Boundary layer suction
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Boundary layer suction is technique in which an air pump is used to extract the boundary layer at the wing or the inlet of an aircraft. Improving the air flow can reduce drag. Improvements in fuel efficiency have been estimated as high as 30%.
The boundary layer
The air molecules at the surface of a wing are effectively stationary (see the no-slip condition). If the flow is smooth, known as laminar flow, the velocity of the air increases steadily as measurements are taken further away from the surface. However the smooth flow is often disturbed by the boundary layer breaking away from the surface and creating a low pressure region immediately behind the airfoil (see flow separation). This low pressure region results in increased overall drag. Attempts have been made over the years to delay the onset of this flow separation by careful design and smooth surfaces.
Use of suction
As flow separation results from the velocity deficit that is characteristic of boundary layers, suction attempts to remove the boundary layer from the surface before it can separate. The technology was first developed by Werner Pfenninger in the Second World War and has been researched almost continuously since. In the 1960s, NASA experimented with this concept with the Northrop X-21, a converted Douglas WB-66D. In the 1990s tests  were done by NASA with a F16XL.
Loek Boermans is researching the technology for use in gliders at the Technical University of Delft. However about 500 watts of power would be needed to drive the pumps and this would mean covering the glider with solar panels and would increase the cost greatly. There are also structural problems to be overcome before the ultimate glider could be manufactured.
- The pusher configuration is an alternative way to re-energize the boundary layer, but is only used on the fuselage.
- NASA F-16XL research aircraft
- Vortex generator
- Boundary layer control
- Circulation control wing
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