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Aviation History
1957
1957 - 1712.PDF
802 Finer/ assembly of Gnats at Hamble, The machine in the foreground is the first for Finland. The small size of the aircraft makes it par- ticularly easy to work on without staging. Note dielectric nose-cap and deletion of rearward-vision windows. GNAT M k I ... ANATOMY OF THE GNAT TO a consideration of the structure of the Gnat one might verywell apply that overworked but effective American term, systems concept. It is a delicate blend of the demands of struc-tural efficiency, with extreme compactness, as the drawing overleaf clearly shows. Although large-scale forging and machining can be expensive inplant and material, they normally pay dividends in structural economy. Though machined skins might be considered the pre-rogative of heavy aircraft, the Gnat contains several examples. Follands point out that, since doubling is required in parts ofthe wing skin, some constructors of Gnats under licence might find it just as cheap to set up a skin-milling plant or chemicaletching equipment as to install a Reduxing plant, which is the alternative. These firms may make their own choice, but Follandsthemselves are turning to machining. Of ease of access (and therefore of repair), Follands have nowhere lost sight; and one ofthe main factors in achieving diis goal is, indeed, the universal simplification of systems—with no attendant lessening of theireffectiveness—which is at the root of the design-thinking behind the whole project. The Gnat, then, is—basically at any rate—simple. The wingof the Mk 1 is in fact almost wholly given over to providing lift and does not house more equipment than is required for under-wing armament and drop-tanks. Neither wheel-wells nor internal tankage complicate it; and even the aileron actuating gear ishoused in the fuselage. But the fuselage is a rather different case. Here every member has more than one function. No one frameis like another, no space is left unused, no panel is unstressed unless it is meant to be removed quickly and frequently. Yetnowhere is accessibility sacrificed. Every component can be reached easily, if only because it has perforce to be installed closeto the outer skin of the structure. The fuselage is something of a technical writer's nightmare, but it may well prove the mainten-ance engineer's dream. A continuous structure passing through the fuselage, the Gnatwing is made up of two main planes permanently joined to a carry-through structure. It is based on a two-spar torsion boxwhich picks up the fuselage by four bolts, disposed longitudinally in order to avoid passing bending moments directly into thefuselage. Ribs both fore and aft of the rear spar are aligned normal to it for ease of production; and those of the leading edge, whichadds to the torsional stiffness of the wing, are normal to the front spar. Extruded Z-section stringers lie spanwise, parallel tothe rear spar; and separator fittings at the rib/stringer junctions take the compressive loads between the wing skins under bendingloads. The material most generally used is D.T.D.546. The spars are simply made up from 12 s.w.g. plate formed by apower press tool into channel section, the inboard portion of the rear spar being stiffened by a second 10 s.w.g. channel sectionriveted to it back-to-back. Interspar ribs are plate pressings, flanged all round and with flanged lightening holes. The span-wise stringers pass through cut-outs in the flanges; and the separator fittings pick up the stringers at these points, beingriveted at the top and secured with blind bolts to weided fittings at the bottom. Spar/rib junctions are riveted. Stringers aft ofthe rear spar, where the wing supports only air loads, are of L section, with their horizontal faces riveted to joggled portions ofthe rib flanges. The trailing edge is formed by a sheet wrapped round the ends of the ribs. The outboard end of the main wingstructure is a pressed closing rib, to which the detachable, built-up tip structure proper is fastened by spigots. Two pylons in each semi-span are based on fork lugs which arepart of magnesium zirconium castings ahead of the front spar. Three bolts in each lug take the load, while further castingswithin the torsion box locate the aft portion of the pylon between special ribs. Pipes and leads are disposed in the leading edgeand are accessible through circular panels adjacent to each pylon. Like the other control surfaces, the ailerons are of conventionalbuilt-up structure, skinned with 22 s.w.g. sheet inboard and 20 s.w.g. outboard. Tubes in the leading edges carry lead mass-balancing and the operating levers are magnesium zirconium cast- ings bolted to the inboard end of each surface.Initially the torsion box skin had a 12 s.w.g. doubler over approximately half of the chord, the doubling being projectedalong stringer and rib lines. The two skins were Reduxed flat and then rolled to contour. Now Follands are to machine a single skinto the same thicknesses and then roll it. All wing-skins are riveted to the supporting structure, 16 s.w.g. with 20 s.w.g. doublingnear the spar being applied to the leading edge and 18 s.w.g., with an 18 s.w.g. doubler in parts, aft of the rear spar. Each wing abuts on a root rib, having sheet webs bolted toextruded T-section booms which are stretch-formed to the aero- foil contour. Skin edges are bolted to these and covered in turnby butt straps formed to take both aerofoil section and anhedral angle. Special fittings carry the stringers through the ribs. Die-forged fittings join the root rib webs with the channel section spars and also hold the bolts attaching the wing to fuselage frames12 and 16. A doubler skin (or machined extra thickness) reinforces the skin aft of 50 per cent chord at each surface of the carry-through structure. The wing picks up the fuselage through the main forgings atframe 16 and the webs of frame 12, by four bolts parallel to the fuselage centre-line. The bolts screw into the fittings in the rootribs but are located inside sleeves which themselves carry a nut at their outer end. These sleeves provide a good tight fit and avoidmovement and fretting under heavy flight loads. The nuts act as extractors as well. Main wing loads are taken by the rear sparand by the forgings at frame 16. The lower butt strap in each root rib is bolted to the magnesium zirconium shear wall in the fuselageFrames and skinning completing the fuselage over the wing are a permanent part of the carry-through structure, the skin beingbolted to adjoining sections fore and aft of it to carry end loads caused by bending moments originating at the tail. There is noaccess to the wing torsion box, but the aileron hinge fittings art detachable and therefore offer easy aileron repair and servicing- The Gnat fuselage is a conventional frame-and-stringer, stressed-skin monococque supported by two vertical shear walls and four main longerons, but many of the frames act also as bulkheads ancionly three of them are anything like identical. Each one earns its keep. Of the original total of 30, one has been eliminated—and.say Follands, it is pure coincidence that this should have beei No. 13. Co-incidentally the elimination occurred at about thetime that the first Gnat orders were placed.
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