Slats are aerodynamic surfaces on the leading edges of wings which allow the wing to operate at a higher angle of attack than would otherwise be possible. A higher coefficient of lift is produced as a product of angle of attack and speed, enabling an aircraft to fly more slowly or take off and land in a shorter distance. Where the leading edge slat is fixed in position, it is more accurately described as a leading edge slot.

 

Air from below the wing can accelerate through the slot towards the low pressure region above the wing, and exit from the slot moving parallel to the upper wing surface. This high-speed flow then mixes with the boundary layer attached to the upper surface and delays boundary layer separation.

 

 

 

 

 

 

 

 

At low angles of attack the airflow through the slot is insignificant. At progressively higher angles, the flow of air through the slot becomes increasingly significant, accelerating from the higher pressure region below the wing to the lower pressure region on top of the wing. At high angles of attack the fastest airspeed relative to the airfoil is very close to  the leading  edge, on the upper  surface.

In this region of high local airspeed, skin friction (viscous force) is very high and the boundary layer arriving at the slot on the upper wing has lost much of its total pressure (or total mechanical energy) due to this friction. In contrast, the air passing through the slot has not experienced this high local airspeed or high skin friction, and its total pressure remains close to the free-stream value.

The mixing of the upper surface boundary layer with air arriving through the slot re-energises the boundary layer which then remains attached to the upper surface of the wing to a higher angle of attack than if the slot were not there.

 

Slots naturally exact a penalty on the aircraft in which they are used, this is because they contribute to drag compared to an unslotted wing. The extra drag at low speed is acceptable because of the beneficial reduction in stall speed and improvement in handling characteristics, but at higher speeds the extra drag contributed by slots is a significant disadvantage, reducing cruising speed and increasing fuel consumption.

 

 

A vortex generator is an aerodynamic surface, consisting of a small vane that creates a vortex in the airflow. Vortex generators delay flow separation and aerodynamic stalling, they also improve the effectiveness of control surfaces. They are typically rectangular or triangular in shape, about 80% as tall as the boundary layer, and run in spanwise lines across the rear portion of the wing leading edge.

Vortex generators are positioned in such a way that they have an angle of attack with respect to the local airflow, creating a vortex which draws energetic, rapidly-moving air, from outside the slow-moving boundary layer, into contact with the aircraft skin.

An energized boundary layer is more resistant to flow separation than a stagnant (laminar flow) boundary layer. The result is that airflow “sticks” to the wing better, permitting flight at lower airspeeds with improved control.