Aircraft control surfaces are components of an aircraft used to control an aircraft. Normally, the control surfaces are movable parts in the plane wings and the tail. The surfaces help the pilot adjust attitude or plane orientation by creating differential pressure on different parts of the aircraft. The difference is pressure produces a force which then acts on the plane to produce the desired effect. Control surfaces employ the principle of lift just like the wings but in different directions. The lift forces produced by a control surface can act independently or in combination with a force from another control surface to produce a resultant force. Some of the control surfaces include rudders, elevators, flaps, and the aileron [1]. This paper discusses the control surfaces and their importance in the control of an aircraft.
Figure 1: An aircraft showing its control surfaces. Source: aviastar.org, n.d.
In earlier aircrafts, control surfaces were mechanically coupled to the controls in the cockpits by a system of levers, pulleys, and chains. As progress was made in the aviation industry, aircrafts became bigger and more sophisticated. As a result, the aerodynamic forces acting on the control surfaces also increased and therefore they could no longer be operated manually. Hence, aero engineers designed alternative control systems, such as the hydromechanical system [2]. The hydromechanical control design consisted of a hydraulic and mechanical circuit that helped reduce the limitations and weight of mechanical control systems.
Electrical control systems
Modern flight control systems are operated by electric motors which are in turn actuated by digital computers. Therefore, there is not direct mechanical connection between the aircraft cabin and the control surfaces. Instead, optic fiber cables, which are also referred to as fly-by wires, transmit signals to the control interface at the cabin and the control surface [3]. However, in some extra-large airlines, the electrical motors are assisted by hydraulic systems.
Figure 2: Schematic diagram of an aircraft electrical control system sued to actuate the control surfaces. Source: Briere, Favre & Traverse, 2001.
In the aviation industry, the term fly-by wire is used to describe the use of electrical signals to transmit the pilot’s inputs to the control surfaces. The fly-by wire provides signals to the actuators equivalent to the angular movement of the control devices such as the joystick and the control wheel. Inbuilt computers systems pick up the pilot’s command from the control devices, and convert them from mechanical movements to scaled electric signals before transmitting them to the motor and the hydromechanical systems [4]. The electrical system enables extensive stability augmentation and also enables the pilot to perform flight envelope limiting. Also, the fly-by wire system desensitizes the control inputs to avoid mishaps due to pilot errors or miscalculation.
The movement of an aircraft in the air is defined by three axes, a vertical axis, lateral axis, and a longitudinal axis as shown in figure 2 below. The rudder, which is the vertical axis control surface, helps the aircraft turn left or right, controls yawing, and gives the aircraft directional stability. The aileron controls the movement of the aircraft along its longitudinal axis [5]. Also, the aileron enables the aircraft to make rolling movements and achieve lateral stability [6]. Finally, the elevator controls the motion of the aircraft along it lateral axis and gives the aircraft longitudinal stability. The individual controls surfaces are discussed in detail below.
Figure 3: The axis that defines the motion of an aircraft in air. Source: FAA.gov, 2005.
Wing control surfaces
The control surfaces located on the wings of an aircraft are the flaps and the ailerons.
Ailerons
The ailerons are attached to the edges of the end of each wing in an aircraft. There are two ailerons in an aircraft and they move in opposite directions to each other to induce a rolling motion along the aircraft’s horizontal axis [7]. Ailerons are connected to a control wheel or stick in the pilot’s cabin by bell cranks, cables, push-pull tubes, or pulleys. When the pilot spins the control wheel to the right, the aileron on the aircraft’s right is deflected upward while the left aileron is deflected downwards [8]. The upward motion of the right aileron reduces lift on the right wing due to reduced camber. Conversely, the downward movement of the left aileron increases camber and hence the lift acting on the left wing [9]. As a result, the reduced lift on the right wing and the increased lift on left wing cause the aircraft to bank to the right.
Figure 4; Image showing the different positions of an aileron for an aircraft in flight [11].
Differential Ailerons
In differential ailerons, there is a difference in the upward and downward distance covered by the two ailerons in an aircraft. As a result, the aircraft experiences a higher drag on the wing moving downward during a roll, which reduced the yawing motion [12]. The greater drag on the descending wing is created by raising the upward deflected aileron attached to it by a higher margin than the downward deflected aileron on the rising wing.
Frise-type ailerons
In fries-type ailerons, the upward deflected aileron is pivoted at an offset hinge. As a result, the front end of the aileron comes into contact with the air flow thereby creating drag, which offsets the drag generated by the downward deflected aileron. Therefore, adverse yaw acting on the aircraft is reduced. Frise-type ailerons also create an air slot on the lowered aileron such that air flows smoothly through it, thereby lowering drags at high angles of attack [13]. However, fries-types ailerons do not completely eliminate drag but only reduces it to a certain extent.
Flaperons
Flaperons have the designed attributes of both ailerons and flaps. As a result, flaperons can control the bank angle and can also be lowered together in unison to act as flaps. In such a case, the pilot has two sets of controls, one for operating the flaperons as ailerons and another for controlling the flaps [14]. A mixer control unit is used to integrate the two aspects of the flaperons.
Flaps
The flaps are thin controls surfaces that are hinged on the trailing side of the wing and adjacent to the aircraft body. Unlike the ailerons that are deflected in different directions, the flaps are both deflected in the same direction. Flaps are used during takeoff and during flight to reduce speed [15]. When flaps are raised, they conform to the shape of the rest of the wings and hence induce no drag or lift. Conversely, when they are lowered, they increase the wings curvature such that air on top of the wing flows faster than beneath the wing. As a result of the differential wind speed, a pressure difference ensues on both surfaces of the wing resulting to a lift force.
During takeoff, the flaps are lowered to generate extra lift to overcome gravity. The flaps are retracted during flight to reduce drag and maximize speed. They are also used to increase drag during deceleration. The flaps can only be used in low and medium wind speeds to generate lift because at fast wind speeds, air moves around them too fast thereby creating drag [16]. Also, in high speed dives, the flap and other control surfaces are unusable as air travels around them too fast and the force required to actuate them is enormous. Therefore, the speed has to be lowered first.
Stabilators
Stabilators are horizontal control surfaces hinged from a pivot located at the mid-width position. As such, a stabilator’s leading and trailing edges can be alternatively raised or lowered. When the trailing edge is lowered, the plane pitches downwards, nose first. Likewise, reversing the stabilator’s orientation such that the trailing edge is raised pitches the aircraft’s nose upwards. Stabilators are very sensitive to aerodynamic loads and control inputs. Therefore, an antiservo tab is attached to the trailing edge to lower sensitivity [17]. The antiservo tabs increase the input force required to operate the stabilators thereby reducing the sensitivity. Also, they have a balance weight that acts as a counterweight to the stabilators movements.
Tail control surfaces
Elevator
The elevators are flat control surfaces that hinged on the horizontal components of the tail. They control the pitching of the aircraft about the lateral axis and stabilize the plane. The elevators create pitch by altering the angle of attack of the tail wings. Also, they can be deflected up or down. Deflection in the upwards direction, also referred to as “up-elevator”, lowers the camber thereby creating an aerodynamic force with a downward direction. The resulting downward force is greater than the tail-down force experienced by the aircraft during straight-and-level flight [18]. As a result, the aircraft’s tail moves downwards while the nose pitches upwards. The craft’s center of gravity acts as a pivot for the pitching motion.
Rudder
The rudder controls the yawing of the aircraft about the vertical axis. The rudder is hinged to the vertical stabilizer and can be deflected to the right or left direction by moving the rudder pedal. Deflecting the rudder into either side induces a horizontal force in the opposite direction. For example, when the rudder is deflected into the left, a forced is created in the right direction. The directional force causes the plane to yaw to the left direction [19]. The effectiveness of the rudder improves as the speed increases. Therefore, a given deflection angle will give a smaller yawing at low speeds than at high speeds.
Forward canards
Forward canards are small wings positioned in front of main wings in fixed-wing aircrafts. Canards are used to reduce the loads acting on the wings, to improve wing airflow control, and to improve maneuverability during stall and at high angles of attack. The canards also influence an aircraft’s static and dynamic stability and its longitudinal equilibrium. In the XB-70 bomber plane developed in the late 1960s, the canards were positioned just behind the pilot cabin on either side. Both canards had a total area of 415.6 square foot and had an all-moving design [2]. The main function of forward canards in the aircraft was to provide variable trim.
In the TU-144 aircraft, the canards were used to generate lift at low speeds. The use of the canards lowered the craft’s landing or approach speed to around 333 Km/h. Also, the canards eliminated the problem of a pitching nose caused by the elevons thereby reducing the nose dive associated with the aircraft [2]. In addition, the low speed lift generated by the canards enabled the TU-44 to take off at lower speed and hence shorter runways were required.
Both the TU-144 and the Concorde designs initially had delta wings. However, TU-144 design did not have conical camber. In a bid to improve their design, Tupolev engineers incorporated a double delta with conical camber to the TU-144 design. However, their design was yet to matchup to that of Conocrde with regard to low speed lift. Therefore, they had to boost their design with a moustache canard to generate more lift at low speeds. Nonetheless, their design never matched the Concorde’s refined wing profile, which gave the Concorde aircraft an approach speed of 287km/h [2]. As a result, the French engineers who designed Concorde did not find any use for the canards.
Evolution of control surfaces
Development of control surfaces began in the 19th century long before the first aircraft was built [6]. Aviation pioneers such as Alphonse Penaud of France and Bratain’s George Cayley experimented with manned gliders and models. They discovered that aircrafts would require mechanism to make maneuvers in the air such as yawing to the right and left and pitching the nose up and down. The discoveries led to the development of the rudder and elevator, which were paramount in controlling the first manned flight.
In 1804, Cayley invented a glider with a jointed tail. The tail had a vertical and a horizontal unit which was rotated up and down to act as an elevator [15]. Cayley’s invention was used for almost a century until it was replaced by the current elevator. The elevator tail invention was later refined by other inventors such as Otto Lilienthal from Germany, John Stringfellow from Britain, Samuel Langley, and the wright Brothers. Also, the wright brothers popularized the use of all-moving vertical tails to act as rudders.
The modern control surfaces were first used in a powered and manned aircraft in 1908 by Léon Levavasseur from France. Leon’s aircraft had a tail with fixed and vertical components with adjustable elevator and rudder surfaces attached to them. The aileron principle was first used by the wright brothers whereby they demonstrated how bending the tip of the wing of an aircraft resulted to change in aerodynamic lift [18].
The development and improvement of control surfaces revolutionized flying. This is because it would be impossible to control and aircraft without control surfaces. Also, takeoff and landing would be very difficult without such movements as pitching, yawing, and rolling. Modern flights would not be possible if the control surfaces were not invented and refined.
Conclusion
Control surfaces enable the handling and control of aircrafts. There are four major control surfaces, the ailerons, flaps, rudder, and elevator. Each of these components is important to the control of an aircraft. The elevator controls the pitching about the lateral axis and stabilizes the plane while the rudder controls yawing about the vertical axis. On the other hand, the ailerons induce a rolling motion about the horizontal axis while the flaps are used to reduce speed during and to create extra lift during takeoff. Stabilators are used to control the pitching motion of the aircraft’s nose during takeoff and descent. The control surfaces are operated by a pilot in the cabin through a computerized control interface that transmits signals through an optic fiber. The control surfaces are actuated by electric motors or a hydraulic system. Big planes employ both electric motors and hydraulic systems in order to overcome the large aerodynamic forces generated at the control surfaces.
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