Aircraft Flight Control System
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Aircraft Flight Control System
Description of the system; functional and schematic diagrams
According to the Federal Aviation Administration, an aircraft control system is assemblage of mechanical and electronic equipment that permits a plane to be flown with excellent accuracy and steadfastness. A control system mainly constitutes cockpit controls, sensors, actuators which may be hydraulic, mechanical or electrical and computers. With improvement in technology, aircraft flight controls vary depending on the type of plane since planes with different feature and sizes have been introduced in the market thus have to be fit with flight control systems that match their capacity. However, the most basic flight control system designs are mechanical and are characterized with the early aircrafts. According to Garg et al (2013, p.60),they involve collective use of different mechanical parts which include rods, cables, pulleys and chains in some designs which play a significant role in transferring forces of flight deck controls to the control surfaces. Though new flight control models have been introduced with advancement in technology, application of mechanical flight controls still continues to date especially in small general and sport classification aircrafts especially where aerodynamic forces are not extreme.
Illustration of mechanical aircraft control system (Garg et al, 2013, p.61)

Aircraft control systems are group into two main divisions namely primary and secondary systems. The primary systems constitute ailerons, elevator and rudder and their main function is to regulate a plane securely during flight.On the other hand, secondary control systems constitute wing-flaps, leading edge devices, spoilers and trim systems and their main functions are to boost the performance of the plane or aircraft (Federal Aviation Administration). Research has proven that aircraft control systems are structured to generate adequate responsiveness to control inputs while allowing a natural feel. At limited airspeeds, the controls feel soft and sluggish thus making the plane respond sluggishly to control applications. However, at high airspeeds, the controls increase in firmness making the plane’s response rapid.
According to Seabridge and Moir (2013, p.206), changes in movement of any primary control surfaces alter the air flow and pressure distribution over and around the airfoil. The alterations affect the lift and drag produced by the airfoil and permits the pilot to regulate the plane about its three axes of rotation. It been noted that design features hinder the amount of deflection of flight control surfaces. Design features play a significant role in avoiding the pilot to unintentionally over control and overstress the plane in the course of normal maneuvers. Control surface inputs trigger movement about the three axes of rotation and the kinds of stability exhibited by a plane are associated to the three axes of rotation.
Ailerons regulate roll about the longitudinal axis and are usually fixed to the outboard trailing edge of each wing and change position in the opposite direction of each other. Ailerons are linked together using cables, bellcranks and pulleys. Ailerons contribute in lateral stability of the plane. To minimize the effects of adverse yaw, innovators have introduced four main kinds of ailerons namely differential, fries-type and couple ailerons (Obert et al, 2009, p.445). Rudder and flaperons are in this category. The major function of the elevator is regulating pitch about the lateral axis thus producing longitudinal stability in the plane. Power thrust line and position of the horizontal tail surfaces on the empennage are factors that contribute to elevator effectiveness controlling pitch.
A stabilator functions as a stabilizer in an aircraft context. It is a one piece horizontal stabilizer that spins around from the central hinge point. According to Pratt (2000, p.20), pulling back the control column elevates the stabiator’s trailing edge pulling the nose of the airplane up and the reverse happens when the control column is pushed forward as it lowers the trailing edge of the stabilator and descends the nose of the airplane. The rudder regulates the movement of an airplane about its vertical axis a motion referred to as yawing. The rudder is movable and is usually attached to the fin. Controlling the rudder is done by moving the right or left rudder pedal. The efficiency of the rudder increases with speed thus implying that large deflections at limited speeds and small deflections at high speeds may be ideal to offer the desired reaction.

Air plane controls, movement, axes of rotation and the types of stability (Garg et al, 2013, p.60).

According to Stengel (2014, p.6), flaps are most frequently used high lift devices used on aircraft and are usually fitted to the trailing edge of the wing. Flaps elevate speed and trigger drag. Moreover, AOA flaps permit conciliation between high cruising speed and low landing speed since they are extendible when necessary and retractable into wing structures when unnecessary. Flaps occur in four major types namely plain, split, slotted and fowler flaps. Leading edge devices mainly consist of fixed slots, movable slats, leading edge flaps and cups and their main purpose is to enhance the lift of the plane since they are all high lift devices (SRM University). Fixed slots channel airflow to the upper wing surface and hinder airflow separation at higher angles of attack. Leading edge flaps elevate CL-MAX and camber of the wings. Spoilers are associated with gliders and some aircrafts and their main purpose is to distort the smooth airflow thus decreasing lift and rising drag. Spoilers on gliders function to control the rate of decent required for precise landing (Roskam, 1998, p.10). On the planes, spoilers main function is roll control.
Fly-by-wire design and its advantage over the mechanical flight control system
The fly-by-wire control system is the current flight control system being applied in the in most planes. It has replaced most manual flight controls especially those with an electronic interface. The changes in position of flight controls are changed to electronic signals and transported via wires hence the fly-by –wire term. According to Airbus (2016), the alteration of the actuators at each control surface to generate the ordered response is established by flight control computers. According to Garge et al (2013, 63),the FBW system permits signals generated by the planes computers to execute functions without the input of the pilot thus the fly-by-wire system functions as a closed feedback loop. The fly-by –wire system is an improvement of the of the mechanical flight control via advancement in technology though it still applies some basics of the mechanical flight control.
Fundamental aspects of the fly-by-wire system (Garge et al, 2013, p.63)

Research has proven that the flight control was first applied in the 1970s and used analog implementations. However, digital fly-by-wire systems were incepted in 1980s and have been in use since then. Civil airliners such as Airbus A319, A330, A380, A340 and Boeing 777 and 787 and the first planes to be fitted with fly-by-wire technology and have proven to be very efficient over the basic mechanical flight control. This is clear from the millions of flying ours that have been recorded from planes with digital FBW system. The safety and integrity of the planes have been ascertained since flights operating using the digital fly-by-wire system have not had any significant hitches that can warrant their discretion from the aircraft field (Federal Aviation Aministration).
Stability of FBW
According to Garge et al (2013, p.63), the fly-by-wire flight control system in attempts to boost the plane’s stability is fitted with three gryoscopes having sensors to detect changes in movements of pitch, roll and yaw axes. Changes in movement especially trigger sending of signals to the control flight computers that in turn send commands to the relevant controls actuators to correct the movements. The corrections happen without the pilot’s knowledge since rarely do the cockpits’ control move. This is a clear indication of the efficiency of the fly-by-wire system.
According to Seabridge and Moir (2013, p.208), the digital fly-by-wire has significant advantages over the mechanical flight model that makes it more efficient and fit for application in the current airplane industry. FBW saves on weight and it can be attributed to the replacement of the heavy mechanical cables thus contributing to profound reduction in fuel consumption. Electrical controls employed in the FBW are easier to keep and are less complex than the mechanical ones thus reducing the maintenance costs. Issuing direct input via electrical signals increase the precision of commands thus enhancing the overall safety of the plane compared to the mechanical control system (SRM University).
Disadvantages of the Mechanical flight control system
According to the SRM University, the mechanical control system has certain features that make it inefficient compared to other flight control systems despite being the basic flight control system. The system is heavy thus demanding careful routing of the flight’s control cables via pulleys, cranks, tension cables and hydraulic pipes. The system necessitates a redundant backup to counter failures that might occur in the plane and this in turn increases the weight of the plane. The system has limited ability to reimburse for changing aerodynamic conditions. The control system has no ability to counter unsafe characteristics such as stalling, spinning and pilot-induced oscillation (Stengel, 2014, p.61). The plane’s stability and structure determine the occurrence of the effects thus implying that the flight control system has some limitations in terms of safety.
Analysis of fly-by-wire flight control interface and mechanical flight control interface
The mechanical flight control system is more involving than the fly-by-wire since in the mechanical design the pilot is responsible for invoking most controls that determine the planes movement especially change in direction, ascending and descending and taking off and landing. In the fly-by- wire design, flight control computers perform most of the tasks performed by the pilot in the mechanical design. This is clear from the signals that are sent automatically by the fly-by-wire flight control to carry out certain functions without the pilot’s contribution (Federal Aviation Administration). The flight control computers in the digital fly-by-wire flight control system are responsible for upholding the flights stability unlike in the mechanical flight control system where it is the responsibility of the pilot to ensure that the plane is in stable condition by regulating the functions of the primary flight controls. The issuance of commands via electrical signals induced by flight control computers in the fly-by –wire design increases precision thus assuring overall safety of the plane unlike in the mechanical flight control where signals are issued manually and rely on the pilot’s keenness rather than a programmed system (Garge et al, 2013, p.63).
Sequential representation of the mechanical flight control system in different modes of operation
Different flight control systems are utilized in different aircrafts. Helicopters for example utilize the mechanical flight control since the pilot regulates different parts to invoke movement in the plane. In a helicopter, the cyclic stick is used to incline the rotor in the preferred direction alongside collective levers to influence rotor pitch and anti-torque pedals to regulate yaw (Federal Aviation Administration).
Elevator
The figure below illustrates how a plane ascends and descends and the techniques involved in the process (SRM University). To rise in the atmosphere, the pilot pulls the control stick backwards making the elevator to be deflected upwards. The deflection makes the airflow oblige the as down and nose up creating a pitch angle and increasing it. Pushing the control stick forward causes the downward deflection which makes airflow to lift the tail and lower the nose decreasing the pitch angle and the overall effect is that the plane descends signifying it is about to land (SRM University).

Ailerons
The figure that follows show ailerons are applied to regulate the roll angle. When the pilot alters the control stick to the right, the left aileron is moves up while the right aileron moves in a downward position thus creating a roll angle which induces a more lift on the left wing and less lift on the right wing (SRM University). The difference in forces created make the plane to incline to the right the vice versa happens when the pilot move the control stick to the left (SRM University).

Adverse yaw
According to Swayne Martin (2015), the yawing effect comes about when the plane is turning either to the left or right and is mainly associated with the ailerons, which are responsible for turning. For effective turning of an airplane, it has to be banked in that the total lift is partitioned into a vertical component that offers support to the weight of the plane and a horizontal component that causes turn. To make a left turn, the aileron control is altered to the left. This in turn makes the right aileron to move in a downward position elevating the camber and lift of the right wing making it to rise. On the other hand, the left aileron adopts an upward position leading to a decrease in the camber making the left wing to descend. The adverse yaw effect comes about since the lift and drag effect are directly proportional.
During the left turn, increased lift of the right wing increases drag and the decreased position of the left wing causes a decreased drag. The difference in drag makes the plane yaw to the right. The vice versa happens when the plane is making a right turn since as the yawing effect on the nose of the plane will take place to the left. In an attempt to reduce the effects of adverse yawing, innovators in the aircraft industry are fitting planes with differential type or fries-type ailerons (Martin, 2015). Altering the differential type aileron makes the up aileron to move more than the down aileron which in turn creates more drag. Changing the position of the fries-type aileron control makes the offset hinge to make the front part of the upward deflected aileron to be exposed beneath the wing generating more drag. Fitting the two type of ailerons and manipulating the aileron control eliminates all or considerable adverse yaw.
Spoilers’ application

The main purpose of spoilers in gliders is to elevate the amount of drag created thus giving the glider pilot a perfect opportunity to lose altitude with gaining extreme speed (Federal Aviation Administration).
Autopilot
It is a flight control system that maintains a plane in level flight or on a set course. The flight control system does not rely on a pilot for navigation since the plane can be fitted with a radio navigation signal. It has similar characteristics of the fly-by-wire control system thus is very safe since the physical and mental demands of the pilot are reduced. The major factors associated with autopilot are altitude and heading hold. According to the Federal Aviation Administration, the most common autopilot systems use gyroscopic altitude indicators and magnetic compasses to regulate the servos connected to the aircraft’s control system. The number of the servos relies on the convolution of the system. More complicated autopilot systems constitute a vertical speed and can include or exclude an air speed hold mode. Complicated autopilot systems are connected to navigational aids via flight directors. Autopilot systems comprise a disconnect safety feature whose main purpose is to disconnect the system automatically or manually depending on the flight’s control system. For a fly-by-wire flight control system, autopilot is an inbuilt feature which is utilized when necessary (Federal Aviation Administration). The main components required to have a profound autopilot control system are inertial navigation systems, global positioning systems and flight computers to regulate the plane.

Redundancy of the Fly-By-Wire System and Implications on Flight in Case of Failure
In a digital flight by wire flight control system, in case flight control computers crash or get damaged or fail because of electromagnetic impulses, the remaining flight control computers overrule the faulty computers and continue flying the plane to safety. In most instances, they usually turn off the computers or reboot them to try and fix. According to Arnold (2016, p.38),bigger planes despite the four major flight control computers running on the primary flight control software, have a fifth back up computer which takes control of the flight incase the four major computers fail. It runs separately and only commanded to function when the four main computers fail (SRM University).
The back-up system eliminates the risk of total flight control system failure which may have a fatal impact in case it occurs since the plane will begin to descend and crash causing a huge loss in lives and assets (Arnold 2016, p.45). The susceptibility of such an event happening is reduced to zero since the general purpose flight software fault has evaded notice in the four main primary computers but is captured in the backup computer. In many plane, flight control redundancy advances their safety (SRM University). Flight control computers whose output differs from other is either ignored or re-booted to fix the error. A plane control system ability to withstand fight damage is one of the major reflections in the plane industry since its failure can have fatal consequences.
According to Arnold (2016, p.45),the fly-by-wire flight control system has boosted the economy in flights significantly by reducing the overall weight of the plane that was brought by increased mechanical and heavy flight control mechanisms. Elimination of the heavy control mechanism has led to increase in space thus increasing the plane capacity in passenger and cargo leading to increased gains economically. The digital fly-by-wire system has automated system responsible for checking on the jet engine’s throttles, air inlets, fuel storage and its distribution. This in turn has a significant impact on the plane’s fuel consumption since it is regulated efficiently making operation of a plane utilizing the fly-by-wire control system more economical than one fitted with a mechanical flight control system.

References
AIRBUS. (2016), FLY_BY_WIRE Retrieved on 26th April, 2016 from http://www.airbus.com/innovation/proven-concepts/in-design/fly-by-wire/
Arnold, P. K. (2016). The UAV Ground Control Station: Types, Components, Safety, Redundancy, and Future Applications. International Journal of Unmanned Systems Engineering4.1 (2016): 37-50
Federal Aviation Administration. Pilot’s handbook of Aeronautical knowledge. Retrieved on 26th April 2016 from http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/pilot_handbook/m edia/ phak%20-%20chapter%2005.pdf
Garg , A. Linda, R. I. & Chowdhury, T. (2013). Evolution of Aircraft Flight Control System and Fly-By-Light Flight control System. International Journal of Emerging Technology and Advanced Engineering. 3(12), 61-64
Martin, S. (2015). Adverse Yaw: What Is It, And How Do You Prevent It? Retrieved on 28th April 2016 from http://www.boldmethod.com/learn-to-fly/aerodynamics/adverse-yaw-what-is- it-and-how-do-you-prevent-it/
Obert, E., Slingerland, R., Leusink, D., Berg, T., Koning, J. & Tooren. (2009). Aerodynamic design of transport aircraft. Amsterdam, the Netherlands Delft: Ios Press Delft University of Technology, Faculty of Aerospace Engineering, Section Design of Aircraft and Rotorcraft.
Roskam, J. (1998). Airplane flight dynamics and automatic flight controls. Lawrence, Kan: DARcorporation.
Seabridge, A. & Moir, I. (2013). Design and development of aircraft systems. Chichester, West Sussex, UK: Wiley.
SRM University. (n.d). UNIT II AIRCRAFT CONTROL SYSTEMS. RETRIEVED ON 26th April, 2016 from http://www.srmuniv.ac.in/sites/default/files/downloads/Aircraft_ctrl_Systems.pdf
Stengel, R. (2014). Aircraft Control Devices and Systems. Aircraft Flight Dynamics, MAE 331. Retrieved on 26th April, 2016 from https://www.princeton.edu/~stengel/MAE331Lecture10.pdf
Pratt, R. (2000). Flight control systems : practical issues in design and implementation. Herts, UK Reston, VA: Institution of Electrical Engineers American Institute of Aeronautics and Astronautics.