Theory of Flight - Secondary Flight Controls
The Secondary flight controls include spoiler/speed brakes, the leading edge slats/flaps, the trailing edge flaps and where fitted, the variable incidence horizontal stabilizer.
Leading edge flaps work in conjunction with the trailing edge flaps. Both flap systems can increase wing lift by 50%. Leading edge flaps are installed to aid the trailing edge flaps in increasing lift. This is particularly useful on take off and landing.
Slats are used on large commercial aircraft to re-energise the boundary layer at slow speeds or high angle of attack. This prevents the aircraft from stalling at slower speeds a greater angles of attack.
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Spoilers are divided into groups of flight spoilers and ground spoiler panels. Flight spoilers operate differentially in conjunction with the ailerons to exercise roll control during high roll commands and also act collectively as speed brakes both in flight and on the ground.
Ground spoilers on the other hand act purely to dump lift both in the air and on the ground.
Just like the leading edge flaps, the trailing edge flaps increase the lift of the aircraft. This is done through the increased camber generated by the flaps deployment. The increased camber increases the drag of the aircraft as well as the lift. Flaps are used for take off and landing with slow speeds and high lift is needed.