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Aviation History
1957
1957 - 1713.PDF
FLIGHT, 22 November 1957 Of the 29 remaining frames, Nos. 4 and 8 mark the limits ofthe pressure cabin, while Nos. 12 and 16 enclose the gun bays, hold the wings, support the main undercarriage and enclose oneof the fuel tanks. The forward pan of the engine is housed between frames 18 and 22, engine loads being taken primarily atNo. 22. At this point the fuselage is broken for engine installa- tion and removal. The Gnat can be carried on a lorry with onlythe wing-tips removed to keep within legal width limits. To form the break, frame 22 is duplicated, the two sections beingcalled 22A and 22B. The latter carries a steel firewall. Eight spigots and nuts serve to join the two halves, the inner four pick-ing up longerons in the main fuselage and the outer four picking up in the fairings. A magnesium zirconium pressed-sheet shearwall extends from frame 12 to 22, enclosing the main fuselage space and leaving the outer skin around the fairings substantiallyunstressed. The strength of the wall obviates a number of stringers in the tank bays in this area, so making the contours ofthe bag tanks less complex. Considerable use is made of magnesium castings. In manyinstances the minimum casting thicknesses are used; and, where even smaller thicknesses would suffice, magnesium saves weight.In all cases magnesium units are given an epoxide stoved finish which is exceedingly tough and durable and should have admirablecorrosion-resistant properties. Frame 26 is inclined to the vertical and picks up the front sparof the fin. Frame 27 picks up the fin centre spar; and diaphragms between the two serve as the inner walls of the two boxes whichsupport the magnesium castings bearing the tailplane pivots, each in two ball bearings. There are four longerons, two of them extending from the nose,along the edges of the tank floor to frame 22 and two more running from frames 17 to 22 at the 10 o'clock and 2 o'clock levels. Allfour hold fuselage-break spigots. The lower segments of frames 10 to 22A are flattened to support the single-piece main tank floor,the outer contour being taken by detachable panels. An under - fuselage trough is thus left clear to take control runs, piping,cables and certain components of the fuel and hydraulic systems. The engine air-intake ducts are made out of hot-pressed halvesof magnesium zirconium, welded together and held in the main frame structure, but bearing no structural loads themselves. Theybegin just forward of frame 9, outside the skin which is the main shear member at this point, and pass inwards through the shearwall between frame 10 and 16, forming a cylindrical section with a central dividing wall up to the engine intake lip at frame 18. Theprofile of the intake fairings is continued aft alongside the fuselage proper to form a housing for the guns and main undercarriage unitsand for two fuel tanks just forward of frame 22A. The fairings are closed transversely by the steel firewall at frame 22B, butcontinue alongside the fuselage right to the tail, providing housing for the tailplane attachment and sundry other equipment. The nose cap and the top of the nose to frame 4 open upwards,the flight loads when closed being taken by toggle fasteners. Frame 4 carries the hinge and forms part of the forward pressure bulk-head. Having a D.T.D.610 web fin thick, it also acts as the forward armour protection for the pilot. The equipment bay isextended aft through the upper portion of frame 4 to provide an extra shelf for equipment; and the vertical pressure bulkhead iscompleted just ahead of frame 5. This unit is termed frame 5A and forms the forward anchorage for the magnesium cast wind-screen frame. Made by Triplex, the windscreen is a conventional laminated,dry-air sandwich unit with an outer layer fin thick, separated by a iin airspace from the bullet-proof and pressure-retaining 1 Jin-thick laminated inner layer. The magnesium frame holds the glass and provides a channel for the inflatable-rubber canopyseal, the whole being supported by extensions of the casting reaching forward to frame 5A, as stated, and downward at an angleto a point on frame 6 on the fuselage datum. Frame 5 is broken at the top to accommodate this casting. The canopy, which is of the clam-shell type and hinged at therear, is of Folland's own manufacture. It is a single |in-thick blown plastic unit, the edges of which are cemented with high-temperature "dough" to glasscloth strips, to which in turn are bolted the metal flanges mating with the canopy seal. Since the The underside of a Gnat wing, showing the tapered torsion box and carrying-through structure, the latter having holes tor spigots locating the tops of fuselage frames 14 and 15. The wing-tips are not shown. Above, a radar-ranging installation shown by Folland at the last S.BA.C. show. R':ght, the forging which, at fuselage frame 16, holds the mainwheel leg and oleo, sequence valve, wing rear spar, aileron servo, aileron-droop pulley and rear gun mounting. windscreen is cantilevered the canopy fits round it and no quarterlights are needed. A tie bar, fitted across the front of the canopy and lying ahead of the screen when the canopy is closed, has thesole function of preventing premature disintegration after jettisoning. An important structural unit, the coaming is built up from platesections and extruded magnesium beams running from the slop- ing rear pressure-bulkhead forward to frame 4. It is continued aftby the longeron running back to frame 12. Two canopy-locking hooks, controlled by a hand lever, are located in castings at thetop of frame 6. Frame 8 merely locates skin and stringers, the main structuraltask at this point being performed by the sloping pressure-bulk- head which supports the canopy hinges, the ejection-seat rollersand the nosewheel leg mounting. Rearward-vision windows have now been deleted. The top of the bulkhead is supportedby the arched panel which forms a stressed cover for the equipment bay. This stressed panel also completes frame 9, which is inter-rupted by the bay opening. The tie bar which crosses the intake to locate the forward end of each gun blast-tube is held by frame 9.The intake fairing is largely unstressed and shear loads in the fuselage are taken by the magnesium shear wall which continuesthe skin line of the forward fuselage back to frame 22. Underneath the equipment bay floor is attached the built-uparch which holds the nosewheel radius arm and jack. The leg itself is a magnesium casting (by British Messier); and the com-pression oleo is attached slightly below the leg pivot, giving a pre- retraction effect as the leg retracts. Being also an airbrake surface,the door is permanently attached and does not move relative to the leg. A smaller fairing door is operated by cable and pulley fromthe nosewheel leg. Follands arrived at a neat anti-shimmy device for the nosewheeland saved 10 per cent in weight in the process. They forged their own nosewheels, with split rims for easy tyre removal, havinga coned web only O.lin thick. The two wheels are located, eight inches apart, on a tapered axle, and tightened home by torque-loaded nuts, without any spline or key, thus avoiding stress con- centrations. Since they are rigidly coupled, the wheels giveexcellent anti-shimmy protection. A pair of them has successfully completed over 10,000 miles on the Dunlop spinning rig. Frame 11 supports the gun barrels and stiffens the No. 1 fueltank bay walls. The lower segments of frame 10 to 18 are flattened to hold the single-piece tank floor, and the space below themprovides stowage for control cables, fuel piping, undercarriage selector valve and fuel-flow proportioner. The long panel whichcovers the whole space is unstressed. The magnesium intake ducts are made in sections betweenframes 9 and 12 and between 12 and 16, where the ducts merge into a single cylindrical section. A flexible joint at this point stopsthe ducting working with the fuselage under load. The cylindrical section has a dividing wall to prevent the engine air entering at oneintake and turning to flow out through the other. Since 13 has been eliminated, frames 14 and 15 are located,evenly spaced, between 12 and 16; and their main task is to locate the ducts and stiffen the tank walls against pressure loads. Theyare not complete at the top, their three webs being located by spigots in holes in the under-surface of the wing carry-through. The ducts pass through the shear wall in the region of frames (Continued on page 806, after double-page drawing of Gnat)
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