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 (stick and rudder) for rotary wing aircraft (helicopter or autogyro) accomplish the same motions about the 3 axes of rotation, but manipulate the rotating flight controls (main rotor disk and tail rotor disk) in a completely different manner.
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.
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.
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.
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.
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.
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. The lifting force, perpendicular to the wings of the aircraft, is tilted in the direction of the intended turn by rolling the aircraft into the turn. As the bank angle is increased the lifting force, which was previously acting only in the vertical, is split into two components: One acting vertically and one acting horizontally.
If the total lift is kept constant, the vertical component of lift will decrease. As the weight of the aircraft is unchanged, this would result in the aircraft descending if not countered. To maintain level flight requires increased positive (up) elevator to increase the angle of attack, increase the total lift generated and keep the vertical component of lift equal with the weight of the aircraft. This cannot continue indefinitely. The wings can only generate a finite amount of lift at a given air speed. As the load factor (commonly called G loading) is increased an accelerated aerodynamic stall will occur, even though the airplane is above its 1G stall speed.
The total lift (load factor) required to maintain level flight is directly related to the bank angle. This means that for a given airspeed, level flight can only be maintained up to a certain given angle of bank. Beyond this angle of bank, the aircraft will suffer an accelerated stall if the pilot attempts to generate enough lift to maintain level flight.
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. 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.
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 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.
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.
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.
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, also called spoilers, are used on high speed aircraft to increase drag. On a typical airliner, for example, the spoilers are a series of panels on the upper surface of the wing which deploy upwards to disrupt airflow over the wing, thus adding drag. The number of panels that deploy, as well as the degree to which they deploy, depends on the regime of flight in which they are used. For example, if a pilot must descend quickly without increasing speed, he may select a speed brake setting for the desired effect. In such a case, only certain spoiler panels will deploy to create the most efficient reduction in speed without overstressing the wing. On most airliners, spoiler panels on the wings mix with aileron inputs to enhance roll control. For example, a left bank will engage the ailerons as well as deploy certain spoiler panels on the down-going wing. Ground spoilers are essentially similar to flight spoilers, except that they deploy upon touchdown on the runway, and include all spoiler panels for maximum "lift dump". After touchdown, the ground spoilers deploy, and "dump" the lift generated by the wings, thus placing the aircraft's weight on the wheels, which accomplish the vast majority of braking after touchdown. Most jet airliners also have a thrust reverser, which simply deflects thrust from the engines forward, helping to slow the aircraft down.
The Wright brothers are credited with developing the first practical control surfaces. It is a main part of their patent on flying. Unlike modern control surfaces they used wing warping. In an attempt to circumvent the Wright patent, Glen Curtis made hinged control surfaces. Hinged control surfaces have the advantage of not causing stresses that are a problem of wing warping and are easier to build into structures.
The first attempt at controlled flight was by Abbas Ibn Firnas, who used artificial wings as a form of flight control in order to be able to change direction and alter his altitude. However, the flight ended in failure, and a practical control surface would not appear until a thousand years later.
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