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Extended flaps. Photo: Shutterstock
Extended flaps. Photo: Shutterstock

Aviation

Why wings extend before take off

Seated near a wing you have probably noticed that the wing expands and changes its form before and after takeoff and landing. The devices that alters the wing size and form are called slats and flaps, and here´s how they work.

With the slats and flaps retracted, the aircraft will have less drag and can fly faster, but at the same time it requires a higher speed to create enough lift to keep the aircraft flying.

With the flaps and slats extended, the maximum speed is limited and a lot of drag is created, requiring extra thrust from the engines if not in a descent. But the aircraft will be able to fly at a lower speed, making it possible to land on and take off from shorter runways.The pilots control the slats and flaps with a handle in the cockpit - here on a 737NG. Photo: Jann Lipka

It is possible to take off and land without slats and flaps, but it requires high speeds and extremely long runways. The limiting factor for high speed on ground is often the tire speed and the ability to stop.

The devices at the leading edge of the wing are called slats, and the ones at the trailing edge are called flaps. The lift devices increase the wing area and improve the airflow around the wing.

The pilots control the slats and flaps with a small handle in the cockpit. They are normally controlled electrically but powered hydraulically. The use of slats and flaps for takeoff is determined in tables or by computers taking into account the engine thrust, aircraft weight, runway length, obstacles and braking action. As soon as you are safely airborne and a certain altitude is achieved, the devices are retracted little by little as the speed is increased.

The use of slats and flaps for landing is also determined well in advance and is influenced by aircraft weight, runway length and go-around capability. When pilots need to lower the speed, they extend the slats and then the flaps. Fuel consumption increases with increased drag, so configuration for landing is made as late as possible, and the use of landing flaps is often reduced if they are not needed to meet appropriate safety margins.

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