Krueger flap

Lore & Background

Krüger flaps were invented by Werner Krüger in 1943 and evaluated in wind tunnels in Göttingen, Germany. One of the earliest civil applications was the Boeing 707, while the Swiss company FFA claimed the first use of the flap in its FFA P-16 fighter which flew in 1955. 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 (the Dash 80) using a fixed flap and a skid on the after-body. After the Boeing test flight on the B-707 prototype on 15 July 1954, Krueger flaps were first used in production for the Boeing 727 which made its maiden flight on 9 February 1963.

The Boeing 707 and Boeing 747 used Krueger flaps on the wing leading edge. Several modern aircraft use Krueger flaps between the fuselage and closest engine, but use slats outboard of the closest engine. The Boeing 727 also used a mix of inboard Krueger flaps and outboard slats, although it had no engine between them. The Krueger flaps developed for the Boeing 747 were constructed from fiberglass material and were designed to be intentionally distorted into a much more efficient aerofoil section on deployment. The Boeing 747-8 wing was redesigned with optimized VCK flap panels similar to those on the original 747. Another airplane that used VCK flaps was the Boeing YC-14.

Reader's Guide

Krueger flaps are significant as a leading-edge high-lift device that enhances low-speed lift production, especially on swept-wing aircraft. They allow smaller-radius wing leading edges better optimized for cruise, while the deployed flap creates a blunt leading edge for improved low-speed handling. The variable-camber Krueger (VCK) flap, invented by James B. Cole and Richard H. Weiland of Boeing in the mid-1960s, uses a fiberglass panel that is deployed and bent to an optimal aerodynamic shape for low-speed flight, with a separate aluminum folding nose deployed tangent to the panel. Boeing conducted test flights in 2015 with a modified Boeing 757 incorporating a variable-camber Krueger flap for landing, as part of research into natural laminar flow (NLF) to reduce fuel burn. Although Krueger flaps had been tried before as insect-mitigation screens, previous designs caused additional drag; the newer design retracts seamlessly into the lower wing surface. The test flights were supported by TUI AG and conducted jointly with NASA under the Environmentally Responsible Aviation (ERA) program.