You might have seen vortex generators - those little fins that protrude from the front of an airliner like the 737-800, or a Short Take Off and Landing (STOL) aircraft like this Top Cub.

737-800

Top Cub

These little fins are amazing; they create vortices just like your wingtips do. How does that help? The vortices pull high energy air into the boundary layer, which delays a stall. They're an integral part of many aircraft to lower stall speed - but did you know that they're also used on transonic aircraft to keep control surfaces effective at high speeds?

Vortex Generators Create Mini Wingtip Vortices To Energize The Boundary Layer

A few weeks ago, we posted about the boundary layer. It's a layer of air right above the surface of your aircraft where skin friction slows down and removes energy from the airflow.

As air flows across your wing, the pressure decreases until it reaches the center of lift - about 25% down the wing's chord. Then, pressure starts to increase again, so the air moves from an area of low pressure to higher pressure - this is called an "adverse pressure gradient." As the airflow moves towards high pressure, it loses energy. Eventually, when it runs out of energy, the airflow separates from the wing.

The air above the boundary layer isn't affected by skin friction, so it has more energy than the air in the boundary layer. If you could pull some of that free-stream air into the boundary layer, you could add energy and delay the boundary layer's separation. That's where the vortex generators come in.

Vortex generators act like tiny wings and create mini wingtip vortices, which spiral through the boundary layer and free-stream airflow. These vortices mix the high-energy free-stream air into the lower energy boundary layer, allowing the airflow in the boundary layer to withstand the adverse pressure gradient longer. Your wing can now operate at a higher angle of attack before airflow separation causes a stall.

Practical Examples

On Short Take Off and Landing (STOL) aircraft, you'll often see vortex generators along the leading edge of the wing. On airliners, you may see them in front of the flaps, where large adverse pressure gradients develop. In both cases, the vortex generators help keep the airflow attached at higher angles of attack, delaying a stall.

Vortex Generators Can Also Delay A High-Speed Stall

When airflow across an airfoil reaches transonic or supersonic speeds, a shock wave forms. Eventually, these shock waves will form at the leading edge of the airfoil, plus at the trailing edge and at any control surface hinge points.

As air moves across the shock wave, it suddenly loses energy. In fact, the energy loss may be so great that the airflow separates from the airfoil behind the shock wave - just like it does in a low-speed stall. If an aileron or elevator lies behind the shock wave, the separated airflow makes the control surface ineffective, and it may make the aircraft impossible to control.

In this high-speed situation, vortex generators can pull in high energy air from outside the boundary layer, mix it with air inside the boundary layer, and prevent separation. They can also disrupt the shock wave, reducing the amount of energy lost as air travels through the wave.

The horizontal stabilizer on a L-39 Albatros is a great example. A horizontal stabilizer is essentially an upside down wing that generates lift downward. Even though the L-39 is a subsonic aircraft, airflow moving over the tail can accelerate to transonic speeds, forming a shock wave. The vortex generators on the bottom of the stabilizer keep the airflow attached to the airfoil as it travels across the elevator, allowing you to maintain pitch control at high speeds.