AN ORGANIC SPITFIRE?

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The state of emergency which  existed in Britain during  the later months of 1940 brought a review of the sources of  raw materials. The fall of France removed as a source of supply some of the largest bauxite deposits in the world and among the measures studied  was the possibility of making aircraft from materials other than  light alloys which could be produced in the UK. In August 1940 Aero Research Ltd were asked to build an experimental Spitfire fuselage in order to determine if synthetic material could be used satisfactorily for such a purpose.

The material selected was Gordon Aerolite, which had been developed by the company as a result of research into the problem of producing a suitable synthetic material for moulded airscrew blades.  Gordon Aerolite was made of untwisted fibres of flax impregnated with phenolic resin and made into bands approximately six inches in width on a machine developed for the purpose. To make sheet material a number of bands were placed edge to edge and overlaid by others placed at right angles to build up the required thickness. The pack of strips was then hot-pressed to bond them into a single sheet.

Tests on the material were made by the Royal Aircraft Establishment at Farnborough and as a result the opinion was expressed that Gordon Aerolite was then the most promising organic sheet material available as a possible stressed-skin covering for aircraft. It was considered probable that, if it could be made available in quantity, it might be used as a direct substitute to replace light-alloy sheet in existing designs. The material had approximately equal strength and stiffness along and across the sheet,  but the strength and stiffness at 45 degrees to the grain was only one-half that along the fibres.  As in plywood, the latter property could actually be an advantage where it was required to bend the material round a sharp curve.

In applying this material for airframe construction Aero Research used it on this basis as a direct substitute. Development of a specialized technique to take advantage of any particular property of the material was not attempted: time was an important factor and it was decided to adhere closely to existing assembly practice in order to avoid any break in production should a changeover to the new material become necessary. Gordon Aerolite sheet was used for all the structural members, including the frames, longerons and stringers and, in general, the design followed closely that of the original Spitfire. Except in the majority of the frames, which were built up on a flat, steam-heated plate with Ardux cement, riveting was used throughout in assembly.

The bottom of the fuselage looking forward from Frame 9, to Frames 8, 7, 6 and 5.

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As a preliminary to the construction of the complete fuselage, the rear section from frames 19 to 25 was made. It was tested at the Royal Aircraft Establishiment against the corresponding section of a metal Spitfire fuselage. As a result of these tests the complete Gordon Aerolite fuselage was stiffened in the region of the opening for batteries at the rear end.

As already stated, all structural parts of the fuselage were of Gordon Aerolite, but cellulose-acetate sheet and cotton-reinforced phenol-formaldehyde sheet were used in a few subsidiary portions where they offered advantages. At the top of the fuselage, the fairing between frames 18 and 19 was made of black cellulose-acetate sheet and the transparent acetate sheet secured with Plastel stainless steel to give a flush finish.

The fuselage in Gordon Aerolite was of the same total weight as the production fuselage in light alloy. The construction of this fuselage may be regarded as having been something in the nature of an extreme insurance policy to cover a fairly remote but nevertheless possible emergency. Although the need for this type of construction never actually arose, the experiment was justified by the circumstances and the results obtained. It affords an interesting example of what can be done with alternative materials and ,incidentally, a convincing demonstration of the determined spirit which informed the country at that time of crisis.

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Part of the forward starboard side with the metal main spar members on Frame 5 to the left.

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The Spitfire fuselage, from the forward main-spar frame (frame 5) to the joint with the tail section frame19) formed the subject of the experiment. The non-structural decking round the pilot’s cockpit was omitted. Frames 12 to 19 were of single built-up channel section and frames 5 to 11 were heavier members made by riveting two single frames together. Spar members of the original material were embodied in frame 5.

Channel sections for the frames were composed of two L-section flanges of 0.6 inches by 0.5 inches by 0.6 inches.  Gordon  Aerolite joined by a flat web of the same material 0.05 inches in thickness.  The web-attachment flange of each L-section was saw-cut at intervals to simplify bending to the contour of the fuselage section. Each frame was made in four sections, top, bottom and two sides that were buttjointed together and reinforced by a strap of Gordon Aerolite 0.05 inches in thickness. Angles and webs were bonded on by Ardux glue on a steam-heated hotplate with pressure applied by hand cramps.

Bottom of the cockpit section showing frames 9 and 10 with the first complete frame 11 on the right.

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The fixture on which the fuselage was assembled. Frame locations were mounted on an internal cantilever structure.

The skin covering consisted of planks or strakes of Gordon Aerolite sheet continuous over the entire length of the fuselage. Each strake decreased progressively in thickness from the forward to the after end. Between frames 5 and 8 the thickness was 0.11 inches and from frames 8 to 19 this thickness was gradually reduced to 0.045 inches. This varying section was obtained by building-up packs of Gordon Aerolite laminations, which were then bonded by being fed progressively through a 70-ton press. In order to obtain maximum torsional rigidity in the fuselage, the grain of the Gordon Aerolite sheet was inclined at an angle of 45 degrees to the edge of each strake.

The problem of obtaining the compound curves on the fuselage was solved in a similar manner to how plywood fuel-tanks were built. One edge of each strake was straight, but the other was cut to a curvature related to that required on the fuselage. By joining the straight edge of one strake to the adjacent curved edge of the next, a composite panel of compound curvature was obtained. A joggle was moulded along the straight edge of each strake to give a flush exterior surface at the lap joints between them. Countersunk rivets were used to keep everything together.

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