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so Ive been looking into airplane flaps, like the longer one for take off-

Is it correct to say that the only reason to have one is for take off, to make it take off easier?

And so when they take off, they just put the flap fully down and take off quicker?

Jonathan Ching
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    @JonathanChing - You seem to be very interested in this stuff. The FAA have produced a book, available as a PDF; The Pilot's Handbook of Aeronautical Knowledge. Chapter 6 is about flight controls such as the ones you are asking about - https://www.faa.gov/sites/faa.gov/files/regulations_policies/handbooks_manuals/aviation/phak/08_phak_ch6.pdf – Dave Gremlin Mar 31 '22 at 10:52

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It is correct to say it makes take off "quicker", and in many cases it is that it makes take off "possible", at least for larger airliners - a light aircraft can often make a safe take off with no flaps extended.

The problem with large, heavy airliners is that the stall speed with no flaps is higher than the speed that can be attained on a given length of runway. In order to take off with more weight, or with a shorter runway, that stall speed needs to be reduced, which is done by increasing the lift produced by the wings, which is done by increasing their surface area and coefficient of lift, which is done by extending the flaps.

Robert DiGiovanni
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Jamiec
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Flaps are used for safe TakeOff and landing on fast aircraft. TakeOff without flaps could be possible, but a longer runway would be required to gather the extra speed. Landing without flaps means a prohibitively high landing speed.

Lift and drag on aeroplanes aree proportional to wing area S, wing profile curvature $C_L$, air density $\rho$, and airspeed squared, the well know lift and drag equations:

$$L = C_L \cdot \frac{1}{2} \rho V^2 \cdot S$$

$$D = C_D \cdot \frac{1}{2} \rho V^2 \cdot S$$

At high airspeed, the lift required can be produced by a relatively small wing area, which then also reduces drag so that less fuel is used for the trip. But with a relatively small wing area, flying slow can become an issue. To maintain enough lift to keep the plane up, the $C_L$ needs to be increased by flying with the nose up, and this has a limit beyond which the wing stalls and the plane will sink.

enter image description here

Flaps increase $ C_L$. The graph above was earlier used in this answer and shows how $C_L$ is increased at every angle of attack. At TakeOff flaps are used (partly deployed) on faster planes, but they are particularly important durimg landing when they are deployed fully. Fast jets often have Fowler flaps, which increase both the $C_L$ and the wing area.

Koyovis
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    Flaps aren't just for fast aircraft. I used to fly a slow, heavy, two-seater spam-can that cruised at 80kn and I always used flaps to take off (and for landing, obviously). Without take off flaps, it could take a week or two of lumbering down the runway before I could coax it into the air :) – Dave Gremlin Mar 31 '22 at 10:58