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Aviation History
1953
1953 - 1381.PDF
535 FLIGHT, 16 October 1953 The Short S.B.4 prototype is finished in the same colour scheme as the S.B.5, namely, with a silver wing and a black fuselage. The span is the same as that of a Vampir?. Note that there are no horizontal tail surfaces. SHERPA TAKES THE AIR Shorfs Latest Research Aircraft: More About the Aero-isoclinic Wing THE prototype Short S.B.4 Sherpa (briefly introduced in our issue of September 11th last) made its first flight on Sunday, October 4th at Sydenham Airport, Belfast. It is the second experimental-wing research aircraft, and the third wholly new design, to have come from Short Brothers and Harland during the past 12 months. The Sherpa is primarily a research tool to assist in the develop ment of wings for faster, very high-flying aircraft. As Rear- Admiral M. S. Slattery, Snort's chairman and managing director, has pointed out, "Sherpas blaze the trail to great heights." The type of wing which the company have in mind was dis cussed in our issue of September 11th. No wing—as we then wrote—can provide high-speed efficiency without suffering some penalties at low speeds or in conditions of high-speed manoeuvre. Tip-stalling and instability caused by aero-elastic distortion are but two of the troubles frequently met, and the delta and the crescent wing both attack these problems in individual ways. But the ideal theoretical approach calls for the aero-isoclinic wing, which, as its name implies, attempts to maintain constant incidence under flexion, i.e., as the wing bends under aerodynamic loads it always meets the air at the same angle. The first indication that the company were considering an aero-isoclinic wing was given in a lecture before the British Asso ciation a year ago by David Keith-Lucas, R.A., M.I.Mech.E., F.R.Ae.S., the company's chief designer. This lecture, which was summarized in our issue of September 26th, 1952, pointed out the numerous advantages which could theoretically be obtained from a constant-incidence characteristic, but did not indicate whether actual experiments were then in progress. In fact, Shorts were then constructing a simple glider, largely of wooden construction, with the aid of which it was hoped that basic data on aero-isoclinic wings would be obtained. Unfortunately, this aircraft met with a mishap while taking off under tow, and it was decided that trials of a completely new wing of this nature could be efficiently carried out only with a powered aircraft. In particular, it was felt that taking off on tow was asking too much of a test pilot. Accordingly the aircraft was re-designed to take two Blackburn Turbomeca Palas turbojets of 350 lb thrust each, mounted in a fireproof dorsal bay and exhausting above either side of the rear fuselage as shown in the accompanying illustrations. At the same time, the airframe was modified to permit operation at much increased weights; and the S.B.4, as it is now flying, is a fully equipped aircraft which will probably be extremely useful in perfecting these high-flying wings. The whole programme is, we believe, largely inspired by Mr. Keith-Lucas himself. The Sherpa is a private venture and the machine is owned entirely by Short and Harland. There is, however, no doubt that the Ministry of Supply is keenly interested, and it is expected that they will be brought into the picture at some future date. The most important part of the Sherpa is, of course, the wing and associated control system. The profile is not that of a particularly high-speed wing, for it was felt that the aero-isoclinic principle could be proved just as fully with a more conventional section, so preserving easier low-speed qualities. Sweep-back on the leading edge is just over 42 deg. Construction is largely of spruce with a plywood covering, although light alloy components are used at strategic points. Split flaps are fitted. The entire wing-tips, with a total area approximately one-fifth of the total wing area, can be rotated either together or in opposi tion to act as elevators or ailerons respectively. The use of this type of control has been dictated by the flexibility of the wing, which in turn is necessary to preserve constant incidence under flexure. It is, however, expected that the rotary-wing-tip con trols will offer unique advantages in the transonic regime and should also increase manoeuvrability at high altitudes. The tips are hinged at about 30 per cent chord, and each carries, on its trailing edge, a small anti-balance tab, the fulcrum of which can be moved by means of an electric actuator. The control system is quite normal and appears to work extremely well. The rudder is quite conventional. In its present configuration, the S.B.4 is not intended to test a number of interchangeable wings. The fuselage is a straightforward stressed-skin structure, with the fuel tankage disposed centrally beneath the carefully-designed air intakes. The Palas is not a suitable engine from which to drive a generator, and the latter accessory is therefore mounted in the nose where it will be driven by a small windmill; this had not been fitted when the accompanying photograph was taken. In the extreme tail is housed an anti-spin parachute, the stream/release mechanism of which consists of a cable running over pulleys in the fuselage to tie-rods which connect with the cockpit control. The undercarriage is fixed. After the" Sherpa's first flight on October 4th the company's chief test pilot, Tom Brooke-Smith, is quoted as saying, '"It was, by any standard, a satisfactory first flight. The Sherpa does not appear to suffer from the longitudinal sensitivity normally associated with the tailless configuration. My initial impressions of the stability and control of the aircraft are very favourable." We learn that Mr. Keith-Lucas himself was in the control tower at Sydenham at the time, and that he was extremely impressed by the lovely picture made by the Sherpa as it cruised in the airfield circuit. In the air it reveals unexpected beauty and has been quoted as "one of the most graceful aircraft now flying." It is to be hoped that testing will be rapid and successful so that the company can apply their knowldge to the design of a machine which should be more controllable at a greater height than any other aircraft. In conclusion, it may be said that the 60-deg-sweep testing of the S.B.5 adjustable wing aircraft is just being concluded. The aircraft will shortly be modified to a 69 deg configuration with the tailplane mounted under the fuselage instead of on top of the vertical tail. The low tailplane is now being made at Belfast and will shortly be shipped to the S.B.5, which is at Boscombe Down.
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