Why are flaps required for takeoff?

Hey IFC,

I know that pilots deploy flaps for takeoff and then retract them once they have enough speed. My question is, why are flaps so crucial to takeoff? I want the explanation physics wise and how deploying flaps helps airplanes climb.

A good example of a crash is Northwest Airlines Flight 255. After takeoff, the plane rolled to the left and stalled. Everyone on board, except for one little girl, were killed. The crash was the effect of the pilots not extending the flaps before takeoff.

How are flaps so crucial to takeoff?

Haven’t seen any other topics that answer my question.


Extending the wing flaps increases the camber or curvature of the wing, raising the maximum lift coefficient or the upper limit to the lift a wing can generate. This allows the aircraft to generate the required lift at a lower speed, reducing the stalling speed of the aircraft, and therefore also the minimum speed at which the aircraft will safely maintain flight. The increase in camber also increases the wing drag, which can be beneficial during approach and landing, because it slows the aircraft. 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.




An airplane’s wings are best optimized for cruise, which is often the longest duration of the flight.
Because cruise is done at high speeds, the lower speeds make it harder to maintain lift on the high-speed optimized wing. When the flaps and slats are extended, it maximizes the aircraft’s lift at lower speeds and allows it to take off and land at slower and more acceptable speeds.


So does this mean that if there were no flaps, rotate speed would have to be higher in order to leave the runway?

How does it maximize the aircraft’s lift at lower speeds when it looks like the flaps and slats would create drag or make the wing heavier? I feel like a noob for asking this. XD

Yes, that is correct; hence the reason flaps are so crucial for takeoff.

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Flaps generate lift from the ground which in order to make the plane fly you need lift, slats and flaps do this. using flaps increase the curvature of the wing, which increse drag and create lift from the underside of the wing

So flaps increase drag and create lift from the underside of the wing, what do slats do?

Apparently, Beline 548 crashed because one of the pilots accidentally pulled in the slats/droops too early on the trident, resulting in a stall.

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Rotate speed would be much higher, instead of like 140-170 knots it could easily increase it to upwards of 200-230.

@QF_Pilot5 I don’t think you are correct.

At lower speeds, a wing that has more distance over the top edge of the wing than the bottom generates more lift.
I suggest you go to Infinite Flight and drop the flaps on an aircraft, you can see by looking at the side that the distance going from the top of the wing is much greater than the bottom. This means more lift is generated at lower speeds.

However, as I said at higher speeds, the characteristics to maximize your lift is different and you don’t need as much of a distance differential.

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So do slats and flaps work together to help generate lift for the airplane?

Also, to me, deploying slats and flaps makes the wing into an upside down “parabola (or something like that).” How does that create lift for the airplane?

Slats increase the aircraft’s wing area by extending forward and when retracted go flush against the wing when at the optimal climb speed/rate, the combination of flaps and slats provide more lift allowing for shorter takeoff distances and lower takeoff speeds.

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@BigBert10 take a read about how lift works: https://en.wikipedia.org/wiki/Lift_(force)

As I said, the distance the air travels over the top wing has to be greater than the distance on the bottom wing to maximize lift.
At lower speeds when you don’t generate as much lift that curve needs to be much greater.
At higher speeds when you generate more lift, that curve just creates more drag and isn’t necessary.

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the slat isn’t a solid surface attached to the wing, by that I mean it essentially a mini wing extending further off creating more lift for the aircraft

diagram for visual reference above

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High pressure below, low pressure above, in simplified form.


Flaps are used for takeoff because it gives the wing more surface area there you have more wind flowing on top of the wing and the underside of the wing. Low/High pressure. With a bigger wing area, the airplane stall speeds decreases since there’s more air flow on the wings being an airplane can lift off faster with flaps deployed vs if the flaps weren’t deployed.! The simple Answer is that flaps are used for takeoff to increase the surface area of the wing for slower takeoff speeds

It helps with lift…




Flaps increase wing surface area therefore increasing lift

Thank you for your comment.

Your friend,

Captain Obvious.

To be clear, I am not @ captain obvious