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91. Tankage

13/01/2014

I thought I’d do an update on the tanks, petrol and oil as they are one of the more complex engineering challenges.

The oil tank (below)  is made of thin brass. It’s not too clear from the photo, but the ends are flanged (the outside edges are bent back 90 degrees) for the rivets, and they are also dished to stiffen them up.

The oil tank made of brass, with about 300 rivets holding it together!

The oil tank made of brass, with about 300 rivets holding it together!

Having had a quick go at them ourselves, we realised it was tougher than we thought, and have asked veteran car engineer Ian Harris to help. I should make clear that it’s the cars that are veteran, not Ian!

He’s managed to make the two ends by gently tapping the brass over a former made of MDF, using a combination of some of the 14 hammers he has in his hammer drawer, together with something that looks like a small cricket bat made of ash. Dishing the ends is more complex, involving heating the middle up so that it expands and bulges out, followed by careful work with an English wheel. The results are beautiful. He’s also spun the cone at the bottom for the outlet, and when that’s rivetted on, the one-piece wrapper plate that goes right round the outside with a single join at the top will be rivetted on.

The oil tank is a constant cross-section, so it’s relatively simple. The petrol tank is the real challenge, for a number of reasons.

The underside of the petrol tank. The two channels are for fixing it to the fuselage longerons, and you can see the horizontal lines of rivet heads that mark where the internal baffle plates are. It's sat on the front end for the picture, which as you can see is wider than the back, and you can see the cone for the petrol outlet that will have to be made up.

The underside of the petrol tank. The two channels are for fixing it to the fuselage longerons, and you can see the horizontal lines of rivet heads that mark where the internal baffle plates are. It’s sat on the front end for the picture, which as you can see is wider than the back, and you can see the cone for the petrol outlet that will have to be made up

 

The first is that the bottom tapers from back to front although the height remains constant, so the wrapper plate isn’t a nice neat rectangle. In fact it isn’t even a section of a cone.

This also means that the flanges on the end plates aren’t at right angles – it’s continuously changing from top to bottom.

Next; the front plate has to be exactly the right size so that the flange fits the engine former. There’s really no room for error here, unlike the oil tank which doesn’t have to fit anything exactly.

The next is that the back plate (at the top in the picture) slopes forward in order to line up with the cabane struts. This means that the shape is a very slight ellipse, with the angle of the flange changing from top to bottom, and not a right angle to be seen.

And then there are the baffle plates… Inside the tank are two baffle plates (you can see the two lines of rivets part way up the tank in the picture), which have to be odd sizes and aren’t an exact circle, and whose flanges are not at right angles, and which have to have large holes cut in them to let the petrol through.

And finally, it’s made of steel, not brass.

So cutting the MDF patterns for these is going to be good fun!

Our cunning plan is this:

We’ll cut out the MDF pattern for the front because it’s a perfect arc of a circle and we can size it to be a perfect fit with the engine former. To start with we’ll cut the edges at right angles.

Next, we’ll make up a pattern for the bottom of the tank, also in MDF. It’s a trapezoid with known dimensions, so that’s easy too.

Then we’ll cut an MDF pattern for the back plate, but upright, not sloping. This way, we know that it’s the arc of a circle and we know exactly what its diameter and height should be.

All three are screwed together firmly, with internal bracing if needed.

Now, we take our bench belt sander and make up surfaces each side of it that can be adjusted so that they on exactly the same plane as the belt.

Once this is done (we can use a scrap piece of timber to check the alignment), we place the back end pattern on the surface and very carefully sand the edges of the front pattern until they line up with the back, making sure we don’t remove any material from the front face.

Now, we make up patterns for each of the baffle plates slightly oversize and mount them on the baseplate in the right locations.

Then, with the front and back patterns on the surfaces each side of the belt sander, we sand the baffle plates down until they are  perfectly in line.

Finally, we remove the rear pattern (the upright one) and replace it with one that’s sloping at the correct angle and slightly oversize, and sand it down, using the front and baffle plate formers as guides.

That’s the plan. We’ll let you know how we get on…

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From → Building, Technical

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