There are a number of mechanisms on every aircraft which enable the pilot to control it. These flight controls are divided into primary and secondary control surfaces.
The primaries direct the aircraft on its three axes (vertical, lateral, and longitudinal). These are the elements that provide the aerodynamic force to move in the desired direction. The elevator works on the lateral axis; the rudder affects the vertical axis; and the ailerons act on longitudinal control.
Secondary control surfaces are complementary to the primary controls, and change the control characteristics and performance of the aircraft. These include compensators, high lift devices, and drag devices.
Primary Control Surfaces
Each primary flight-control system regulates the movement of the aircraft on a different axis, and their function is to change the aircraft’s orientation.
Controlling the aircraft on its lateral axis, this causes the aircraft to climb or descend, and it’s located on the rear wings, usually consisting of two parts, one on each side, behind the horizontal stabiliser.
The elevators move at the same time and rotate the aircraft relative to its centre of gravity. If deflected downwards, the increased lift raises the nose of the aircraft. Conversely, if the elevators are deflected upwards, the decreased lift will raise the tail of the aircraft and the nose of the aircraft will point downwards. This movement is called pitch.
Located on the trailing edge of the wings, these are responsible for controlling the aircraft on its longitudinal axis. If the left aileron deflects downwards, the right aileron deflects upwards and vice versa. The downward-facing aileron generates lift and the reverse is true for the opposite aileron. They will then cause the aircraft to move in the longitudinal plane, a movement known as roll.
Usually a single piece located on the trailing edge of the vertical stabilizer (at the tail of the aircraft, just above the horizontal stabilizer), the rudder as mentioned above controls the aircraft and its movement on the vertical axis. If the rudder is turned to the left, it generates lift, which will move the aircraft to the left (and vice versa). The resulting movement of the rudder is called yaw.
Secondary Control Surfaces
High Lift Devices
These are used to increase aircraft lift at low speed. Here are several of them:
- Flaps are located on the trailing edge of the wing and change its overall curvature. Retracted, they align with the wing profile. Conversely, when they are extended, the wing has its greatest curvature. The slats increase lift, perfect for manoeuvres such as landing and takeoff.
- Slatsre typically installed on large aircraft, on the leading edge of the wing. Like the flaps, they are movable and designed to change the overall curvature of the wing in order to increase lift.
- Vortex generators are small fixed fins on the upper part of the wing, and pull high-energy air into the boundary layer to lower stall speed (the minimum speed at which an aircraft flies).
Their purpose is to assist in relieving pressure on the pilot when controlling altitude. Imagine a pilot having to hold controls and press pedals for hours at a time; this would cause overexertion and affect concentration. Compensators (fixed and movable) are mechanisms to keep the controls in the position set by the pilot.
Designed to increase drag in certain manoeuvres, these decrease speed and slow the aircraft, either in flight or while landing and braking on the ground. They’re also often referred to as spoilers and speed brakes.
Photo | Orbon Alija