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I was travelling recently in a Boeing 737 and I noticed one thing that I didn't understand. After the takeoff the wing flaps started retracting into/under the wing, making it narrower. The opposite happened before the landing.

You can see what I mean in this video:

The question is:
Why are the flaps hiding into/under the wing?

I understand that the bigger wing area is better during takeoff and landing. But why is the wings' size reduced during the cruise? Shouldn't bigger wings provide better lift during the entire flight?

Some clarification after comments:
I'm asking specifically why are the flaps retracted and not just remain parallel to the wing

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    $\begingroup$ Possible duplicate of Why and when to use flaps? $\endgroup$ – fooot Sep 2 '16 at 16:08
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    $\begingroup$ @ymb1 Judging from the last paragraph, it sounds like he's just asking why the flaps are retracted for cruise, which seems like a duplicate of the other question. $\endgroup$ – reirab Sep 2 '16 at 16:17
  • $\begingroup$ Yes, @ymb1 is right. I knew that flaps in the position not parallel to the wing would increase drag. However, I didn't realise that bigger wing itself results in more drag too. $\endgroup$ – pajonk Sep 2 '16 at 16:19
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    $\begingroup$ @pajonk So, you're asking why they're retracted rather than just raised to be parallel with the wing? Some flaps do exactly that, actually, just hinging on a fixed point instead of retracting forward underneath the wing. That's pretty common on light aircraft. That kind is called plain flaps, while the kind you see more commonly on airliners are called Fowler flaps (or some variant thereof, at least.) $\endgroup$ – reirab Sep 2 '16 at 16:23
  • $\begingroup$ @reirab Yes, that's right. I'll try to clarify that in the question. $\endgroup$ – pajonk Sep 2 '16 at 16:25
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Well, yes, bigger wings to provide better lift, but the also produce more induced drag in the process.

The wings on an airliner are optimized for cruise in high subsonic and transonic flight where a slender, swept wing works well. While this is great for cruise flight, the trade-off is this style of wing requires a very high approach speed for landings which in turn require very long runways to accelerate the airplane on to reach rotation speed for takeoff or to decelerate the aircraft on once it has landed.

The Boeing Company successfully addressed these problems in the early 1960 with the development of the 727 airplane as a regional airliner. It made use of a type of flaps called Fowler flaps (see Fig 1) in concert with leading edge extensions. Fowler flaps. These style of flaps consist of a series of segments attached to tracks or support linkages running chordwise, allowing the flaps segments to extend and retract by rolling along said tracks.

enter image description here

Fig 1. Typical Fowler flap installation

When deployed these give the effect of changing the airfoil shape from a slender, slightly cambered airfoil into a wide airfoil with a large camber. Fowler flaps have an additional advantage to them in that partial deployment creates a large increase in lift with limited additional drag, very useful for takeoff, while when fully deployed they create a lot of drag in addition to higher lift.

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  • $\begingroup$ but not to the extent which the 727 did. The reduced approach speeds and ability to get into short airfields from that was one of its chief selling point. $\endgroup$ – Carlo Felicione Sep 2 '16 at 16:44
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    $\begingroup$ Induced drag depends on the lift and speed and the lift is always the same as in level flight it needs to exactly balance the weight. So leaving the flaps extended would not increase the induced drag. It would increase the parasite drag because the camber would be too large for the amount of lift needed and the wetted area would be larger. $\endgroup$ – Jan Hudec Sep 2 '16 at 20:08
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Bigger wings also produce more drag. Instead, flying faster (in cruise) produces the required lift.

For any given object, the bigger it is, the more drag it produces.

Since a plane spends most of its time in cruise, the wings are designed with a lift-to-drag ratio that suits cruising.

For slow flying (take-offs and landings), high-lift devices are then used, they come in many flavors.

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Flaps are a way of changing the shape of the wing so that it is able to provide more lift at lower speeds and higher angles of attack. This is important both on take-off and, especially, landing where the aircraft is moving relatively slowly.

The trade off is that flaps dramatically increase the drag.

Bear in mind, that commercial airliners spend most of their flight time cruising at, more or less, constant speed and altitude. At cruise, there is no value in 'more' lift, they need exactly enough lift to support their weight at an efficient cruising speed. So, the wing shape is designed to do this with the minimum possible drag.

Flaps increase the lift at low speed so the aircraft still has enough lift to support its own weight when approaching for landing without needing to be traveling at a speed which would make safe landing more difficult and require a very long runway to slow down after touchdown.

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The "flaps up" configuration represents the wing's base shape. This is the shape designed to produce the intended flight characteristics including required lift at minimum drag under the aircraft's designed cruising flight conditions (airspeed/altitude).

Take-off and landing is best performed at the lowest speeds which are practically attainable (take-off and landing distances increase dramatically and non-linearly with speed). The "flaps down" configurations (note the plural) increase the effective wing area, but also change its profile. Extending flaps increase lift but also increase drag. Most aircraft have multiple flap settings, providing a progression between "clean" (flaps up) and full flaps. Full flaps are only used for landing where high drag is desirable because it helps slow the plane down after touchdown. Intermediate settings are used for take-off because they aid lift with a minimal drag penalty and are used in the early stages of approach as the aircraft is slowing down.

Flaps aren't simply enlarging the wing. Most flap designs leave gaps when fully or mostly extended. These gaps allow some air from under the wing to flow up through the gap and join the air moving over the top of the wing. This helps the airflow remain "attached" to the top surface of the flap. Without this feature, air on the top surface can "detach", become turbulent, and no longer aid lift.

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