How do flaps move at startup

Why and when to use flaps?

Flaps change the camber of the wing profile. This in turn changes the angle of attack of the zero lift, increases the maximum lift potential and, in the case of poultry flaps, the wing area.

Commercial aircraft use flaps both when taking off and landing and are usually only partially deflected. The full deflection is only set on the final approach in order to reduce the landing speed as much as possible.

If simple fall flaps are used, the increase in air resistance with small deflections is negligible (± 10 ° for a 20% flap) and can be used for both take-off and landing. Other types of flaps are designed to increase air resistance (split flap, zap flap) and should only be used for landing. If the airspeed remains constant, flap deflection will only change the induced drag if it redistributes lift across the wingspan. Modern gliders use lintel flaps on board and flaperons (a combination of ailerons and flaps) on board, and changing the flap settings does not affect the induced drag.

In general, the purpose of flaps is to shift the minimum wing drag to the desired coefficient of lift cL. . Laminar wings in particular have a pronounced range of lift coefficients with low air resistance. The pilot should try to stay in this area with minimal drag, and flaps give him the means to achieve this. Below is an example of an airfoil profile with a Reynolds number of 1 million.

Flaps to increase buoyancy work the same way, but change the camber a lot more. Usually they need to be combined with leading edge devices (fins) that change the camber at the nose of the airfoil to achieve their full potential. Below are some examples of flaps in combination with suitable leading edge devices. Note that the solid outline of the poultry flap is the position for landing, while the dashed outline is the position for takeoff. δ is the key's relative chord.

Junker flaps were the best way to control large aircraft in the days before more sophisticated force balancing mechanisms were developed, and split and zap flaps were very popular in the 1930s. They are still used in smaller aircraft today. The simple Fowler flap is used on the C-130, and commercial aircraft use slotted flaps like those in the bottom two rows of the table above.

In addition to increasing air resistance, powerful flaps alter the wing's pitching moment by shifting the center of lift backwards, which requires adequate tail size and steering power. In commercial aircraft, the incline of the full horizontal tail can be adjusted to generate enough downforce to counteract the pitching moment. Notice in the picture of the An-70 below that the stabilizer has a fin that works opposite that of the wing.


I wish I could give you 10 votes. Good answer! :) :)

Peter Kämpf

@Lnafziger: Thanks for the kind words! I was hesitant at first about posting a long answer to an old question, but the line "Flaps increase both lift and induced drag" in the only answer that bugged me so far.

Electric pilot

@Peter Kämpf, can we say that flaps also deflect the airflow downwards and in this way increase the lift? In other words, are they introducing some kind of Newton's 3rd law principle?

Peter Kämpf

@ParadigmPilot: Not just the flaps, the whole wing does this all the time as it moves. The flaps only increase this effect.