123 Oil Tank
While Theo and I were busy with the linen covering, Rick was also hard at work with Ian Harris on the oil tank. The result is, frankly, a work of art, and it’s criminal that it, along with the majority of the internal structure, will be hidden from view on the completed aircraft. So feast your eyes on this wonderful piece of work while it’s still available. It’s been made exactly as the drawing stipulated, and Ian has even managed to acquire from somewhere the original brand of filler cap – a Rotherham’s no. 2.
The front and back are made by forming the flanges over a wooden template. The straight bottom flange is easy, of course, but the curved one has to be gently tapped with a lightweight curved wooden bat, making sure that you never let a crease form. The result is that the brass actually gets thicker towards the edge of the flange.
The ends are slightly dished to stop them from ‘panting’, and Ian’s method of doing this is frankly genius, and I won’t give the game away! It’s finished off in an English wheel.
The sump at the bottom is made using a mandrel – a steel template machined from solid. A disc of brass is clamped to it in the lathe, and the brass is ever so gently forced around the shape of the mandrel using a wooden paddle. By the time you get to the top of the sump, the brass is considerably thicker than than it started out.
There are lots and lots of rivets. Each one has to be cut to exactly the right length, and peened over. Getting exactly the right strength and direction of tap is a real art, and Ian and Rick got down to about 2 minutes per rivet by the end. Even so, its a monster workload.
When that’s done, you apply solder to the joints. For all except the front end, you can apply the solder from both inside and out, but the final joint has to be done ‘blind’, and it’s a considerable worry to know whether it has formed a perfect seal. Happily it did, and the tank has survived its pressure test, which means applying an air pressure of 1.5psi to the inside and wiping a 50:50 mix of water and washing up liquid all over it. Any leaks will show up as bubbles.
The next stage is to mount it onto the aircraft, and here, I admit, we aren’t following the original drawings. The Scout was famous for leaking oil tanks (here is one example, but there are lots more).
The reason is that the tank had originally been located behind the pilot, where it was accessible. It was decided to move it forward because of oil starvation problems, and in the haste to do this, they hid it under the plywood cover forward of the cockpit. The ply cover is almost impossible to remove, making the tank almost impossible to access. The tank had channels rivetted and soldered to the underside of the tank which fitted over the fuselage longerons, and they recognised that vibration of the longerons could cause leaks where the channels were attached to the tank because they fitted bits of rubber between the top and inside of the longerons, and the channels. but there wasn’t room for rubber on the outside of the longerons, so the whole exercise was a bit of a waste of time.
Since the whole thing is out of sight – even if you poke your head in under the instrument panel – we’ve decided to adopt modern technology and bond the channels to the tank via strips of rubber – much like an exhaust mounting bracket on a car. This should reduce or eliminate the risk of these leaks.
One of the compromises we’ve had, reluctantly, to make, to ensure the aircraft is practical to operate.
Incidentally, on the later Scout D they fixed the problem by making a combined oil and petrol tank which filled more or less the whole space between the cabane struts and didn’t have a ply cover, so if you’re thinking of building a Scout D you won’t have this particular issue!
And now, Ian and Rick have moved on to the petrol tank, which is a good deal more complicated, since it’s a sort of conical shape – wider at the front – and because its made of tinned steel, not brass. It also has a couple of baffle plates fitted internally. On the plus side, the petrol tank isn’t covered up with ply, so the craftsmanship will be permanently on display.