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Aviation History
1976
1976 - 2831.PDF
LIGHT COMMERCIAL happy with the size and weight of their aeroplanes and, leaving costs aside, did not want to see significantly bigger and heavier versions. Also, by staying with the basic Gulf- stream II design, some certification effort could be saved, many of the existing fixtures and jigs could be used, and there was a possibility of substantial equipment common ality to attract buyers. The firm therefore examined an airframe with the Gil's basic fuselage, tailplane and engines but having an en tirely new wing embodying advanced drag-reducing aero dynamics developed by Dr Whitcomb at Nasa Langley. Grumman placed a contract with its associate company, Grumman Aerospace at Bethpage, Long Island, to adapt basic Whitcomb aerodynamics for the new aeroplane. The latter organisation has far more extensive wind-tunnel and computer facilities than its southern associate, and also has plenty of experience of low-speed, high-lift aerodynamics, stemming from years of building carrier aircraft for the US Navy. The Bethpage work showed that the required performance could be achieved with a relatively small airframe stretch by embodying Nasa-developed aero dynamics. The basic seating capacity for 16 passengers remains unchanged, but to provide extra comfort on long overwater flights the fuselage has been extended by means of a 4ft plug, and there are two extra windows. The Rolls-Royce Spey Mk 511, its nacelles, and the empennage and rear fuselage are identical to those of the earlier Gulfstream. The profile of the front fuselage has been improved so as to reduce aerodynamic noise at the higher cruising speed, and for the same reason a wrap-round windscreen has been substituted. New aerodynamics But the G.III owes its performance principally to the new wing. The wetted area of the stretched Gulfstream is about five per cent greater than that of its predecessor. The wing's efficiency therefore had to exceed that needed to give the principal performance improvements—a 35 per cent increase in NBAA certificated IFR range and a 12 per cent higher long-range cruise Mach number—to offset the extra profile drag. Maximum take-off weight had to be kept to 70,0001b in order to keep within the design limits of the existing undercarriage and its attachment structure. Significant weight increases would also have reduced field performance to well below G.II levels, particularly in view of the reduced-thrust, noise-abatement take-off procedure devised by Grumman for the G.III. The Gill's performance improvements arise from a change to the supercritical wing section, and from the increase in effective aspect ratio from 6 to 8 produced by lengthening the wing from 68ft to 84ft 9in and fitting special tip surfaces. The new section has the same 12*2 per cent root thickness/chord ratio as that used on the G.II, but tapers to 1112 per cent at the tip, compared with the Gil's 9J2 per cent. The tip fins and a higher aspect ratio combine to improve the lift/drag ratio from 14-5 to 19. The remaining range growth is achieved by increasing fuel capacity from 23,0001b to 26,9901b. The front wingtip extension, or winglet, is about 16in long and angled downwards, the rear one about 6ft long and pointing upwards. Both are cambered in the same sense as the wing—if they were bent so as to lie in the same plane as the wing their camber would lie on the same side as that of the wing. The forward tip is washed by the high-pressure outward flow under the wing, and generates a forward reaction analogous to the propulsive thrust developed by a yacht sail when tacking against a headwind. The rear winglet likewise produces a forward force from the inflowing air, but is angled upwards to take advantage of the different floiv pattern over the upper surface of the wing. Effective aspect ratio is increased about as much as it would have been if the tips had simply been extended by the length of the longest winglet. But these surfaces have the advantage in that they do not in- FLIGHT International, 4 December 1976 crease the wing-root bending moment, resulting in a much lighter structure. This increased efficiency raises specific range (air miles per pound of fuel) by 20 per cent. The winglets are opti mised for long-range cruise; at 45,000ft the indicated air speed corresponding to Mach 0-84 is only 210kt. The wing is therefore having to work quite hard, operating at a lift coefficient of around 0-5, and nearly 50 per cent of the drag is lift-induced. The wing has rather more dihedral than Grumman would like, something dictated by the desire to retain a simple gravity-feed fuel system (all fuel is carried in the wing) and by the need to provide adequate tip clearance during landing and take-off. Yaw-damping will be standard. The engines are throttled mechanically to about 9,0001b thrust for take-off to comply with the proposed 1979 noise regulations, particularly those covering sideline noise. The reduction in thrust from 11,4001b to 9,0001b, combined with an increase in gross take-off weight from 62,0001b to 66,9001b, would have reduced field performance to well below that of the G.II were it not for the greater efficiency of the new wing. As it is, the FAA take-off distance (ISA) has increased from 5,000ft on the Gil to 5,900ft. The wing has single-slotted Fowler trailing-edge flaps, with the upper-surface spoilers moving to follow the con tour when the flaps are in the drooped position. This tech- 6000 4000 IS 2000 1500 2000 2500 3000 3500 Range, n.m. 4000 4500 nique was used on the F-14 fighter and improves the lift/ drag ratio. Grumman hopes that satisfactory field perform ance can be achieved without having to resort to leading- edge devices. These supercritical sections have a generous leading-edge radius to give a high lift coefficient, but the company admits to Flight that their performance has not been thoroughly explored at full-scale Reynolds numbers. Kruger flaps will be fitted across part of the span if tunnel tests indicate a risk of a serious performance short-fall without further lift-augmentation. The lower-drag wing not only confers a better rate of climb, but also permits the G.III to reach cruising altitude —43,000ft (ISA) or 41,000ft (ISA plus 10°C)—directly after take-off. It will thus be able to overfly airways and make use of random jet routing procedures to achieve substantial savings on longer journeys. The first, high-speed phase of wind-tunnel testing was completed last week in the Calspan facility of Cornell University at Buffalo, New York. A one-eighth-scale model spent 60hr in the variable-density tunnel there and the data will be used mainly to check the wing planform and the engine nacelle/trailing-edge overlap. The model is now being modified by the addition of flaps for low-speed tests due to begin this month, at Reynolds numbers up to 75 per cent of full scale. Probably the biggest unknown is the influence on engine behaviour of the flow field around the centre section. Much of the Grumman Aerospace work at Bethpage has been given over to optimising the shape of the centre section and the wing/body junction in order to prevent interference with the engine. The company will take a go-ahead decision next March, by which time market research and tunnel tests will have checked out the Gill's technical and commercial pros pects. If these results are favourable, Grumman plans to fly the first G.III in 1979 (and not 1978 as stated on page 1476 of Flight for November 20), with certification and customer deliveries beginning in 1980. •
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