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Leading edge slots are a great way to increase the critical angle of attack, but they come with a hefty cruise performance penalty. To overcome the drag pitfalls, engineers designed slats.

What's the difference? Slats are the same as slots - except they open and close.. In fact, slots are often called slats - though technically they're a "fixed slat."

Check out the video below. It's a great example of how opening and closing the slat affects airflow over the wing.

Slats are categorized into three types: fixed (a slot), automatic, and powered.

Automatic Slats - Let The Wind Do The Work

Automatic slats open and close aerodynamically. They're not managed by the pilot - they're managed by airflow.

When air approaches the leading edge of an airfoil, it divides - some flowing over the top of the wing, and some flowing over the bottom. The spot where the airflow splits is called the "stagnation point."

When the airfoil's at a low angle of attack, the stagnation point is on the leading edge. As you can see below, it pushes the slat closed.

Boldmethod

When the airfoil is at a high angle of attack, the stagnation point moves below the leading edge and behind the slat. Air flowing up and over the wing pushes the slat open.

Boldmethod

Powered - The Modern Slat

Powered slats appear on many large aircraft and provide the same benefit of an automatic slat. However, they're electrically or hydraulically powered - increasing reliability. The flight crew sets the slat position via cockpit controls. The slats below are on an MD-80, but the design is found on nearly every airliner. Check it out next time and during takeoff and landing, and you'll see the same thing.

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