Figure 6-17. Five common types of flaps.
Plain flap
Basic section
Split flap
Slotted flap
Fowler flap
Slotted Fowler flap
Figure 6-18. Leading edge high lift devices.
Leading edge cuff
Leading edge flap
Movable slot
Fixed slot
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.
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 6-18]
Fixed slots direct airflow to the upper wing surface and delay
airflow separation at higher angles of attack. The slot does not
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 C L. 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.
Figure 6-19. Spoilers reduce lift and increase drag during descent
and landing.
variables. Trim systems are used to relieve the pilot of the
need to maintain constant pressure on the flight controls, and
usually consist of flight deck controls and small hinged devices
attached to the trailing edge of one or more of the primary flight
control surfaces. Designed to help minimize a pilot’s workload,
trim systems aerodynamically assist movement and position of
the flight control surface to which they are attached. Common
types of trim systems include trim tabs, balance tabs, antiservo
tabs, ground adjustable tabs, and an adjustable stabilizer.
Trim Tabs
The most common installation on small aircraft is a single
trim tab attached to the trailing edge of the elevator. Most trim
tabs are manually operated by a small, vertically mounted
control wheel. However, a trim crank may be found in some
aircraft. The flight deck control includes a trim tab position
indicator. Placing the trim control in the full nose-down
position moves the trim tab to its full up position. With
the trim tab up and into the airstream, the airflow over the
horizontal tail surface tends to force the trailing edge of the
elevator down. This causes the tail of the aircraft to move
up and the nose to move down. [Figure 6-20]
If the trim tab is set to the full nose-up position, the tab moves
to its full down position. In this case, the air flowing under the
horizontal tail surface hits the tab and forces the trailing edge
of the elevator up, reducing the elevator’s AOA. This causes
the tail of the aircraft to move down and the nose to move up.
In spite of the opposing directional movement of the trim
tab and the elevator, control of trim is natural to a pilot. If
the pilot needs to exert constant back pressure on a control
column, the need for nose-up trim is indicated. The normal
trim procedure is to continue trimming until the aircraft is
balanced and the nose-heavy condition is no longer apparent.
Pilots normally establish the desired power, pitch attitude,
and configuration first, and then trim the aircraft to relieve
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 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 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 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.
Leading edge cuffs, like leading edge flaps and trailing edge
flaps are used to increase both C L-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 penalty.
Spoilers
Found on some fixed-wing aircraft, high drag devices called
spoilers are deployed from the wings to spoil the smooth
airflow, reducing lift and increasing drag. On gliders, spoilers
are most often used to control rate of descent for accurate
landings. On other aircraft, spoilers are often used for roll
control, an advantage of which is the elimination of adverse
yaw. To turn right, for example, the spoiler on the right wing
is raised, destroying some of the lift and creating more drag
on the right. The right wing drops, and the aircraft banks
and yaws to the right. Deploying spoilers on both wings at
the same time allows the aircraft to descend without gaining
speed. Spoilers are also deployed to help reduce ground roll
after landing. By destroying lift, they transfer weight to the
wheels, improving braking effectiveness. [Figure 6-19]
Trim Systems
Although an aircraft can be operated throughout a wide range
of attitudes, airspeeds, and power settings, it can be designed to
fly hands-off within only a very limited combination of these