Fundamental Facts About Aircraft Flight Control Systems (Types, Components,& Functions)
Before understanding the Aircraft flight control systems you need to know that an aircraft is a complicated assembly of several systems working together to ensure safe and optimal flight, maneuvering, and functionality.
One of such systems is the flight control system, a combination of components that take control commands from the pilot, relay the commands to flight control surfaces mechanically, hydro-mechanically, or hydro-electrically and then make the aircraft move in the desired direction.
Think of it as the combination of a car steering wheel, gearshift lever, and all other parts of the car that enable the car to move in the direction chosen by the driver. However, this is much more complicated as there are a lot more forces at play, aerodynamic and otherwise.
This article covers all you need to know about aircraft flight control systems:
- Types of aircraft movements
- Components of flight control systems and their functions.
- Types of aircraft flight control systems
Types of Aircraft Movements
Before we proceed further regarding Aircraft flight control systems, there is a need to understand the various types of Aircraft movements. All aircraft have imaginary lines about which they move. These lines are called “axes of rotation”. The three axes of rotation of an aircraft are the longitudinal, lateral, and vertical axes.
All three axes have a 90° inclination to each other and meet at the center of gravity (CG) of the aircraft. In order to control the aircraft’s movement, the pilot uses the primary flight controls to move about the axes.
- The longitudinal axis is a straight line that cuts through the nose and tail of the aircraft. If you find it hard to remember, this might help: the longitudinal axis is the long line cutting through the aircraft, going all the way from the front to the back. Movement about the longitudinal axis is called the roll, sometimes referred to as banking. It is the up-down movement of the wings.
- The lateral axis is a straight line that cuts through the wings and CG of the aircraft. Movement about this axis is called pitch. It is the up-down movement of the aircraft’s nose.
- The vertical axis is a vertical line that cuts through the CG of the aircraft. Movement about it is called yaw. It is the side-to-side movement of the aircraft.
Each axis of rotation has a single primary flight control which is used to control movement about it. However, movement about one of the axis can induce movement about another axis, so more than one primary flight control may need to be used at once.
Components of Aircraft Flight Control Systems and Their Functions
This section of the paper will be further broken down into:
- Primary Flight Controls
- Secondary Flight Controls
- Pilot Command Inputs
Primary Aircraft Flight Control Systems
This is one of the main components of Aircraft Flight Control Systems. As said in the previous section, these are the main components that control movement about each axis of the aircraft.
They are:
- Elevator: it controls the pitching of the aircraft. There is one on each side of the horizontal stabilizer at the back of the aircraft. The elevator deflected downwards causes the aircraft nose to pitch down, while the elevator deflected upwards causes the aircraft nose to pitch up.
- Aileron: it controls the rolling of the aircraft. There is one on each wing and they are deployed in opposite directions to each other. If the aileron on the left wing is down and the aileron on the right is up, the left wing goes up while the right-wing goes down.
- Rudder: it controls the yawing of the aircraft and is located on the vertical stabilizer. The rudder is deflected to the left causing the nose of the aircraft to turn left, while the rudder is deflected to the right causing the nose of the aircraft to turn right.
How the Primary Aircraft Flight Control Systems Operate
The flight controls work based on the principles of aerodynamics. This means they typically act to increase or decrease lift on the part of the aircraft they are found in such that it influences the movement of the entire aircraft.
For an aircraft to rise above the ground or maintain an altitude, enough lift must be generated to overcome the weight or balance it out. The lift force always acts perpendicular to the wing. It is created by the flow of air over an aerofoil, which in this case is the control surface.
The positively cambered aerofoil shape in aviation causes air to flow faster at the top than at the bottom, thereby decreasing the dynamic pressure at the top following Bernoulli’s Principle. This difference in pressure between the top and bottom creates a lift force on the aerofoil.
The angle of attack (AOA) is the angle between the wing chord and the relative wind. The greater the AOA, the greater the airflow separation between the top and bottom of the aerofoil. This results in a higher pressure difference and therefore, a larger lift force. The flight control surfaces work by increasing or decreasing the AOA of the aircraft.
In the former case, more lift is produced. In the latter, a negative lift can be produced. This is ‘lift’ in the downwards direction. Some flight control surfaces also work by increasing the camber of the aerofoil. An increase in camber would mean the airflow over the top would be faster and its pressure even lower. This results in more generations of lift.
For example, the left aileron being deflected downwards increases the camber of the left-wing. The airflow at the top would have to pass faster and with a lower pressure than the one at the bottom.
The combination of the high pressure at the bottom and low pressure at the top generates a lift force that pushes the wing upwards. The opposite will be the case for the right-wing with the aileron deflected downwards. Just remember the wing moves in the opposite direction to the aileron.
As for pitch and yaw, since the elevators and rudders are located at the back, what happens is different. Here, the elevator being deflected downwards causes the aircraft’s nose at the front to go down as well, but the aircraft tail where the elevator is going up.
As the elevator is deflected downwards, more lift is generated at the tail causing the tail to go upwards. Now, imagine a see-saw. If one end goes up, the other end goes down. In the same way, the trail going down means the nose goes up and vice versa.
In the same way, the aircraft’s nose turns in the same direction as the rudder. Imagine the case with the rudder like a side-ward lift. The rudder deflected to the right causes a lift force from behind the rudder toward the left side. The trail then turns left while the nose turns right.
Secondary Aircraft Flight Control Systems
Secondary flight controls to support and improve flight performance.
Some examples of secondary Aircraft Flight Control Systems are:
- Flaps: Flaps, located on the wings’ trailing edges, increase lift during take-off and landing so that the aircraft can be flown at slower airspeeds. They also increase the drag acting on the aircraft which helps to reduce the speed, in turn reducing the amount of runway needed to get to a complete stop. The flaps are also sometimes used in conjunction with the ailerons to roll the aircraft. Some aircraft have a component called a flaperon which is a combination of a flap and an aileron.
The flaps generate lift by increasing the camber of the wing, thereby increasing the airflow separation and the pressure difference between the top and bottom of the wing. As more lift is generated, more lift-induced drag is also generated therefore, a maximum deflection of the flaps is typically avoided.
- Spoilers: they are found on the wings. When stowed, they lie flat on the upper surface of the wings. When deployed, they raise up at an angle into the airflow above the wing, making the flow above the trailing edge of the wing more turbulent. This results in reduced lift. When flying at low speeds, the spoilers operate together with the ailerons so that the lateral stability of the aircraft can be maintained. On the wing whose aileron is raised up, the spoiler is also raised up, further decreasing the corresponding lift and making the wing bank more. On the wing whose aileron is extended downwards, the spoiler is not deployed. As the speed of the aircraft increases, the ailerons become sufficient to maintain control of lateral movement so the spoilers will no longer be connected to the ailerons.
Spoilers can also be used as speed brakes when the spoilers on both wings are both deployed fully. They cause the lift to reduce and the drag to increase, thereby rapidly reducing the speed of the aircraft. There are actual speed brake panels on some wings with the same function, however, at low airspeed, these do not work together with the ailerons as spoilers do.
- Trim tabs: though they also exist in larger aircraft, they are only usually considered secondary flight controls in lighter aircraft. I will explain the operation of smaller aircraft in this article.
Trim tabs are located on the trailing edges of the rudder and elevators and allow for the aircraft to be controlled with less input from the pilot. During take-off, the trim tab will be neutral meaning it is exactly aligned with the elevator.
During the climbing phase, the aircraft might be trimmed to tackle excess control pressure so that the desired attitude is maintained. The trim can also be adjusted to maintain a certain desired airspeed.
The trim tab deflects in the opposite direction to that of the elevator, allowing it to trim out any unbalanced condition about the lateral axis. The ground adjustable tab of the rudder is adjusted directly by the pilot before the flight. It helps to combat the left-turning tendencies of the aircraft during take-off.
- Slats: these are basically flaps, but at the leading edges of the wings. Not all aircraft have slats but in larger aircraft, they can help produce even more lift. They do not automatically deploy when the leading edge flaps are extended and can, therefore, be operated separately using a control in the cockpit. However, the inboard slats are deployed at the same time as the flaps.
When deployed, they create a slot (space) between the trailing and leading edges of the wing. This slot is useful at high AOAs as they promote laminar (smooth) flow on the upper surface of the wing. This means that the critical angle of attack will be higher. The aircraft will be able to be flown with a slower stall speed and maintain control.
Pilot Command Inputs
These are interfaces for the pilots to introduce their own commands to the flight control system.
Some of them are:
- Yoke: This is similar to the steering wheel of a car. There are two in the cockpit –for both pilots – and the movement of one causes the other to move in the same direction as they are mechanically linked. The yokes are connected to the ailerons and elevators. When the yokes move in the desired direction, the linkage between them and the flight controls causes the latter to deflect or retract.
When a yoke is turned to the left, it causes the left aileron to move up, resulting in the aircraft banking to the left. The opposite happens when the control yoke is moved to the right. When the pilot pulls the control yoke forward, the elevator is to deflect upwards, causing the nose of the aircraft to pitch upwards and the aircraft to start climbing. The opposite happens when the yoke is pushed back and the elevator deflects downwards so that the aircraft descends.
- Rudder pedals: as the name implies, these are used to send commands to the rudder. When the pilot steps on the rudder pedal, the linkage connected to the rudder pedal causes the rudder to move. If it is the left rudder pedal that is pushed, the rudder moves to the left so that nose of the aircraft yaws to the left. The opposite happens when the right rudder pedal is pushed.
There are two rudder pedals on each side of the cockpit (for both pilots), making four rudder pedals in total.
- Trim wheel: this is used to control the trim tabs on the elevator. The amount of trim can be adjusted.
- Flap lever: A lever is pushed down by the pilot to the desired level/degree of deflection. The maximum level of deflection is labeled “FULL” and is 30° in most civil aircraft. The linkage between the lever and flap then actuates the flap in the desired direction.
Types of Aircraft Flight Control Systems
Over the years, developments in technology and engineering have resulted in changes in the workings of flight control systems.
Here are four types of flight control systems in the order they were developed:
- Mechanical flight control system: in this type of flight control system, there are mechanical linkages between the pilot command inputs and the various flight controls. This was the first type of flight control system developed and is still in use today in some light aircraft. It requires much of the physical strength of the pilot to move the flight controls, which means it cannot be used in larger aircraft where significant aerodynamic forces are at play. It is also called a reversible flight control system because the yoke must be held in place, otherwise, the stick force will cause the flight control to go back to its former position.
The mechanical linkages in this system are a network of cables moving over pulleys, rods, sprockets, and chains. Each control surface has its own mechanical network. Mechanical flight control systems are very heavy because of all the components involved, especially the stainless steel cables. However, they are cheaper and have a relatively simple design.
- Hydromechanical flight control system: As mentioned earlier, heavier aircraft are affected by much larger aerodynamic forces. The heavier structure must therefore come with a more powerful flight control system. This calls for the use of hydraulics to provide the needed power which cannot possibly be supplied by the pilot. Working based on Pascal’s Law, hydraulic systems ensure that a small pilot input force can result in a much larger force at the control surface’s end.
Hydromechanical systems use the combination of a mechanical network and hydraulic power. The device which acts as the pilot’s control input is linked to the hydraulic system through cables or push-pull rod linkages while the hydraulic system is connected to the control surface on the actuating end.
Since the primary control surfaces are hydraulically powered, little to no effect is felt on the pilot when s/he uses the control inputs. For this reason, an artificial feel system (Arthur Q unit) must be incorporated into the system to provide feedback and prevent the pilot from overcontrolling the aircraft.
- Hydro-electric flight control system: this type of flight control system has no mechanical linkages. Instead, it is electrically controlled by means of electric cables which connect the pilot’s input to the hydraulic actuator by sending electric signals. It is also called an electro-hydraulic flight control system. The electrical cables used in this system are much lighter than mechanical cables and the electrical signals are much quicker, allowing for a shorter response time.
This system makes use of solenoid valves to open and close the servo-control valves in the servo-actuators. The solenoid valves are controlled by electrical switches from the pilot’s end.
Pressure transducers (LVDTs) relay feedback electrical signals to the pilot about the position of the control surfaces, making it a closed-loop system. Electric motors control some valves.
- Fly-by-wire (FBW) flight control system: As the name implies, fly-by-wire, makes use of electrical wiring. There are almost no mechanical linkages between the pilot command inputs and the flight control surfaces. Instead, they are electronically controlled and electrically or hydraulically powered. The system is operated by computers which act as interfaces between the pilot’s commands and the control surfaces. Electrical signals are converted to electronic/digital signals which the computers quickly process. The FBW system is the quickest and most accurate type of flight control system. It is also self-stabilizing.
In the system, there are multiple motion sensors that serve as feedback of the aircraft’s angular and linear movement to the computers. This means that FBW systems are closed-loop systems. Air data sensors supply information concerning the aircraft’s altitude and airspeed to the computer(s). The pilot is able to control the aircraft via the flight control computer. Between the pilot’s command input and the computers, there are transducers that convert the movement energy to electrical energy, i.e., a corresponding electrical signal which is later amplified and manipulated.
Conclusion
Having known the detail of Aircraft flight control systems, their types, their components, and their various Functions through this post, please give us feedback regarding areas that need improvement and where the information you seek is not mentioned here regarding this topic.
References:
- Youtube, 2020. Aircraft Systems-02-Flight Controls. Available at: <https://www.youtube.com/watch?v=WhQ8Ai4fa_Q>
- Cfinotebook.net. 2020. Flight Control System. [online] Available at: https://www.cfinotebook.net/notebook/operation-of-aircraft-systems/flight-controls
- Moir, I., Seabridge, A. and Jukes, M., 2013. Civil Avionics Systems. 2nd ed. Sussex: Wiley and Sons.
- Anson Aviation Production, 2018. Available at: https://www.youtube.com/watch?v=1bNTVdMzZuU
