Aileron droop is a flight-control configuration in which the ailerons deflect downward whenever the pilot extends the wing flaps. These devices function primarily as ailerons, but when the flap lever is moved they droop, acting simultaneously as roll-control outboard ailerons and as improvised flaps. By tilting to nearly the same angle as the inboard flaps, the drooped ailerons add camber to the outer wing sections, increasing lift on the area that possesses no dedicated flaps. STOL airplanes and many retro-fit STOL kits exploit this principle to shorten take-off and landing distances.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is the meaning of aileron droop?
Aileron droop is the condition in which the ailerons on both wings deflect downwards when the flaps are extended. Some commercial aircraft have this possibility. On the MD-11 they are named deflected ailerons. Because pilots do not have direct control over the activation process, the motion appears automatic when wing flaps increase lift generated by the wings at low speed and extending them allows reduction in takeoff speed.
What is the purpose of aileron droop?
Aileron droop lowers both ailerons when the wing flaps are extended for takeoff or for slower speeds. By moving the trailing edge down, the aileron acts like a small flap, so directing airflow creates lifting force added to lift of the wing. The extra lift lets the aircraft fly at a smaller angle of attack, giving the pilot a wider margin before the wing stalls. At the same time the downward-deflected aileron adds energy to the boundary layer, helping the airflow stay attached to the outer wing. This action makes the aileron more effective at high angles of attack and preserves the differential deflection capability that provides roll control at low speed.
Because both surfaces droop together, roll movement around the aircraft’s longitudinal axis is still available when the pilot applies roll input to stick; one aileron simply moves less down or moves up from the drooped position, while the other moves farther down. The imbalance in lift between the two wings is created with less surface travel, so ailerons reduce the force needed by the pilot and decrease stick force. Thus, aileron droop lets the same control surfaces serve two functions: they act as miniature flaps to increase lift generated by wings at low speed, and they remain ready to make roll movements that guide the aircraft during the take-off and approach phases.
Do ailerons droop?
Ailerons droop in modern aircraft but not in the original Camels. Selecting the first flap notch moves the ailerons to a 5-degree downward angle, transforming them into temporary flaps. On the MD-11 this function is named deflected ailerons, and the design provides the aircraft with full-length flaps. Aileron positions are shown on the EICAS status display, and the AIL LOCKOUT light illuminates to indicate a fault in the aileron lockout system.
What is an aileron anti-droop?
Ground spoiler control involves all spoilers (full extension) and ailerons and this pairing is called Aileron Anti Droop. When the wheels touch, the spoilers rise and the ailerons simultaneously droop a few degrees downward. The extra surface acts like a mini spoiler on each wing, so lift is destroyed faster and braking is more effective. Pilots do not have direct control over the activation process. The droop happens automatically as part of the ground spoiler function. Aircraft manufacturers add weight forward to the hinge to keep the linkages simple and reliable. On the A320 family the anti-droop function improves the landing performance, while on the MD-11 the same surfaces are named deflected ailerons.
