DLR tests Krüger flap for laminar wing

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European aerodynamicists are working on Krüger flaps as a high-lift device for laminar-flow wings on next-generation aircraft in a EU-funded project led by Germany's aerospace research centre DLR.

The objective is to avoid sharp edges, gaps and rivet heads as found in conventional slat designs in the leading edges of aircraft wings, which would disturb the air stream around a laminar-flow aerofoil even when the devices are retracted.

Krüger flaps have been employed on various aircraft in the past, such as the Boeing 747. But DLR now says that the researchers have "newly interpreted" the concept whereby the high-lift device is "perfectly" integrated into the wing to minimise any turbulence.

While the leading edge is lowered for take-off and landing in a conventional slat design, the Krüger flap involves a section of the wing skin on the underside of the aerofoil swinging forward ahead of the leading edge. The respective skin panel is hinged at its forward edge.

On the 747, that panel is made of fibreglass and slightly bent by the retraction mechanism to assume an aerofoil shape in the extended position.

The aerodynamicists of the research project - dubbed "design, simulation and flight Reynolds number testing for advanced high-lift solutions" - build a wind tunnel model that demonstrated fuel burn savings of up to 7% "without any compromises", says Jochen Wild of DLR's institute of aerodynamics and flow technology.

The tests were undertaken at the European transonic wind tunnel in Cologne.

The four-year project - which has been finished in 2013 - was conducted in reverse order to save costs and time, says DLR. While the scientists usually build a wind tunnel model first to gather data that is subsequently evaluated in computer-based studies, the researchers initially started with virtual designs to produce a refined physical specimen.

That model comprised a complete high-lift system "from aerodynamic to the mechanic [features]", says DLR.

A follow-up project called "active flow-loads and noise control on next generation wing" has now been started that will involve building a functional model of the new Krüger flap design.