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Aviation History
1957
1957 - 0430.PDF
432 FLIGHT, 5 April 1957 The Sherpa at Cranfield Continuing Work on the Aero-Isoclinic Wing By F/L. A. .1. McDONALD* THE Short Sherpa once again takes the air on a series of experi-mental nights for the College of Aeronautics. It may be remembered that this aircraft made its first flight on October 4,1953, since when it has flown at the Farnborough Air Display and subsequently, having finished its flight trials, was handed to theCollege for further experiments and thesis work. The Sherpa was originally built to combat torsional instabilityand tip-stalling characteristics of conventional swept wings, with the associated tendency to aileron reversal. A characteristic of swept-back wings is that they bend upwardsunder air loads and consequently lose incidence at the tips. This condition is mainly found when pulling out of dives: the loss ofincidence at the tips acts like an upward deflection of the elevator, and the aircraft pulls out more sharply than was intended. In theR.A.F. this effect is known as "pitch up" and on high-speed air- craft can give rise to a form of dynamic instability which is some-times fatal. The wing designed for the Sherpa was aero-isoclinic, with a42 deg sweep. The term "aero-isoclinic" describes a wing having an angle of incidence that remains constant regardless of flexure.This is achieved by placing a torsion box well back in the wing, so that the air loads, acting in the region of the quarter-chord line,have a considerable moment arm about it. This condition is not easily achieved in an ordinary wing, where the torsional stiffness isdictated by considerations of aileron reversal; but the Sherpa has all-moving wing-up controllers which, known as elevons, act asthe ailerons and elevators. Being well to the rear of the wing, these controllers make it possible to dispense with the tailplane, thussaving weight and drag. They also prevent the tendency to tip stalling at low speeds, due to the fact that the wing-tip incidenceis reduced on a backward movement of the control columns. Mr. M. C. Wilson, A.F.C., B.Sc, D.C.Ae., A.F.R.Ae.S., super-vising the research programme, says: — "Trailing-edge ailerons have been in vogue now for severaldecades, but it seems probable that their days are numbered. * Test pilot, College of Aeronautics, Cranfield. The extreme positions of the Sherpa's elevens in this photograph indicate low-speed flight and high angle of attack. Present-day fighter aircraft, although having a phenomenal rate ofroll at high speeds, tend to suffer from poor initial response; and at the other end of the scale, certain high-aspect-ratio swept-wingbombers have been trying out spoiler-type lateral controls. "In both these fields the Sherpa type of all-moving tip appearsto score heavily. It appears that the initial response is excellent, and the design ensures that twisting effects on a high aspect-ratiowing are kept to a minimum. This should permit an all-round weight saving, due to the relaxation of wing-stiffness requirements. "The rolling performance of the Sherpa is being investigated byone of the second-year students at the College of Aeronautics for his experimental thesis. A detailed analysis will be made from theresults and comparisons made with more conventional aircraft. The standards of instrumentation for this type of test are very high;but with the equipment available at the College no difficulties are anticipated, and we look forward to some interesting findings. "This investigation into lateral response is the first step in whatis hoped will be a long series of tests of a basic nature. The second stage, which is being considered at the moment, may consist ofnew methods of flight flutter-testing, followed in all probability by studies of the dynamic longitudinal stability. Without doubt, theSherpa is an ideal research vehicle for second-year experimental thesis work at the College of Aeronautics." The Sherpa is a very pleasant aircraft to fly in fine weather andhas ample control at low speeds (i.e., just above the stall) but tends to become a bit of a handful in bumpy conditions. The test flightenvelope is between 70 and 130 knots, with a maximum g of 2\. It is hoped that this envelope will be extended as the test flightresults are investigated. The power units are two Blackburn Turbomeca Palas which,although sufficient for the work, call for rather judicious handling. This aircraft is not capable of high speeds; but the lessonslearned from the tests should form a basis of design for future high- speed civil and military aircraft. DOUGLAS 1940 (continued from page 430) Lycoming LTC4A-1 (T55), a scaled-up version of the T53described in Flight for April 27, 1956. The new engine, which is described and illustrated at the foot of this page, is light,compact and of free-turbine configuration, with an automatic fuel control. It will be installed as a powerplant, removablethrough clam-shell doors in the underside of each semi- monocoque nacelle. Three-bladed, high-solidity propellers willbe used, with a diameter of 13ft and a minimum clearance of 38in from the fuselage and 78in from the ground. Blades, spinners andengine intakes will be de-iced electrically. Integral wing tanks will house a total of 4,000 U.S. gal (3,331Imp. gal). All fuel will be taken aboard through a single pressure- fuelling socket, with a flow rate of 400 U.S. gal/min, near thefront of the starboard undercarriage fairing. Everything possible is being done to allow the aircraft to achieve new standards ofhigh availability and quick turn-round. At present Douglas have no firm plans to go ahead with the1940, but it has completed the engineering project stage and is ready for detail design and tunnel testing. Should the marketappear sufficiently favourable Douglas would probably sell the aircraft as the DC-10. No price has yet been quoted but it isunderstood that certificated production aeroplanes could be made available three years from now, at an overall cost (assuming bulkorders) of rather more than $lm, i.e. about £400,000. 1,500 s.h.p. LYCOMING POWERPLANT of the new Douglas transport,the T55 free-turbine engine is the second gas tur- bine for aircraft propulsion to be designed by ateam under Dr. Anselm Franz for the Lycoming Division of Avco, of Stratford, Connecticut. Itcomprises an axial/centrifugal compressor, a reverse-flow annular combustion chamber andmechanically independent compressor and power turbines. Data beiow refer to the fixed-wing ver-sion, the commercial version of which (LTC4A) powers the Douglas 1940. The rotary-wing ver-sion is rather more powerful and, having a simpler gearbox, weighs but 600 lb. In external appearance the Lycoming LTC4A-1 (T55) strongly resembles its smaller predecessor, the T53. The latter was described in our issue of April 27 last year. Dimensions and weight: overall length, 58.8in;nominal diameter, 24.25in; dry weight with basic equip- ment, 665 Ib. Performance: maximum rating for take-off, 1,500$.h.p.(1,651 e.h.p.) at 1,200 r.p.m. on the output shaft; corres- ponding s.f.c, 0.657 Ib/hr/e.h.p.; maximum continuousrating, 1,383 e.h.p. with s.f.c. of 0.688.
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