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Aviation History
1954
1954 - 0264.PDF
124 FLIGHT, 29 January 1954 THE AVON-SABRE Latest Particulars from Australia: Fuselage Sixty per cent New A lively and informative air- to-air view of the prototype Avon-Sabre, first flown by F/L. bill Scott, R.A.A.F. SINCE the review "Sabres and Furies" in the issue of January 15th new information has come from Australia concerning the version of the Sabre produced by the Commonwealth Aircraft Corporation Pty., Ltd. An official description says that externally the machine is very little different from the F-86F, though substitution of the Rolls- Royce Avon RA.7 for the General Electric J47, and replace ment of the six 0.5in guns by "cannon of heavier calibre" [30 mm—Ed.], required so many changes that only about 40 per cent of the original fuselage structure has been retained. As the Avon consumes some 25 per cent more air than the General Electric, it was necessary to increase the size of the intake duct correspondingly. At the outset this appeared to present serious difficulties, but a simple solution was achieved in that the front fuselage was split horizontally and a "wedge" of structure inserted, thus dropping the lower line of the fuselage some 3iin at the front. In order to utilize as much of the original structure as possible the fuselage frames in the region of the nose had in most cases extension pieces let into their sides, and completely new frames were necessary only where dictated by the change in armament. The split in the duct was covered by a wedge-shaped strip of skin and the gap in the outer surface was covered by extensions of the skin panels. This comparatively simple modification left undisturbed the complex cockpit arrangements and the nosewheel-fittings in the lower half of the fuselage. A secondary benefit was derived in that take-off per formance was improved, since the new arrangement lowered the nosewheel pivot with respect to the wing datum, thus increasing the static angle of attack. Although the Avon has much the same dimensions as the J47, the mounting points are very different and the weight, according to the Australian description, "much reduced." Consequently, the engine had to be moved aft in order to preserve the original e.g. position, and, so that it could be completely supported within the forward part of the fuselage, it was necessary to move the trans port joint rearwards to the new location of the engine-mounting trunnions. Thus, the rear part of the fuselage was shortened by some 26in and the forward part lengthened by a correspond ing amount in order to preserve the original fuselage length. The Avon brought with it many problems of cooling and ventirfg, as well as the mere physical problems of access and installation; thus, from the rear spar aft to the transport joint an entirely new structure—though within the original mould lines— was required. Moreover, a gearbox separate from the engine was necessary, as provision had to be made for either electric or liquid combustion starting—thus utilizing all the space within the engine nose fairing which had been occupied, in the case of the J47, by the hydraulic pumps and generator. "In the train of this change," runs the Australian description, "followed changes in the shapes of the fuselage fuel tanks (and con sequently in the fuel gauging system) and the repositioning of numerous services. Cooling and venting of the Avon engine for all flight and ground conditions was a major problem, as it was desired, in the interests of retaining maximum strength in the fuselage material, to limit all structural temperatures to a maxi mum of 200 deg F. In the zone forward of the firewall there is no great source of heat as this portion of the engine is comparatively cool. However, it is necessary to ventilate the zone to remove all fuel and oil fumes that might accumulate to form an explosive or combustible mixture. Thus, all drain lines from fuel pumps and filters have been carefully led overboard and the zone has been ventilated in flight by small N.A.C.A.-type flush entries in the outer skin, exhausting through an opening on the top centre line. To cover the case of ground static conditions where no rammed air is available through the side entries a cooling flow is induced by means of the ejector pump action of the balance piston vents exhausting through the exit opening on the top centre-line. Aft of the firewall there are great sources of heat in the combustion chambers, turbine shrouds, exhaust cone and tail pipe, and all important structural members such as longerons and major frames have been fabricated of corrosion-resistant steel. Skins and minor frames are fabricated of 75S aluminium alloy, while all control cables and hydraulic services have been isolated in stainless steel tunnels carefully ventilated with cool air. This zone is cooled by a flow of air between the engine-tailpipe com bination and the fuselage structure, such air flow entering through three flush entries in the outer skin and exhausting through an annulus around the jet-pipe nozzle. In flight, the ram pressure recovery of the entries provides a sufficient cooling flow, but a light air-pump has had to be designed to provide sufficient cooling air flow in the case of engine running at full thrust on the ground and during take-off. The air pump is simple in principle, consisting of a line bleeding air from a high pressure stage of the axial-flow compressor and delivering this air to a tube bent into a circle and positioned in the cooling air exhaust annulus, co-axial with the jet-pipe. A series of gin diameter holes in this tube project streams of high pressure air rearwards, thus entraining sufficient air from the annulus to pro duce the necessary cooling air flow within the fuselage. This pumping action is necessary whenever the aircraft is stationary with engine running or at very low speeds. Thus, it has been arranged that the bleed line is open only when the undercarriage is extended and consequently no valuable compressor air is wasted, when maximum thrust is needed in flight. As there were so many unknowns in the design of the cooling system, a dummy fuselage incorporating all cooling provisions was constructed and shipped to England and, with the co-opera tion of Rolls-Royce, was tested in a static rig with an Avon engine under all conditions of engine thrust. Structure temperatures were measured and adjustments were made until completely satisfactory cooling arrangements were achieved." The Avon, it is pointed out, differs from the J47 in not being able to support the tailpipe in cantilever fashion. Thus, the shortened rear fuselage had to be redesigned to support the jetpipe in such a manner tfiat neither tailpipe inertia loads nor empennage flight loads were transferred to the engine; addi tionally the jet-pipe supports had to provide for easy removal of the rear fuselage for inspection of the engine and tailpipe. For security reasons the Australian description says little about the armament changes, except that great attention has been paid to servicing problems and "an exceedingly low turn-round time, covering flight servicing and re-arming, has been aimed at." AERODROME OWNERS' A.G.M. T HE reverses suffered at a number of non-State-owned air fields as a result of the closure of the Reserve schools are among the subjects likely to be discussed at the forthcoming annual general meeting of the Aerodrome Owners' Association. The meeting is to be held at the A.O.A.'s headquarters at Londonderry House on February 25th, and in the evening the annual dinner will be held at the Hyde Park Hotel. Both the retiring chairman (Aid. R. Mould-Graham, O.B.E., T.D.) and the chairman-elect (Mr. Charles M. Newton) have recendy had honours bestowed on them: the former a Deputy- Lieutenancy of the County of Northumberland and the latter an M.B.E. in the New Year Honours List.
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