The visible fuselage frames
Sunday, 15th September, 2013
Fuselage Frames 7 to 12 are all prominent within the Spitfire’s cockpit, so they needed to be uncompromisingly accurate. However, they also had to double both as exterior fuselage formers and as landings for composite balsa wood and styrene structural panels that would enclose the cockpit area. The only practicable solution I could think of was to make them from three laminated plywood layers – the middle being cut to full exterior fuselage dimensions and the two flanking layers reduced by a nominal amount (skin thickness) so as to create shoulder rebates either side. Finally, the entire piece would be laminated with styrene sheet and detailed in replication of the original. The trick was choosing materials of a thickness such that the three layers summed together equalled the frame thickness, minus an allowance for the styrene. For the heavier double frames, this solution appeared robust; less so for the narrower frames, particularly Frame 9.
If this sounds confusing, I hope the pictures make things reasonably clear. They show how, until the composite skin could be installed, the entire assembly was held in situ on the upper and lower longerons and by the artifice of a shallow four-ply ‘stringer’ at the bottom of the fuselage, a relic of the longitudinal web of ply over which the entire fuselage structure is built. I chose high quality mahogany square section for the longerons, which, like the frames themselves, were laminated with styrene sheet of sufficient thickness to allow for corner radii where specified and secured fore and aft by small wood screws reinforced by two-part epoxy glue.
The biggest challenge in the detail was the many lightening holes that characterise most of the individual frames. Clearly, these were cut in the styrene skin but the underlying ply also had to be cut out and by a greater diameter to make the piece look convincing.
While all the main features of Frames 8 to 11 are given in the Monforton drawings, I was fortunate enough to have what appears to be the Supermarine GAs, as published in the book, Dutch Spitfires, by Theo Melchers. Used in combination with photographs, these were invaluable when adding the finer detail made up from sundry styrene and litho plate parts.