What are aircraft slats and how slats work?

What Are Aircraft Slats ? How Do They Work?

What are aircraft slats - Wikimedia

The main function of the slats of the aircraft are to increase the critical angle of attack of the aircraft and prevent the aircraft from early stalling. 
The critical angle of attack of an aircraft is the angle of attack at which an aircraft starts stalling. Stalling of an aircraft is the condition when an aircraft looses sufficient lift force to continue flight. Technically speaking stalling is the condition when the boundary layer of the air flow separates from the wing surface i.e air flow separates from wing surface and hence they cannot produce and lift. As the air flow separates, the wing surface looses its lift. So, what slats do is, they keep the air flow in contact with the wing surface and delay the onset of stalling.
Aircraft Stalling flow separation - Wikimedia

Now, lets see how slats do this!

How Slats work?

Actually slats are an extended part of the wing. When slats are extended they move a little ahead of the wing and leave little space between them and the wing leading edge.

                                                       Condition I - No slats
As the air flow approaches the wing it gets divided into two paths. One above the wing and one below it. The point at which the air flow gets divided into two paths is known as the stagnation point. As angle of attack of an aircraft is increased, its stagnation point moves toward the bottom of the wing. And energy of the air flowing over the top of the wing reduces. And hence the air flow separates from the top of the wing. This condition is known as boundary layer separation. As air flow separates aircraft looses it's lift.

Now the aircraft will stall at certain angle of attack known as critical angle of attack. Stalling of an aircraft can be delayed by increasing critical angle of attack of the aircraft. To make this happen the air flow should remain attached to the wing surface for a longer time. This is what slats do


                                                       Condition II - Slats Extended

Extended Slats - Wikimedia

When the slats are extended and angle of attack is increased, air from the bottom part of the wing enters to the top part of the wing. This speeds up the air flowing over the upper surface of the wing and increases its kinetic energy. This keeps the air in contact with the wing surface for a longer time and hence results in delaying the onset of stalling.

For more info, check out  ' Coanda Effect '
    Flow over extended slat Airfoil - Wikimedia

In such a way stalling of the aircraft can be delayed with the help of slats.

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