fredag 9. februar 2018

Boeing 737-10 230-seter med mange bestillinger - AW&ST

Boeing says market interest in its 737-10, the 230-seat stretch of the MAX twinjet, is gathering momentum as the design passes the firm configuration milestone and potential operators gain confidence in the ruggedness of the novel landing gear redesign underpinning the fuselage extension.
Since launching the 737-10 with 325 orders at the Paris Air Show in June, Boeing has seen the backlog grow to 416 commitments from 18 customers, the majority of whom are in the Asia-Pacific region. “The -10 plugged a hole that we had,” says Randy Tinseth, Boeing Commercial Airplanes marketing vice president. “That hole was we didn’t have as many seats as the Airbus A321. With this airplane, we carry the same number of passengers and fly a little bit farther than the A320neo today. The magic of that airplane is really about the weight savings. It weighs 5,500 lb. less than the competition, which means they need 5,000 lb. more thrust and 7,000 lb. more takeoff weight to fly the same mission.”
  • Operators gain confidence in new gear design
  • Configuration of 230-seat 737-10 stretch completed
Although Boeing acknowledges that some cannibalization of the current 737-8 and -9 backlog is inevitable, the company believes this will be more than compensated for by the incremental business generated by the -10. Overall, “60-65% of demand will be for the 737-8 size, 20-25% of demand will be in the upper end served by the -9 and -10, and 10% on the lower end of the market,” says Tinseth.
The 737-10, stretched by 66 in. more than the 737-9, will have an overall length of 143 ft.—the additional 40 in. will be in the forward fuselage, 26 in. will be in the aft. Due to enter service in 2020, the key design change is a completely revised taller main landing gear that still fits within the existing wheel well yet can extend to raise the body by a further 9 in. The design combines a telescoping feature to shorten the leg and a semi-levered lower element to move the aircraft takeoff rotation point aft.
Describing the evolution and features of the novel landing gear design, Keith Leverkuhn, 737 vice president and general manager, says: “The -10 is about the gear. It has to be reliable, it has to function and be maintainable and go through the production system with the same ‘takt’ time [the average time between the start of production of one unit and the start of production of the next] as other 737 variants.” In addition to the standard 8-in. nose leg extension on all MAX versions, to accommodate the added ground clearance for the larger fan of the CFM Leap 1B engine, the main landing gear requires an additional 9 in. of height to give the longer 737-10 the same body rotation angle as other MAX models.
Stretched by 66 in. to 143 ft. overall, Boeing's 737-10 will be the longest and final MAX derivative. Credit: Boeing
To shoehorn the longer gear into the same physical space without major surgery to the wingbox, the main landing gear incorporates two key new elements. “On the lower end, we introduced a semi-levered gear where we added an additional shock strut. On rotation, the spring force in the second oleo will keep the wheels on the ground as the aircraft rotates up. On rotation, the lever will step up and move the pivot point aft,” says Leverkuhn.
When landing, the motion of the semi-levered gear is limited by a bump stop as it closes up. “A big concern of the engineers was whether—on a limit landing at 10 ft./sec.—the design would be able to maintain the load path. The answer was ‘no,’ so we had to make changes to the forging to carry that extra load,” says Leverkuhn. Boeing’s research and technology unit worked to dynamically model and analyze the mechanism, which was then continuously tested for limit landing conditions in a special rig. For a while “it was a big concern for the team, but now it is no longer a worry,” he says.

The company took its inspiration from several naval fighter designs to also modify the upper end of the landing gear with a “shrink link” mechanism that enables it to be drawn in and shortened while being retracted. “The initial concepts weren’t so good and had lots of moving parts, but then we came up with a clever, simple solution,” says Leverkuhn. “We have a retraction actuator and a walking beam which translates as you bring the gear up. The beam is linked to the actuator and shock struts so that when the gear moves, the beam rotates and pulls up the upper end of the leg,” he adds.

Before Boeing hit on the idea of translating mechanical links, other original concepts under consideration included an additional hydraulic actuator and a self-contained hydraulic actuator system. “Compared to those, this is beautiful. We quickly rejected the additional hydraulics because it could have fundamentally changed the aircraft’s hydraulics system; the self-contained system could have been a maintenance nightmare,” Leverkuhn says.

Boeing will also be adding new brakes and wheels for the -10 and has a separate development program for this. For the packaging of the new configuration, which also involves relocating some elements of the existing main landing gear design, a full-scale mockup of the gear and wheel well is being developed.

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