Boundary layer suction
A technique using suction to remove boundary layer and reduce drag.
Boundary layer suction is a method of managing airflow over an aircraft, using an air pump to remove the slow-moving boundary layer from a wing or engine inlet. By improving the airflow, drag is reduced, and fuel efficiency can improve by up to an estimated 30%.
The boundary layer itself consists of air molecules that are nearly stationary right at the wing's surface, a phenomenon known as the no-slip condition. In smooth, laminar flow, air velocity increases steadily with distance from the surface. However, this smooth flow often breaks down when the boundary layer separates from the surface, creating a low-pressure area behind the airfoil. This flow separation increases overall drag. Over the years, designers have tried to delay separation through careful shaping and smooth surfaces.
Because flow separation stems from the velocity deficit within the boundary layer, suction works by removing that layer before it can detach. The concept was first developed by Werner Pfenninger during World War II and has been studied almost continuously since. In the 1960s, NASA tested it using the Northrop X-21, a converted Douglas WB-66D, and in the 1990s, further tests were conducted with an F-16XL research aircraft. Research into using suction on gliders is ongoing at the Technical University of Delft. However, powering the pumps would require about 500 watts, likely necessitating solar panels covering the glider, which would greatly increase costs.
- Fuel efficiency improvement estimate
- as high as 30%
- Power required for glider pumps
- about 500 watts
- First developer
- Werner Pfenninger
- Era of initial development
- Second World War
- Test aircraft 1960s
- Northrop X-21 (converted Douglas WB-66D)
- Test aircraft 1990s
- F-16XL
Lore & Background
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 using the Northrop X-21, a converted Douglas WB-66D. In the 1990s, tests were done by NASA with an F-16XL research aircraft. Research is ongoing for its use in gliders at the Technical University of Delft, though about 500 watts of power would be needed to drive the pumps, which would mean covering the glider with solar panels and would increase the cost greatly.
Reader's Guide
Boundary layer suction addresses flow separation, which results from the velocity deficit characteristic of boundary layers. By removing the boundary layer from the surface before it can separate, the technique aims to maintain smooth laminar flow and reduce overall drag. The article notes that improvements in fuel efficiency have been estimated as high as 30%, indicating significant potential for aircraft performance. However, practical application faces challenges, such as the power requirement of about 500 watts for pumps on gliders, necessitating solar panel coverage and increased cost. The technology has been researched since its development by Werner Pfenninger in the Second World War, with notable tests by NASA in the 1960s and 1990s. Ongoing research at the Technical University of Delft suggests continued interest, particularly for gliders, though cost and power constraints remain. The article also references alternative methods like the pusher configuration for re-energizing the boundary layer on the fuselage, and mentions vortex generators and turbulators as related concepts.
Did You Know?
- NASA tested the concept in the 1960s with the Northrop X-21, a converted Douglas WB-66D.
- About 500 watts of power would be needed to drive pumps for boundary layer suction on a glider.
- Fuel efficiency improvements from boundary layer suction have been estimated as high as 30%.
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