Boundary Layer Control Devices

BOUNDARY LAYER CONTROL DEVICES 
The maximum value of CL is limited by the AOA at which boundary layer separation occurs. If airflow separation can be delayed to an AOA higher than normal stalling AOA, a higher CLmax can be achieved. Both CLmax and CLmax AOA increase with the use of Boundary Layer Control (BLC) devices.
Slots operate by allowing the high static pressure air beneath
the wing to be accelerated through a nozzle and injected into the boundary layer on the upper surface of the airfoil. As the air is accelerated through the nozzle, its potential energy is converted to kinetic energy. Using this extra kinetic energy, the turbulent boundary layer is able to overcome the adverse pressure gradient and adhere to the airfoil at higher AOAs.
There are generally two types of slots, fixed slots and automatic slots. 
Fixed slots are gaps located at the leading edge of a wing that allow air to flow from below the wing to the upper surface. High pressure air from the vicinity of the leading edge stag- nation point is directed through the slot, which acts as a nozzle converting the static pressure into dynamic pressure. The high kinetic energy air leaving the nozzle increases the energy of the boundary layer and delays separation. This is very efficient and causes only a small increase in drag.
Slats are moveable leading edge sections used to form automatic slots. When the slat deploys, it opens a slot. Some slats are deployed aerodynamically. At low AOA, the slat is held flush against the leading edge by the high static pressure around the leading edge stag- nation point. When the airfoil is at a high AOA, the leading edge stagnation point and associat- ed high pressure area move down away from the leading edge and are replaced by a low (suction) pressure which creates a chordwise force forward and actuates the slat. Other auto- matic slots are deployed mechanically, hydraulically or electrically.
Since slats and slots on their own effect no change in camber, there is no change to CL at low AOA. The higher value of CLmax is achieved at a higher AOA, i.e., the stall is delayed to a higher AOA.
A simple form of BLC is achieved by vortex generators, which are small vanes installed on the upper surface of an airfoil to disturb the laminar boundary layer and induce a turbulent bound- ary layer. This ensures the area behind the vortex generators benefits from airflow that ad- heres better to the wing, delaying separation.