Fowler flap
A split flap that slides backwards before hinging downward, thereby increasing first chord, then camber. It may or may not form part of the upper surface of the wing.
Fowler flap: a flap that slides back before dropping down
A Fowler flap is a trailing-edge control surface that moves in two distinct phases. First, it slides rearward along the wing chord, extending the wing's effective area. Only after sliding backward does it then rotate downward to increase camber. This two-step motion contrasts sharply with simpler flap types that hinge downward immediately without horizontal travel.
The sliding motion is mechanized by small auxiliary wheels or rollers mounted on a track, or by a cam-and-track system that converts the rotational input from the flap actuator into combined linear and angular motion. As the flap extends, it may or may not remain flush with the upper wing surface, depending on the specific design. Some Fowler installations maintain aerodynamic fairness by having the flap panel form part of the upper surface during the slide; others create a small step or discontinuity.
The key advantage is efficiency. By extending the wing area first, a Fowler flap generates significant lift increase at modest angles of attack, making it valuable for takeoff and landing operations where both low speed and reasonable drag are needed. A simple hinged flap of the same deflection angle would add more drag for the same lift. At cruise speeds, Fowler flaps are retracted fully flush, reducing drag.
Complexity and maintenance burden
Fowler flaps demand more maintenance than fixed flaps or simple hinged flaps because the track, rollers, and sliding mechanism are subject to corrosion, jamming, and wear. Salt spray, dust, and water ingress can seize the tracks or bind the rollers. Lubrication schedules are strict, and worn rollers or track misalignment produce uneven deployment, which generates asymmetric lift and control problems during the flap extension cycle.
The mechanism also adds weight and complexity to the wing structure. The interior must house the track and actuator rod, reducing usable internal volume and increasing manufacturing cost. For these reasons, smaller general-aviation aircraft often use simpler flap types, while larger transport category aircraft, where the efficiency gain justifies the cost and weight, favor Fowler or similar high-lift devices. Modern designs sometimes replace the roller track with sealed ball-screw or linear actuator systems to reduce maintenance.