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 airplane. Early efforts at airplane design succeeded in generating sufficient lift to get the aircraft off the ground, but once aloft, the aircraft proved uncontrollable, often with disasterous 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.
Axes of motion
- Vertical - 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.
- Longitudinal - 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.
- Lateral - 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.
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Main Control Surfaces
The main control surfaces are attached to the airframe on hinges 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 back 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.
- Elevators - 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.
- Rudder - The rudder is mounted on the back edge of the fin in the tail. 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.
Secondary effects of controls
- Ailerons: The ailerons primarily control bank. Whenever lift is increased, induced drag is also increased. When the stick is moved left to bank the airplane 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 airplane 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.
- 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. Out of all the control inputs, rudder inputs creates the greatest amount of an adverse affect. 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.
Turning the aircraft
Unlike a boat, turning an aircraft is not normally carried out with the rudder. Instead the ailerons are used to bank the aircraft. The forces on the plane cause the aircraft to turn in the same direction as the bank, with a steeper bank causing a faster turn. While this is happening the nose of the aircraft has a tendency to drop, and the aircraft may also yaw, so the nose is not pointing in the direction it is flying. The elevators are used to counteract the first, and the rudder to counteract the second.
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.
Secondary control surfaces
Trimming
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.
- 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.
- 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.
- 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.
Whilst carrying out certain flight excersises a lot of trim could be required inorder 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.
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 affects of slip stream, or to counter the affects of the centre of gravity being to one side. This can be caused when there is a larger weight on one side of the aircraft compared to the other, such as if one fuel tank has a lot more fuel in it then the other, or when there is heavier people on one side of the aircraft then the other.
Other Controls
- 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 airpeed. Spoilers are sometimes called "lift dumpers". Spoilers, that can be used asymmetrically are called spoilerons and are able to effect an aircraft's roll.
- Flaps - Flaps are mounted on the back edge 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.
- 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 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.
- Air brakes - these 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 additonal 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 over heat 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. Reversing the direction of the engines helps to obtain this speed.
See also
References
- Private Pilot Manual; Jeppesen Sanderson; ISBN 0-88487-238-6 (hardcover, 1999)
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.
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