A morphing wing leading-edge that smoothly changes shape to minimize drag and noise has been tested by the German Aerospace Center (DLR) in a windtunnel at the Russian Central Aerohydrodynamics Institute (TsAGI) in Moscow.
The droop nose would replace the traditional leading-edge slat high-lift device, the flexible surface deflecting downwards by up to 20° during take-off and landing with almost no loss of lift, says DLR. Operation and performance of the system was tested between Aug 27 and Sep 7 in one of Europe's largest tunnels at TsAGI's Zhukovsky research center.
While it has to be flexible enough to deflect smoothly, the morphing leading-edge must be rigid enough to carry up to a third of the aircraft's weight during landing - while remaining flat enough to maintain drag-reducing laminar flow over the wing. Any waviness in the flexible surface would cause airflow to go turbulent and increase wing drag.
After experimenting with various composites, DLR says it selected a glass fiber-reinforced plastic material. The skin is curved, and not stretched, to minimize stresses and laminate layers are laid up so as to tailor the rigidity distribution. The flexible leading edge is morphed using integrated actuators and support elements to stabilize the structure.
The German research center says the next steps are to develop the droop-nose leading edge to meet requirements such as lightning protection, de-icing and birdstrike resistance.
The project is part of a wider research effort at DLR aimed at improving aircraft efficiency so as to reduce fuel consumption and emissions. The LamAIR project, for example, is developing the technology for a short/medium-range airliner with a natural laminar-flow forward-swept wing and a hybrid laminar-flow control T-tail.
To achieve laminar flow on a conventional aft-swept wing there is a limit on leading-edge sweep of about 20° - beyond that point cross-flow instabilities cause the airflow over the wing to become turbulent. This limits cruise speed to about Mach 0.75. But with forward sweep, DLR says, instabilities are delayed and cruise speed can be increased to Mach 0.78, with the ability to reach 0.80 without major penalties.
The LamAIR project is also working on a vertical stabilizer with suction-pump system to achieve laminar flow by removing turbulent air through microscopic holes in the tail surface. The plan is to fly the system on DLR's A320 testbed.