: Why do planes takeoff with flaps down ?


From what i understand, flaps are used to increase the lift produced by the wing. I also read that flaps slow down the plane, which why they are used to land. All explanations on the internet that I have been able to find boil down to : they increase the lift of the wing. They do not address the fact that flaps slow down the plane. That’s what I as wondering why keeping the flaps down doesn’t make takeoff less efficient, since the engines have to work harder in order to achieve minimum takeoff speed. So, : why do planes takeoff with flaps down ? Thanks

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The minimum takeoff speed is with the flaps down. If flaps were up, less drag, higher min take off speed. Someone did the math and figured it’s more efficient to take off with flaps down to reach 150mph to take off. Because with flaps up engine probably works even harder to get to 200mph the min take off speed with flaps up. Numbers made up.

There are 2 types of drag when you’re talking about aerodynamics

1. Parasitic drag which is drag that doesn’t do any work, it doens’t produce lift.

2. Induced drag which is drag that’s generated by producing lift. No matter what, when you create lift, you do so at the expense of drag. Extending your flaps increases the amount of lift you make at the expense of drag, but it also reduces your take-off speed and the extra drag you produce is easily overcome by the engines being at the max power setting.

If an airplane wing produces a lot of lift it will also produce a lot of drag. An airplane need lots of lift to take off and land at reasonable speeds. Jetliner needs low drag wings to fly fast. Flaps and slats are used to temporarily turn low drag low lift wing into high lift high drag wings.

Beyond simply providing drag, flaps are used to reduce the speed at which an airplane’s wing stalls. Wings that are designed to be highly efficient during cruise will stall at speeds that are much higher than is desirable for either takeoff or landing.

Most airplanes are fitted with fowler flaps (or slotted flaps, which work similarly). During extension, fowler flaps move backwards for some distance, then dip down into the airflow at a steeper angle.

So when partially extended for takeoff, fowler flaps tend to increase effective wing area and reduce stall speed without substantially increasing drag. This reduces takeoff speed and shortens takeoff distances. Most airplanes are designed so that extended flaps cause the airplane to assume a less nose up attitude relative to the airflow than with retracted flaps. This reduces drag produced by the airplane’s fuselage, increasing initial climb angle (the climb’s steepness) so the airplane can avoid obstacles at low altitude.

During landing, flaps are fully extended to minimize the stall speed and maximize drag. More drag means the airplane can maintain a steeper descent angle without speeding up. The reduced stall speed yields a lower approach speed, which minimizes energy that brakes must dissipate and shorten’s landing distances.