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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Lernkarten
Explain the pitching moment coefficient (cm0) of a cambered aerofoil
For a cambered aerofoil, cm0 is usually negative (which means that for positive lift the centre of pressure lies behind the aerodynamic centre)
What happens to the compressibility and the a.c. at Mach numbers above 0.5 - 0.6?
- compressibility effects become important
- a.c. starts to move backwards, slowly at first, then quite rapidly when zones of supersonic flow appear on the aerofoil (transonic regime). In supersonic flow, the position of the a.c. tends to 50% of the chord.
Explain "Ground loads"
- for taxiing, take off ground roll, landing
- through landing gear (reduce loads to a level that can be absorbed without damage)
What parts of the A/C do the ground load conditions influence?
- design of the landing gear
- structure of the A/C
- attachment points
List some of the main load conditions to be considered according to airworthiness regulations.
- level landing condition
- lateral drift landing condition
- one-gear landing condition
- side load condition
- braked roll conditions
- forward acting horizontal load (during towing)
Explain "ultimate load"
The maximum load that the test specimen can withstand before the material fails.
What is the common unit of the "aircraft lift curve slope" [CLα]?
The shear and bending loads are independent of speed.
What happens to a beam (fixed on both ends) when its heated/cooled?
It will expand (when heated) or shrink (when cooled).
- Expandation will create a compression load
- Shrinking will create a tension load
When can thermal induced loads become significant?
At supersonic speed.
What are the minimum load factor limitation of commuter and normal aircrafts?
Positive loads: 2.9 to 3.8
Negative loads: -1.2 to -1.6
How do you calculate the ultimate loads?
limit loads multiplied by a safety factor of 1.5
What must the aircraft structure withstand while testing it?
The aircraft structure can withstand the ultimate load for at least 3 seconds without failure (permanent deformations are allowed).
What do fast aircraft that are designed to fly at low altitude in heavy turbulence have, to reduce the load factor increments due to gust?
a relatively high wing loading and a short wing span
(-> the lighter the aircraft, the higher the load factors due to gusts)
What is Aeroelasticity?
the subject that describes the interaction of aerodynamic, inertia and elastic forces for a flexible structure and the phenomena that can result
What does the Collar’s scheme look like?
Torsional divergence: What happens to the angle of attack and the lift distribution for a straight wing?
- the angle of attack of the wing sections towards the tip will increase more than close to the root
- spanwise redistribution of lift: c.p. will move outward and the bending moment at the wing root will increase.
What is the formula for the total energy absorbed by the landing gear?
E = 0.5 * m * v2
What happens in the case of a combined bending and twisting oscillation when maximum twist occurs at zero bending and vice versa?
During the wing upward motion, at the position of zero bending the twisting of the wing causes a positive geometric incidence and therefore an aerodynamic lift force in the same direction as the wing motion. A similar but reversed situation occurs when the wing is moving downwards.
What is Vf?
Flutter Speed
What happens above the speed Vf?
the oscillation will diverge and destroy the wing structure
What is "Aileron Buzz"?
- a form of flutter associated with shock waves oscillating back and forth on the upper and lower wing surfaces and interacting with the boundary layer
- The sharp changes in pressure distribution cause a high frequency oscillation of the aileron
What ist "Buffeting"?
the most common form of buffeting is that of the tail, which can experience severe vibrations when it is invested by the turbulent wake of the stalled wing
What is "Dynamic response"?
an elastic aircraft in an accelerated flight condition may experience loads which are much greater than those predicted assuming a perfectly rigid structure. External loads, such as gust and landing loads, excite oscillations of the structure. The additional inertia forces associated with these oscillations can produce dynamic load overshoots.
Name the five components of the rear fuselage section.
- Pressure Dome
- Cabin Floor
- Frame
- Stringers
- Skin
During landing, the maximum vertical velocity at touchdown is defined by requirements and specifications, depending on the class of the aircraft. What is the max. vertical velocity for civil aircrafts?
3.05 m/s (10 ft/s) at maximum landing weight (MLW)
Why is the landing gear load factor important for the landing gear sizing?
the higher its value, the shorter the landing gear compression required to absorb the vertical kinetic energy at landing
What are the advantages and disadvantages of the old taildragger arrangement landing gears?
- + lower weight (nose gear is much heavier)
- + less drag (if the gear is not retractable)
- + more space available in the aircraft nose (particularly if the gear is retractable)
- + better suited for operation on rough fields
- - laterally unstable when rolling
- - heavy braking may bring the danger of tipping the aircraft over on its nose
What are the advantages of the tricycle landing gear?
- the aircraft does not need to be aligned with the runway (a crab landing is possible)
- it is much easier for the pilot to keep it on the centreline, even with strong side gusts
- offers better visibility for the pilots during taxiing
- easier loading and unloading of the aircraft (and more comfort for the passengers) thanks to the nearly horizontal floor
- low drag during take off acceleration
- is more forgiving when landing with insufficient or no flare (the aircraft tends to reduce the angle of attack after touchdown, and is less prone to bouncing back in the air)
What are the advantages and disadvantages of the bicycle gear?
- + lower weight
- + less drag (when gear is extended)
- + used when not enough space for langing gear
- - difficult take-off and landing
- - In crosswind conditions, care must be taken not to land on one of the outrigger gear legs, which are not designed to take substantial landing loads.
What is the maximum take-off mass for a single wheel per main gear leg?
23,000 kg (although for safety reasons it would be better to fit twin wheels)
For which take-off mass are there usually two wheels per main leg?
23,000 - 68,000 kg (This configuration is sometimes found also on heavier aircraft with a take-off mass of up to 100,000 kg)
For which take-off mass is the four-wheel bogie common?
90,000 - 180,000 kg
What types of shock absorbers exist?
- spring type
- bungee type
- oleopneumatic shock absorbers (oleo struts)
Explain the simple spring type shock absorber.
- gear legs are elastically deformed under load
- typical of small general aviation aircraft
- The vertical kinetic energy is transmitted gradually to the structure
- practically no damping effect, except for the energy dissipated by lateral friction of the tires (when the gear deflects, the wheels move sideways)
- aircraft tends to bounce back in a hard landing
Explain the simple bungee type shock absorber.
- the main gear legs are hinged at the side of the fuselage and connected by rubber bungees (bundles of rubber
strands packed in a cloth tube) that stretch as the gear deflects - common in early light aircraft
- In addition to the lateral friction of the tires, some energy is dissipated by rubber internal friction and hysteresis
Explain the oleopneumatic shock absorbers (oleo struts).
- efficiently dissipate the vertical kinetic energy at landing and smooth out the bumps on uneven terrain
- ombine a spring effect, created by a variable chamber filled with compressed air or nitrogen, with the damping effect of a piston which forces oil through small orifices
- The three most common configurations are: the telescopic gear, the triangulated gear, and the levered (or trailing link) gear
Explain the oleo strut configuration telescopic gear.
- the main structure of the gear leg is formed by the oleo strut itself
- simpler design and has a lower number of components
- must provide the full required amount of compression, and be sized to take the whole lateral and longitudinal loads
- quite heavy
- The bending associated with these loads can cause serious wear on the oil seals
Explain the oleo strut configuration triangulated gear.
- conceptually similar to the bungee type: when the gear leg deflects, an oleo strut is compressed
- the wheel lateral and longitudinal loads are carried by the solid gear legs, and a shorter and smaller oleo strut can be used
- Maintenance is easier (oleo strut can be replaced or overhauled without removing the wheel assembly and the landing gear leg)
- configuration is usually a little heavier than the telescopic one
- the gear deflections causes lateral friction on the tires, which shortens their life
Explain the oleo strut configuration trailing link gear.
- variant of the other two configurations
- the wheels travel slightly backwards when the oleo strut is compressed; this makes it easier for a wheel to overcome a bump
- very robust
- heavier than the telescopic leg type
- well suited to aircraft that operate on aircraft carriers or to short take off and landing aircraft designed to take off and land from unprepared runways
