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Krueger flaps deployed from the leading edge of a Boeing 747 (top left and right in photo)

Krueger flaps, or Krüger flaps, are lift enhancement devices that may be fitted to the leading edge of an aircraft wing. Unlike slats or droop flaps, 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.

Slotted flap Is the most popular flap for large ones. Slotted flaps increase the lift coefficient significantly more than plain or split flaps. When the slotted flap is lowered, a high energy air from the lower surface is ducted to the flap’s upper surface. The slats and flaps would continue to operate, but at half speed What flap/slat configurations correspond to position 1 on the FLAPS lever and how do they differ? - CONF 1+F is used for takeoff and provides both slats (position 1) + Flaps.

Operation[edit]

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.^[1] 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. Leading edge Krueger flaps enhance wing's low speed lift production especially on swept wing aircraft. ^[2]

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.^[3]

Flap and slat positions are indicated by two pointers in the flap position indicator. There are L and R pointers for the left and right wing flaps and slats. The right pointer is normally hidden from view by the left pointer. In the flaps 1 position, the flaps and slats move. Flaps 1, 5, 15, 20 are takeoff flap positions. Leading-Edge Flaps vs Slats. A leading-edge flap is a generic term for any moveable high-lift device on the front edge of the wing. If non-moveable devices like slots are included, these items are known as “leading-edge devices.” Krueger flaps are sometimes referred to as “leading-edge flaps” since their design is similar to a plain flap. As you all know, the DC-8 doesn't have slats or Kreuger flaps on the wing leading edge, unlike most jetliners. And maybe you remember one of the differences between the original DC-8-11 and the -12 was that the latter had slots- inboard of each nacelle, says the Putnam, and it says they were uncovered by something sliding off them when the flaps were lowered.

History[edit]

Krüger flaps were invented by Werner Krüger in 1943 and evaluated in the wind tunnels in Göttingen, Germany.^[4] One of the earliest civil applications was the Boeing 707 whereas swiss FFA claimed the first use of the flap in its FFA P-16 fighter which flew in 1955.^[5] 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 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.^[6] After the Boeing test flight on the B-707 prototype on 15 July 1954, Krueger flaps were first used in production for the B-727 which made its maiden flight on 9 February 1963.^[7]

Krueger flap operation
Slat operation

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.^[8] 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 reduce 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.

References[edit]

Wikimedia Commons has media related to Krueger flaps.

Notes[edit]