A/C SYS 2017
Aircraft System 2017
Aircraft System 2017
Why is the efficiency of a real propeller much lower than the propulsive efficiency predicted by Froude’s theory?
- the distribution of thrust on the propeller disc is not uniform
- Most of the thrust is generated in the outer part of the blade (rotational velocity component is higher)
- The energy lost in these tip vortices corresponds to the mechanical work needed to overcome the induced drag of the blades.
- Other sources of energy loss are the parasite drag of the rotating blades, of the spinner and of the hub, and the angular momentum of the slipstream behind the propeller.
¨How do you calculate the velocity due to rotation of a propeller?
\(\pi*n*D\)
- n = propeller speed in revolutions per second
- D = propeller diameter in meters
What are the principal factors affecting propeller efficiency?
the advance ratio J and the blade angle \(\beta\)
What is feathering?
When the propeller pitch is set to a value that reduces drag to a minimum (in the event of an engine failure)
How are the volume surrounding the engine and momentum flow considerations of a jet engine defined?
What is HL?
the fuel lower heating value [J/kg]
If only 30% of the fuel thermal energy is converted into kinetic energy of the exhaust,
what happens to the rest?
- A small fraction is used to overcome internal friction in the engine and to drive accessories, such as the fuel pump and the oil pump.
- The rest is left in the exhaust gas thermal energy. Indeed, the exhaust gas temperature of this engine at maximum static thrust was higher than 1000C.
What is a ramjet?
A jet engine with no compressor and turbine
What are Booster pumps?
- sometimes called engine feed pumps
- are used to boost the fuel pressure in the engine feed lines. One of the reasons for this is to prevent aeration and cavitation at high altitudes
What is aeration?
the presence of air bubbles in the fuel lines that could cause an engine flame-out
What is cavitation?
- the combination of low pressure, relatively high fuel temperature and high engine demand causes the formation of vapour bubbles in the fuel
- drastic reduction in fuel flow to the engine
- can cause a flame-out
- For this reason, engine manufacturers usually impose the requirement that the fuel feed pressure must remain at least 5 psi above the true vapour pressure at all times.
Hydraulic systems can be subdivided in four stages. Name them.
generation, control, transfer and utilization
What are the main advantages of hydraulic systems?
- excellent power to weight ratio;
- high power transmission efficiency;
- flexibility in installation;
- resitance to overload conditions;
- high reliability;
- fast response;
- precise position and load control.
What is the formula for Pressure [Pa]? (Hydraulic Actuator)
\(P = {F \over A}\)
F= Kraft
A= Fläche des actuator
What is the formula for Power [W]? (Hydraulic Actuator)
\(P = {W \over t} = {F*d \over t} \)
F= Kraft
d= Distanz
How do you calculate the area of a circle?
\(A = {\pi *r^2}\)
What is the formula for the TSFC?
\(TSFC = {m_f \over F}\)
What is the formula for the specific thrust?
\(spec. thrust = {F \over m_0}\)
Explain the ideal Otto cycle.
The theoretical Otto cycle comprises an isentropic compression (process 1-2), a constant volume heat addition (process 2-3), an isentropic expansion (process 3-4) and a constant volume heat rejection (process 4-1).
What is the formula for the piston velocity?
2*s*n
In classical aircraft engineering, an aircraft is subdivided in which three groups?
airframe, engine and systems
What are the three system groups of an A/C?
Structure, vehicle system, avionic system
What is the function of the airframe?
- complex and integrated set of structural components
- supports the mass of systems and PAX
- carries loads and stresses
- also known as the “platform”
What is the function of the vehicle system?
- also known as general or utility system
- enables the A/C to fly safely
- provides an adequate environment for crew and PAX
What is the function of the avionic system?
provides information, control and communication functions that enable the aircraft to fulfil its operational role
What is the Flight control system?
- nowadays usually a Fly-by-Wire (FBW) system
- pilot control input is elaborated on the basis of air data, A/C attitude etc. to issue the proper control surface deflections that will effect the desired aircraft motion
What is the Engine control system?
in most recent aircraft a Full Authority Digital Engine Control (FADEC) system
What is the Air data system?
provides airspeed, altitude, air temperature, rate of climb or descent information
What is the Attitude system?
provides information on pitch, yaw and roll angles, rates and accelerations
What is the Electrical system?
provides electrical power for flight and engine control computers, air data system, attitude systems, hydraulic servo-valves and (on some A/C) electrically driven hydraulic pumps
What is the Hydraulic system?
provides motive power for the actuators that move the flight control surfaces, or the engine thrust reversers
What are essential functions?
The functions whose loss would reduce the capability of the A/C or its crew to cope with adverse operating and environmental conditions (i.e. loss of cabin pressurization or loss of the anti-icing system)
What are critical functions?
Functions which could cause the loss of the aircraft or fatal injuries to the occupants (i.e. a complete loss of engine power)
How does the severity of a failure condition relate to its probability?
The severity of a failure condition must be inversely proportional to ist probability
What must be considered to evaluate the severity of a failure condition?
- Effects on the airplane
- Effects on the crew members
- Effects on the occupants
Explain the Classification of Failure Condition and the Allowable Qualitative Probability.
What are the four probability terms and how are they defined?
1. Probable: occur one or more times during the entire operational life of each A/C
2. Remote: may occur several times when considering the total operational life of a number of airplanes of this type
3. Extremely Remote: may occur a few times when considering the total operational life of all airplanes of this type
4. Extremely Improbable: not anticipated to occur during the entire operational life of all airplanes of one type
What are the probability ranges for commuter and transport aircraft?
Probable: 10-5 to 10-3
Remote: 10-7 to 10-5
Extremely remote: 10-9 to 10-7
Extremely Improbable: lower than 10-9
Which main design concepts are applied in aviation?
- Safe-Life
- Fail-Safe
- Fault-Tolerant
Explain the concept "Safe-Life"
systems or components will survive a specific design life with no failures
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Flashcards
How do Oleopneumatic shock absorbers work?
- A torsion link keeps the alignment between cylinder and piston, and prevents it from dropping out of the cylinder.
- Airborne: the air (or nitrogen) pressure in the inner cylinder, and the weight of the wheel, tire and brakes, causes the shock absorber to fully extend, and the oil fills the piston cavity.
- Touchdown: the ground reaction pushes the piston inside the cylinder, and the oil is forced to flow into the snubber tube through the metered orifice, then into the inner cylinder through the flapper valve.
- The flow pressure losses caused by the metered orifice provide the compression damping effect. When max. compression is reached, the increased air pressure in the cylinder tries to extend the shock absorber by pushing the oil back into the piston, but the flapper valve closes and the oil is forced to flow back slowly through the tiny oil return holes.
- The extension damping caused by the pressure drop through these holes is generally higher than the compression damping. This prevents a rapid extension of the strut, which would make the aircraft bounce back into the air after a hard landing.
When the designers lay out the landing gear geometry (tricycle), what are the first requirements that they consider?
The ground pitch and roll angles that may occur in operation. At these angles there must be no contact between any part of the aircraft and the ground.
- Pitch angle: at least equal to the angle of attack at lift off plus a reserve (ca. 10° - 15°)
- Roll angle: at least 8 for large transport aircraft, and up to 15 for light aircraft
What is a slipstream?
the flow which passes through the actuator disc of a propeller.
How is propulsive power defined?
as the product of thrust and aircraft speed
Why is the efficiency of a real propeller much lower than the propulsive efficiency predicted by Froude’s theory?
- the distribution of thrust on the propeller disc is not uniform
- Most of the thrust is generated in the outer part of the blade (rotational velocity component is higher)
- The energy lost in these tip vortices corresponds to the mechanical work needed to overcome the induced drag of the blades.
- Other sources of energy loss are the parasite drag of the rotating blades, of the spinner and of the hub, and the angular momentum of the slipstream behind the propeller.
¨How do you calculate the velocity due to rotation of a propeller?
\(\pi*n*D\)
- n = propeller speed in revolutions per second
- D = propeller diameter in meters
What are the principal factors affecting propeller efficiency?
the advance ratio J and the blade angle \(\beta\)
What is feathering?
When the propeller pitch is set to a value that reduces drag to a minimum (in the event of an engine failure)
How are the volume surrounding the engine and momentum flow considerations of a jet engine defined?
What is HL?
the fuel lower heating value [J/kg]
If only 30% of the fuel thermal energy is converted into kinetic energy of the exhaust,
what happens to the rest?
- A small fraction is used to overcome internal friction in the engine and to drive accessories, such as the fuel pump and the oil pump.
- The rest is left in the exhaust gas thermal energy. Indeed, the exhaust gas temperature of this engine at maximum static thrust was higher than 1000C.
What is a ramjet?
A jet engine with no compressor and turbine
What are Booster pumps?
- sometimes called engine feed pumps
- are used to boost the fuel pressure in the engine feed lines. One of the reasons for this is to prevent aeration and cavitation at high altitudes
What is aeration?
the presence of air bubbles in the fuel lines that could cause an engine flame-out
What is cavitation?
- the combination of low pressure, relatively high fuel temperature and high engine demand causes the formation of vapour bubbles in the fuel
- drastic reduction in fuel flow to the engine
- can cause a flame-out
- For this reason, engine manufacturers usually impose the requirement that the fuel feed pressure must remain at least 5 psi above the true vapour pressure at all times.
Hydraulic systems can be subdivided in four stages. Name them.
generation, control, transfer and utilization
What are the main advantages of hydraulic systems?
- excellent power to weight ratio;
- high power transmission efficiency;
- flexibility in installation;
- resitance to overload conditions;
- high reliability;
- fast response;
- precise position and load control.
What is the formula for Pressure [Pa]? (Hydraulic Actuator)
\(P = {F \over A}\)
F= Kraft
A= Fläche des actuator
What is the formula for Power [W]? (Hydraulic Actuator)
\(P = {W \over t} = {F*d \over t} \)
F= Kraft
d= Distanz
How do you calculate the area of a circle?
\(A = {\pi *r^2}\)
What is the formula for the TSFC?
\(TSFC = {m_f \over F}\)
What is the formula for the specific thrust?
\(spec. thrust = {F \over m_0}\)
Explain the ideal Otto cycle.
The theoretical Otto cycle comprises an isentropic compression (process 1-2), a constant volume heat addition (process 2-3), an isentropic expansion (process 3-4) and a constant volume heat rejection (process 4-1).
What is the formula for the piston velocity?
2*s*n
