Hogeschool van Amsterdam Amsterdamse Hogeschool voor techniek
Aviation studies
Project Flight Controls
ALAGroup: 2A1Q
Amsterdam, March 19 2008Wiecher de Klein
Jasper Schoen
Rogier Stoelman
Bill de Vries
Jelle van Eijk
Sander Groenendijk
Robbin Habekotte
Rick de Hoop
List of contents
Summary 1
Introduction 2
1. DEFINITION FLIGHT CONTROLS 3
1.1 Aerodynamics 3
General laws The wing
1.2 Primary flight controls 6
Elevators Ailerons Rudders Trim
1.3 Secondary Flight Controls 10
Flaps
Leading edge devices Spoilers
1.4 Regulations and demands 12
Regulations Demands
1.5 Functional analysis 15
2. FLIGHT CONTROL ANALYSIS 16
2.1 Hydro mechanical system 16
Input Transport Convert Output Power Supply
Control laws & limitations
2.2 Fly-By-Wire system 19 Input Convert Processing Transport Convert Output Power Control laws
2.3 Pro’s and Con’s 23
Pro’s and Con’s per system Pro’s and Con’s table
2.4 Comparison 24
Weighing factors table System compared 3. Modification plan 26 3.1 System overview 26 Modifications Construction overview 3.2 Designing aspects 28 Safety Maintenance Environment Planning
3.3 Cost and Benefits 29
Cost Benefits Break-even-point 3.4 Conclusion 30 3.5 Recomendation 31 Bibliography 32
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SUMMARY
The project contains the definition and analysis of the flight controls as found in the Boeing 737 Next Generation (NG) and Airbus A320. When this is done, an actual modification and recommendation is made to the customer.
The flight controls works on an aerodynamic force. The flow of air around a subject creates forces; these forces are ex-plained with help of Bernoulli’s law and the law of continuity. Important characteristics of the wing are the leading edge, the trailing edge, chord, mean camber line, camber, thickness and nose radius. The length or positions of these characteristics are important for the lift and drag. The airplane can move around three different axles; the longitudinal axis, the normal axis and the lateral axis with help of the primary flight controls. The elevator is used to turn the airplane around the lateral axis. The ailerons are the control surfaces mounted on the rear of each wing and are designed to make the airplane roll around its longitudinal axis. The rudder is the control surface on the tail of the airplane that can make the airplane yaw around its nor-mal axis. There are also trim functions, these are used for lighten the control for the pilot. Altering the shape of the wings, which can be done by flaps, slats and spoilers can change the aerodynamic forces of the wings. These controls are named secondary flight controls. Flaps are used during take-off and landing. They increase the lift so the airplane can fly at a lower speed. There are two different types of spoilers; flight spoilers and ground spoilers. They enhance the working of ailerons and can be used as speed brakes. The system must meet the regulations by the European Aviation Safety Agency (EASA). This contains forces, controls and safety. The demands are made by the constituent they can be flexible and fixed. The flexible demands are wanted but not necessary. The fixed demands should always be met.
The system of the Boeing 737NG and Airbus A320 consists of seven steps: input, convert, transport, process, convert, transport and output.
First the analysis of the Boeing 737NG. There are two inputs; one of the pilot, using the steering column, and one of the autopilot. The signal of the pilot is mechanically transported by means of steel cables and pulleys. The autopilot signal is analog electrical. A Power Control Unit (PCU) amplifies the mechanical movement. The signal of the autopilot is transported to a servo. Both these signals move the control surface. The system receives its power of the engines during flight and the Auxiliary Power Unit (APU) on the ground.
The Airbus A320 also has two inputs: the pilot and the autopilot. Both inputs are electrical and transported by the ARINC 429 protocol. They are transported to the flight computers that correct the input values. After the conversion step the signal is converted into a hydraulic signal and it is transported. The movement of the hydraulic fluid is converted into a movement, which is done by means of the actuators. Airbus programmed the flight computers to protect the pilot from abnormal control inputs, but this also gives the pilot more control in abnormal situations.
To make a good overview, a pro’s and con’s table is set up. The hydro mechanical system of the Boeing 737NG scores 64%. The fly-by-wire system scores 71.5%. These scores are based on safety, maintenance, fuel efficiency, comfort, marketing and feedback.
In order to advice Amsterdam Leeuwenburg Airlines (ALA) whether or not they should modify their fleet of 737’s, all aspects that are changed are known. Numerous systems, like the cable systems, must be removed and/or modified. Also the whole function analysis has been made for the modified fly-by-wire system. The new fly-by-wire system of the Boeing 737’s has been tested and certified. Because of the modification, the maintenance procedures for the Boeing 737’s change. The new fly-by-wire system makes the Boeing 737’s more fuel-efficient, so there is less carbon dioxide emission. From a marketing perspective, this is a positive aspect. Also the complete period of the modification process is planned. The feasibility of the modification of the Boeing 737’s depends on the costs and benefits. Therefore the costs of the modification need to be ana-lyzed. Besides the costs, the modification to a fly-by-wire system also has benefits. Once the fly-by-wire system has been implemented, the maintenance costs drop. The fly-by-wire computers also improve the fuel efficiency, which saves ALA 3% on fuel costs.
Introduction
The airliner Amsterdam Leeuwenburg Airlines (ALA) has gathered a project group to create a way to make a modification from the hydro mechanical flight control system to fly-by-wire. ALA wants to modify their Boeing 737NG to use fly-by-wire just like an Airbus A320. The project group exists out of eight students of the Amsterdam School of Technology. The students in this project group have been carefully selected to realize this modification.
An airplane uses flight controls to adjust and control the airplane’s flight attitude. These controls exist of surfaces on the outside of the airplane that are moveable. The pilot can move these surfaces by means of cables that are attached to the surfaces outside the airplane. The older airplanes used to fly with this conventional flight system. The problem with every-thing in the world of engineering is that some systems get old and that there are newer better systems like fly-by-wire. The project group has chosen a Cessna 172 as an example to get an understanding of the basics of flight controls. With the use of this information, learned from the Cessna, the Airbus A320 fly-by-wire system will be analyzed to find out how it works. With the knowledge from these two systems it is possible to modify a Boeing 737NG to realize a fly-by-wire system.
The report is divided into three chapters.
In order to draw up the report the project group has first examined the aerodynamic forces that work on the control surfaces and the way those forces come to be. Knowing what a flight control really is, the project group has taken a Cessna 172 as an example. This helped to find out that the flight controls consist of primary and secondary flight controls. It is also important to know what kind of restrictions and demands there are on flight controls. To know what kind of parts there are used in the flight controls, a functional examination (1) is made.
In the second chapter a comprehensive description is given about how a hydro mechanical and a fly-by-wire system work. Using this information the pro’s and con’s for the hydro mechanical and fly-by-wire system are determined. The advantages and drawbacks of each system have been put in a pro’s and con’s table in order to compare them to each other (2).
In the final chapter a modification plan is created to fit a fly-by wire system in a Boeing 737NG. First, a system overview is made to look at what components are replaced and what can remain. The design of this system needs to have some de-mands that are put into designing aspects. With all the new parts that are ordered, the crews that need to be hired to make this modification possible cost money. An assessment on how much the entire system costs is made and in order to know if it’s worth to replace the system or not, a recommendation to ALA (3) is given.
For the project the book of Siers (2004) is used for structure. Also the HvA weekberichten are used. For a complete list of information sources, refer to page 32. In the appendix map the complete assignment given by ALA (Appendix I) and the pyramid model (Appendix II) can be found.
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1
Definition of flight Controls
The airplane has a system that controls the movement around the three axis: the flight control system. In order to understand how the flight controls work, you need to understand which forces and laws of physics work on the controls (1.1).
There are two types of flight controls. First the primary flight controls, consisting of rudder, ailerons, elevator and trim (1.2),
and secondary flight controls, the slats, flaps and spoilers (1.3).
Before the flight controls are modified, they have to meet the requirements of the law and the demands of ALA (1.4). There are two flight control systems, the hydro mechanical and the fly-by-wire system. These systems consists of various sub functions which are important to describe in analyses (1.5).
The main sources that are used are: Vliegtuigen B1 en B3, The air pilot’s manual 4 / The aeroplane-technical and the web-sites from FAA and the EASA.
1.1
Aerodynamics
Before the flight controls are described, the theory behind the Aerodynamics is explained. The Aerodynamic laws (1.1.1) are the basics behind the flight controls. The theory behind the wing (1.1.2) is needed to understand the how the flight controls let the airplane move.
1.1.1 General laws
There are two general laws for flight controls: the law of continuity (1.1.1a) and Bernoulli (1.1.1b). These two laws have effect on the lift of the plane and the way it handles. These two laws combined is called a venturi effect (1.1.1c). An airplane reacts on the center of gravity (1.1.1d). Some forces act in the center of pressure (1.1.1e).
1.1.1a Law of continuity
In a stationary air stream the product of the surface and speed is constant. When the surface (A) decreases the speed (v)
increases, this works the other way around too. This effect is referred as the Continuity law [1] and is also called: the law of containment of volume.
1.1.1b Law of Bernoulli
The law of Bernoulli [2] states that the energy per volume unit is constant. In other words the product of speed and static pressure is equal. [2] v= speed (m/s) c = constant ρ = air density (kg/m3) p = Pressure (n/m2) [1] A = surface (m2) ρ = air density (kg/m3) v = speed (m/s) c = constant
1. Venturi throat
2. Incoming air (m/s)
3. Pressure difference (N/ m2) 4. Static pressure (N/m2)
Figure 2.2 Wing profile
The law of Bernoulli can only be used if it meets these requirements:
• The air is incompressible. When reaching high speeds the air becomes compressible (around mach 0.5). Bernoulli is only useable at low speeds.
• Bernoulli's equation is not applicable when there is viscosity. Viscous means friction between different air layers so there mustn’t be any friction between the air layers.
• The airflow is stationary. This means that each air particle flies in the same direction and at the same speed.
• The airflow is adiabatic. Adiabatic airflow can’t receive incoming energy or outgoing.
Bernoulli gives the relationship between speed and static pressure. When the speed, which is kinetic energy, increases the static pressure, which is potential energy, decreases. This works the other way around too.
1.1.1c Venturi effect
The law of continuity and Bernoulli’s law together is called the venturi (Figure 1.1) effect. This means that in a converging air stream (1), the surface between the different air streams becomes smaller. When this occurs the speed of the incom-ing air (2) increases and the static pressure decreases. In this case the pressure difference is measured by the tube under the venturi. The fluid in the tube goes up (3) (because of the pressure difference) when the static pressure (4) becomes higher.
1.1.1d Center of gravity
Every object on earth stays on this planet because of gravity. Every little molecule of that object has got his gravity. For calculations we use the center of gravity. That is a point in the object where in theory the gravity grabs the object. This point is important for the balance of the airplane.
1.1.1e Center of pressure
Not all the forces are going through the center of gravity, the sum of the total aerodynamic forces are going through the center of pressure. Because there is no distance between the force and this point there is no moment.
1.1.2 Aerodynamic profiles
Every wing has got its own characteristics (1.1.2a). Small adjustments on the shape length etc. can lead to a different beha-vior of the airflow around it. But before looking at these differences between wings, it is examined how the air flows around a wing profile. Also the angle of attack and the boundary layer (1.1.2b) are explained. The boundary layer is split up in laminar and turbulent airflows (1.1.2c). The change of airflow is calculated with a Reynolds number (1.1.2d).
1.1.2a Lift and Drag
Important sections of the wing (Figure 1.2) are, the leading edge (1), trailing edge (2), chord (3), mean camber line (4), camber (5), thickness (6) and nose radius (7). The length or positions (8) of these sections are important for the lift and drag. The most important function of the wing is to provide the airplane with lift. This is realized by a difference in pressure above and below the wing.
Figure 1.1 Venturi
1. Leading edge
2. Trailing edge
3. Chord
4. Main camber line
5. Camber
6. Max. Thickness
7. Position on chord
8. Nose radius
__________________________________________________________________________________ Figure 1.4 Turbulent and laminar boundary layer.
When there is no viscosity, the air molecules do not disturb each other. This means the speed of layers of air does not de-crease or inde-crease when the adjacent layer is faster or slower. In this situation there is no lift and drag. When this profile is placed in a narrow angle with the incoming airflow, the air travels faster at the trailing edge at one side of the wing. However, without viscosity the air flows to the other side and creates a pressure point. The pressure point found at the leading edge is in the opposite direction of the one at the trailing edge, so the two pressure points cancel each other out, resulting in no lift.
In presence of viscosity, the flow on the end is not there so there is only one force, which is lift. This force is separated in a horizontal and vertical force, the lift and drag. This depends on:
1. Angle of attack 2. Wing characteristics
Ad 1 Angle of Attack
The definition (Figure 1.3) of the angle of attack (1) is the angle that the chord (2) of the wing makes with the incoming air-flow (3). This angle can increase or decrease the lift of the wing. This also effects the drag. A greater angle creates more drag.
Ad 2 Wing characteristics
To compare wings of different sizes, realistic forces cannot be used because the surface of the wing is needed as part of calculating this force. To be able to compare wings of different sizes, a realistic number with no dimension is needed so the size is not important. To create this, the surface and the dynamic pressure are equaled out [3]. A Cl and a Cd coefficient are
left. This indicates the lift and drag for different wing profiles so they can be compared.
1.1.2b Boundary layer
If a wing is placed in an air flow, the air close to the surface is influenced by resistance from the wing. On the surface of the wing the speed of the air is reduced to zero. The speed of the higher layers in the airflow increases until the speed of the undisturbed airflow is reached. This layer is called the boundary layer. The dynamic viscosity plays a significant role here, without it the airplane would not experience friction and flying would become impossible.
There are two types of boundary layers (Figure 1.4), laminar (1) and turbulent (2). A laminar layer is a layered stream of air moving over a surface, where the space between each layer is equal. The longer the laminar flow is, the more air layers it influences.
A laminar boundary layer turning into a turbulent layer is called the change of air flow. The reason why this happens is that the air particles on a wing profile accelerate at the leading edge. When the air particles passes the point of the lowest pres-sure they slow down. When the low kinetic energy air particles reach the trailing edge, the higher static prespres-sure at the trail-ing edge causes the flow to go from laminar to turbulent.
CF = F / q.S [3]
When you replace the Lift or drag for force you get a Lift and drag coefficient
CL = L / q.S and CD = D / q.S
1. Angle of attack
2. Chord
3. Direction of incoming airflow
Figure 1.3 Angle of attack
1. Laminar boundary layer
1.1.2c Comparison of the laminar and turbulent layer
The two boundary layers (Figure 1.5) have different properties. It is important to know which flow goes over the surface of an object because it affects the aerodynamic characteristics. For example a laminar layer has lower friction then a turbulent layer. This shows that for lesser fuel consumption a laminar layer is better than a turbulent layer.
Laminar layer Turbulent layer
A layered profile. The speed profile shows the true speed. The speed profile shows the average speed.
It is a thin layer. It is a thick layer.
Low speed close to the wing surface. Higher speed close to the wing surface.
An equal increase of the speed. A very high speed increase
Little friction. Much friction.
The average speed of the air particles in the boundary layer is low. The boundary layer has little kinetic energy.
The average speed is high. The particles in the boundary layer have more kinetic energy.
1.1.2d Reynolds number
It is possible to calculate the change of air flow with the Reynolds formula [4]. When the Reynolds number is greater than 530.000, the air stream changes from laminar to turbulent. At low speed the length (l) of the laminar air flow is greater than at high speed. At greater height the dynamic viscosity (µ) increases which influences the length (l) of the laminar to turbulent air flow.
1.2
Primary flight controls
The primary flight controls are the most important controls of an airplane, without them the airplane would be uncontrollable. There are four types of primary flight controls; the elevator (1.2.1) for the control around the lateral axis, the ailerons (1.2.2)
to move the airplane around the longitudinal axis, the rudder (1.2.3) to move the airplane around the normal axis and finally the trim tabs (1.2.4) to maintain a direction and/or height.
1.2.1 Elevator
The elevator is used to turn the airplane around the lateral axis. In the Cessna 172 the elevator is controlled via a system of cables and pulleys. They connect the yoke to the control surface located on the horizontal stabilizer on the back of the air-plane.
[4]
ρ = air density (kg/m3)
v= speed (m/s)
l = length of the air flow (m) µ = Dynamic viscosity (N/ρ)
__________________________________________________________________________________ 1. Yoke
2. Lever
3. Cable for upward motion
4. Cable for downward motion
5. Elevator
6. Horizontal stabilizer
Figure 1.6 Elevator control on a Cessna 172
Figure 1.7 The workings of the ailerons
Figure 1.8 Aileron controls
1. Control wheel 2. Cables 3. Pulleys 4. Ailerons 1. Left aieron 2. Increased lift 3. Right aileron 4. Decreased lift 5. Rolling direction 1. Purpose 2. System Ad1 Purpose
The purpose of the elevator is to create a moment of forces along the lateral axis of the airplane (Appendix VI). This move-ment is created by the change in lift on the horizontal stabilizer. When for example the elevator moves up, the camber of the profile changes thus providing more lift. In the case of an elevator with negative camber (like the Cessna). This means the elevator creates a downward force pushing the nose of the plane upwards.
Ad2 System
The movement is controlled by the pilot and is transported via cables and pul-leys (Figure 1.6).
When the pilot moves the yoke (1), the movement is transported via a lever (2), to the cables (3 & 4). There is one cable for the upward, and one for the down-ward motion. These cables run through the airplane guided by pulleys and are connected to the elevator (5) which is located on the back of horizontal stabi-lizer (6).
1.2.2 Ailerons
The ailerons are the control surfaces mounted on the rear of each wing and are designed to make the airplane roll around it’s longitudinal axis. They are controlled by the wheel on the yoke in the cockpit and linked to that wheel via a mechanical sys-tem of wires and pulleys. Also there are many different kinds of ailerons, they all are different designs to counter the side effect that occurs (which is yawing caused by a difference in drag between the left and right wing when the ailerons are used), thus making the airplane more controllable.
1. Purpose
2. System
3. Variations
Ad1 Purpose
The ailerons (Figure 1.7) are the control surfaces at the rear of each wing. They serve only one purpose and that is to make the airplane roll controlled around its longitudinal axis. If the pilot wants the airplane to roll to the right for example, he or she turns the wheel on the yoke clockwise. This makes the aileron. On the left wing (1) move downward, which increases the effec-tive camber of the left wing which in its turn increasing the lift (2), while the aileron on the right wing (3) moves upward, decreasing the effective camber and thus the lift (4) on the right wing. The difference in lift between the left and the right wing makes the airplane roll (5). There is a limit set to the maximum angle the ailerons can handle, this varies on different types of airplane, but on a Cessna its 30° upward and 30° downward from the hori-zontal position.
Ad2 System
The control surfaces are linked to the controls in the cockpit using a system of cables and pulleys (Figure 1.8). When the pilot turns the control wheel (1), two cables (2) guided by a series of pulleys (3) move in opposite direction of each other. Each cable is attached to an aileron (4). Because the cables move in opposite direction of each other, the one attached to the left aileron moves in the opposite direction of the one attached to the right aileron, thus one aileron is pulled downwards and the other aileron is pulled upwards.
Figure 1.9 Frise-ailerons Figure 1.10 Yaw 1. Left pedal 2. Rudder 3. Lift 4. Yaw Ad3 Variations
When an aileron moves downwards the drag in produces increases, while the drag of the up going aileron is reduced. Be-cause of the difference in drag between the left and right wing the airplane starts to roll in the opposite direction of the in-tended turn. This side effect is called adverse yaw. There are several options for an airplane designer to choose from to counter this effect, the usage of differential ailerons, the usage of frise-type ailerons or a system where the rudder and aile-rons are combined.
Differential ailerons are designed to minimize adverse yaw by increasing the drag on the up going aileron (which is the down going wing). This is achieved by deflecting the up going aileron through a greater angle than the down going aileron. Be-cause of this the difference in drag between the left en right wing is decreased which eliminates most of the adverse yaw. With this type of aileron not all adverse yaw is eliminated. To counter all the adverse yaw the rudder needs to be used.
Frise-type ailerons (Figure 1.9) are another solution to the problem of adverse yaw. Frise-type ailerons also increase drag on the up going aileron to make the difference in drag between the left and the right wing as small as possible. Because of the shape of frise-ailerons and the positioning of the hinge a part of the aileron protrudes into the airstream beneath the wing when the aileron moves upward, causing more drag.
On the other wing that part does not protrude in to the airstream causing no extra drag. Frise-type ailerons may also be designed to operate like differential ailerons, eliminating adverse yaw all together.
The rudder and ailerons also can be linked together so that when the pilot starts to bank the airplane using the ailerons, the rudder automatically compensates for the adverse yaw.
1.2.3 Rudder
The rudder is the control surface on the tail of the airplane that can make the airplane yaw around its normal axis. The rudder is controlled by two paddles found at the feet of the pilot in the cockpit. If for example the pilot pushes the right pedal forward, the left pedal moves the same distance in the opposite direction. The pedals are connected to the rudder by the same kind of system of pulleys and wires used for the elevator and ailerons. When the pilot pushes the right paddle forward, the rudder moves to the right thus yawing the airplane to the right. A side effect occurs when using the rudder which is the airplane starting to bank in the direction of the yawing motion the airplane is doing.
1. Purpose
2. System
Ad1 Purpose
The rudder (Figure 1.10) is mounted on the rear end of the airplane. Its purpose is to make the airplane yaw, which means rotating around its normal axis. This works by for example pushing the left pedal (1), which moves the rudder to the left (2). This alters the rudder airfoil from a symmetrical shaped airfoil to a positively cambered one, creating sideways lift (3) so the airplane starts to yaw (4).
The effectiveness of the rudder increases with the airplane’s speed. When flying slowly, a large rudder deflection is needed to get the same effect as a small rudder deflection at high speeds. This is because the amount if lift generated (in this case sideways) also depends on the speed of the air flowing around an airfoil. In propeller driven airplane where the propeller is mounted in front of the nose (so also in front of the rudder), the slipstream from the propeller flowing over the rudder also increases its effectiveness.
__________________________________________________________________________________ Figure 1… Trim control on a Cessna 172
1. Trim wheel
2. Cable for upward motion
3. Cable for downward motion
4. Pulley
5. Trim tab
Figure 1.12 Elevator trim control
1. Elevator
2. Trim/balance tab
Figure 1.13 Balance tab
1. Pedal
2. Cable
3. Pulley
4. Rudder
Figure 1.11 Ruddercontrols
Ad2 System
The system (Figure 1.11) to link the pedals in the cockpit to the rudder on the back of the airplane is similar to the system used to control the elevator and ailerons. When a pedal (1) is pushed forward, it pulls on the cable (2) it is attached to. There is one cable fitted to each of the pedals. Those cables are guided by a series of pulleys (3) and move in an opposite direction of each other. The cable attached to the left pedal is attached to the left side of the rudder (4) and the cable on the right pedal to the right side of the rudder. Because of this the rudder is pulled on one side and pushed on the other, thus moving it.
When an airplane is yawing to the left for example, the right wing is traveling faster than the left wing. Because the amount of lift a wing generates is also dependent on air speed, the right wing starts generating more lift than the left wing, thus banking the airplane to the left. The only way to counter this is to use the ailerons to counter the rolling motion.
1.2.4 Trim
Trim tabs are devices on the trailing edge of the control surfaces of an airplane. Trim is used to lighten the controls for the pilot, by holding the control surfaces in a position which normally required an input force by the pilot. Most airplanes come with a trimmable elevator and a trimmable rudder, larger passenger planes also come with aileron trim.
1. Purpose
2. System
3. Variations
Ad1 Purpose
Trim is used to lighten the controls for the pilot. By moving the trim on the elevator the pilot can hold the elevator and thus the airplane in level flight, controlled climb or controlled descend without the need of an input force. When installed on the rudder, trim is used to hold a steady course when changing the power setting on a single engine airplane or in the event of crosswind. The aileron trim is used to hold the airplane straight along its longitudinal axis when for example the airplane is unbalanced due to a difference in weight between the left and the right side of the airplane.
Ad2 System
The movement of the elevator trim tab is controlled by a system of cables and pulleys (Figure 1.12).
When the pilot moves the trim wheel
(1) the movement is transported via two cables (2 & 3). There is one cable for upward motion (2) and one for downward motion (3). These cables run through the airframe and horizontal stabilizer on pulleys (4) and are con-nected to the trim tab (5).
Ad3 Variations
The trimming of an airplane is done with the help of various devices.
• The most common way to lighten the control, and trim the airplane is with the use of tabs (Figure 1.13). In the case of a balance tab the elevator or rudder (1)
as well as the balance tab (2) are connected to the controls, they work in opposite direction of each other. When used as a servo tab, only the balance tab is connected to the controls. In this case the tab is driven by a servo. Controlling the elevator or rudder with the help of tabs reduces the input force needed, and holds the elevator in the desired position without the need of a constant input force. The controls are much lighter because the tab gives the elevator more or less lift, pulling the control surface up or down.
1. Pivot point
2. Weight
3. Servo tab
Figure 1.15 Weight balanced stabilizer
Figure 1.14 Inset hinge
1. Hinge
2. Aerodynamic force
• Another way to lightening the controls is with the use of an inset hinge (Figure 1.14) or horn balance. Whit an inset hinge (1), the elevator or rudder is pushed in the desired direction by the
air dynamic force (2). The force is created by the air flowing around the profile, which pushes on the back of the control surface. In the case of a horn balance there is only a small part with an overlay on the edge of the elevator or the top of a rudder.
• A third way of lightening the controls is with weight balancing, this method is often used on airplanes with a stabilizer (Figure 1.15). This design uses the whole horizontal stabilizer as an elevator. The stabi-lizer rotates along an axis (1), the weight (2) keeps the stabilizer in balance. In this example there is al-so a tab installed (3).
1.3
Secondary flight controls
Secondary flight controls are flight controls that are not essential for flying an airplane. The main function of secondary flight controls is to improve control of the airplane. The aerodynamic forces generated by the wings can be changed by altering the shape of the wings, which is done by flaps (1.3.1), slats (1.3.2) and spoilers (1.3.3).
1.3.1 Flaps
Flaps are used during take-off and landing. They increase lift so the airplane can fly at a slower speed. There are various types of flaps that are used on airplanes. Each type of flap has its own Cl-α and Cl-Cd diagram (Appendix VII). On take-off
and landing, an airplane uses different flap settings.
1. Purpose
2. Types of flaps
3. Operation
Ad1 Purpose
The purpose of flaps is to increase lift at a lower airspeed. By extending the flaps, which are located at the trailing edge of the wings, the camber increases so the wing produces more lift at the same speed or the same amount of lift at a lower speed. Besides increasing lift, extended flaps also increase drag. In the first stages of extension the lift increases more than the drag, while in the last stages of extending the drag increases more than the lift. Depending on the type of flap used the lift/drag ratio varies.
Ad2 Types of flaps
There are four common flap types.
• The plain flap (Figure 1.16a) is the simplest type of flap. The flap (1) rotates around a hinge which is located partly inside the wing (2). This increases the wing camber and results in a increase in both lift and drag. The surface of the wing decreases when the flap is extended.
• The split flap (Figure 1.16b) (1) deflects from the lower surface of the wing (2). Like the plain flap this type increas-es the lift but producincreas-es even more drag. When the flap is extended the camber increasincreas-es but the surface of the wing stays equal.
• The slotted flap (Figure 1.16c) is almost the same as the plain flap. The difference is that the flap (1) does not only rotate but also moves backwards. This is an advantage compared to the plain flap because when extended, the slot (2) between the flap and the airfoil makes it possible for high energy air from beneath the wing (3) to flow to the upper surface. There it accelerates the boundary layer which results in a delay in air separation and thus increases lift. Large airplanes use double or even triple slotted flaps (Appendix VIII).
__________________________________________________________________________________ 1. Slats up
2. Slats extended
3. Slats fully extended
Figure 1.17 Slats
• The Fowler flap (Figure 1.16d) (1) slides backwards on tracks from underneath the wing (2). This increases both camber and wing area. In the first stages of extension the lift increases more than the drag. Like the slotted flap the Fowler flap has a slot (3) which delays airflow separation.
Ad3 Operation
In the cockpit the pilot operates the flaps by selecting the degree of flap with a lever or switch. In smaller airplanes the flaps can also be operated manually by a handle which is connected by means of cables to the flaps.
Before take-off, the flaps are lowered to the required take-off position. By doing this the wings produce more lift so the air-plane takes-off at a lower speed and therefore uses less runway. After take-off the flaps can only be retracted when the airplane has gained enough speed. When the flaps are retracted at a too low speed, the airplane may stall.
On approach the flaps are lowered to generate more lift so the airplane can have a lower approach speed. The flaps can only be lowered if the airplane is flying below the maximum flap extension speed (VFE), otherwise the flaps are damaged.
When the flaps are lowered a side effect called ballooning occurs, unless the pilot lowers the pitch attitude. If an airplane is ballooning, the extension of flaps causes a short period of much more increased lift then drag. The result is an unpleasant short climb. Shortly after the climb the drag increases which slows the airplane down.
With flaps extended, the angle of attack which causes the airplane to stall is lower than when the flaps are retracted.
1.3.2 Leading edge devices
An airplane has several devices on the leading edge of the wing. The purpose of this devices is to prevent the airplane from stalling when flying slowly. The slats come out on the front of the wings and increase the camber, nose radius and some-times the surface of the wing. There are also “flaps” on the leading edge. These are the Krueger flaps and they also increase the camber, nose radius and surface of the wing. There are also nose flaps and fixed slots to increase the angle of attack. Leading edge devices cannot be used without the trailing edge devices because then the airplane’s tail would touch the ground during take off and landing and the pilot would only be able to see the sky and not the airstrip. The Cl-α graph shows
the angle of attack (Appendix IX).
1. Slats
2. Krueger flaps
3. Nose flaps
4. Fixed slots
Ad1 Slats
A Cessna is a small airplane and does not have slats. Every big airplane like a Boeing or Airbus is equipped with slats. These slats increase lift and camber, nose radius and the surface of the wing. Slats are controlled by the pilot or by the auto slat computer. The slats are not used during cruise flight, only during take-off and landing. With slats extended an airplane can fly slower without stalling or it can fly with the same speed but with an bigger angle-of-attack. The slats have three settings
(Figure 1.17): up (1), extended (2) and fully extended (3).
The slats are operated by a hydraulic system. Retracting and extending is done using hydraulic actuators. When the airplane is near a stall, auto slat computers or the pilot gives a signal, which activates the servo valves. When this happens, pressure is applied to the main pumps. This results in extending of the slats and prevents or delays a stall.
1. Flap
2. Wing
1. Flap
2. Wing
Figure 1.16a The plain flap Figure 1.16b The split flap
1. Flap 2. Wing 3. Slot 1. Flap 2. Wing 3. Slot
Figure 1.16c The slotted flap Figure 1.16d The Fowler flap
1
2
Figure 1.20 Spoilers
Figure 1.20 Fixed slots
Figure 1.19 Nose flap Figure 1.18 Krueger flap
Ad2 Krueger flaps
The Krueger flaps drop down from the underside of the wing at the leading edge (Figure 1.18). They in-crease the nose radius of the wing and the camber line.
Ad3 Nose flap
The nose flap (Figure 1.19) lowers the leading edge so it increases the camber of airfoil by increasing the mean camber line.
Ad4 Fixed slots
The fixed slots are not adjustable (Figure 1.20). These are made to increase the angle of attack. But with the fixed slots the surface is not
extended but air flows through the gap so it provides more energy to the bound-ary layer to delay a stall.
1.3.3 Spoilers
Commercial airplanes have two kinds of spoilers: flight spoilers which are used during flight and ground spoilers which are only used on ground or during landings. The ground spoilers also have an important function; this is to counter the ground effect.
1. Flight spoilers
2. Ground spoilers
3. Ground effect
Ad1 Flight spoilers
Flight spoilers (Figure 1.20) are located on the upper wing surface of the airplane (1). They enhance the working of ailerons and can also be used as speed brakes. If the spoilers are made to improve the working of the ailerons, they must work in the opposite way of the ailerons. For example, if the control wheel is moved right, the left aileron deflects downwards and the right aileron deflects upwards. To increase this effect, the right flight spoilers deflects when aileron deflection exceeds 10°. As a result of the deflection, the right wing looses lift and gains drag. The right wing falls down and the airplane rolls to the right. Flight spoilers can also be used as speed brakes, to reduce speed or to increase the descent angle while maintaining the same speed.
Ad2 Ground spoilers
Ground spoilers are also located on the upper wing surface, next to the flight spoilers. Ground spoilers are used as speed brakes when the airplane is on the ground during its roll-out after landing. Just before the airplane makes contact with the ground, the ground spoilers deflect. This removes the lifting properties of the wing so all the weight of the airplane is on the wheels, increasing the wheel brake effectiveness.
Ad3 The ground effect
When the airplane lands there is a lot of air flowing around it. This air acts like a cushion between the ground and the wings. This prevents the airplane from touching the ground. To make the airplane touch the ground the spoilers are deflected. This enables the air between the airplane and the ground to escape through the wings, thus eliminating ground effect.
1.4
Regulations & Demands
Before designing a multifunctional fly-by-wire system, the system is tested to meet specific regulations (1.4.1). Also the constituent has made some demands (1.4.2) to make the design a success.
1
__________________________________________________________________________________ 1.4.1 Regulations
The modification of the flight controls needs to comply with numerous regulations. These regulations can be found in the documents of the European Aviation Safety Agency (EASA), the parts which are relevant to the modification are found in the CS 25 (Appendix X). The regulations of the CS 25 suit the certification specifications for large airplanes on which every airplane must comply. All paragraphs mentioned in the CS25 can be divided in several factors like forces (1.4.1a) or cockpit controls (1.4.1b) but also safety (1.4.1c). The regulations mentioned by the Federal Aviation Authority (FAA) are almost full integrated in the regulations made by EASA.
1.4.1.a Forces
As described in the CS25.397 the flight controls need to comply with several (feedback) forces (table 1). Also the forces which come up when designing flight controls are calculated, CS25.395 states that the forces are calculated at 25% above the maximum forces. In the CS25.457 is described how wing flaps, their operating mechanisms and their supporting struc-tures are designed for critical loads. When designing the flight control system, all flaps and those operating mechanisms are checked and recalculated for the existing and possible new installed components. All the cables, pulleys, turnbuckles or fairleads which may have given greater forces when the system is redesigned, have to comply with the regulations in CS25.689. The used cables must also match these regulations. When the flaps or slats are operated, they are internected to each other as written in CS25.701. This compensates unsymmetrical loads or prevents damage to the flight con-trols. These regulations comply when programming software for the fly-by-wire flight contol system.
Primary Flight Controls Control: Maximum Pilot Forces: Minimum Pilot Forces: Aileron - Stick - Wheel* 445 N 356 DNm 178 N 178 DNm Elevator - Stick - Wheel 1112 N 1335 N 445 N 445 N Rudder 1335 N 578 N
Secondary Flight Controls
Control: Limit Pilot Forces:
Miscellaneous: Crank, wheel, or lever.
Not less than 222 N nor more than 667 N. Dependent of kind of control.
Twist 15 N
Push-Pull To be chosen by applicant.
Table 1
*D = wheel diameter in m
1.4.1.b Cockpit controls
During the design, the regulations in CS25.399 must be respected in order to meet the safety requirements of EASA. The cockpit is designed with the right controls and correct labels. The flap handle for example should be labeled as “flaps” and the motions are labeled as “up” for flaps up and “down” for flaps down. In CS25.779 the motions and effects of the flight controls are mentioned. The primary and secondary flight controls respect the regulations in order to meet the safety re-quirements. All the motions of the designed flight controls match the effects described (table 2). When the controls in the cockpit are integrated, it is considered that the controls must comply with the regulations in CS25.781; the controls must be placed in the right order and labeled correctly. The knobs must have the described dimensions and should have enough room to move.
Primary Flight
Controls: Motion and effect:
Aileron Right (clockwise) for right wing down.
Elevator Rearward for nose up.
Rudder Right pedal forward for nose right.
Trim tabs Rotate to produce similar rotation of the airplane about an axis parallel to the axis of the control.
Secondary Flight Controls
Controls: Motion and effect:
Flaps Forward for wing-flaps up; rearward for flaps down.
Table 2 1.4.1c Safety
All the regulations mentioned in the CS25 are made for safety reasons. CS25.671 describeswhich safety requirements apply when the flight controls jam. Even when the engines fail, the airplane must still be controllable. When designing the flight control system, it is considered necessary to install a back-up system. CS25.865 also mentions that essential flight controls and other flight structures, located in designated fire zones or in adjacent areas, which would be subjected to the effects of fire in the fire zone, are constructed of fireproof material or shielded, so they are capable of resisting the effects of fire.
1.4.2 Demands
The ALA staff would like to know if the 737NG fly-by-wire modification has enough benefits and is financial reliable. All de-mands required, flexible or desirable, are assessed (table 3).
ALA Demands R eq u ire d F le xib le D es ira b le
The newly designed system needs a better in-flight comfort than the old system. X
The newly designed system must fly more fuel-efficient than a hydro-mechanical flight control sys-tem.
X
Reduction of CO2 emission. X
The design is at least as safe as the regular Boeing 737NG or Airbus A320 X
Maintenance costs are lower than the maintenance costs of a hydro mechanical system. X
The break-even point is reachable before the aircraft is taken out of service. X
The newly designed system should be more reliable and far more durable than the hydro mechani-cal system.
X
The fly-by-wire systems are as uniform as possible, to insure lower maintenance costs. X
The implementation costs of the fly-by-wire system are as low as possible. X
__________________________________________________________________________________
1.5
Function analyses
The flight control system consists of a range of steps which are carried out to let the system function. The main function is controlling the movement of the airplane during flight. This main function is divided in five steps, the so called sub functions. These sub functions are all different, each step has a different function, and together they for fill the task of the main function. The different functions are placed in a function-block-diagram (Appendix XI). The sub functions are the following
1. Input 2. Convert 1 3. Transport 1 4. Process 5. Transport 2 6. Convert 2 7. Output 8. Feedback Ad1 Input
When the pilot wants to change direction by using the pedals, yoke and/or control wheel, he creates an input.
Ad2 Convert 1
The input is converted to a binary signal so it can be transported through electrical wires. A binary signal cannot move the flight controls, therefore it is converted back to a motion.
The input can also be a motion and is converted to another motion to amplify the signal or to make it more easy to transport.
Ad3 Transport 1
The binary signal is transported through electrical wires.
The Motion signal is transported threw a series of cables and pulleys.
Ad4 Process
In the case of a Binary signal, a computer corrects the input signals, maximizing performance or preventing dangerous ma-noeuvres.
The computer in the 737 process the information of all the instruments on the airplane when the pilot exceeds the limits an alarm go’s off.
Ad5 Transport 2
The transport method is for both systems mechanical.
Ad6 Convert 2
A binary signal can’t move the flight controls therefore it is converted back to a motion. The motion is converted to an other motion that amplify the signal
Ad7 Output
The movement of the flight control leads to a change/movement in one of the three axes of the airplane.
Ad8 Feedback
The fly-by-wire system has no physical feedback. There for sensors are placed on the flight control to tell the computer that it’s moved.
2
Flight Control Analysis
Before ALA can be advised in changing their fleet 737 planes flight control systems, it is important to know how the conven-tional hydro mechanical systems works (2.1).
To change the flight control system into a newer and possibly more efficient fly-by-wire system the workings of this fly-by-wire system and its computers is critical (2.2).
When changing the flight control system it is very important to know the benefits and possibly drawbacks of the system change (2.3).
The final step to the modification is the comparison in which the advantages and disadvantages are discussed (2.4). The system used to control the elevator is used as a reference. The main sources used to make this chapter are the ma-nuals of the Boeing 737-NG and the Airbus A320 as well as the website smartcockpit.com.
2.1
Hydro mechanical system
The hydro mechanic system as found in the Boeing 737NG can be divided in several subsystems. This system splits up in a input (2.1.1) from the pilot or the auto pilot. These signals are transported (2.1.2), this is done mechanical, electrical or hy-draulically. These signals are not strong enough for the output so they are amplified. (2.1.3) This motion eventually becomes an output (2.1.4). The system is powered by hydraulics and electricity (2.1.5) and it has certain limitations (2.1.6).
2.1.1 Input
Every system has an input and output. To explain the hydro mechanical system of the flight controls in a Boeing 737NG, we use the elevator as a reference. All other flight controls work the same way. In the Boeing 737 the flight control system has 2 inputs: the pilot (2.1.1a) and the autopilot (2.1.1b).
2.1.1a Steering column
The pilot and co-pilot have their own control column (Figure 2.1). This column (1) can move forwards and backwards. On the same column the yoke (2) is situated. The steering column is connected with the other column, so when the pilot moves it, the column of the copilot moves in the same direction. This movement makes the pulley (3) move. This rotation of the pulley in turn moves the steel cable (4). That transfers the input to the control surfaces to the back of the aircraft.
2.1.1b Autopilot
The autopilot is the second input for the elevators. Unlike the steering column this input device is analog electrical. The auto-pilot creates an input value for the autoauto-pilot actuators. The autoauto-pilot compares the data from the pitot tubes and gyroscopes with the inputted values in the flight control unit or flight management computer. The autopilot calculates how much the ele-vator, rudder and ailerons must move to hold the aircraft in the desired direction.
2.1.2 Transport
After the input from the pilot or autopilot this motion or value is transferred for a correction or amplification. In a Boeing 737NG there are three ways to transfer a signal: electrical (2.1.2a), mechanical (2.1.2.b) and hydraulic (2.1.2.c).
1. Steering column
2. Yoke
3. Pulley
4. Steel cable
__________________________________________________________________________________ Figure 2.2 Rotating pump
2.1.2a Electrical
For the electrical part of the system electrical wires are used to transport the signal from A to B. This signal is binary or ana-log. In the Boeing 737NG there are only analog electrical connections.
That means that the signals are not encoded with a data bus. With this method only one signal can be sent at a time through a cable. An analog signal is a variation in voltage of the signal. A binary signal has got two options; on or off. This means one or zero. The combination of zero’s and one’s gives the computer information.
2.1.2.b Mechanical
Mechanical transport needs some basic parts. These are cables, pulleys, springs and other mechanical connections. Cables are used to transfer a force between two parts of the mechanical system. Pulleys are used to change the direction of the force, and guide the control cables trough the airplane. They can also be used to amplify the force. The mechanical part of the system is spring loaded to bring the controls and control column in a neutral position when no force is applied.
2.1.2.c Hydraulic
For transport and controlling of forces, a hydraulic system is used. In the Boeing 737NG there are three separate systems
(Appendix XII) for safety reasons. There are two different parts:
1. Rotating pumps
2. Actuator
Ad 1 Rotating pumps
Rotating pumps (Figure 2.2) use sprockets. These pumps create hydraulic pressure with the help of a mechanical motion. Both sprockets (1) rotate in a different direction. The rotation of the wheels transports the liquid from the left (2)
to the right side (3) of the pump.
Ad 2 Actuator
An actuator is filled with hydraulic fluid. When there is more pressure on one side of the actuator, the volume of that part increas-es, which makes the actuator rod move. When every side is connected with a sepa-rate hydraulic system there is a backup system to make sure the actuator is still operational. There are actuators that move hydraulically one way and are pushed back mechanically the other way. Another type
(Figure 2.3) is moved in both directions by hydraulic pressure.
In this type there are two hydraulic systems; system A (1) and system B (2). When the pressure and volume of system A increases (3) and gets greater than system B (4), the actuator rod (5) moves to the right and give a mechanical movement to, for instance, an elevator (6).
2.1.3 Convert
In the Boeing 737NG the amplification of the elevator controls is done by means of the mechanism close to the elevator itself. The control columns are used to control the elevator; the movement of those columns is transported and amplified. This amplification process is done by several hydro mechanical components such as a power control unit (PCU) (2.1.1.a)
and a mach trim actuator for trimming the horizontal stabilizer (2.1.1.b). 2.1.3.a Power Control Unit
The PCU in the Boeing 737NG is used for several flight controls such as the rudder and the elevator.
A PCU (Figure 2.4) is based on the principle of leverage. Due to the use of hydraulics, the pilot has to apply little force to move the elevator.
1. Input rods 2. Piston 3. Actuator rod 4. Elevator 1. System A 2. System B
3. Increased pressure side A
4. Less pressure than side A
5. Actuator rod 6. Elevator Figure 2.3 Actuator 1. Sprockets 2. Incoming fluid 3. Outgoing pump
The pilot’s input is transferred by means of the input rods (1) to the mechanical system that is used for the elevator. The hydraulic unit has two pistons (2) on an actuator rod (3) that moves the elevator (4). Powered by both of the hydraulic sys-tems, the elevator can be used at full strength at all time.
Also the advantage of using both hydraulic systems is when system A fails, system B can take over. That way the pilot never has to use excessive force to control the elevator on the aircraft and the elevator can never jam in a certain position.
2.1.3.b Trim actuator
In the Boeing 737NG the horizontal stabilizer can also be used as a trim surface. Because of this, there is no need for a trim surface on the ele-vator. The trim actuator of the hori-zontal stabilizer is used to correct the movement or unbalance in the aircraft. It is also used to amplify the working of the elevator; however it’s not common on a regular flight. When trimming the horizontal
stabi-lizer, the pilots can use the control knob positioned on the column wheel (Figure 2.5). When the knob (1) is pressed, the servo (2) of the trim actuator is activated and it adjusts the angle of attack of the horizontal stabilizer (3).
2.1.4 Output
The Boeing 737NG has two mechanical units to control the elevator, one mechanism and elevator on each side of the rud-der, also there is one mechanical system located in the tail of the aircraft that controls the movement of the horizontal stabi-lizer.
2.1.5 Power supply
Electric power is very important. Some components of the system work electrically, so it is important to have electrical power at all times. Therefore there are multiple energy sources available.
Electric power is normally supplied by one of the two main generators driven by the engines. When the airplane is on the ground an external power source or the generator driven by the auxiliary power unit (APU) is used. A Boeing 737NG has got an APU in the back of the tail. This APU is a small gas turbine which provides the aircraft with electricity and hydraulic pres-sure. This gas turbine is installed in a fireproof and sound-reducing shroud. To start this turbine, electricity from the aircraft battery and fuel from the primary tank is used. This APU is used to start the engines of the airplane. The APU provides the engines of compressed air. This compressed air and fuel starts the engine.
Each of the three generators can power the whole electric system of the aircraft when needed. But normally either the APU, or two engines drive the system. All three generators provide the same output and are individually connected to a generator control unit (GCU), which provide a stable voltage and frequency.
When all three generators are out of order power from the main batteries are used to supply the aircrafts main systems, or the ram air turbine is used to power the most critical systems on board.
The generators in the Boeing provide alternating current (AC), but important systems like the computers need direct current (DC) to operate. Therefore the Boeing is equipped with transformers, which convert AC the DC. In a situation where no generators are available there are static inverters, which convert DC to AC.
2.1.6 Control laws and limitations.
The Boeing 737 can be fully operated manually; there are almost no limitations which the flight control computers (FCC) can set. The pilot is always able to set all functions of the flight controls, even in normal conditions. The FCC gathers all informa-tion from the sensors, which monitor the posiinforma-tion of all flight controls and other flight informainforma-tion. The FCC checks the posi-tion and circumstances of the airplane for possible faults or miscalculaposi-tions from the pilot. Then the FCC gives the pilot in-formation about the conditions of the flight controls and how the airplane performs. The FCC can also check the hydraulic and electrical systems involving with the flight controls.
1. Trim control knob
2. Servo motor
3. Horizontal stabilizer
__________________________________________________________________________________
When for example there is a leak in the hydraulics, the low pressure lights are lit to inform the pilot (Figure 2.6). The pilot then can reroute the hydraulic current to still maintain control over the used flight controls.
There are multiple checklists available in the Boeing 737 Quick Reference Ma-nual (QRM). The most common used checklist when a abnormal situation does occur is the Non-Normal Checklist (NNC). The NNC explains which functions come available in specific mentioned conditions. All of the pilot’s controls do have multiple functions, for example if the flight spoilers become jammed, the control wheel of the First Officer now can control the ailerons. This way the pilot and the first officer maintain full control over the flight controls at any time.
Also there are some limitations of the flight controls, these are found in the Air-craft Flight Manual. For example some in flight limitations;
- Max flap extension altitude is 20,000 ft.
- Holding in icing conditions with flaps extended is prohibited.
- Do not deploy speed brakes in flight at radio altitudes less than 1000 ft.
2.2
Fly-by-wire
The fly-by-wire system, as found in the Airbus A320, starts with input from the side stick or the autopilot (2.2.1). The output signal from the side stick is converted using an A/D converter (2.2.2). The binary signal is transported to the flight computers where the input values are corrected (2.2.3). After the conversion step, the signal is converted to a hydraulic signal and transported (2.2.4). The movement of the hydraulic fluid is converted to a movement (2.2.5) which is done with the help of actuators. The last step is the output (2.2.6), in this case of the elevator. The system is powered by hydraulics and electricity
(2.2.7), and it works with different control laws (2.2.8) in various situations
2.2.1 Input
The input in the airbus is done through one of the two side stick’s (2.2.1a), or by the auto-pilot (2.2.1b). The side stick con-verts the movement from the pilot to an analog electrical signal using a transducer, which is converted to a binary signal. The autopilot directly puts out a digital signal.
2.2.1a Side stick
The side stick (Figure 2.7) is a method to convert the move-ment of the pilot into a usable analog electrical signal for the computers used in the fly-by-wire system. The side stick replaces the function of the steering column and control wheel on a hydro mechanical system.
Unlike the hydro mechanical system, the side sticks are not linked to each other. Therefore the priority button is used to select the side stick used to control the aircraft. When both side sticks are used, the sum of the two inputs goes to the computer. To prevent this there are several warning sys-tems to signal the pilots. The stick (1) is connected to an
universal joint (2) which separates the movements in forward (3) and sideward motion (4). The axes are connected to rods
(5) which are connected to the transducer unit (6). This unit converts the movements from the rods to an electrical signal using pot meters.
2.2.1b Autopilot
The second input method is the autopilot. Data is put in directly by the pilot via the flight control unit (FCU) or indirectly via the flight management computer (FMC). Since the output of the autopilot is already a binary signal, it is directly linked to the computers without the need of a converter.
The FCU is used to select an altitude, heading or course and airspeed of the airplane. When for example the pilot wants the airplane to fly 280 knots at an altitude of 30.000 ft. with a heading of 210°, he simply puts in the values after which the air-plane controls itself.
1. Stick 2. Universal joint 3. Forward motion 4. Sideward motion 5. Connection rods 6. Transducer units
Figure 2.7 Side stick unit
The FMC is a more sophisticated version of the FCU. A big advantage of the FMC over the FCU is that the route can be programmed after which the airplane follows this route.
For safety there are three autopilots available in the A320, which are selected by the buttons on the FCU (Appendix XIII).
2.2.2 Convert
Since the signals from the side stick’s transducer are analog electrical, they are converted to binary signals. This conversion is done by an A/D converter.
The A/D converter converts the analog input signal into a binary output code which is used by the computers in the airplane. The A/D converter splits the input signal in little steps, the accuracy depends on the number of bits used for this process. For example a four bits converter splits the signal in 24 =16 steps, and an eight bit converter does the same only with 28=256
steps.
2.2.3 Processing
The Airbus A320 has a very advanced computer system on board, used to control the airplane. This system makes the airplane different than planes like the Boeing 737. The airplane has seven primary flight control computers installed:
• Two elevator aileron computers (ELAC)
These computers controls the elevator and ailerons.
• Three spoiler elevator computer (SEC)
These computers controls the spoilers, the trim system and it is the backup computer for the elevator.
• Two flight augmentation computer (FAC) These computers control the rudder.
Only the ELAC’s are discussed. The computers contain several chips and processors (2.2.2a). These processors work to-gether and are part of the elevator system (2.2.2b).
2.2.3a Computer content
The ELAC’s contain three processors, the main processor, the ARINC processor and the co-processor.
The main processor receives the commands from the pilot and deals with the servo controls. The ARINC processor is dedi-cated to deal with the ARINC 429 data busses, which is necessary for the transport. Last, there is the co-processor. This processor deals with the flight control laws (2.2.8). The software for these computers is placed on ‘On-Board Replaceable Modules’ (OBRM’s). These modules are slotted in the back of the computer and can easily be pulled out (like a USB-Stick) so the software can easily be updated.
2.2.3b Electrical flight controls
The computers are part of a bigger system (Figure 2.8). If the pilot wants to go up for example, he pulls on the side stick (1). The signal is sent to the computers main processor (2). Then the signal is sent to the co-processor. The co processor checks the signal. If the system is within the limit of the flight envelope the signal is sent back to the main processor. If it is not within limits, the signal is corrected. After the correction the signal is sent back to the main processor. Finally the signal is sent to the ARINC processor, which converts the signal to be sent using the ARINC 429 protocol (2.2.4a), to the servo (3). The servo deflects the elevator upwards, which makes the airplane pitch up. The sensors read the new angle of attack and they send a feedback signal (4) back to the computers so the computers know the position of the elevator.
2.2.4 Transport
There are two types of transport in the flight control system of the Airbus 320. The transport of digital electric signals (2.2.4a),
and hydraulic fluids (2.2.4b). Figure 2.8 Electrical flight controls
1. Side stick
2. Main processor
3. Servo
__________________________________________________________________________________ 2.2.4a Electric transport
Transport from the computers to the hydraulic system is done using electrical cables. To limit the amount of cables needed the signals are transported via the ARINC protocol. This means that the packages sent by the system are send via the same cable with pauses in between.
The ARINC data bus system consists out of senders and receivers. The packages are called data words, the data bus sends data words with an address so only the receiver with the right address receives the package. The ARINC 429 system used by airbus is a one way system, so for feedback to the computers there are still two lines needed where a newer ARINC system like the 629 only uses one cable for two directions.
2.2.4b Hydraulic transport
The hydraulic pressure is transported via aluminum or rubber tubes. They connect the actuators to the hydraulic reservoir. The tubes are made of special high strength material because the pressure in them is high; normally 3000 psi, or 2500 psi when powered by de ram air turbine.
2.2.5 Convert
The last conversion is from a movement of the hydraulic fluids into a movement of the actual control surface. This conversion is done using actuators, which is very similar to the hydro-mechanical system. The only big difference to the cylinders used in the Boeing 737 is the way the actuators are controlled. Since the airbus is a fly-by-wire airplane, the cylinders are driven by servo valves controlled by the computers.
The actuators in the airbus system are the only devices connecting the control system to the elevator. In case of a complete hydraulic failure the elevators are uncontrollable and pitching is done with the mechanical trim system.
2.2.6 Output
In the case of the elevator, the output is done by the elevators. The airbus has two elevators, both connected to two actua-tors. One actuator per side of the airplane is active while the other acts as a damper. In case of an emergency when one of the actuators has failed, the other one can take over while the broken one can still act as a damper.
The actuator of the Airbus is very similar to that of the Boeing, the only big difference is that the valve controlling the flow of hydraulic fluids is a servo valve.
2.2.7 Power
The flight control system, as described above needs power to operate. Power is available in two different variations, hydrau-lic (2.2.7a) and electric power (2.2.7b).
2.2.7a Hydraulic power
The hydraulic system in a fly-by-wire airplane like the A 320 is very similar to the hydraulic system used in the Boeing 737. There are three hydraulic systems and the control surfaces are driven by actuators.
In the airbus there are three hydraulic systems like used in the 737, a blue labeled system driven by engine one, an electric driven blue system, and a yellow system driven by engine two (Appendix XIV). In case of an emergency the yellow system can also be driven by engine one and vice versa or the blue system can driven by the ram air turbine.
2.2.7b Electric power
Electric power is very important for the working of the fly-by-wire system because the key components of the system are working electrically. To ensure electrical power at all times there are multiple energy sources available. The electrical system of the Airbus is build very similar to that used in the Boeing. There are three generators installed in the airplane, which al have the same output, so that the fly-by-wire system can work on any generator. The Airbus has two batteries for supplying power when the generators are turned off. When all systems fail, the ram air turbine can supply electrical power at speeds above 100 knots.
