A lot of planes would deploy the flaps before takeoff and landing. Flaps are a kind of high-lift device and everything you need to know about them is explained in this video.
If we want a plane to produce the same amount of lift but with a lower speed, we need to increase either the area of the wing or its camber or its angle of attack. This is when flaps come in handy. When flaps are deployed, the wing’s area gets bigger. Plus, the camber increases because flaps change the shape of the wing by adding more curvature to it. Not only that, the angle of attack increases because the effective chord line, which runs from the leading edge of the wing to the trailing edge of the flap, pivots up. Now, the angle between the new chord line and the relative wind gets bigger.
So why do we want the plane to produce the same amount of lift with a lower speed? Two instances where this is ideal are takeoff and landing. During takeoff, the additional lift from the flaps will help the plane to reach the minimum amount of lift to start flying, faster, hence, reducing the length of runway needed. As a plane approaches a runway to land, it needs to slow down but not too slow as it will stall. With flaps, the plane’s approaching speed and hence, its speed as it touches down on the runway, can be even lower, which will reduce the risk of an accident. It also means the aircraft needs a shorter runway to land as it can come to a full stop quicker. In some aircraft configurations, a useful side effect of flap deployment is a decrease in aircraft pitch angle, which lowers the nose thereby improving the pilot's view of the runway over the nose of the aircraft during landing.
Just like everything else, nothing comes for free. With the additional size and lift, comes the additional drag as well. That is why flaps are retracted mid-flight. The drawback of having that extra drag outweighs the benefit of the extra lift. It is also the reason why takeoff and landing use different flap settings. During takeoff, the flaps are not fully extended to reduce the extra drag. For landing, however, they are typically extended to their maximum setting. The extra drag actually allows you to fly a steeper descent angle to the runway. This means the aircraft can stay at cruising altitude and speed longer before starting the descent.
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