Spitfire in my workshop. David Glen BSc (Hons) MSc: Model Maker, Journalist David Glen

Pneumatic system 1: Air tanks

Sunday, 29th June, 2014

Come June, I shifted direction entirely and resumed work on the long abandoned fuselage interior, focussing on the section immediately aft of the cockpit between frames 11 and 13. Until the detailed work here is complete I am unable to install the top half of frame 11 or extend the adjacent skin upwards from the waist to the rear glazing.

The dominant features are the two big compressed air bottles on the port side, and that is where I started. They feed the aircraft’s pneumatic system, which works – among other things – the brakes and guns. I have many photographs of them in various extant aircraft, and it is clear that in some restorations, sleek modern tanks are fitted. I wanted to fit the original domed type with their very prominent weld seams.

The need for two tanks immediately suggested casting from a pattern. Luckily I have some thick-walled alloy tube of exactly scale diameter, so I capped one end with a scrap of waste resin and turned an accurate dome shape – which is easier than it may sound: I simply filed a 90-degree arc into one end of a scrap 3/64-in. mild steel. Mounted in the tool post of my lathe, this simple tool had edge enough to cut the resin nicely. Before removing the work from the chuck, I turned two minute grooves in the aluminium to mark the path of the upper and lower weld seams, and with these as a guide built up the welds themselves from blobs of extra-thick superglue dabbed on with a fine wire applicator. I think I produced the completed pattern in no more than an hour or so.

Resin casting techniques require little regurgitation here; suffice it to say that by next day I had two nice resin blanks ready to re-chuck and shape the two remaining dome ends with my make-shift cutter. I was pleased with the weld effect, particularly since by installing the tanks rotated relative to one another the welds appear subtly different. All that remained now was to create the pipe union stacks at both ends of each cylinder, and these were easily made up from various sizes of hexagon brass turned in the lathe.

The bottles are attached to the airframe at their lower ends by two small straps around the pipe unions, but I decided to ignore these since they are all but invisible. Instead, I drilled a hole about a third of the way up from the bottom of each cylinder for a peg of 1/8-in brass designed as a push fit into matching holes in the fuselage sides ­– a simple ruse but one that ensured a firm anchorage. However, the two prominent straps at waist longeron level and the formed aluminium plate onto which they hinge could not be ignored. This proved problematic since I had no drawing of the component, and my various photographs showed only tantalisingly fragmented glimpses of it. Having scoured about every source available, I was forced to make a best guess as to the exact shape, and set about marking out and cutting a steel former over which to bend the flanged edges from a litho-plate blank. The job proved easier than anticipated, and I was rather pleased with the outcome with its little lightening hole in the centre. I formed the two paired straps from litho plate with tiny slots in their hinged ends, added liners of similar gauge red-brown plastic – a tedious rather than difficult job – and attached them as in the original with tiny bolts and nuts. The next day I was on the Internet and scrolling through the matchless photo archive chronicling the Ardmore Mk V restoration in New Zealand, and there was the plate and its attendant straps photographed demounted and in impeccable detail. My chagrin was dampened only by the fact that I had got it very nearly right!

With the mounting plate spray painted and bolted onto the airframe and the straps tightened, the tanks fitted snugly and firmly, but all was not quite right: one sat very slightly but noticeably higher than its neighbour – an error that required removal and replacement of one of the brass pegs and a repositioned hole in the fuselage side.

I have finished the tanks with a spray coat of Halfords aluminium, a close match to several independent references; I have also generated art work on my Apple Mac for the prominent stencilled data on the sides and a set of dry transfers are in production as I write.

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The pattern for the cast resin compressed air tanks is made from aluminium tube capped with an off-cut of resin from a previous job. I turned the dome-shaped end in the lathe, using an ad hoc cutter made from a scrap of mild steel. The same cutter was used to turn the reverse ends of the paired castings.
The twin tanks complete with their pipe unions and simulated weld seams. The little valve is part of the pneumatic system.
The mounting plate and straps and the steel former used to make the flanges. The strap liners are made from red plastic.
With a coat of aluminium paint and the application of the data stencils, the tanks have now been installed permanently and plumbed in at top and bottom.
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