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Sometimes, you really need more lift. Short runways and high terrain can put the pinch on takeoff and landing performance. That's where STOL aircraft - Short Take Off and Landing - come into play. They shorten their takeoff and landing distance through trailing edge flaps, which nearly all aircraft have; and leading edge devices, which are less common. The most basic leading edge device is a "slot," and while it lets you generate more lift at slower speeds, it carries a price in drag.

The Problem - Delaying the Stall

If you're trying to decrease takeoff and landing distance, generating lots of lift at slow speeds is key. And to generate lots of lift at a slow speed, you'll need a high angle of attack. However, most general aviation aircraft stall at an angle of attack around 15 degrees. Slots delay the stall by increasing the stalling angle of attack (called the critical angle of attack), often past 22 degrees.

The Method - Moving Energy From The Bottom To The Top

A wing stalls because the air moving across it's upper surface separates, causing a decrease in lift. This separation happens when the air loses too much energy and is essentially sucked off of the wing.

As air travels across the upper surface of the wing, it moves from ambient pressure to low pressure, and then back to ambient pressure. Air naturally flows from high pressure to low pressure, but on the aft portion of a wing's upper surface, the air travels backwards - from low pressure to higher pressure. As it moves away from the low pressure, it loses energy as that low pressure "sucks" against it. Once the air loses too much energy, it separates from the wing and that area stops producing lift.

In the images below, lower pressure appears as a darker blue color, while ambient appears white. In the streamline, high energy air appears dark black, while low energy air appears light grey. Notice how the low angle of attack image has a more even pressure gradient, and how the streamline maintains enough energy to stay attached to the wing. In the high angle of attack image, notice how the pressure gradient is large, and the streamline loses a lot of energy, separating as it turns light gray.

Boldmethod

Boldmethod

So - how do you keep the air moving across the top of the wing energized? Inject some of the higher pressure air from the bottom of the wing into the airflow on the top of the wing - via a slot.

At high angles of attack, slots allow higher pressure air to flow from the underside of the wing's leading edge into the air moving over the top of the wing, adding energy. Now, the air flowing over the top of the wing can oppose suction and stay attached longer, and the the wing can operate at a higher angle of attack. The performance increase is so great that a slot can often increase a wing's maximum lift by up to 40%!

Boldmethod

With A Gain Comes Pain - Drag

In aerodynamics, everything comes with a penalty. In a slot's case, it's drag, capping your airplane's cruise speed and efficiency. Since slots are always open, the drag is always there. More complex devices, like leading edge slats, solve this problem. We'll cover those during another article.

The Zenith STOL - Slots in Action

The Zenith CH750 is a great example of an aircraft which uses slots. Check out this video to get the full rundown of the 750's leading edge devices. (Note: In the video, they call the device a slat. Slots are sometimes referred to as "fixed slats.")