How does the use of Leading Edge Root Extension help in increasing stall angle? Leading Edge Root Extensions were first used in Northrop F-5A/B Freedom Fighter, which first flew in 1959. After that, many fighter air-crafts have used it, some of which are mentioned after the question. The reason behind using such extension is delay of stall at very high angles of attack. How does the use of Leading Edge Root Extension help in increasing stall angle?

Northrop F-5A/B Freedom Fighter


author unknown, public domain (source)

Sukhoi Su-27


photo by Dmitriy Pichugin, GFDL 1.2 (source)

McDonnell Douglas F/A-18 Hornet


photo by LCPL John McGarity, USMC, public domain (source)
The idea is to generate a vortex near the fuselage. In most cases we do not want any vortexes, as the unnecessary air movement always causes an increase in drag. In this case however, you need them.

As the air flow separates from this small triangle wing while at large angles of attack, it gains a rotary motion (if you look at the left wing standing in front of the aircraft it will rotate clock-wise). This vortex inducts further swirling movement across the wing. 

Now, if you are familiar with fluid dynamics, you can imagine that this will decrease air pressure at the top of the wing (not much impact on the bottom side), which is what we want - higher lift is generated. 

What is more important in this the case, is the boundary layer. If you provide it with energy, making it more turbulent, it will pay off with sticking to the wing surface even while flying at large angles of attack. Obviously the larger is the angle without stall, the higher lift can be generated. Therefore an airplane with wing extensions can perform sick maneuvers like cobra and other more useful in aerial combat. Of course it requires a huge amount of spare thrust, but you do not have to worry about that in military jets.