you posted.

Anyway, having selected this method of making hinges, you would think it would make sense to do the same for the rudder, which also has a tubular construction.

But that would be too simple.

The rudder, assembled but not welded (the orange strap is just to hold it together for the photograph)

Close-up of the machined rudder hinges

Manufacturing the rudder hinges. On the left is the part-machined bit, with the finished article (well, actually a test piece) on the right

You can get the idea from the lower picture.

But at any rate, having decided on the use of machined parts for the rudder, they used something similar for the other part of the hinges, where it attached to the rear of the fuselage.

Didn’t they?

Well… no, actually.

Tailpost fitting

This is a detail from the fantastic parts list, published in 1915, together with pictures illustrating the various component parts.

This part goes right at the back of the fuselage. You need to imagine the four main pieces of wood that run the length of the fuselage (called longerons) being drawn together and fitting in the brackets on the right of the picture.

The vertical tube forms the very back of the fuselage, and the rudder is hinged from the back of it. But if you thought it would be simplicity itself to make more ‘figure eight’ parts and slot them over the tube, you thought wrong.

Tailpost detail showing rudder hinge. This is at the top left of the picture above, and shows the smaller diameter bush that holds the hinge pin next to the tailpost, which is the larger diameter tube. (The hole to the right of the tube is for attaching rigging wires to, and needn’t concern us now).

 

Bear with me, because this gets a little complicated if you aren’t allowed to wave your hands about and demonstrate with salt cellars and pencils.

The bush for the rudder hinge is fixed alongside the vertical tailpost tube, and welded in place. The top two brackets that hold the wooden longerons are actually made from a single piece of metal which goes round the tailpost and the bush before joining on to the bracket on the other side.

Tailpost from on top, showing where the metal strip has to go

I’ve tried to show this idea in the bottom picture, with a dark line showing where we’re supposed to get this piece of metal to go.

Having somehow managed that, it’s all supposed to be magically welded in place.

I don’t know if you’ve ever tried welding, but it’s one of the few areas of engineering that still requires a very high degree of skill; I remember an instructor saying that you can’t teach someone to weld unless they’ve got a high degree of innate ‘feel’ for it. For most of us – and that includes me – we can never be a good welder, no matter how much instruction we get.

I haven’t spoken to our welder, Allen Haseldine yet, but I have a feeling that even he, who is pretty much a black belt welder, will struggle to understand how we’re supposed to do this.

We’ll keep you posted!

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From → Building, Technical

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