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Pilot's Handbook of Aeronautical Knowledge
Flight Controls

Flight Control Systems

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




The plain flap is the simplest of the four types. It increases
the airfoil camber, resulting in a significant increase in the
coefficient of lift (CL) at a given AOA. 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

The split flap is deflected from the lower surface of the airfoil
and produces a slightly greater increase in lift than the plain
flap More drag is created because of the turbulent air pattern
produced behind the airfoil. When fully extended, both plain
and split flaps produce high drag with little additional lift.

The most popular flap on aircraft today is the slotted flap.
Variations of this design are used for small aircraft, as well
as for large ones. Slotted flaps increase the lift coefficient
significantly more than plain or split flaps. On small aircraft,
the hinge is located below the lower surface of the flap, and
when the flap is lowered, a duct forms between the flap well
in the wing and the leading edge of the flap When the slotted
flap is lowered, 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 While
there are many types of slotted flaps, large aircraft often
have double- and even triple-slotted flaps These allow the
maximum increase in drag without the airflow over the flaps
separating and destroying the lift they produce.

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. 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.

Leading Edge Devices
High-lift devices also can be applied to the leading edge of
the airfoil. The most common types are fixed slots, movable
slats, leading edge flaps, and cuffs. [Figure 5-18]

Fixed slots direct airflow to the upper wing surface and delay
airflow separation at higher angles of attack. The slot does
not increase the wing camber, but allows a higher maximum
CL because the stall is delayed until the wing reaches a
greater AOA.

Movable slats consist of leading edge segments, which 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 high pressure
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

Leading edge high lift devices.
Figure 5-18. Leading edge high lift devices.

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 greater amounts of flaps
are extended, drag increases at a greater rate than lift.

Leading edge cuffs, like leading edge flaps and trailing edge
flaps are used to increase both CL-MAX and the camber of
the wings. Unlike leading edge flaps and trailing edge flaps,
leading edge cuffs are fixed aerodynamic devices. In most
cases leading edge cuffs extend the leading edge down and
forward. This 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. The fixed nature of leading edge
cuffs extracts a penalty in maximum cruise airspeed, but
recent advances in design and technology have reduced this