The Krüger flap is a leading-edge high-lift device used on aircraft wings to increase lift during takeoff and landing. Invented by German engineer Werner Krüger in 1943, this aerodynamic innovation has become a vital feature in modern commercial and military aircraft design. Its clever integration with the wing’s leading edge allows it to improve low-speed performance without significantly compromising aerodynamic efficiency during cruise.
Design and Operation
Unlike trailing-edge flaps that extend downward and backward, the Krüger flap is a leading-edge device that deploys forward and downward from the underside of the wing’s front section. When deployed, it effectively increases the wing’s camber (curvature) and surface area, both of which contribute to higher lift coefficients.
The Krüger flap typically consists of a rigid panel that rotates out from the wing’s leading edge. In its retracted position, it forms part of the wing’s smooth contour. When extended, it creates a slot that allows airflow to energize the boundary layer on the upper surface, delaying flow separation and reducing the risk of stall.
Aerodynamic Benefits
The key aerodynamic advantages of Krüger flaps include:
1. Increased Lift:
By increasing the wing’s camber, the flap enables greater lift at lower speeds, allowing for safer and shorter takeoffs and landings.
2. Improved Stall Characteristics:
The enhanced airflow control over the wing’s upper surface delays flow separation, improving the aircraft’s stability and stall margin.
3. Low Drag Penalty in Cruise:
When retracted, Krüger flaps fit seamlessly into the wing, maintaining a clean aerodynamic profile with minimal drag increase.
4. Structural Simplicity and Durability:
Compared to slats, Krüger flaps are simpler to construct and maintain, as they deploy from the lower surface rather than sliding along tracks.
Applications in Modern Aircraft
Krüger flaps are most commonly found on large commercial jetliners, particularly on the inboard leading edges of wings, where the engine nacelles and pylon mounts limit the use of conventional slats. For example:
Boeing 747, 777, and 787 aircraft employ Krüger flaps near the fuselage to enhance lift while accommodating engine placement.
Some Airbus models also incorporate Krüger flaps inboard, complementing outboard slats for an optimized lift distribution.
Comparison with Leading-Edge Slats
While both devices serve the same purpose—improving lift at low speeds—they differ in mechanism and application:
Feature Krüger Flap Leading-Edge Slat
Deployment Direction Rotates forward from lower surface Slides forward on tracks
Common Location Inboard wing sections Outboard wing sections
Structural Complexity Simpler More complex
Typical Use High-lift with minimal mechanical complexity Maximum lift and stall control
Conclusion
The Krüger flap represents a fine balance between aerodynamic performance and structural simplicity. By providing significant lift benefits without major drag or weight penalties, it remains a crucial component in modern aircraft design. Werner Krüger’s innovation continues to contribute to safer, more efficient flight operations—an enduring testament to elegant engineering in aviation.