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Aviation History
1998
1998 - 1867.PDF
HEADLINES Boeing studies triple-engined 777X GUY NORRIS/LOS ANGELES BOEING IS considering die use of a third, tail-mounted engine in the 777-200X/300X that would also double up as an auxiliary power unit. The virtually unprecedented use of a dirusting APU is one of a wide range of changes being stud ied to improve the field perfor mance of die aircraft as the Seattle-based airframe builder attempts to find a launch customer for die aircraft after being beaten to several key orders by die latest ver sions of die Airbus A340. The study of what is dubbed an "auxiliary power and thrust unit" (APTU) is considering dual-role powerplants covering the 31- 66.7kN (7,000-15,0001b) thrust range, including the Allison AE3007, BMW Rolls-Royce BR710 and General Electric CF34-8. GE has also proposed a civil version of die F414 fighter engine, but Boeing says noise con siderations will make it "more like ly to go with an engine of a higher bypass ratio". The added thrust of the tail engine would augment the main engines for take-off as well as for initial climb, but would otherwise "provide the capabilities of today's APU" says 777 preliminary design chief engineer Mike Burtle. Tail-mounted actuated inlets would "open and close like a hinged door" and be closed flush widi the skin during die cruise. "It seems a little less conventional at first but in reality, when traded against more dirust under wing, it could trade favourably well," according to Burde. John Monroe, the 777X deriva tives deputy programme manager adds: "We also have to look at it from an airline's point of view from an operability standpoint. It will use more fuel and we just have to balance all those things out. It could have higher operating costs, more weight and mean more drag. It's a humungous business deci sion" he adds. Boeing plans to hold initial discussions on the APTU concept widi airvvordiiness regula tors in the next mondi. To increase take-off thrust Boeing 777-200/300X performance improvement studies FLIGHT i _ >#uS7hrpuTr unit (APTU) ' exhaust Conceptual APTU "pop up" inlet Semi-levered main gear with brake torque increase (300X only) Inboard Krueger Rescheduled inboard Large raked tip flap aft flap or blended winglet Boeing is working widi GE, Pratt & Whitney and Rolls-Royce to improve engine flat rating temper atures so that higher thrust levels will be available at a wider range of ambient temperatures. It also con tinues to "trade against the poten tial for more thrust on the main engines" says Burtle. Outside the possible powerplant changes, the -200X/300Xstudies are concentrated on three areas: lift/drag improvements for better take-off, improved stopping and added dirust. For enhanced take-off, Boeing is studying gapped outer ailerons which wou*ld improve outboard span loading and reduce induced drag. The aileron would be slight ly drooped with a gap along its leading edge to help produce lift at low speeds. Boeing is also looking at rescheduling die inboard double slotted Fowler flaps to reduce the take-off angle of attack slightly. The change, in concert with the aileron changes, would produce a more even span loading, thereby reducing drag, says Burde. Boeing is also studying changing the inboard leading edge slat, from the engine to the fuselage, to a Krueger flap such as diose used on die 7 3 7 and 747. The change could lead to an improvement in die par asitic drag of the leading edge, says die company. lb improve aerodynamic brak ing, it is also looking at changing the sequencing of surface move ments to "get more drag, more quickly" for rejected take-offs. This would include deflecting die outboard ailerons and flaperons up sooner. "It's essentially tuning of the fly-by-wire system" adds Burtle. Thanks in part to the gapped outboard aileron study, Boeing is also revisiting the use of a larger raked tip for die wing, such as diat designed for the 767-400. "We ruled out such a device last year," says Burde, who adds diat a hybrid constant-chord extension and raked tip was adopted as a result. The improved wing tip aerody namic flow produced by the revised aileron design means diat a larger raked tip is now a potential option though span will still be restricted to a maximum of 65m (213ft). Boeing says it is also "going back and looking at winglets and com paring diem to extensions" follow- Addition of a thrusting auxiliary power unit could improve field performance ing die recent in-flight experience gained by die 737-800 test team working on a possible winglet configuration for the Boeing Business Jet (BBJ). "They have really picked up good knowledge widi the BBJ" says Monroe. Anodier potential improvement "still on the list" is the semi-lev ered, or articulated, gear for die longer -300X. This would make the aircraft stand higher at rota tion, thereby enabling die take-off speed and runway length to be reduced. "It has the potential to increase the maximum take-off weight [MTOW] by 10,000- 15,0001b [4,540-6,800kg] out of a given field length," says Burtle. The 777-200X as now configured has an MTOW of 333,700kg and 324,600kg for the- 300X, although the gear change could raise diis to "almost die same level" says Boeing. To improve braking, Boeing is studying an increase in die torque of the existing main gear brakes. "We could increase the area of die pistons or increase die heat in die [brake] stack itself," says Burtle. "These are performance improvement studies diat we believe will make die 777X a better aircraft. It is our plan to begin going dirough die individual busi ness cases on diese studies, diough we are not committed to any of diem," cautions Monroe. Most of the improvements are aimed at the -200X and -300X, whilst some are also being consid ered for the baseline -200ER. "We think the -200X has the right range, but we diink it needs better take-off performance" says Burtle. Meanwhile, Boeing has briefed several potential -200X/-300X users on die study package, includ ing Air France, American Airlines, British Airways and Emirates. "We're also getting ready to talk to Malaysia Airlines," says Monroe who adds: "We're really at a point in the programme where we will soon be signing up definitive agreements widi customers." The studies will be completed by the end of die year to meet the first quarter 2000 firm configuration target date. • FLIGHT INTERNATIONAL 15 - 21 July 1998 5
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