Krueger flap

Krueger 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. 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.

The Krueger flaps developed for the Boeing 747 were constructed from fiberglass honeycomb material and were designed to be intentionally distorted into an aerofoil section on deployment.

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 deHavilland 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.

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