Flight controls
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Aircraft flight controls allow a pilot to adjust and control the aircraft's flight attitude.
Development of an effective set of flight controls was a critical advance in the development of the 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.
This article describes controls 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 for rotary wing aircraft (helicopter or autogyro) are completely different.
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[edit] Axes of motion
An aircraft is free to rotate around three axes which are perpendicular to each other and intersect at the plane's center of gravity (CG). To control position and direction a pilot must be able to control rotation about each of them.
[edit] Vertical axis
The vertical axis passes through the plane 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.
[edit] Longitudinal axis
The longitudinal axis passes through the plane from nose to tail. Rotation about this axis is called bank or roll. Bank changes the orientation of the aircraft's wings with respect to the downward force of gravity. The pilot changes bank angle by increasing the lift on one wing and decreasing it on the other. This differential lift causes bank rotation around the longitudinal axis. The ailerons are the primary control of bank. The rudder also has a secondary effect on bank.
[edit] Lateral axis
The lateral axis passes through the plane from wingtip to wingtip. Rotation about this axis is called pitch. Pitch changes the vertical direction the aircraft's nose is pointing. The elevators are the primary control of pitch.
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.
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[edit] 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.
[edit] 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 plane to bank left and begin to turn to the left. Centering the stick returns the ailerons to neutral maintaining the bank angle. The plane will continue to turn until opposite aileron motion returns the bank angle to zero to fly straight.
[edit] Elevator
An Elevator is mounted on the back 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 control tab within the elevator surface to aerodynamically backdrive 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.
[edit] Rudder
The rudder is typically mounted on the back edge of the fin in 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 right. Centering the rudder pedals returns the rudder to neutral and stops the yaw.
1. Winglet
2. Low-Speed Aileron
3. High-Speed Aileron
4. Flap track fairing
5. Krüger flaps
6. Slats
7. Three slotted inner flaps
8. Three slotted outer flaps
9. Spoilers
10. Spoilers-Air brakes
[edit] Secondary effects of controls
[edit] Ailerons
The ailerons primarily control bank. Whenever lift is increased, induced drag is also increased. When the stick is moved left to bank the aircraft to the left, the right aileron is lowered which increases lift on the right wing and therefore increases 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 downgoing aileron deflects less than the upward-moving one, reducing adverse yaw.
[edit] Rudder
Using the rudder causes one wing to move forward faster than the other. Increased speed means increased lift, and hence rudder use causes a roll effect. Also, since rudders generally extend above the aircraft's center of gravity, a torque is imparted to the aircraft resulting in an adverse bank. Pushing the rudder to the right not only pulls the tail to the left and the nose to the right, but it also "spins" the aircraft as if a left turn were going to be made. Out of all the control inputs, rudder input creates the greatest amount of adverse effect. For this reason ailerons and rudder are generally used together on light aircraft. When turning to the left, the control column is moved left, and adequate left rudder is applied. If too much left rudder is applied the aircraft could enter a skid and then enter a spin. However, rudder inputs are also a good method to alter course in a light aircraft instead of aileron inputs as they free the pilots hands, so he/she can carry out more tasks, or check any navigational charts which are required on a cross country flight.
[edit] Turning the aircraft
Unlike a boat, turning an aircraft is not normally carried out with the rudder. With aircraft, the turn is caused by the horizontal component of lift. In actual practice, the horizontal component of lift is altered by using the ailerons and rudder in coordinated flight. As the angle of turn or bank angle is increased, less lift is available to support the aircraft. This requires increased elevator to increase the angle of attack and utimately match the lift with the weight to maintain level flight. This process cannot continue indefinitely beyond a certain bank angle (60 degrees in most aircraft). An accelerated aerodynamic stall will occur with excessive bank angle even though the airplane is above its stall speed. Pilots take diligence to limit the bank angle in actual flying practice.
[edit] Alternate main control surfaces
Some aircraft configurations have non-standard primary controls. For example instead of elevators at the back of the stabilizers, the entire tailplane may change angle. Most supersonic aircraft will have a fully-moving tail. Some aircraft have a tail in the shape of a V, and the moving parts at the back of those combine the functions of elevators and rudder. Delta wing aircraft may have "elevons" at the back of the wing, which combine the functions of elevators and ailerons.
[edit] Secondary control surfaces
[edit] Trim
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.
[edit] 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 in order to maintain the desired angle of attack. This mainly applies to slow flight, where a lot of trim is required to maintain the nose up attitude. 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 disturbences such as gusts or turbulence will be dampened over a short period of time and the aircraft will return to its level flight trimmed airspeed.
[edit] Trim tabs
Trim Tabs are used to adjust the position of an associated main control surface. They are often hinged to the back edge of the control surface with a control in the cockpit. Some trim tabs on light aircraft are fixed sheets of metal that can be bent while the aircraft is on the ground but cannot be controlled in flight. Both types function by redirecting the air stream to generate a force which holds the main control surface in the desired position. Because they are furthest from the pivot point of the main control surface, their small aerodynamic effects are magnified by leverage to achieve the deflection of the main surface. An aircraft in trim in a given axis has no net tendency to rotate about the center of gravity of that axis.
[edit] Trimming Tail Plane
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.
[edit] 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.
[edit] 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.
[edit] Rudder and Aileron Trim
Trim doesn't only apply to the elevator, as there is also trim for the rudder and ailerons. The use of this is to counter the effects of slip stream, or to counter the effects of the centre 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, or when there are heavier people on one side of the aircraft than the other.
[edit] Other Controls
[edit] Spoilers
On very high lift/low drag aircraft like sailplanes, spoilers are used to disrupt airflow over the wing and greatly reduce the amount of 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 are able to affect an aircraft's roll.
[edit] Flaps
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 and produce additional lift, and also reduce the stalling speed of the wing. They are used during low speed, high angle of attack flight like descent for landing. Some aircraft use flaperons instead, which can also be used for roll control.
[edit] Slats
Slats 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 on most airliners, allow higher lift on take off, but retract for cruising.
[edit] Air Brakes
Air brakes are used on high speed aircraft and are intended to increase the drag of an aircraft without altering the amount of lift. Airbrakes and spoilers are sometimes the same device - on most airliners for example, the combined spoiler/airbrakes act to simultaneously remove lift and to slow the aircraft's forward motion. Ground spoilers, which are a combination of airbrakes/flight spoilers along with additional panels are deployed upon touchdown to assist braking the aircraft by applying positive downward forces which also ensures that the aircraft remains planted firmly on the ground. Conventional brakes, used in cars, are often ineffective at the high speeds of modern aircraft as they will overheat and lose efficiency. Therefore increasing the drag of an aircraft with air brakes and spoilers will eventually slow the aircraft down to a speed at which conventional brakes become effective.
Most jet airliners also have a thrust reverser, which simply deflects thrust from the engines forward, slowing the aircraft down.
[edit] See also
- Aircraft flight mechanics
- Visual flight
- Aircraft engine controls
- Aeronautics
- V-tail
- Aileron
- Flaperon
- Stabilator (stabilizer + elevator in one part)
[edit] References
- Private Pilot Manual; Jeppesen Sanderson; ISBN 0-88487-238-6 (hardcover, 1999)
[edit] External links
- A clear explanation of model aircraft flight controls
- See How It Flies By John S. Denker. A new spin on the perceptions, procedures, and principles of flight.
- Semiautonomous Flight Director (SFD) US Patent 6,460,810
