Krueger flaps, or Krüger flaps, are lift enhancement devices that may be fitted to the leading edge of an aircraft wing. Unlike slats or drooped leading edges, the main wing upper surface and its nose is not changed. Instead, a portion of the lower wing is rotated out in front of the main wing leading edge. Current Boeing aircraft, and many others, use this design between the fuselage and closest engine, where the wing is thickest. Outboard of the engine, slat flaps are used on the leading edge. The Boeing 727 also used a mix of inboard Krueger flaps and outboard slats, although it had no engine between them. Most early jet airliners, such as the Boeing 707 and Boeing 747, used Krueger flaps only.
While the aerodynamic effect of Krueger flaps may be similar to that of slats or slots (in those cases where there is a gap or slot between the flap trailing edge and wing leading edge), they are deployed differently. Krueger flaps, hinged at their foremost position that once deployed actually become their trailing edges, hinge forwards from the under surface of the wing, increasing the wing camber and maximum coefficient of lift. It produces a nose-up pitching moment. Conversely, slats extend forwards from the upper surface of the leading edge. Also, when deployed, Krueger flaps result in a much more pronounced blunt leading edge on the wing, helping to achieve better low-speed handling. This allows smaller-radius wing leading edges, better optimized for cruise.
Krüger flaps were invented by Werner Krüger in 1943 and evaluated in the wind tunnels in Göttingen, Germany. One of the earliest applications was the Boeing 707 in 1954. The flap was added to prevent wing stall with an extreme attitude take-off with the tail dragging on the runway, a scenario that had already caused two de Havilland Comet accidents. A preliminary flight test had been made on the Boeing 367-80 using a fixed flap and a skid on the after-body.
Boeing commenced a series of test flights on 17 March 2015 with a modified Boeing 757, incorporating new wing-leading-edge sections and an actively blown vertical tail. The left wing has been modified to include a 6.7 m-span glove section supporting a variable-camber Krueger flap which will be deployed during landing and which protrudes just ahead of the leading edge. Although Krueger flaps have been tried before as insect-mitigation screens, previous designs caused additional drag; the newer design being tested is variable-camber and designed to retract as seamlessly as possible into the lower wing surface. Increasing the use of natural laminar flow (NLF) on an aircraft wing has the potential to improve fuel burn by as much as 15%, but even small contaminants from insect remains will trip the flow from laminar to turbulent, destroying the performance benefit. The test flights have been supported by the European airline group TUI AG and conducted jointly with NASA as part of the agency’s Environmentally Responsible Aviation (ERA) program.
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