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Aviation History
1954
1954 - 3304.PDF
T. The first prototype S.A.4 Sperrin, showing the pairs of superimposed engines. SHORT SPERRIN Noteworthy Features of a British Four-jet Bomber Design THE fourteenth official specification for the year 1946 was intended to produce a jet-propelled successor to the Lincoln, which was then the standard heavy equipment of Bomber Command. Short Brothers and Harland, Ltd., submitted a design to the specification, in the form of a shoulder-wing machine powered by four of the new Rolls- Royce axial-flow Avon turbojets mounted in unusual double- banked nacelles,-*tbe jet-pipes passing above and beneath the wing. ^^*^V. The resulting aircraft received die company designation S.A,4 —and has recently been named Sperrin, after a lofty mountain range dividing the counties of Londonderry and Tyrone (by, coincidence a picture of these mountains appeared in the Decern-' ber 10 issue of our associated journal The Autocar). The speci fication called for a medium range, good altitude performance and a crew of five. Design went forward from 1947 until the first prototype flew on August 10th, 1951. Another prototype was completed shortly afterwards, and yet a third airframe was built for structural testing. The development and testing of this aircraft has been accompanied by a most extensive research programme, many of the results of which are manifest in the] Sperrin today. Last week the Ministry of Supply relaxed the security restric tions which had until then applied to the Sperrin, and we have accordingly prepared the accompanying drawings and the follow ing account of the principal features of the aircraft. No one would pretend that the Sperrin is a beautiful machine to look at, or that it represents die latest thought in bomber design. On the other hand, it is the newest aircraft of its type about which any thing approaching a full description can be published, and the following notes are accordingly of considerable interest. Airframe. The structure of the Sperrin is a conventional stressed-skin, light-alloy assembly, in which a number of pro cesses developed by the manufacturers—such as hot dimpling and nvet-head milling—have resulted in external finish of a very high order. The fuselage consists of nose, centre and tail sections, but is built as a unit. The nose section incorporates a crew compartment and a nose tunnel for die air bomber, all of which are pressurized to 9 Ib/sq in. The centre fuselage carries four heavy transverse hoomsjjto which the port and starboard mainplanes are attached. The tail section carries the fin and dihedral tailplane. The mainplane has a torsion-box spar, and is tapered in plan, with a slight overall angle of sweep-back. Between the fuselage and die nacelles, and between the latter and the ailerons, are fitted tour simple flaps, the outer units incorporating split air-brakes which op;n above and below die trailing edge. The flaps and air brakes are operated hydraulically, an independent system being provided to lower the flaps to 40 deg in emergency. The tail unit is quite conventional. The cockpit has dual con trols, connected by cable and roller chain to screw-operated servo tabs on the trailing edges of the control surfaces (including me ailerons), the tabs moving the latter aerodynamically without the need for any additional power assistance. Automatic injection of "feel" is provided/and die cable runs are fitted widi automatic tensioners to compensate for changes in temperature. The elevator control also incorporates a g-restriction unit which applies a load opposing the pilot's stick-force when the vertical acceleration imposed on the aircraft reaches a pre-determined magnitude. Power Units. Each wing carries a pair of Rolls-Royce Avon RA.3 turbojets mounted one above the other in a triangulated tubular structure attached at four points to built-up fittings in the region of the front spar girder. Each Avon is secured by spherical bearing-blocks, two on each side, and the joint between the mount ing and the wing is held by double-ended bolts, conical bushes and expanding sleeves. The lengtiiy jet-pipes are carried on guide rails above and below the wing (the structure of which is continuous through the nacelles), the large, substantially unstressed nacelles fairing the entire installation. In the nacelles, the powerplant space is sub divided by vertical and horizontal firewalls, as shown in the draw ing at die top of p. 873. Each Avon is connected, via a universally- jointed shaft, to a Rotol accessory gearbox mounted in the swept-forward fairing to the leading edge of the wing where the latter meets the nacelles. Each gearbox carries a supercharger for die cabin air system, two D.C. generators for die electrical system and two pumps for die hydraulic system. The Avons have self-contained lubricating systems of the wet-sump type. P-type electric starter motors are used, faired by a nose "bullet" on each engine. \ Undercarriage. In our Farnborough Report Number for 1951 referred to die "towering undercarriage" of the S.A.4, terming it "Snimpressive piece of engineering." Of British Messier design, it cori8i«|s of a steerable and castoring two-wheel nose unit and a pair of ro»Srwheel bogies with a track of 32ft 2in. The three baits are raised and lowered together by the main 4,000 Ib/sq inhy*toulic system, the nose unit retracting to die rear through twin dobisjnto an unpressurized box in the forward fuselage, and the main "units folding inwards to lie between the wing spars inboard of the nacelles. A safety device consisting of a ram-air pressure switch is inserted in the circuit to the electric/ hydraulic selectors to prevent retraction until die aircraft is flying at a predetermined airspeed. The selectors are housed in the bomb-bay and are controlled by a push-button unit mounted on die right-hand side of the first pilot's instrument-flying panel. The nose unit is hydraulically steerable from the aileron hand-wheels, though not until a hydraulically operated clutch has been engaged by operating an on/off switch on the flying-control-lock panel forward of the power levers. Widi the clutch disengaged, the nose wheels are free to castor.
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