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Module 15 Gas Turbine Engine study materials
Module 15 Gas Turbine Engine study resources

Module 15 Gas Turbine Engine practice questions

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Question 1Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

In the standard gas turbine station numbering system, which station represents the inlet to the high-pressure compressor?

  1. Station 1
  2. Station 3
  3. Station 2 Correct answer.
  4. Station 4

Explanation

Station 1 is the engine inlet (freestream), and station 2 is the compressor inlet face after the intake. Station 3 is the compressor exit and station 4 is the combustor exit, so the HP compressor inlet is station 2.

Question 2Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

Which thermodynamic cycle is used as the ideal reference model for a turbojet engine?

  1. Rankine cycle
  2. Otto cycle
  3. Brayton cycle Correct answer.
  4. Diesel cycle

Explanation

The ideal gas turbine cycle is the Brayton (constant-pressure) cycle, comprising isentropic compression, constant-pressure heat addition, isentropic expansion and constant-pressure heat rejection. The Otto and Diesel cycles are reciprocating engine models, and Rankine applies to steam plant.

Question 3Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

In a pure turbojet, thrust is produced primarily because of which change across the engine?

  1. An increase in the momentum of the working fluid Correct answer.
  2. A reduction in the mass flow through the core
  3. A decrease in the exhaust gas temperature
  4. A rise in static pressure at the nozzle exit

Explanation

Thrust follows from Newton's second law: accelerating the airflow rearward increases its momentum, and the reaction pushes the engine forward. The net thrust equals mass flow times the difference between exhaust and flight velocities, so momentum change is the governing quantity.

Question 4Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

What is the principal reason a high-bypass turbofan is more fuel efficient at subsonic cruise than a pure turbojet of similar core size?

  1. It burns fuel at a higher peak combustor temperature
  2. It accelerates a larger mass of air to a lower velocity Correct answer.
  3. It eliminates the turbine section entirely
  4. It operates at a higher exhaust velocity for the same fuel flow

Explanation

Propulsive efficiency improves when a large mass of air is accelerated by a small velocity increment, which is what the bypass fan provides. A turbojet accelerates a smaller mass to a much higher velocity, wasting energy in the high-velocity jet.

Question 5Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

In a twin-spool turbofan, what is the mechanical relationship between the fan and the low-pressure turbine?

  1. They are geared to the high-pressure compressor
  2. They are connected by a single shaft and rotate at the same speed Correct answer.
  3. They are connected only through the high-pressure spool
  4. They are on separate shafts with no mechanical link

Explanation

In a twin-spool arrangement the low-pressure turbine drives the fan (and booster) through the LP shaft, so fan and LP turbine share one rotational speed. The HP turbine separately drives the HP compressor on a concentric shaft.

Question 6Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

A turbofan has a core mass flow of 40 kg/s and a bypass mass flow of 360 kg/s. What is its bypass ratio?

  1. 9.0 Correct answer.
  2. 1.1
  3. 8.0
  4. 0.11

Explanation

Bypass ratio is bypass mass flow divided by core mass flow, so 360 divided by 40 equals 9.0. This is typical of a modern high-bypass civil turbofan.

Question 7Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

Which station number conventionally marks the turbine exit and nozzle inlet in a single-spool turbojet?

  1. Station 7
  2. Station 3
  3. Station 4
  4. Station 5 Correct answer.

Explanation

Station 4 is the combustor exit or turbine inlet, station 5 is the turbine exit and exhaust nozzle inlet, and station 7 is the nozzle exit. The sequence 3-4-5-7 tracks compressor exit, combustor exit, turbine exit and nozzle exit.

Question 8Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

In the ideal Brayton cycle, thermal efficiency depends only on which parameter?

  1. Pressure ratio Correct answer.
  2. Turbine inlet Mach number
  3. Mass flow rate
  4. Combustor exit temperature

Explanation

For the ideal air-standard Brayton cycle, efficiency is 1 minus the reciprocal of the pressure ratio raised to the specific heat ratio exponent. It is independent of the peak temperature, which instead limits specific work output.

Question 9Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

The compressor pressure ratio of a gas turbine is defined as which quantity?

  1. Compressor exit total pressure divided by inlet total pressure Correct answer.
  2. Ambient pressure divided by combustor pressure
  3. Compressor exit static pressure divided by inlet static pressure
  4. Turbine inlet total pressure divided by compressor exit total pressure

Explanation

Pressure ratio is conventionally quoted using total (stagnation) pressures across the compressor, exit total pressure over inlet total pressure. Total pressure accounts for the dynamic head and is the correct measure of compression work input.

Question 10Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

Which expression gives the net thrust of a turbojet with separate exhaust nozzle, ignoring pressure thrust?

  1. Exhaust velocity divided by flight velocity
  2. Mass flow times exhaust velocity
  3. Mass flow times the velocity difference between exhaust and flight Correct answer.
  4. Mass flow times flight velocity

Explanation

Net thrust equals mass flow multiplied by the difference between jet velocity and aircraft velocity. When the aircraft is stationary, this reduces to mass flow times jet velocity, the static thrust case.

Question 11Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

A turbofan cruises at 250 m/s with core jet velocity 550 m/s. What is the specific thrust of the core stream, neglecting fuel mass and pressure thrust?

  1. 550 N per kg/s
  2. 2.2 N per kg/s
  3. 800 N per kg/s
  4. 300 N per kg/s Correct answer.

Explanation

Specific thrust is the velocity difference between jet and flight speed, here 550 minus 250 equals 300 N per kg/s. Multiplying by core mass flow would give the core thrust contribution.

Question 12Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

Why does a convergent nozzle become choked when the pressure ratio across it exceeds the critical value?

  1. The turbine stops extracting work
  2. The flow reaches sonic velocity at the throat and mass flow is fixed by upstream conditions Correct answer.
  3. The nozzle walls heat the gas until it ignites
  4. The gas viscosity rises sharply and blocks the flow

Explanation

Once the throat reaches Mach 1, downstream pressure disturbances cannot propagate upstream, so further pressure ratio increase does not raise mass flow. The mass flow then depends only on upstream total pressure and temperature and throat area.

Question 13Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

Which engine layout uses two concentric shafts so the low-pressure turbine drives only the fan and booster?

  1. Twin-spool turbofan Correct answer.
  2. Single-spool turbojet
  3. Ramjet
  4. Free-turbine turboshaft

Explanation

A twin-spool turbofan separates the LP spool, linking fan and booster to the LP turbine, from the HP spool linking HP compressor and HP turbine. This lets each spool run at its own optimum speed.

Question 14Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

In an ideal turbojet, which component produces the highest total pressure loss?

  1. Intake
  2. Compressor
  3. Nozzle
  4. Combustor Correct answer.

Explanation

The combustor involves heat addition with a deliberate total pressure loss, typically a few percent, which is the largest pressure loss in the cycle. The intake and nozzle losses are small by comparison, and the compressor raises pressure rather than losing it.

Question 15Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

What does the term 'ram drag' represent in the thrust equation?

  1. The friction loss in the compressor bearings
  2. The pressure loss in the combustor
  3. The drag of the aircraft wing
  4. The momentum of the incoming air that must be overcome to accelerate it Correct answer.

Explanation

Ram drag is the product of mass flow and flight velocity, representing the momentum the engine must supply to bring incoming air to jet velocity. Net thrust is gross thrust minus ram drag.

Question 16Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

Which statement correctly describes the energy conversion sequence in a turboshaft engine?

  1. Chemical energy becomes shaft work directly in the combustor
  2. Thermal energy becomes electrical energy in the turbine
  3. Kinetic energy becomes chemical energy in the compressor
  4. Chemical energy becomes thermal energy, then kinetic energy, then shaft work Correct answer.

Explanation

Fuel chemical energy is released as heat in the combustor, the gas generator converts part of that into kinetic energy, and the power turbine extracts it as shaft work. The compressor reverses the kinetic-to-pressure conversion on the incoming air.

Question 17Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

A gas turbine compressor delivers air at a total pressure of 1200 kPa from an inlet total pressure of 100 kPa. What is the pressure ratio?

  1. 12 Correct answer.
  2. 11
  3. 120
  4. 1.2

Explanation

Pressure ratio is exit total pressure divided by inlet total pressure, so 1200 divided by 100 equals 12. This value is typical of a modern high-pressure compressor in a large turbofan.

Question 18Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

Why is the turbine inlet temperature a limiting parameter in gas turbine design?

  1. It is fixed by the compressor pressure ratio
  2. It must equal the compressor exit temperature
  3. It is bounded by the temperature capability of the turbine blade materials and cooling system Correct answer.
  4. It is limited by the fuel tank capacity

Explanation

Raising turbine inlet temperature improves specific work and efficiency, but the first-stage nozzle guide vanes and blades must survive it, so material capability and cooling technology set the ceiling. This is why blade alloys and cooling schemes are central design drivers.

Question 19Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

Which station numbering convention applies to the bypass duct of a turbofan?

  1. Stations 4 to 5
  2. Stations 2 to 3
  3. Stations 12 to 15 or 16 Correct answer.
  4. Stations 7 to 9

Explanation

In many turbofan conventions the bypass (fan) duct is numbered from station 12 at fan exit to station 15 or 16 at the bypass nozzle exit. The core stream keeps the traditional 2-3-4-5-7 sequence.

Question 20Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

An engine produces 60 kN of gross thrust at a flight speed of 200 m/s with an air mass flow of 100 kg/s. What is the net thrust?

  1. 80 kN
  2. 40 kN Correct answer.
  3. 20 kN
  4. 60 kN

Explanation

Ram drag is mass flow times flight speed, 100 times 200 equals 20 kN, which is subtracted from gross thrust. Net thrust is therefore 60 minus 20 equals 40 kN.

Question 21Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

In an ideal turbojet, the turbine work extracted exactly equals which quantity?

  1. The nozzle kinetic energy
  2. The compressor work only Correct answer.
  3. The combustor heat release
  4. The compressor work plus the fan work

Explanation

In a single-spool turbojet the turbine drives only the compressor, so at steady state the turbine work equals the compressor work. The remaining enthalpy is left in the gas to produce jet kinetic energy in the nozzle.

Question 22Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

Which combination correctly pairs a turbofan station with its physical location?

  1. Station 13 with the bypass duct Correct answer.
  2. Station 3 with the turbine exit
  3. Station 13 with the core compressor exit
  4. Station 5 with the fan inlet

Explanation

In the common convention station 13 lies in the bypass duct downstream of the fan, while the core uses stations 2, 3, 4, 5 and 7 for compressor inlet, compressor exit, combustor exit, turbine exit and nozzle exit. Pairings must respect this split.

Question 23Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

For a turbofan, propulsive efficiency is highest when which condition is approached?

  1. Jet velocity is much greater than flight velocity
  2. Jet velocity is close to flight velocity Correct answer.
  3. Flight velocity is zero
  4. Mass flow is minimised

Explanation

Propulsive efficiency approaches unity as the jet velocity approaches flight velocity, because less kinetic energy is left in the wake. This is why large bypass ratios with modest jet velocities are favoured for subsonic transport.

Question 24Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

Which component in a turbofan converts the highest proportion of the fuel energy into useful propulsive work at subsonic cruise?

  1. The high-pressure turbine
  2. The bypass fan Correct answer.
  3. The exhaust nozzle of the core
  4. The combustor liner

Explanation

At high bypass ratio the fan moves most of the air and therefore generates most of the thrust, so it delivers the largest share of useful propulsive work. The core contributes a smaller, higher-velocity jet.

Question 25Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

In gas turbine performance, which statement best defines net thrust for a turbojet installed on a test stand?

  1. Gross thrust plus the momentum of the fuel flow
  2. The difference between compressor work and turbine work
  3. Gross thrust minus the ram drag of the incoming air Correct answer.
  4. The product of mass airflow and exhaust gas temperature

Explanation

Net thrust equals gross thrust (momentum of the exhaust jet plus any pressure thrust) minus the ram drag associated with bringing the incoming air to rest relative to the engine. Fuel flow momentum is a small additional term normally accounted for within gross thrust calculations.

Question 26Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

Specific fuel consumption (SFC) for a turbofan is normally expressed in which units in SI-based engineering practice?

  1. Kilograms of fuel per hour per newton of thrust Correct answer.
  2. Newtons of thrust per kilogram of fuel burned
  3. Joules per kilogram of fuel consumed
  4. Kilograms of fuel per newton per second squared

Explanation

SFC quantifies how much fuel is burned to produce a unit of thrust over time, commonly kg/(h·N) or lb/(lbf·h). A lower value indicates better fuel efficiency for the same thrust output.

Question 27Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

An engine is operating at a fixed throttle setting while the aircraft accelerates. As ram pressure ratio increases with forward speed, what happens to net thrust?

  1. Net thrust falls to zero once ram pressure ratio reaches unity
  2. Net thrust decreases because ram drag rises faster than gross thrust at constant fuel flow Correct answer.
  3. Net thrust stays constant because gross thrust is unaffected by speed
  4. Net thrust increases in direct proportion to true airspeed

Explanation

At a fixed throttle, gross thrust grows only modestly with ram pressure ratio, while ram drag increases with speed. The net result is that net thrust falls as forward speed rises, which is why thrust ratings are quoted at zero forward speed.

Question 28Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

Which ambient condition most directly reduces available takeoff thrust for a given throttle setting?

  1. A high aircraft gross weight at rotation
  2. A high outside air temperature at sea level Correct answer.
  3. A strong direct headwind during the takeoff roll
  4. A low relative humidity at sea level

Explanation

High ambient temperature lowers air density, reducing mass flow through the engine and therefore thrust. This is why hot-day takeoff performance is limiting and why flat-rated engines are temperature-derated.

Question 29Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

A turbofan is flat-rated to a fixed thrust value below its maximum capability at low ambient temperature. What is the primary operational benefit of this flat rating?

  1. It increases bypass ratio automatically as temperature falls
  2. It preserves engine life by capping turbine temperature and thrust at low ambient temperatures Correct answer.
  3. It allows the engine to exceed its certified maximum thrust on cold days
  4. It eliminates the need for any takeoff derating on hot days

Explanation

Flat rating caps thrust at a design value so that on cold days the engine is not run at excessive turbine temperatures, protecting hot-section life. Above the flat-rating break temperature, thrust then falls with rising temperature.

Question 30Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

Assumed temperature (flex) takeoff derating works by:

  1. Selecting a higher assumed outside air temperature so the FADEC schedules reduced thrust Correct answer.
  2. Increasing fuel flow to shorten the takeoff distance
  3. Lowering the certified maximum takeoff weight of the aircraft
  4. Reducing the number of engines operating during the takeoff roll

Explanation

By entering an assumed temperature above actual, the crew causes the engine control system to schedule a lower thrust level consistent with that warmer day. This reduces hot-section wear while still meeting the required field and climb gradients.

Question 31Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

Which parameter is most commonly used as the thrust-setting parameter on modern turbofan engines?

  1. Compressor bleed valve position
  2. Exhaust gas temperature alone
  3. Oil pressure in the bearing sumps
  4. Engine pressure ratio or fan speed (N1) Correct answer.

Explanation

Thrust is inferred from a parameter that tracks it closely, typically engine pressure ratio (EPR) or fan/low-pressure spool speed (N1). EGT and oil pressure are monitored for limits but are not primary thrust-setting parameters.

Question 32Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

Two engines produce the same net thrust, but engine A has a higher bypass ratio and lower exhaust velocity than engine B. Which statement about propulsive efficiency is correct?

  1. Propulsive efficiency depends only on fuel flow, not on exhaust velocity
  2. Engine B has higher propulsive efficiency because its exhaust velocity is greater
  3. Both engines have equal propulsive efficiency because net thrust is the same
  4. Engine A has higher propulsive efficiency because it accelerates a larger mass of air to a lower velocity Correct answer.

Explanation

Propulsive efficiency improves as exhaust velocity approaches flight velocity, which is achieved by moving a larger mass of air at lower acceleration. High-bypass turbofans therefore achieve better propulsive efficiency than low-bypass designs at the same thrust.

Question 33Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

During a climb at constant indicated airspeed, thrust must generally be increased compared with level flight at the same speed because:

  1. SFC improves as altitude increases
  2. A component of thrust is required to balance the weight component along the flight path Correct answer.
  3. Exhaust gas temperature falls with altitude
  4. Ram drag decreases as altitude increases

Explanation

In a climb the thrust vector must overcome both drag and the rearward component of weight along the flight path. This additional demand explains the higher thrust requirement and reduced climb rate at high altitude.

Question 34Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

An operator wishes to compare the fuel efficiency of two engines of different rated thrust. Which metric provides the fairest comparison?

  1. Exhaust gas temperature at takeoff
  2. Specific fuel consumption expressed per unit of thrust Correct answer.
  3. Total fuel flow in kilograms per hour
  4. Maximum rated thrust alone

Explanation

SFC normalizes fuel flow by the thrust produced, allowing engines of different size to be compared on efficiency. Absolute fuel flow alone would favor the smaller engine regardless of output.

Question 35Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

An engine's thrust is observed to decrease more steeply with altitude than density ratio alone would predict. Which factor best explains the additional loss?

  1. Reduced Reynolds number and lower compressor efficiency at low density Correct answer.
  2. Increasing ambient temperature with altitude
  3. Higher fuel heating value at altitude
  4. Increased ram pressure ratio at high altitude

Explanation

Beyond the direct density effect, low Reynolds numbers at high altitude degrade compressor and turbine efficiency, and the operating line shifts. These effects cause thrust lapse to be steeper than a simple density-ratio scaling.

Question 36Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

A pilot applies takeoff derating on a runway that is limiting in field length. What is the most important precondition before using reduced thrust?

  1. The reduced thrust must still satisfy all certified field-length, climb-gradient, and obstacle requirements Correct answer.
  2. The outside air temperature must be below standard day
  3. The aircraft must be lighter than maximum takeoff weight
  4. The runway must be dry and level

Explanation

Derating is only permitted when performance calculations confirm that the reduced thrust still meets every required gradient and distance. If any limit is not met, full rated thrust must be used.

Question 37Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

Which statement correctly distinguishes gross thrust from net thrust?

  1. Net thrust includes only pressure thrust, while gross thrust includes momentum thrust
  2. Gross and net thrust are identical when the aircraft is stationary only if fuel flow is zero
  3. Gross thrust is always smaller than net thrust in forward flight
  4. Gross thrust is the momentum thrust at the nozzle, while net thrust subtracts inlet momentum drag Correct answer.

Explanation

Gross thrust represents the momentum and pressure forces at the exhaust, while net thrust accounts for the momentum of the incoming air that must be decelerated relative to the engine. In forward flight, net thrust is therefore lower than gross thrust.

Question 38Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

An engine's thrust-specific fuel consumption is 0.35 lb/(lbf·h) at cruise. If net thrust is 8,000 lbf, what is the approximate fuel flow in lb/h?

  1. 8,350 lb/h
  2. 22,857 lb/h
  3. 2,800 lb/h Correct answer.
  4. 350 lb/h

Explanation

Fuel flow equals SFC multiplied by thrust: 0.35 × 8,000 = 2,800 lb/h. This direct multiplication is the standard use of SFC in mission planning.

Question 39Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

Why does a high-bypass turbofan typically have a lower SFC than a low-bypass turbojet at subsonic cruise?

  1. It has a higher exhaust velocity for the same thrust
  2. It eliminates the need for a compressor
  3. It achieves higher propulsive efficiency by accelerating a large mass of air to a modest velocity Correct answer.
  4. It burns fuel at a higher peak turbine temperature

Explanation

High bypass ratio improves propulsive efficiency by producing thrust through a large mass flow at lower velocity, reducing wasted kinetic energy. Combined with good thermal efficiency of the core, this lowers SFC at subsonic speeds.

Question 40Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

Which of the following best describes a 'rating' in the context of gas turbine engine performance?

  1. The fuel flow at idle conditions
  2. A defined thrust or power level with associated limiting parameters and time limits Correct answer.
  3. The maximum rotational speed of the high-pressure spool
  4. The pressure ratio across a single compressor stage

Explanation

A rating specifies an approved operating level, such as takeoff, maximum continuous, or cruise, together with limits on parameters like EGT and time at that level. Ratings govern how long and under what conditions thrust may be produced.

Question 41Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

During a maximum continuous thrust rating, which limitation is typically most binding on a hot day?

  1. Fuel tank venting capacity
  2. Oil tank quantity
  3. Turbine inlet temperature and hot-section material limits Correct answer.
  4. Bypass duct pressure

Explanation

Maximum continuous thrust is limited largely by turbine temperature and hot-section durability over sustained operation. On hot days the engine reaches these temperature limits at lower thrust, reducing the available continuous rating.

Question 42Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

Which effect explains why net thrust at a fixed throttle generally rises as ambient pressure increases at constant temperature?

  1. Higher ambient pressure reduces exhaust nozzle area
  2. Higher inlet density increases mass flow through the engine Correct answer.
  3. Higher ambient pressure lowers fuel heating value
  4. Higher ambient pressure reduces compressor pressure ratio

Explanation

At constant temperature, higher ambient pressure raises air density, increasing mass flow and therefore thrust for the same non-dimensional operating point. This is why sea-level thrust is higher than at altitude.

Question 43Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

An engine is flat-rated at 22,000 lbf up to a break temperature of 30 °C, above which thrust decreases. If thrust falls approximately 1% per 2 °C above the break temperature, what is the approximate thrust at 40 °C?

  1. Approximately 17,600 lbf
  2. Approximately 22,000 lbf
  3. Approximately 24,200 lbf
  4. Approximately 20,900 lbf Correct answer.

Explanation

The temperature excess is 10 °C, or five 2 °C increments, giving about 5% loss. Five percent of 22,000 lbf is 1,100 lbf, so thrust is approximately 20,900 lbf.

Question 44Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

Why do engine manufacturers publish thrust ratings at specific reference conditions such as sea level, standard day, static?

  1. To eliminate the need for takeoff performance calculations
  2. To provide a common baseline so engines can be compared and installed performance predicted Correct answer.
  3. Because thrust is independent of ambient conditions
  4. Because engines cannot produce thrust at any other condition

Explanation

Reference conditions give a standardized baseline for comparing engines and for scaling to actual conditions using lapse models. Actual thrust then depends on temperature, pressure, and speed at the time of operation.

Question 45Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

In an axial compressor, what is the primary function of the row of rotor blades?

  1. To bleed boundary-layer air from the casing before the combustion section
  2. To straighten the flow leaving the preceding stator row into a purely axial direction
  3. To diffuse the air and convert velocity into static pressure without adding work
  4. To add kinetic energy to the airflow and turn it in the direction of rotation Correct answer.

Explanation

Rotor blades are the only compressor elements that receive shaft work; they accelerate and swirl the air, raising its kinetic energy. The following stator row then diffuses that flow, converting velocity into static pressure. Diffusion alone is the stator's job, so the rotor must be identified as the work-input element.

Question 46Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

Why does an axial compressor normally use several stages rather than one?

  1. Because a single stage cannot raise the airflow velocity above the speed of sound
  2. Because multi-stage rotors allow the shaft to run at a lower rotational speed
  3. Because the pressure rise per stage is limited by diffusion and boundary-layer separation Correct answer.
  4. Because the air must be cooled between stages to keep the compressor work positive

Explanation

Each stage can only diffuse so much before the adverse pressure gradient separates the boundary layer on the blades. Limiting the per-stage pressure ratio and repeating many stages keeps the flow attached and the efficiency high. This is why axial compressors need many stages for a modest overall pressure ratio.

Question 47Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

What is the essential difference between a compressor stall and a compressor surge?

  1. Stall is a steady loss of efficiency, whereas surge is a gradual loss of pressure ratio over many hours
  2. Stall occurs only at low rotational speed, whereas surge occurs only at design speed
  3. Stall is a local breakdown of flow on some blades, whereas surge is a whole-engine oscillation of flow that can reverse through the compressor Correct answer.
  4. Stall affects only the turbine, whereas surge affects only the compressor

Explanation

Stall begins as a local separation on individual blades or a group of stages and may be steady or rotating. Surge is an unstable system-level condition in which the whole compressor flow oscillates and may momentarily reverse. Recognizing this local-versus-global distinction is fundamental to diagnosing the two.

Question 48Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

What does the compressor map's surge line represent?

  1. The locus of points where the compressor efficiency is highest
  2. The line along which corrected mass flow is constant for all speeds
  3. The boundary of stable operation above which the compressor stalls or surges Correct answer.
  4. The maximum pressure ratio achievable at the design rotational speed

Explanation

The surge line marks the high-pressure, low-flow boundary of the stable operating region on the compressor characteristic. Operating to the left or above it causes stall and surge. Designers keep an adequate surge margin between the operating line and this boundary.

Question 49Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

Which construction feature is used on many axial compressor rotors to reduce vibration and allow blade replacement?

  1. Riveted blade attachments to the casing
  2. Welded drum rotors with integral blades
  3. Dovetail or fir-tree blade roots in slotted discs Correct answer.
  4. Solid forged discs with machined integral blades

Explanation

Blades with dovetail or fir-tree roots slot into the disc rim, which allows individual blade replacement and provides damping through root friction. This is the standard attachment for axial compressor and turbine blades. Integral and welded constructions sacrifice replaceability.

Question 50Fundamentals (thermodynamics, thrust, engine layouts, station numbering)

What is the purpose of variable inlet guide vanes on an axial compressor?

  1. To filter foreign objects from the inlet air before the first stage
  2. To modulate the swirl entering the first stage so the compressor matches off-design operating conditions Correct answer.
  3. To increase the compressor pressure ratio at design speed
  4. To provide structural support for the front bearing housing only

Explanation

Variable inlet guide vanes change the angle of the air entering the first rotor stage. This adjusts the incidence on the front stages so the compressor can operate efficiently at part speed and avoids front-stage stall. They are a matching and stability device, not a filter or a pressure-ratio booster.