Sucking off / blowing out the boundary layer
Sucking off / blowing out the boundary layer are processes for active boundary layer and flow control on wings . They influence the relationship between dynamic lift and drag .
Mode of action
The flow accelerated by the curvature on the upper side of the wing starts running against the pressure gradient at the end of the surface after passing the maximum curvature and is slowed down again in the process. The boundary layer loses kinetic energy , becomes thicker and begins to detach itself from the surface. At the transition point, the laminar boundary layer turns into a turbulent flow . The result is that the lift produced by the surface decreases while at the same time the drag increases.
In order to ensure a laminar flow in a wide flight area over as large a part of the wing as possible, the transition of the boundary layer from laminar to turbulent must be prevented. Suctioning and blowing out in the boundary layer are two possible active methods of influencing.
Suction
With suction, the smallest openings are installed in the critical areas of the surface, through which the turbulent boundary layer is suctioned off. The still undisturbed flow, but displaced outwards by the turbulent boundary layer, can again lie against the surface in a laminar manner. A laminar boundary layer is created behind the suction zone.
Blow out
When blowing out, additional accelerated air is introduced into the boundary layer. The air blown out brings additional energy into the boundary layer, which stabilizes it. It can flow laminarly against the pressure gradient at the end of the surface for a longer period without turning over. The system ("boundary layer control", BLC) was u. a. used in the Lockheed F-104 "Starfighter" to reduce the minimum landing speed.
technology
To implement the principles described, a large number of small openings must be made in the surface. Bleed air obtained from the engines can be fed to the openings for blowing out. Compressors or blowers can generate negative pressure for suction at the openings. The extracted air can flow out again through a nozzle to generate additional thrust.
The use of additional units or the use of bleed air reduces the thrust of the engines that can be used for propulsion. A little additional thrust can be achieved by flowing the extracted air out of nozzles. The overall efficiency improves, however, primarily through the reduction in drag, so that less engine power is required for the same flight performance .
The main problem is the care of the small openings on the wing. Small impurities such as dust, rain or icing quickly reduce the efficiency of the system. This makes maintaining the system very complex.
Alternatives
Further possibilities to stabilize the flow above the wing, especially above the outer area:
Examples
Already during the Second World War , attempts were made with boundary layer suction and a copy of the first modern military transport aircraft , the Arado Ar 232 , was equipped with it. The Focke-Wulf Fw 58 and the Fieseler Storch were also used for such tests at the same time.
The principle of suction was also tested with the test aircraft Northrop X-21 . Because of the high maintenance and repair costs, this technology has not yet been used in series aircraft. Other methods (engine optimization, improved calculation of aerodynamics, lighter construction methods) lead to greater improvements in flight performance.