The methods or mechanisms used to control an aircraft's movements in roll, pitch, and yaw.
Flight controls are used to, as the name suggests, control an aircraft in roll, pitch, and yaw. Part of what makes an airplane an airplane (rather than another type of aircraft) is the ability to control its motions around each of its three "principal axes" (the roll axis, which passes longitudinally through the aircraft from nose to tail; the pitch axis, which passes horizontally through the aircraft from left to right; and the yaw axis, which passes vertically through the aircraft from top to bottom), rather than around only one or two of these axes.
Primary flight controls
These are the main flight controls used to control the plane during normal flight, usually taking the form of a set of control-surfaces mounted on various parts of the aircraft which deflect in response to the pilots' control inputs.
- ailerons are mounted on outer portions of the wings, and are used to roll the aircraft; the aileron(s) on one wing deflect downwards, and those on the other wing deflect upwards, raising the former wing and lowering the latter.
- spoilers are used by some aircraft for roll control in addition to (or, occasionally, instead of) ailerons; to roll the aircraft, the spoiler(s) on one wing deploy, reducing the lift generated by that wing and causing the aircraft to roll towards the wing with the deployed spoilers. A spoiler used for roll control is known as a spoileron or roll spoiler. Spoilers are especially useful for roll control at both very low and very high airspeeds, having less of an advantage in the middle of the speed range.
- The elevators provide pitch control; they are usually mounted on the trailing edge of the horizontal-stabilizer in the tail section of the aircraft, but can also be mounted on the front portion of the aircraft (known as a canard configuration). To pitch the aircraft up or down, the elevators on both sides of the aircraft deflect upwards or downwards, causing the tail (for conventional aircraft) or the nose (for canard aircraft) to rise or fall, and, thereby, pitching the aircraft in the desired direction.
- elevons are used on most delta-wing aircraft; as delta-winged aircraft usually do not have a separate horizontal tail, the control surfaces at the rear of the triangular wings do double duty as ailerons (they go up on one side and down on the other to roll the aircraft) and elevators (they either go up on both sides or go down on both sides, pitching the aircraft up or down).
- The rudder is a vertical surface, almost always mounted on the tail, and usually attached to the trailing edge of the vertical-stabilizer; it deflects to the left or right to yaw the aircraft in the desired direction.
- A few aircraft have a v-tail configuration; these use ruddervators mounted on the V-tail to provide both pitch and yaw control. To pitch the aircraft up or down, both ruddervators deflect upwards-and-in or downwards-and-out; to yaw left or right, the ruddervator on one side deflects upwards-and-in, while the other deflects downwards-and-out.
- Weight shifting, used by hang-glider and parachute pilots, is exactly what it sounds like; the pilot shifts their position in (or, more usually, below) the aircraft, altering its weight distribution and causing it to roll or pitch in the desired direction.
- wing-warping was used in many early aircraft and still hangs on in higher-end parachutes; instead of using discrete control surfaces, the wings and stabilizers themselves are twisted manually in flight to change the aerodynamic forces acting on the aircraft, causing it to roll, pitch, or yaw in the desired direction.
Helicopter flight controls
A helicopter has a very different set of flight controls from an airplane:
- The collective adjusts the angle-of-attack of all of the helicopter's main-rotor blades symmetrically, changing the amount of lift generated by the main rotor and causing the helicopter to climb or descend.
- The cyclic increases the angle of attack of the rotor blades on one side of the main rotor disk and decreases the angle of attack of the blades on the other side, causing the helicopter to tilt towards the low-angle-of-attack side; this causes the lift from the main rotor to point slightly forward or backward or sideways rather than lifting straight up, and, thus, causes the helicopter to move horizontally through the air in the desired direction.
- The helicopter's antitorque mechanism (usually a tail-rotor) exerts a sideways force on the helicopter's tail to keep the main rotor's torque from making the helicopter spin like a top; the amount of force exerted by the antitorque mechanism can be varied slightly to yaw the helicopter to the left or right. (Helicopters that use tip-jets to drive the main rotor have no antitorque mechanism, and must use some other method to yaw the helicopter left or right.)
- Most helicopters also have a tail-mounted rudder, and, sometimes, an elevator as well, for use in forward flight, which function in the same manner as their counterparts on an airplane.
Secondary/backup flight controls
These are not normally used to control the aircraft's motions directly, but can be used to roll, pitch, or yaw the aircraft if necessary (for instance, if the primary flight controls are malfunctioning or disabled).
- trim systems, which work by moving small tabs mounted to the primary control surfaces (or, in the case of pitch trim on larger aircraft, by adjusting the angle-of-attack of the horizontal-stabilizer), are usually used to aerodynamically "balance" the aircraft and reduce or null out the control forces needed to keep it in steady flight, but can be used as flight controls if the primary controls are out of commission (or, sometimes, if the required control authority is greater than what the primary controls can provide on their own).
- The throttles, which control the amount of thrust generated by each individual engine, can be (and often have been) used for control if the primary flight controls are unavailable.
- For pitch control, all the throttles are advanced or retarded symmetrically; advancing the throttles causes the airplane to accelerate and pitch up, while retarding the throttles causes it to decelerate and pitch down. (This effect is most pronounced for airplanes with engines mounted under the wings, or someplace else similarly low down; if the engines are mounted high up on the airplane, as with most regional-jet models and almost all private-jets, advancing the throttles will initially pitch the airplane down, taking several seconds to accelerate enough to pitch back up, and vice versa.)
- For yaw and roll control, the throttles on one side are advanced, while those on the other side are retarded; the resulting thrust asymmetry will yaw the airplane towards the side with the throttles retarded, generating a sideslip which (for most airplane layouts) rolls the airplane towards the throttles-retarded wing. Yawing and rolling an airplane in this manner is known as throttle steering.
- flaps are usually used as high-lift-devices rather than as flight controls; however, extending the flaps changes the aircraft's pitching moment (usually causing it to pitch down, but occasionally up instead; it depends on the specific type of aircraft), and can be used for some crude pitch control in an emergency. Extending the flaps also usually increases the aircraft's inherent stability, which is beneficial in many scenarios where the primary flight controls have been disabled.
- spoilers are usually used as high-drag devices (or sometimes for roll control; see far above), but they also change the aircraft's pitching moment when extended (usually creating a tendency to pitch up, although spoiler extension on a few types of aircraft actually causes them to pitch down), and, thus, can be used to aid with crude pitch control (if whatever disabled the primary controls didn't also take out the spoilers).
- landing-gear and (for aircraft so equipped) airbrakes increase the aircraft's drag when extended, causing it to slow down and pitch down; extending the landing gear also has a direct pitch-down effect (as the landing gear are lower down than the aircraft's center-of-mass).
The systems used to actuate an aircraft's flight controls vary in complexity and technology according to the aircraft's mission. Examples include:
- Mechanical linkages - Pushrods, pulleys, torsion bars, and other mechanical members are used to deflect control surfaces; used in all early aircraft, and small-to-midsize modern aircraft, as well as for backup-systems (something known as manual reversion) in many larger aircraft in case hydraulic or electric linkages fail.
- Hydraulic controls - The pilots' control inputs don't move the control surfaces directly, but, instead, actuate valves in the aircraft's hydraulic-systems, which then provide hydraulic force to move the desired surfaces.
- fly-by-wire - Pilot input is translated into electrical signals, which are then sent to actuators that move the control surfaces; used primarily in large to very large transport aircraft.
- Fly-by-light - Similar to fly-by-wire, but signals are sent via fiber-optic cables instead of electrical wires; not widely used (yet), but projected to have similar applications to fly-by-wire.
No matter how the flight controls are actuated, there are almost always multiple ways to actuate any given control (for instance, duplicated control cables, or multiple hydraulic actuators driven by different hydraulic systems, or a hydraulic actuator for normal operation plus a mechanical linkage for manual-reversion operation, to give a few of the most common systems), in order to provide redundancy.
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