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A/C SYS 2017

Aircraft System 2017

Aircraft System 2017

Kartei Details

Karten 88
Lernende 36
Sprache English
Kategorie Physik
Stufe Universität
Erstellt / Aktualisiert 23.09.2016 / 11.01.2021
Lizenzierung Keine Angabe
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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

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?

  1. the distribution of thrust on the propeller disc is not uniform
    1. Most of the thrust is generated in the outer part of the blade (rotational velocity component is higher)
    2. The energy lost in these tip vortices corresponds to the mechanical work needed to overcome the induced drag of the blades.
  2. 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?


  • 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?