FLIGHT
CONTROL SURFACE
Aircraft flight control surfaces allow
a pilot to adjust and control the aircraft's flight attitude.
Development of an
effective set of flight control surfaces was a critical advance in the
development of aircraft. Early efforts at fixed-wing aircraft design succeeded in generating sufficient lift
to get the aircraft off the ground, but once aloft, the aircraft proved
uncontrollable, often with disastrous results. The development of effective
flight controls is what allowed stable flight.
Axes of motion
Rotation around the three axes
An aircraft is free to rotate around three axes that are perpendicular to each other and intersect at its center of gravity (CG). To control position and direction a pilot must be able to control rotation about each of them.
Lateral
axis
The lateral
axis, also known as transverse axis, passes through an aircraft from wingtip to
wingtip. Rotation about this axis is called pitch. Pitch
changes the vertical direction that the aircraft's nose is pointing. The elevators are the
primary control surfaces for pitch.
Longitudinal
axis
The longitudinal axis passes through the aircraft from nose to
tail. Rotation about this axis is called roll. The angular displacement about this axis is
called bank. The pilot changes bank angle by increasing the lift on one wing
and decreasing it on the other. This differential lift causes rotation around
the longitudinal axis. The ailerons are the primary control of bank. The rudder also has a secondary effect on bank.
Vertical
axis
The vertical axis passes through an aircraft from top to bottom.
Rotation about this axis is called yaw. Yaw
changes the direction the aircraft's nose is pointing, left or right. The
primary control of yaw is with the rudder. Ailerons also have a secondary
effect on yaw.
It is important to note that these axes move with the aircraft,
and change relative to the earth as the aircraft moves. For example, for an
aircraft whose left wing is pointing straight down, its "vertical"
axis is parallel with the ground, while its "lateral" axis is
perpendicular to the ground.
Main
control surfaces
The main control surfaces of a fixed-wing aircraft are
attached to the airframe on hinges or tracks so they may move and thus deflect
the air stream passing over them. This redirection of the air stream generates
an unbalanced force to rotate the plane about the associated axis.
Flight control surfaces of Boeing 727
Ailerons
Aileron surface
Ailerons are mounted on the trailing edge of each wing near the wingtips
and move in opposite directions. When the pilot moves the stick left, or turns the
wheel counter-clockwise, the left aileron goes up and the right aileron goes
down. A raised aileron reduces lift on that wing and a lowered one increases
lift, so moving the stick left causes the left wing to drop and the right wing
to rise. This causes the aircraft to roll to the left and begin to turn to the
left. Centering the stick returns the ailerons to neutral maintaining the bank angle. The aircraft will continue to turn until opposite aileron
motion returns the bank angle to zero to fly straight.
Elevator
The elevator is a moveable part of the horizontal stabilizer, hinged to the back of the fixed part of the
horizontal tail. The elevators move up and down together. When the pilot pulls
the stick backward, the elevators go up. Pushing the stick forward causes the
elevators to go down. Raised elevators push down on the tail and cause the nose
to pitch up. This makes the wings fly at a higher angle of attack, which generates more lift and more drag.
Centering the stick returns the elevators to neutral and stops the change of
pitch. Many aircraft use a fully moveable horizontal stabilizer called stabilator or all-moving tail. Some aircraft, such as an MD-80, use a servo tab within the elevator surface to aerodynamically move the main
surface into position. The direction of travel of the control tab will thus be
in a direction opposite to the main control surface. It is for this reason that
an MD-80 tail looks like it has a 'split' elevator
system.
In the canard arrangement, the elevators are hinged to the rear of a
foreplane and move in the opposite sense, for example when the pilot pulls the
stick back the elevators go down to increase the lift at the front and lift the
nose up.
Rudder
The rudder is typically mounted on the trailing edge of
the vertical stabilizer, part of the empennage. When the pilot pushes the left pedal, the rudder deflects
left. Pushing the right pedal causes the rudder to deflect right. Deflecting
the rudder right pushes the tail left and causes the nose to yaw to the right.
Centering the rudder pedals returns the rudder to neutral and stops the yaw.
Secondary
control surfaces
Spoilers
On low drag aircraft such as sailplanes, spoilers are
used to disrupt airflow over the wing and greatly reduce lift. This allows a
glider pilot to lose altitude without gaining excessive airspeed. Spoilers are
sometimes called "lift dumpers". Spoilers that can be used
asymmetrically are called spoilerons and can affect an aircraft's roll.
Flaps
Flaps are
mounted on the trailing edge on the inboard section of each wing (near the wing
roots). They are deflected down to increase the effective curvature of the
wing. Flaps raise the Maximum
Lift Coefficient of the
aircraft and therefore reduce its stalling speed. They are used during low speed, high angle
of attack flight including take-off and descent for landing. Some aircraft are
equipped with "flaperons", which are more commonly called
"inboard ailerons]. These devices function
primarily as ailerons, but on some aircraft, will "droop" when the
flaps are deployed, thus acting as both a flap and a roll-control inboard
aileron.
Slats
Slats, also known as leading
edge devices, are extensions to the front of a wing for lift augmentation,
and are intended to reduce the stalling speed by altering the airflow over the
wing. Slats may be fixed or retractable - fixed slats (e.g. as on the Fieseler
Fi 156 Storch) give excellent slow speed and STOL capabilities,
but compromise higher speed performance. Retractable slats, as seen on most
airliners, provide reduced stalling speed for take-off and landing, but are
retracted for cruising.
Air
brakes
Air brakes are used to increase drag. Spoilers might act as air
brakes, but are not pure air brakes as they also function as lift-dumpers or in
some cases as roll control surfaces. Air brakes are usually surfaces that
deflect outwards from the fuselage (in most cases symmetrically on opposing sides)
into the airstream in order to increase form-drag. As they are in most cases
located elsewhere on the aircraft, they do not directly affect the lift
generated by the wing. Their purpose is to slow down the aircraft. They are
particularly useful when a high rate of descent is required or the aircraft
velocity needs to be retarded. They are common on high performance military
aircraft as well as civilian aircraft, especially those lacking reverse thrust
capability.
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