What do flaps do on planes?

Answer 1

primary use is to increase lift which allows to land or takeoff at slower speeds - but flaps also increase drag which slows an airplane and requires more power and thus more fuel to fly so flaps are retractable for normal flight - because of the drag flaps can be used as a brake to slow an aircraft or at least to keep it from speeding up while on approach for landing - but the motors and gears that operate the flaps are not very strong (to keep them light) so the flaps can only be used at slow speeds and used as a brake one must be very careful so not to break them - jamming or breaking the flap gears in flight, though by itself is not likely to cause a crash, is still a dangerous situation

A little additional info...In the jet age, flaps also perform another function in addition to providing extra lift. While a portion of the flaps provide lift, extending them further adds drag which then allows the jet engines to maintain a higher power setting. This is necessary as the time to spool up the engine from idle to maximum power for jets is much greater than for prop type aircraft. During an approach, if the engine is unspooled (low or idle RPM) and a go around is necessary, the delay in reaching full power is unacceptably long. With flaps extended beyond the lift range and into the drag range, the aircraft can fly the approach with the engines spooled up and have power immediately available in case it's needed for a missed approach. Most flaps have two retraction speeds. One that quickly brings them out of the drag range for the missed approach, and a slower one that raises them slowly so as not to decrease lift too quickly or cause an abrupt change in aircraft attitude.
The flaps on an airplane change the aerodynamics of the wings for one of two purposes.

1. To allow the airplane to descend at a steeper angle without increasing airspeed.

2. To allow the airplane to takeoff and/or fly at lower airspeeds.

On landing approach it is very important to touchdown at the lowest speed safely possible. However, it is obviously necessary to descend down to ground level before you can touch down. Generally when you lose altitude by diving, you gain airspeed. Extending the flaps will allow you to maintain a steeper descent without increasing airspeed.

On takeoff the goal is to get the airplane off the ground in as little runway as possible. In this case the flaps are used at a shallower setting in order to increase the wing-chord (the aerodynamic profile of the wing) to allow it to produce greater lift at lower speeds. Thereby allowing the airplane to lift off in a shorter distance.