[Ref Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25B Page 6-8]
Flaps are surfaces attached to the trailing edge and/or leading edge of the wing. Flaps are one of the secondary flight control surfaces as well as the most common high-lift devices, which used on aircraft to increase both lift and induced drag for any given AOA.
How’s trailing-edge flaps work?
Trailing-edge flaps may be extended when needed and retracted into the wing’s structure when not needed.
There are two common principles used trailing-edge flaps:
- In general, flaps increases the airfoil camber, which gives result in a significant increase in the coefficient of lift (CL) at a given AOA. Some types of flaps even increase the airfoil area and so the overall lift increases. However, induced drag is also increased and moves the center of pressure (CP) aft on the airfoil, resulting in a nose-down pitching moment.
- For some types on modern jet, the extension of flaps causes high energy air from the lower surface is ducted to the flap’s upper surface. The high energy air from the slot accelerates the upper surface boundary layer and delays airflow separation, which further increase to a higher CL.
How’s leading-edge flaps work?
Leading-edge flaps may be fixed or movable, depending on the type.
There are two common principles used leading-edge flaps:
- Some direct airflow to the upper wing surface and delay airflow separation at higher angles of attack
- Some increase both CL-MAX and the camber of the wings.
Four (4) common types of trailing-edge flaps:
- plain flaps (↑ camber ⇒ ↑ CL)
- split flaps (↑ camber ⇒ ↑ CL)
- slotted flaps (delay airflow separation ⇒ ↑ CL)
- Fowler flaps (↑ camber + ↑ area + delay airflow separation ⇒ ↑ CL)
Four (4) common types of leading-edge flaps:
- fixed slots (delay airflow separation ⇒ ↑ CL)
- movable slats (delay airflow separation ⇒ ↑ CL)
- leading edge flaps (↑ camber ⇒ ↑ CLMAX)
- cuffs (↑ camber ⇒ ↑ CLMAX)
Plain flap is the simplest one. It increases the airfoil camber, resulting in a significant increase in the coefficient of lift (CL) at a given AOA. However, at the same time, it greatly increases drag and moves the center of pressure (CP) aft on the airfoil, resulting in a nose-down pitching moment.
Split flap is deflected from the lower surface of the airfoil and produces a slightly greater increase in lift than the plain flap. However, more drag is created because of the turbulent air pattern produced behind the airfoil.
Slotted flap Is the most popular flap for large ones. Slotted flaps increase the lift coefficient significantly more than plain or split flaps. When the slotted flap is lowered, a high energy air from the lower surface is ducted to the flap’s upper surface. The high energy air from the slot accelerates the upper surface boundary layer and delays airflow separation, providing a higher CL. Thus, the slotted flap produces much greater increases in maximum coefficient of lift (CL-MAX) than the plain or split flap. Large aircraft often have double- and even triple-slotted flaps.
Fowler flaps are a type of slotted flap. This flap design not only changes the camber of the wing, it also increases the wing area. Instead of rotating down on a hinge, it slides backwards on tracks. In the first portion of its extension, it increases the drag very little, but increases the lift a great deal as it increases both the area and camber. Pilots should be aware that flap extension may cause a nose-up or down pitching moment, depending on the type of aircraft, which the pilot will need to compensate for, usually with a trim adjustment. As the extension continues, the flap deflects downward. During the last portion of its travel, the flap increases the drag with little additional increase in lift.
Fixed slots does not increase the wing camber, but it directs airflow to the upper wing surface and delay airflow separation at higher angles of attack. Thus, slots allows a higher maximum CL because the stall is delayed until the wing reaches a greater AOA.
Movable slats consist of leading edge segments that move on tracks. At low angles of attack, each slat is held flush against the wing’s leading edge by the high pressure that forms at the wing’s leading edge. As the AOA increases, the highpressure area moves aft below the lower surface of the wing, allowing the slats to move forward. Some slats, however, are pilot operated and can be deployed at any AOA. Opening a slat allows the air below the wing to flow over the wing’s upper surface, delaying airflow separation.
Leading edge flaps, like trailing edge flaps, are used to increase both CL-MAX and the camber of the wings. This type of leading edge device is frequently used in conjunction with trailing edge flaps and can reduce the nose-down pitching movement produced by the latter. As is true with trailing edge flaps, a small increment of leading edge flaps increases lift to a much greater extent than drag. As flaps are extended, drag increases at a greater rate than lift.
Cuffs are fixed aerodynamic devices extending down and forward from the leading edge. In most cases, leading edge cuffs causes the airflow to attach better to the upper surface of the wing at higher angles of attack, thus lowering an aircraft’s stall speed. In other words, cuffs are used to increase both CL-MAX and the camber of the wings.
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