Primary Flight Control Surfaces
Almost every surface
on the airplane is designed to serve an aerodynamic purpose. In this section we
are going to discuss primary flight control surfaces. These surfaces are those
that are considered required for safe and controlled flight. Primary flight
controls include the ailerons, elevators, rudders, or various combinations
thereof.
Figure 1-12, Basic primary flight controls
Tags:
ailerons, elevators, rudder, elevons, ruddervators, stabilators, differential
stabilizers, trimming stabilizer ,canards, flaperons, secondary flight control
surfaces, speed brakes, wing flaps, slats , trim tabs, balance tabs, control
tabs
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AILERONS of an
aircraft.
An aileron, Figure 1-12, is a movable control
surface. They are found in pairs located in or attached to the outer wing’s
trailing edge on both sides of the aircraft. Their primary purpose is to
control the aircraft around the longitudinal axis – or roll – by creating
unequal or opposing lifting forces on opposite wings.
ELEVATORS of an
aircraft.
Attached to the
horizontal stabilizer, the elevator, Figure 1-12, may be one piece or two. If
two pieces, they are located on either side of the centerline of the aircraft.
Elevators operate together, both going up or down at the same time and are used
to control the pitch of the aircraft around the lateral axis.
RUDDER of an
aircraft.
The rudder, Figure
1-12, is an upright control surface used to control the aircraft about the
vertical axis. It is hinged to the trailing edge of the vertical stabilizer and
is deflected left and right to induce yaw.
ELEVONS of an
aircraft
Elevons, Figure 1-13, are often found on
flying wing and delta wing aircraft. They combine the function of the elevators
and the ailerons into one control. They act independently for roll and together
to control pitch. Elevons can be found on the B-2 bomber.
Figure
1-13, B-2 Bomber with elevons
RUDDERVATORS of an aircraft
Ruddervators are a
“V”, Figure 1-14, or butterfly tail that replaces the standard tail
configuration with two surfaces set at an angle to the horizontal. These
surfaces control pitch by working together and yaw by working asymmetrically.
The F-117A and the refueling boom on the KC-135 are examples of this
configuration.
Figure
1-14, KC-135 boom with “V” tail
STABILATORS of
an aircraft
As aircraft capability and speed increase,
especially up to and beyond the speed of sound, a wider range of control is
required than can be obtained with simpler types of controls. With the advent
of the stabilator, the entire horizontal stabilizer is built in one piece and
is pivoted for pitch control. Examples include the century series fighters like
the F-100 and F-105, the F-4 and the T-38. The stabilator set the stage for the
high performance fighters we have in the current AF inventory.
DIFFERENTIAL STABILIZERS of an aircraft
Are used on
high-speed aircraft such as the F 15, F-16, and B-1 (see figures 1-15 and
1-16). A differential stabilizer acts as a stabilator to control the pitch of
the aircraft; however, each side is also able to move independently of the
other. This allows the “stabs” to assist ailerons with roll control. This is
significant at high speeds where it provides flight control functions normally handled
by the controls on the wings, and allows us to further combine flight controls on
the wings for weight reduction (see “flaperons” below).
Figure
1-15, Differential Stabilizers
TRIMMING STABILIZER of an aircraft
(with elevators). Figure 1-17. Also known
as variable incidence stabilizers, these controls are used on aircraft that
need a wider range of control forces for different flight conditions. In such
cases, the horizontal stabilizer is built to allow changes in the angle of
incidence for the entire horizontal stabilizer while
maintaining
integrated conventional elevators. Aircraft with this configuration are exclusively
heavy airframes that include the C-141, C-5, C-17, KC-10, and KC-135.
Figure
1-17, Trimming stabilizer
CANARDS of an
aircraft
A canard is a horizontal stabilizer mounted
forward of the wing instead of aft. See Figure 1-18. At high angles of attack,
the canard creates high-energy vortices that wash over the wing delaying
boundary layer separation hence stall.
Figure
1-18, Canards on an X-29A
FLAPERONS.
The F-16 and the new
Joint Strike Fighter use these controls. The flaperon is a combination of the
flap and aileron. See Figure 1-19. Flaps are secondary flight controls and are
described below. Flaperons can be operated individually or separately and they
control movement about the longitudinal axis.
Figure 1-19, Flaperons deployed
Secondary Flight Control Surfaces
Secondary flight controls assist primary
flight controls and give the pilot additional control capabilities. They
include spoilers, speed brakes, flaps, slats, slots, and tabs.
SPOILERS. Spoilers are located on the upper surface
of the wing. There purpose is to decrease, or “spoil,” lift on that wing.
Spoilers are considered secondary flight controls, although they typically work
in unison with ailerons. The only exception is on the B-52 they are used in
place of the ailerons, thus acting as a primary flight control surface. They
are also used after landing to offset any lift that remains after touchdown so
that the full weight of the aircraft is resting on the tires. This increases
friction on the tires thereby increasing braking effectiveness.
SPEED BRAKES.
Speed brakes, figure
1-20, can be located any where on the aircraft and they operate much like
spoilers. There sole purpose is to extend into the air stream to create extra
drag and slow the aircraft.
Figure
1-20, Speed brake
WING FLAPS of
an aircraft
Flaps, figure 1-21, are the flat or curved
surfaces that extend from the leading or trailing edge of the wing. Flaps are
used to reduce the minimum landing speed. They do this by increasing camber and
wing area which lowers the velocity required to keep the aircraft aloft. Flaps
also increase drag allowing a steeper descent into small fields.
There are many types of trailing edge flaps.
Some simply pivot down from the underside of the wing only increasing camber
while others extend backward and downward increasing the area as well. Some of
these are slotted to reduce BLS like slots or slats. Leading edge flaps are
used to augment the action of the trailing edge flaps.
Figure
1-21, Flap configurations
SLATS of an
aircraft
A slat (figure 1-22) is a small movable
airfoil fitted to he leading edge of the wing. At high AOA, they deploy and
then are ahead of the wing’s leading edge. The AOA of the slat will be slightly
less than that of the main airfoil. Smooth, laminar air will then flow around
the slat and smooth the turbulence, which begins forming on the wings. Slats
then delay BLS on the wing and delay stall to a greater AOA. Slats are often
located near the wing tips to help improve overall lateral stability. A slat
will also increases the wing’s camber and surface area to some extent.
SLOTS of an aircraft
A slot (figure 1-22) is a fixed opening a short distance from the wing’s leading edge. At
high angles of attack, they serve to guide air over the upper surface of the
wing. Slots serve to smooth turbulence on the wing and delay BLS at high AOA in
much the same manner as slats. However, the fixed slot comes with a penalty of increased
drag at higher speeds.
Figure
1-22, Effect on airflow w/slats and slots deployed at a high AOA
TABS of an
aircraft
These secondary controls make the aircraft
easier to fly by reducing the amount of force required to move the primary
controls and to hold them in position. Tabs are also used to help correct any
imbalance in the aircraft. These tabs are small controls attached to the
trailing edge of the primary controls. They are moved by an electric motor and
move in the opposite direction of their associated control surface. Tabs then
utilize aerodynamic forces to reduce the amount of mechanical force needed to move
the primary control surface.
Trim Tabs of an
aircraft
The trim tab is an adjustable surface
located on the trailing edge of the primary flight control. It is used to
reduce or eliminate the force required to hold the aircraft in a fixed
position.
Balance Tabs of
an aircraft
Balance tabs are movable surfaces on
primary flight controls used to reduce the force required to move the surface.
They are moved by the primary flight control.
Control Tabs of
an aircraft
The
control tab is a movable surface located on the trailing edge of the primary flight control and
directly linked to the pilot’s control in the cockpit. This leaves the primary
control hinged but free moving. This provides the aircrews a means to move the
flight control manually, which can be used as a backup or emergency