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Questions and Answers
When an electrical de-icing system is operating, what happens to thrust?
What is the purpose of a divergent pitot intake?
What method is used for anti-icing inlet guide vanes?
How is a divergent intake characterized?
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What operates the variable inlet guide vanes?
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What does a well-designed intake do with forward speed?
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What is the primary design goal of intakes?
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What is the function of the stator vane assembly in an axial flow compressor?
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In an axial flow, dual compressor forward fan engine, which component turns at the same speed as the fan?
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Which measurement unit is typically used to assess aircraft noise levels?
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What component is essential for the efficient burning of gases in a gas turbine engine?
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What characterizes a turbojet engine compared to a piston engine?
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Where is the jet nozzle located in a turbojet engine?
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Which of the following correctly describes the Brayton cycle?
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In a ducted fan engine, what drives the fan?
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Which theorem is related to the energies present in fluid flow?
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What is the major function of the turbine assembly in a turbojet engine?
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What advantage do shrouded blades provide in gas turbines?
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What is the principle of jet propulsion based on?
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What effect does forward speed have on the efficiency of a jet engine?
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What type of failure is most likely when turbine blades are subjected to excessive heat stress?
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How does thrust change with high temperatures in a jet engine?
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What purpose do nozzle guide vanes serve in a gas turbine engine?
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What is creep in the context of turbine operation?
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The efficiency of converting kinetic energy into propulsive work is known as what?
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Which condition is most likely to lead to creep in turbine blades?
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What happens to thrust when an aircraft's forward speed increases and the engine experiences ram effect?
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What is a common method to compare engine efficiencies?
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What is the primary role of the turbine section in a jet engine?
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How does the passage between adjacent nozzle guide vanes in an impulse-reaction turbine function?
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Which component experiences the highest heat in contact within a jet engine?
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At what Mach speed does ram pressure recovery typically begin to take effect?
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What is the maximum practical angle through which the gas flow can be turned during thrust reversal?
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What happens to pressure and velocity in the first part of the exhaust duct when an exhaust cone is placed aft of the turbine?
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What is the consequence of having an exit area of the nozzle that is too large?
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How can exhaust noise be effectively reduced?
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What should be done if damage is found to the reverse thrust cascade vanes?
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Through what angle are exhaust gases turned in a clamshell type thrust reverser?
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What type(s) of main bearing oil seals are typically used with turbine engines?
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Why are oil seals pressurized in turbine engines?
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Study Notes
Gas Turbine Engine Fundamentals
- Axial flow, dual compressor forward fan engines have a fan that rotates at the same speed as the low-pressure turbine.
- Turbine discharge pressure is often identified using Pt7.
- Turbojet and turbofan engines accelerate a smaller mass of air, while turboprop engines accelerate a larger mass of air.
- Effective perceived noise decibels (EPNdB) are used to measure aircraft noise.
- The combustion section is responsible for mixing fuel and efficiently burning gases.
- Turbojet engines are smoother than piston engines because they lack reciprocating parts.
- The jet nozzle is located in the exhaust section of a turbojet engine.
- The Brayton cycle is a constant pressure cycle.
- The highest pressure in a gas turbine occurs just after the last compressor stage, before the burner.
- In a ducted fan engine, the fan is driven by the turbine.
- Bernoulli's theorem relates kinetic, pressure, and potential energy in a fluid flow.
- The Brayton cycle describes the continuous combustion cycle in a gas turbine engine.
- High bypass ratio turbofan engines have smaller compressors and combustion chambers compared to turbojet engines.
- Jet propulsion is based on the principle of action and reaction.
Engine Performance
- Ram effect increases engine efficiency due to aircraft forward speed.
- Ram effect does not change engine thrust.
- The highest heat to metal contact in a jet engine occurs at the turbine inlet guide vanes.
- Thrust increases with low temperature.
- Specific fuel consumption is used to compare engine efficiencies.
- Mechanical efficiency measures the conversion of kinetic energy into propulsive work.
- Engine rating plugs control thrust rating on FADEC-controlled engines.
- With increasing forward speed, engine thrust decreases slightly but recovers due to ram effect.
- Thrust increases at low temperatures.
- Ram pressure recovery generally takes effect at speeds between Mach 0.1 and 0.2.
- To maintain selected RPM at altitude, fuel is automatically reduced during climbs.
- Ram Recovery is a measure of intake efficiency.
- Gas turbine efficiency is greatest at cold temperatures.
Engine Inlet
- Electrical de-icing systems do not affect thrust.
- A pitot intake is divergent to maximize ram compression.
- Inlet guide vanes are anti-iced using engine bleed air.
- A divergent intake is divergent from front to rear.
- Variable inlet guide vanes are operated by fuel pressure.
- Intakes are designed to convert kinetic energy into pressure energy through forward speed.
- Intakes decelerate the free air stream flow.
- De-icing systems break up ice formations on turboprop engine nose cowls.
Engine Compressors
- A bypass engine LP compressor supplies more air than needed for combustion.
- A bypass ratio of 5:1 signifies the bypass flow is five times the hot stream.
- During high RPM operation, variable guide vanes are at minimum swirl and bleed valves are closed.
- Stator vanes straighten airflow after the axial flow compressor to reduce turbulence.
- Variable guide vanes and compressor bleed valves help stabilize compressor airflow.
- The boundary layer is the thin layer of air that adheres to a surface.
- The boundary layer flow is most affected by the surface along the leading edge, parallel to the edge.
Engine Turbines
- The turbine assembly provides power to rotate the compressor in a turbojet engine.
- Shrouded blades allow for thinner, more efficient blade sections.
- Shrouded turbine rotor blades reduce vibration and improve airflow characteristics.
- Excessive heat stress in turbine blades can lead to stress rupture.
- Shrouding rotor blade tips minimizes vibration.
- Nozzle guide vanes are fitted to increase the velocity of gas flow.
- Turbine rear struts straighten the gas flow.
- Creep is a permanent deformation of turbine blades.
- Creep can occur due to prolonged over-temp with excessive centrifugal loads.
- Cracking is a common occurrence in the turbine section of a jet engine.
- The turbine section converts dynamic pressure into mechanical energy.
- The passage between nozzle guide vanes in an impulse-reaction turbine forms a convergent duct.
- Secondary creep is a slow and constant growth in a turbine blade.
Engine Exhaust
- The exhaust cone assembly straightens and collects exhaust gases into a solid jet.
- The maximum practical angle for thrust reversal is 135 degrees.
- An exhaust cone increases pressure and decreases velocity in the first part of the exhaust duct.
- A nozzle exit area that is too large lowers exit velocity, reducing thrust.
- Exhaust noise can be reduced by increasing the mixing rate.
- Replace damaged reverse thrust cascade vanes with new ones that match the original part.
- Clamshell door thrust reversers redirect the exhaust stream 45 degrees to the thrust line.
Bearings and Seals
- Main bearing oil seals in turbine engines are typically labyrinth and/or carbon rubbing types.
- Indentations on bearing races caused by high static loads are called brinelling.
- Oil seals are pressurized to minimize oil loss.
- Some labyrinth seals control the outflow of air at the turbine.
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