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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 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.
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.
The German Bf-109 fighter used automatic slats to improve slow-speed performance. However, pilots had to check the slats for debris before takeoff. If a slat stuck and refused to open or close, the results could be catastrophic.
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.
Aleks is a Boldmethod co-founder and technical director. He's worked in safety and operations in the airline industry, and was a flight instructor and course manager for the University of North Dakota. You can reach him at firstname.lastname@example.org.