What Are the Different Types of Flaps on a Plane?

The architecture of an aircraft is extremely dynamic, with several components moving in unison to produce stable and comfortable flight. Any devices involved in the aircraft's aerodynamics are labeled as flight control surfaces, and in this category, there are many. Falling under the primary control surface category are elements such as the ailerons, elevators, and rudder. Meanwhile, secondary control surfaces include spoilers, slats, and air brakes. While each of these aerodynamic devices plays a critical role in facilitating flight, this blog will focus on flaps, which serve several objectives and come in a variety of designs.

Flaps are a type of secondary flight control surface found on the trailing edge of the aircraft's wing, commonly in between the body and ailerons. Although they come in several configurations, all flaps share the primary purpose of reducing stalling speed. Unlike the name suggests, stall speed has nothing to do directly with airspeed, but rather the vehicle’s angle of attack. However, at lower speeds, the angle of attack must be higher to generate the same magnitude of lift. This is where flaps come into play.

The most general equation for lift is L=1/2(p*V^2*S*Cl), where p is density, V is airspeed, S is the area of the wing, and Cl is the lift coefficient. While most of these are intuitive and require no definition, the lift coefficient is somewhat harder to comprehend. This value is a dimensionless coefficient that is determined by an airplane’s shape and angle of attack. If one were to plot the lift coefficient and angle of attack on a graph, it would become apparent that the two would increase together linearly. However, once the angle of attack reaches a particular threshold, they begin to diverge, and the lift coefficient drops. With this in mind, we may now explore how flaps aid aircraft by increasing lift.

The first variable affected by flaps is their surface area, which quickly rises when deployed. Additionally, flaps create a more optimal lift coefficient, thereby increasing lift even further. As flaps are diverse in design, this means that what is ideal for one aircraft may be inefficient for another. Listed below are some of the most common flap designs which are likely to be encountered during routine operations.

Plain Flaps

Plain flaps are the oldest and most straightforward of the designs listed. Their configuration consists of a hinged flap that rotates down to a set point. Earlier designs combined flaps and ailerons into a single unit, but this idea was soon replaced with the separation that is seen today.

Split Flap

Unlike plain flaps which fully deflect down from the wing when engaged, split flaps contain an immobile top portion that remains still and in line with the rest of the wing. While these flaps produce a higher amount of drag than most plain flaps, they also incur an unacceptable amount of drag, making their modern use all but obsolete.

Slotted Flaps

These flaps contain a slightly disconnected hinge which allows the flap to disengage almost entirely from the rest of the wing when deployed. The gap that is created from the extension allows for air to flow through, further increasing lift.

Fowler Flap

These unique flaps slide backward from the wing before facing down, thereby significantly increasing the surface area and lift. As a result, Fowler flaps are found on many commercial and cargo aircraft today. Additionally, some products integrate slots into their design, providing the same benefit as mentioned prior.


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