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Aviation History
1983
1983 - 2143.PDF
clearance of 28in on the -300, compared with one of 30in on the -200. Mark Gregoire contends that the ratio of inlet height to engine diameter is the most critical parameter, however, and by this yardstick the 737-300 falls somewhere between the 757 and 767. According to Gregoire neither of these twins has experi enced creeping vortices. He says that the A300 engine installation is more critical. Another factor in foreign object damage is debris thrown up by the nosewheel. The geometry of both 737 types is such that spray patterns clear inlets by comfortable margins, and no problem is foreseen. Clearance between engine and ground is smallest at some distance behind the lip (18in on the -300, and 20in on the -200). The similarity is possible because the bulk of the CFM56-3 sits forward of the wing leading edge, while a JT8D straddles the chord. The CFM56-3's tight fit is exempli fied by the side-mounted accessory gear box, a feature typically found directly below an engine. This layout gives the 737-300's nacelles an oval appearance when viewed from the front. The juncture between engine and wing receives a great deal of design attention and windtunnel testing in any airliner, but arguably never more so than on the 737- 300. Aerodynamicists aim to avoid or reduce interference drag while simultane ously using engine exhaust as a favourable influence on wing lift. Time will tell whether Boeing has the close-coupled nacelle right, but for now the company is basing its confidence on many hundreds of hours in the windtunnel, as well as flight trials of the sister engine on a 707. In the interests of commonality, Boeing has retained the 737-200's basic wing design while making it slightly stronger. It has altered leading-edge contour and slat design, however, thereby reducing cruise fuel-burn and approach speed. The contour change is all forward of the front spar and extends between pylon and tip. It comprises a 4-4 per cent chord-increase plus a blunter, slightly drooped, aerofoil. The modification is a clever way of alter ing airflow over the wing, particularly the top surface, so that it approaches that of an aft-loaded or supercritical section. The result is a 4 per cent improvement in aero dynamic efficiency, equivalent to an increase in cruise Mach number of 0-02 and a fuel saving of 1 • 5-3 per cent at ranges of 200-1,500 n.m. Improved slat design reduces the 737-300's approach speed by 7kt, to a figure only 4-5kt higher than that of the lighter -200. Other benefits are similar handling for both types and an improved buffet-margin. When Boeing conceived the 737-300 it envisaged mass-balance probes protruding from the leading edge of each wingtip. The 737-300 in Design go-ahead CFM56-3 certification Roll out First flight FAA certification + first airline delivery CA-A certification perspective March 1981 September 1983 January 1984 March 1 984 November 1984 January 1985 A flight management system (FMS), seen here on the front left-hand side of the centre pedestal, distinguishes the cockpit from that of its twin. Other differences include a new digital flight control system and dual laser-gyroscopes for inertial reference probes were to increase the natural frequency of the new wing/engine combi nation, and thus keep flutter at bay. To avoid alterations to the leading-edge slat, each wingtip was extended by 14in to support a probe. But in February this year Boeing aerodynamicists concluded that the probes could safely be eliminated. The tip extensions remain, however, providing a small increase in wing area. The -300's tailplane also sprouts tip extensions, but this time it is 30in on each side and their purpose is different. Improved stability is the design driver, but better "loadability" is also achieved, thanks to a e.g. limit that is now 3 per cent further aft. The only other visible external difference is that the fin is preceded by an extended dorsal spine. When Boeing introduced the 757 and 767, it used improved aluminium alloys in the wings, as well as composites in second ary structures such as control surfaces, engine cowlings, and fairings. In each of these new twins, new alloys and composites accounted for 12 per cent of maximum empty weight. Boeing has managed to engineer the same proportion in its latest twin, but this time roughly a half of the composite element is glassfibre-reinforced plastic (GRP), used in the new cabin trim and in the wing/fuselage fairing. On the 757 and 767 nearly all of the composites used are carbonfibre-reinforced plastic (CFRP), Kevlar reinforced plastic (KRP), or a ^f <• FLIGHT International, 26 November 1983 1427
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