The flap is the simplest high lift device used on a wing, consisting of a hinged portion of the trailing edge which can be lowered in order to increase the lift coefficient and thus the lift produced by the wing. Flaps are most commonly used during takeoff and landing, when additional lift is needed to get off the ground with a full fuel load or to land at a low speed while keeping enough lift to flare.

The increase in lift coefficient is due to an increase in the downward change in momentum caused by placing the flap in the airflow. This increases the circulation, thus increasing lift (by the Kutta-Joukowski lift theorem). The effect can also be viewed as a simultaneous increase in both camber and angle of attack, because the trailing edge is being moved so it is below the leading edge, causing the wing to appear pitched with respect to the flow, when in fact it is not.

There are a number of types of flaps, depending on the application. The simple flap described above is a trailing flap, which is used on smaller aircraft where a large lift boost is not needed and simplicity is of utmost importance. As needs increase for larger aircraft, more complex flap devices are used, such as the split, slotted, and Fowler flaps. Each of these offers a greater maximum lift coefficient than the previous design. Note that a wing with no flaps has a CLmax of about 1.4.

``` C_L_max       _________________                        Trailing Flap
_-                 ---------__________
2.4      (______________________________________\
\
\

_________________                        Split Flap
_-                 ---------_____________
2.6      (______________________________________\
\
\

_________________                        Slotted Flap
_-                 ---------______________
2.9      (______________________________________/
\
\

_________________                        Fowler Flap
_-                 ---------_____________
3.0      (______________________________________|__  O
\
\
```

So a wing with an extended Fowler flap, as you might find on an airliner at takeoff, could produce twice as much lift as it does at cruise. However, the extended flap incurs a significant drag penalty. By extending far out into the flow, the parasite drag of the wing is increased greatly, reducing not only the efficiency of the wing but also its maximum speed before stall. Thus, the pilot will retract the flaps fairly quickly after taking off in order to accelerate to cruising speed. On the other hand, this apparent drawback is a boon when attempting to land, because the increased drag will help slow the plane to a safe landing speed.