Chapter 13,2
Module 13 – Aircraft aerodynamics, structures and systems
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Module 13 – Aircraft aerodynamics, structures and systems
• The design concepts, used to project and to construct aircraft structures and components, can be classified in three categories:
1. • The fail safe concept 2. • The safe life concept
3. • The damage tolerance concept
Fail safe concept
• The fail safe criterion sets that the structure shall be able to have a certain residual strength, even if the failure of a component
happens.
• If an element is damaged other structural members must support the load of the failed component
Foundamentals of structural systems
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Module 13 – Aircraft aerodynamics, structures and systems
• The landing gear of an airplane is made of a series of components, the element that absorbs the energy during the tuch down is the shock absorber.
• The landing gear of airplanes and helicopters has the scope to
support the weight of the structure, when the aircraft isn’t in flight, but it is on ground.
• The choice about the typology of landing gear, which must be
installed on airplanes and helicopters, is function of the type and of the main employ of the machine.
• Regarding the landing gears with wheels, the most basic configuration is the tricycle landing gear:
• fore try-cycle (the most common) and rear try-cycle
Landing gears of airplanes and helicopters
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Module 13 – Aircraft aerodynamics, structures and systems
The landing gear of an airplane is made of a series of components:
• The shock absorber
• The brake
• The wheel
• The tire
• The torque link (is a typical element of landing gear with wheels)
• Some possible devices of extraction and retraction
Landing gears of airplanes and helicopters
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Module 13 – Aircraft aerodynamics, structures and systems
Landing gears of airplanes and helicopters
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Module 13 – Aircraft aerodynamics, structures and systems
Airworthiness
The airworthiness certifies the ability of an aircraft to fly. Airworthiness refers to the status of an aircraft, which is congruent with the approved standards, modified according to specifications approved by the
authority or which is in accordance with the mandatory maintenance and has no inadequate parts installed. All these conditions are
mandatory.
The certificate of airworthiness attests that, in a specific moment, the aircraft has been checked and declared able to fly by an assigned
subject. An aircraft with an expired certificate must not fly.
Fundamentals of structural systems: regulations
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Module 13 – Aircraft aerodynamics, structures and systems
• In the frame location system the main manufacturer’s reference system includes 3 principal coordinates:
1. Station line: reference point near the aircraft nose
2. Buttock line or butt line: reference point on the longitudinal axis 3. Water line: reference line near the lower part of the fuselage
Zonal and station identification systems
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Module 13 – Aircraft aerodynamics, structures and systems
• The bonding is the electrical connection of two or more conducting objects.
• The main aim of the bonding is that to eliminate the potential
differences among different points of the structure, making it at the same potential
• Moreover, all electrical systems, present on board of an aircraft, must be adequately ground connected, with the aims:
1. To protect aircraft and personnel from hazards of lighting discharge 2. To protect personnel from shock hazards
3. To prevent the development of potential radio interferences
Electrical bonding
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AUTOFLIGHT
Chapter 13,3
Module 13 – Aircraft aerodynamics, structures and systems
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Module 13 – Aircraft aerodynamics, structures and systems
• A fundamental component of the autopilot system is the Flight Director (FD), which is generally connected to the Flight Control Computer (FCC).
• The FCC has the function to examine the aircraft position and the aircraft orientation.
Flight Director and FCC
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Module 13 – Aircraft aerodynamics, structures and systems
• The Attitude Heading Reference System (AHRS) is made of a series of 3-axis sensors that provide information about the heading, the attitude and the yaw of the aircraft, measuring the attitude, the angular and linear movements
• A modern AHRS is a strap-down system that exploits solid state gyros and accelerometers. A strap down system is a system in which the sensors are in agreement with the aircraft axes without the
presence of hinges
• Other input data to the autopilot system can be inserted in a
manual way directly by the pilot, through the Control Display Unit (CDU).
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Module 13 – Aircraft aerodynamics, structures and systems
• The primary flight controls are operated through Power Control Unit (PCU), utilizing the hydraulic power to activate the electro-hydraulic actuators. It is possible to design these units so that they respond to signals of the AFCS.
• The PCU includes an external input link and an internal control valve (called servo-valve) which directs hydraulic pressure to drive the actuator. The mechanical input is sent to the PCU from the pilot control through rods and cables, and, in modern installation,
through the Fly-By-Wire (FBW) system electrical signals. The input link positions the control valve which directs pressure to the main piston to give the powered output.
PCU
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Module 13 – Aircraft aerodynamics, structures and systems
• A system is called fail-passive (or fail-soft), when it is able to
withstand a malfunctioning, without endangering passenger safety and without producing excessive deviations from the flight path.
• A system is called fail-active (or fail-operational), if its
malfunctioning doesn’t reduce the total functionality of the system.
In a fail-active system a failure can occur, but it leaves the entire system still working, without degrading its performances
.
AFCS classification
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Module 13 – Aircraft aerodynamics, structures and systems
Roll channel
• The roll channel, connected to ailerons of the aircraft, controls the movement around the longitudinal axis
Pitch channel
• The pitch channel, connected to elevator of the aircraft, controls the movement around the lateral axis, analyzing the commands that are generated by the FCC (Flight Control Computer) and that determines when and how the elevator will be moved
Yaw channel
• The yaw channel, connected to rudder of the aircraft, controls the movement around the vertical axis.
• The yaw channel, connected to the rudder, receives two signals that determine when and how much the rudder will be moved. Autopilot control in the yaw axis and is not required in many small aircraft.
Roll channel, pitch channel and yaw channel
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Module 13 – Aircraft aerodynamics, structures and systems
• The auto throttle system is an electronic circuit that controls the engine thrust within engine design parameters
• The auto throttle system is independent to the autopilot system, but it is typically coupled with it
• Basically, the auto throttle system mainly controls the RPM (Rate Per Minute), the fuel consumption of the engine, and the EPR (Engine Pressure Ratio).
Auto throttle systems
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Module 13 – Aircraft aerodynamics, structures and systems
• In the THRUST mode, the engine is maintained at a fixed power setting according to the different flight phases. For example, during the take-off, the auto throttle maintains a constant take-off power until take-off phase is finished
• In the SPEED mode the throttle is positioned in order to reach a set speed. This mode controls aircraft speed within safe operating
margins. For example, if the pilot selects a speed which is slower than stalling speed, or a speed faster than maximum speed, the auto throttle will maintain a speed closest to the set speed that is within the safe range of speeds
Thrust mode – speed mode
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Module 13 – Aircraft aerodynamics, structures and systems
• The modern autopilot systems are also able to manage fully the landing procedure, in completely automatic way. To do this, they utilize the signals of the Instrument Landing System (ILS).
• In the case in which the system isn’t able to couple the ILS signal, the pilot will see the warning “Autoland fault”.
• If the auto-land system notices some data inconsistencies, an
indicator will signal to pilot the writing “Approach only”, informing him about the impossibility to do the auto-land.
Automatic landing systems
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Module 13 – Aircraft aerodynamics, structures and systems
• The autopilot systems for the helicopters are different from those of the airplanes, because the commands, that permit the flight conduct, are different
• According to the number of commands, which the system is able to control and to manage, it can have:
Tri-channel systems, with 3 control channels (channel of lateral cyclic, channel of longitudinal cyclic and channel of the rudder bar)
Quadri-channel systems, with 4 control channels (channel of lateral cyclic, channel of longitudinal cyclic, channel of the rudder bar and collective pitch channel).
Helicopter autopilot
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Module 13 – Aircraft aerodynamics, structures and systems
• The Stability Augmentation Systems (SASs) can operate in coupling with the Flight Director system.
• The SASs increase the stability and the maneuverability of the helicopter in presence of wind and turbolence and maintain constant the helicopter attitude.
• The stabilization is obtained through some electro-mechanical actuators, positioned in series of the cyclic pitch and the control of the tail rotor
SAS
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