This paper describes the control surfaces used on a fixed-wing
aircraft of conventional design. Other fixed-wing aircraft configurations may
use different control surfaces but the basic principles remain. The controls
(stick and rudder) for rotary wing aircraft (helicopter or auto gyro) accomplish the same motions
about the three axes of
rotation, but manipulate the rotating flight controls (main rotor
disk and tail rotor disk) in a completely different manner.
PRIMARY 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.
Ailerons:
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:
An elevator is mounted on the trailing edge of the
horizontal stabilizer on each side of the fin
in the tail. They 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 stabilator a moveable horizontal stabilizer in place
of an elevator. 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.
Rudder:
The ailerons primarily control roll. Whenever lift is increased, induced
drag is also increased. When the stick is moved left to roll the aircraft
to the left, the right aileron is lowered which increases lift on the right
wing and therefore increases induced drag on the right wing. Using ailerons
causes adverse
yaw, meaning the nose of the aircraft yaws in a direction opposite to the
aileron application. When moving the stick to the left to bank the wings,
adverse yaw moves the nose of the aircraft to the right. Adverse yaw is more
pronounced for light aircraft with long wings, such as gliders. It is counteracted
by the pilot with the rudder. Differential ailerons are ailerons which have
been rigged such that the down going aileron deflects less than the upward-moving
one, reducing adverse yaw.
SECONDARY CONTROL SURFACES
Spoilers:
In aeronautics, a spoiler (sometimes called a lift dumper)
is a device intended to reduce lift
in an aircraft. Spoilers are plates on the top surface of a wing which can be
extended upward into the airflow and spoil it. By doing so, the spoiler creates
a carefully controlled stall over the portion of the wing behind it,
greatly reducing the lift of that wing section. Spoilers differ from air brakes in that air brakes are designed to
increase drag making little change to lift, while spoilers reduce lift as well
as increasing drag.
Spoilers fall into two categories: relatively small spoilers
that are deployed at controlled angles during flight to increase descent rate,
and much larger spoilers that are fully deployed immediately on landing to
greatly reduce lift ("lift dumpers") and increase drag.
Flaps are mounted on the trailing edge of each wing
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
"flapperons", 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.
Types of Flaps
Plain flap:
The rear portion of airfoil rotates downwards on a simple hinge
mounted at the front of the flap. Used in this form as early as 1917 (during
World War I) on the widely produced Breguet 14 and possibly
earlier on experimental types. Due to the greater efficiency of other flap
types, the plain flap is normally only used where simplicity is required. A
modern variation on the plain flap exploits the ability of composites to be
designed to be rigid in one direction, while flexible in another. When such a
material forms the skin of the wing, its camber can be altered by the geometry
of the internal supporting structure, allowing such a surface to be used either
as a flap or as an aileron. While most currently use a complex system of motors
and actuators, the simplest such installation uses ribs that resemble bent
carrots - when the bend is nearly horizontal, there is no deflection, but when
the carrot is rotated so the bend is downward, the camber of the airfoil is
changed in the same manner as on a plain flap.
Split flap:
The rear portion of the
lower surface of the airfoil hinges downwards from the leading edge of the
flap, while the upper surface stays immobile. Like the plain flap, this can
cause large changes in longitudinal trim, pitching the nose either down or up,
and tends to produce more drag than lift. At full deflection, a split flaps
acts much like a spoiler, producing lots of drag and little or no lift. It was
invented by Orville
Wright and James M. H. Jacobs in 1920 but only became common in the
1930s but was quickly superseded.
Slotted flap:
A gap between the flap and the wing forces high pressure air
from below the wing over the flap helping the airflow remain attached to the
flap, increasing lift compared to a split flap. Additionally, lift across the
entire chord of the primary airfoil is greatly increased as the velocity of air
leaving its trailing edge is raised, from the typical non-flap 80% of free
stream, to that of the higher-speed, lower-pressure air flowing around the
leading edge of the slotted flap. Any flap that allows air to pass between the
wing and the flap is considered a slotted flap.
Fowler flap:
Split flap that slides backwards flat, before hinging
downwards, thereby increasing first chord, and then camber. The flap may form
part of the upper surface of the wing, like a plain flap, or it may not, like a
split flap but it must slide rearward before lowering. It may provide some slot
effect but this is not a defining feature of the type. Invented by Harlan D. Fowler in 1924, and tested by Fred Weick at NACA in 1932. They were first used on the Martin 146 prototype in
1935, and in production on the 1937 Lockheed Electra, and is still in widespread use on modern
aircraft, often with multiple slots.
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 needs to be retarded. They are common on high performance military
aircraft as well as civilian aircraft, especially those lacking reverse thrust
capability.
OTHER CONTROL SURFACES
Trim controls:
Trimming controls allow a pilot to balance the lift and drag
being produced by the wings and control surfaces over a wide range of load and
airspeed. This reduces the effort required to adjust or maintain a desired
flight attitude.
Elevator trim:
Elevator trim balances the control force necessary to maintain
the aerodynamic down force on the tail. Whilst carrying out certain flight
exercises, a lot of trim could be required to maintain the desired angle of
attack. This mainly applies to slow flight,
where maintaining a nose-up attitude requires a lot of trim. Elevator trim is
correlated with the speed of the airflow over the tail, thus airspeed changes
to the aircraft require re-trimming. An important design parameter for aircraft
is the stability of the aircraft when trimmed for level flight. Any
disturbances such as gusts or turbulence will be damped over a short period of
time and the aircraft will return to its level flight trimmed airspeed.
Except for very light aircraft, trim tabs on elevators are
unable to provide the force and range of motion desired. To provide the
appropriate trim force the entire horizontal tail plane is made adjustable in
pitch. This allows the pilot to select exactly the right amount of positive or
negative lift from the tail plane while reducing drag from the elevators.
Control horn:
A control horn is a section of control surface which projects
ahead of the pivot point. It generates a force which tends to increase the
surface's deflection thus reducing the control pressure experienced by the
pilot. Control horns may also incorporate a counterweight
which helps to balance the control and prevent it from "fluttering"
in the airstream. Some designs feature separate anti-flutter weights.
In RC model aircraft, a "control horn" is an arm
similar to a bell crank that connects to a control rod linkage.
Typically one end of each rod connects to one control horn, sometimes called
the servo arm, rigidly attached to the shaft of the RC servo, and the other end of the rod
connects to another control horn rigidly attached to the control surface.
Spring trim:
In the simplest cases trimming is done by a mechanical spring
(or bungee)
which adds appropriate force to augment the pilot's control input. The spring
is usually connected to an elevator trim lever to allow the pilot to set the
spring force applied.
Rudder and aileron trim:
Trim often does not only apply to the elevator, as there is also trim for the rudder
and ailerons in larger aircraft. The use of this is to counter the effects of
slip stream, or to counter the effects of the center
of gravity being to one side. This can be caused by a larger weight on one
side of the aircraft compared to the other, such as when one fuel tank has a
lot more fuel in it than the other.
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