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Question 47-1 : A turbofan engine with an inlet air mass flow of 300 kg/s and a bypass mass flow of 250 kg/s has a bypass ratio of ? [ Understand DGAC ]
5.
Img939.bypass ratio = bypass mass flow / hp compressor mass flow..inlet = 300.bypass = 50.hp compressor mass flow 300 50 = 250..bypass ratio = 250 / 50 = 5
Question 47-2 : A turbofan engine with a bypass mass flow of 888 kg/s and a hp compressor mass flow of 111 kg/s has a bypass ratio of ?
8.
Img939..bypass ratio = bypass mass flow / hp compressor mass flow..888 / 111 = 8
Question 47-3 : Bleeding air from the hp compressor of a turbofan, whilst maintaining thrust, increases ?
Egt and fuel flow.
Oscarjp.if there is less air flow coming in, egt will rise in function of that and because there is a loss on efficiency due to the bleed more work is demanded to the system and so fuel flow increases.
Question 47-4 : A twin spool engine with an inlet air mass flow of 500 kg/s and a bypass mass flow of 250 kg/s has a bypass ratio bpr of ?
1.
Img939..bypass ratio = bypass mass flow / hp compressor mass flow..inlet = 500..bypass = 250 > compressor = 250..250 / 250 = 1.
Question 47-5 : Compressor stall is due to ?
An excessively high angle of attack of the rotor blades.
Like the stall of a wing, compressor stall is caused by a breakdown of airflow..if your wing is suitable for high speed, and you fly at low speed and large angle of attack, the stall is most likely to occur...a compressor stall is most likely to occur when the air flow stagnates in the rear stages of the compressor. a compressor blade is hitting the oncoming airflow at an angle of attack that is determined by a combination of the rotational speed and the incoming airflow..when the incoming speed is low the angle of attack will be high. when the angle of attack gets too high the blade stalls. so when the axial flow in a compressor backs up the blades stall pressure increase in the rear stages this slows down the flow..you can prevent this by dumping some late stage air to atmosphere via bleed valves.
Question 47-6 : A turbofan engine with an inlet air mass flow of 1000 kg/s and a hp compressor mass flow of 111 kg/s has a bypass ratio of ?
8.01.
Img939.bypass ratio = bypass mass flow / hp compressor mass flow..bypass mass flow = 1000 111 = 889..889 / 111 = 8.01
Question 47-7 : Which of these statements about an reaction turbine are correct or incorrect.i. the pressure drops across the nozzle guide vanes..ii. the pressure remains constant across the rotor blades. ?
I is correct, ii is incorrect.
There is a pressure drop and a velocity increase across the stator of a reaction turbine.
Question 47-8 : The electronic engine control of a fadec gas turbine engine checks ?
All input and output data.
The full authority digital engine control system fadec receives all the necessary data for engine operation and takes over virtually all of the steady state and transient control functions, which includes compressor airflow control vigv's, vsv's, etc.
Question 47-9 : Jet engine total efficiency indicates the efficiency at which ?
Chemical power in the fuel is transformed into propulsive power t x v.
Question 47-10 : Which statement is correct for a gas turbine engine with a constant speed propeller and free power turbine, when the power setting is decreased.i. the hp spool speed decreases..ii. the egt decreases. ?
I is correct, ii is correct.
Hp spool speed, egt and gas generator speed all decrease for a gas turbine engine with a constant speed propeller and free power turbine, when the power setting is decreased.
Question 47-11 : The fadec of a gas turbine engine consists of ?
An electronic engine control unit and its peripherals fuel metering, actuators, electrical wiring, sensors.
Question 47-12 : The bypass ratio..i is the ratio of inlet air mass flow to exhaust air mass flow..ii can be determined from the bypass air mass flow and the hp compressor mass flow. ?
I is incorrect, ii is correct.
Img939..bypass ratio = bypass mass flow / hp compressor mass flow...the bypass ratio can be determined from the inlet air mass flow and the bypass mass flow.
Question 47-13 : The fadec of a gas turbine engine normally includes ?
An engine overspeed and/or an egt protection function.
Question 47-14 : In a gas turbine engine, the propelling jet is generated by ?
Expansion of hot gases resulting in a conversion of potential energy pressure into kinetic energy velocity.
The process of energy conversion in a multistage turbine consists of a number of successive processes in the individual stages...compressed and heated gas at an initial velocity passes through the blade to blade channels of the nozzle assembly where, in the course of expansion, part of the available heat drop is converted into kinetic energy of the exhaust flow. further gas expansion and conversion of the heat drop into useful work take place in the blade to blade channels of the rotor...the gas flow acting on the rotor blades develops torque on the turbine's shaft. as a result, the absolute velocity of the gas is reduced. the lower this velocity, the larger the part of the available gas energy that is converted into mechanical work on the shaft...the rotor blades receive the forces produced by a change in the direction of the gas velocity flowing around them impulse flow effect and by acceleration of the gas flow relative to the movement in the blade to blade channels reaction flow effect.
Question 47-15 : The internal geometry of a turbofan intake for a subsonic commercial aeroplane is ?
Divergent in order to reduce airflow velocity and increase static pressure in front of the fan.
Img953..in a divergent intake, the flow speed decreases and the static pressure increases..dynamic pressure decreases and total pressure remains unchanged.
Question 47-16 : The turbine in a gas turbine engine generates shaft power using ?
Expansion of hot gas followed by conversion of kinetic energy velocity into mechanical work.
The expansion is the drop of pressure. like a venturi tube, the velocity must increase to keep the same mass flow. this kinetic energy of the hot gases is converted into useful rotational, mechanical energy by the turbine. this increased velocity kinetic energy , is what drives the shaft.
Question 47-17 : The principle of aeroplane propulsion is to generate a propelling force by ?
Accelerating air or gas in order to obtain a reaction force.
Question 47-18 : Which statement is correct for a gas turbine engine with a constant speed propeller and free power turbine, when the power setting is increased.i. the gas generator speed decreases..ii. the egt remains constant. ?
I is incorrect, ii is incorrect.
Hp spool speed, egt and gas generator speed all increase for a gas turbine engine with a constant speed propeller and free power turbine, when the power setting is increased.
Question 47-19 : Which statement is correct for a gas turbine engine with a constant speed propeller and free power turbine, when the power setting is increased.i. the gas generator speed decreases..ii. the egt decreases. ?
I is incorrect, ii is incorrect.
Hp spool speed, egt and gas generator speed all increase for a gas turbine engine with a constant speed propeller and free power turbine, when the power setting is increased.
Question 47-20 : The primary performance indicator cockpit instrument for the thrust of a turbofan engine is ?
Either epr or n1.
Primary engine indications.n1, epr and egt are the primary engine indications on a turbofan engine...secondary engine indications.n2, fuel flow, oil pressure, oil temperature, oil quantity, and engine vibration are the secondary engine indications...a turbofan engine is a dual rotor, axial flow turbofan. the n1 rotor consists of a fan, a low pressure compressor and a low pressure turbine. the n2 rotor consists of a high pressure compressor and a high pressure turbine. the n1 and n2 rotors are mechanically independent..epr, is engine pressure ratio. it compares the pressure in the engine inlet, with pressure in the exhaust nozzle..n1 is the speed of the first stage engine compressor. this is used as a power setting in most jet engines.
Question 47-21 : If air is tapped from a gas turbine hp compressor, the effect on the engine pressure ratio epr and the exhaust gas temperature egt is that ?
Epr decreases and egt increases.
If you bleed air from the compressor, you are reducing the pressure inside the compressor, therefore turbine outlet pressure will be reduced. if inlet pressure remains the same, and outlet pressure is reduced, then epr decreases..since hot air is tapped directly from a compressor stage, the temperature will rise in the turbine, egt increases.
Question 47-22 : An abortive hung start is the term used to describe an attempt to start in which the engine ?
Lights up, but fails to accelerate sufficiently.
After the normal light up sequence until the starter 'cut out' , a hung start is identified by light off followed by abnormally slow acceleration and rpm stabilization below idle. a hung start may be result of fuel scheduling being either too lean or too rich. a lean hung start is associated with low fuel flow and proportionally low egt. a rich condition can be recognized by a high fuel flow and an egt rise which may tend to develop into an over temperature condition and possible compressor stall.... hung starts may be caused by .. starter air pressure too low to accelerate engine to self sustaining speed.. premature starter deactivation... fod foreign object debris to compressor... faulty pressurizing valves not opening at scheduled fuel pressure setting... incorrect scheduling of hp compressor igv and variable stators... turbine section damage.
Question 47-23 : On a fadec jet engine, the meaning of f.a.d.e.c. on a gas turbine engine is ?
Full authority digital engine control.
Question 47-24 : The blades in an axial flow compressor decrease in size from the ?
Low pressure to the high pressure stages in an attempt to sustain axial velocity.
Img914..the blades size reduces from the inlet until the output of the compressor.
Question 47-25 : Regarding a jet engine..i. the maximum thrust decreases slightly as the pressure altitude decreases...ii. the specific fuel consumption increases slightly as the pressure altitude decreases at constant tas. ?
I is incorrect, ii is correct.
Jet engines are more efficient at higher altitudes because less fuel is needed for a given thrust, due to reduction in lift and drag, and lower temperatures through the inlet cause a significant reduction in specific fuel consumption, up to the tropopause..specific fuel consumption is the amount of fuel burnt per unit of thrust. for jets, it reduces with altitude up to the tropopause...notice as the pressure altitude decreases means at a lower altitude.
Question 47-26 : Which of the following statements is correct about the flow in a divergent subsonic gas turbine engine intake..i. the speed decreases in flow direction...ii. the total pressure decreases in flow direction. ?
I is correct, ii is incorrect.
In a divergent intake, the flow speed decreases and the static pressure increases. dynamic pressure decreases and total pressure remains unchanged.
Question 47-27 : Which of the following statements is correct about the flow in a divergent subsonic gas turbine engine intake..i. the static pressure increases in flow direction...ii. the total temperature increases in flow direction. ?
I is correct, ii is incorrect.
Question 47-28 : Both gas turbine and piston engines use a cycle made up of induction, compression, combustion and exhaust phases. which of these statements about a gas turbine are correct or incorrect.i. the process in a gas turbine engine is intermittent..ii. theoretically the combustion occurs at constant volume. ?
I is incorrect, ii is incorrect.
.piston engine the combustion process is intermittent, governed by the rythm of the back and forth movements of the piston in a cylinder. the combustion occurs at constant volume in the cylinder...turbine engine the combustion process is continuous and occurs at constant pressure.
Question 47-29 : Which of the following statements is correct about the flow in a divergent subsonic gas turbine engine intake.i. the speed remains constant in flow direction..ii. the total pressure remains constant in flow direction. ?
I is incorrect, ii is correct.
There is a velocity decrease as air flows from a small inlet to a larger outlet.
Question 47-30 : Which statement is correct about the flow in a divergent jet engine inlet.i. the static pressure remains constant in flow direction..ii. the total temperature remains constant in flow direction. ?
I is incorrect, ii is correct.
Question 47-31 : Which of these statements about compressor stall are correct or incorrect..i. a compressor stall is most likely to occur in a compressor designed for a high pressure ratio operating at low rpm...ii. a compressor stall is most likely to occur when the air flow stagnates in the rear stages of the ?
I is correct, ii is correct.
Like the stall of a wing, compressor stall is caused by a breakdown of airflow..if your wing is suitable for high speed, and you fly at low speed and large angle of attack, the stall is most likely to occur...a compressor stall is most likely to occur when the air flow stagnates in the rear stages of the compressor. a compressor blade is hitting the oncoming airflow at an angle of attack that is determined by a combination of the rotational speed and the incoming airflow..when the incoming speed is low the angle of attack will be high. when the angle of attack gets too high the blade stalls. so when the axial flow in a compressor backs up the blades stall pressure increase in the rear stages this slows down the flow..you can prevent this by dumping some late stage air to atmosphere via bleed valves.
Question 47-32 : In a compressor stage, if the axial air velocity increases more than the rotor blade velocity the ?
Stall margin increases.
.compressor stall occurs when the compressor sucks too much air for the turbine wheels to handle. excessive pressure then builds aft of the compressor, and the compressor blades stall..a compressor blade is hitting the oncoming airflow at an angle of attack that is determined by a combination of the rotational speed and the incoming airflow..when the incoming air flow speed is high the blade angle of attack will be low, the stall margin increases there is less risk to encounter a compressor stall.
Question 47-33 : Which of these statements about compressor stall are correct or incorrect..i. a compressor stall is most likely to occur in a compressor designed for a high pressure ratio operating at high rpm...ii. a compressor stall is most likely to occur when the air flow stagnates in the rear stages of the ?
I is incorrect, ii is correct.
Like the stall of a wing, compressor stall is caused by a breakdown of airflow..if your wing is suitable for high speed, and you fly at low speed and large angle of attack, the stall is most likely to occur...a compressor stall is most likely to occur when the air flow stagnates in the rear stages of the compressor. a compressor blade is hitting the oncoming airflow at an angle of attack that is determined by a combination of the rotational speed and the incoming airflow..when the incoming speed is low the angle of attack will be high. when the angle of attack gets too high the blade stalls. so when the axial flow in a compressor backs up the blades stall pressure increase in the rear stages this slows down the flow..you can prevent this by dumping some late stage air to atmosphere via bleed valves.
Question 47-34 : With regard to a twin spool turbofan, an abortive start hung start will result in ?
N2 speed stabilising at a lower than normal rpm and a subsequent increase in egt.
.after the normal light up sequence until the starter 'cut out' , a hung start is identified by light off followed by abnormally slow acceleration and rpm stabilization below idle. a hung start may be result of fuel scheduling being either too lean or too rich. a lean hung start is associated with low fuel flow and proportionally low egt. a rich condition can be recognized by a high fuel flow and an egt rise which may tend to develop into an over temperature condition and possible compressor stall....hung starts may be caused by.. starter air pressure too low to accelerate engine to self sustaining speed.. premature starter deactivation... fod foreign object debris to compressor... faulty pressurizing valves not opening at scheduled fuel pressure setting... incorrect scheduling of hp compressor igv and variable stators... turbine section damage.
Question 47-35 : Which of the following statements is correct about the flow in a divergent subsonic gas turbine engine intake.i. the dynamic pressure increases in flow direction..ii. the static temperature increases in flow direction. ?
I is incorrect, ii is correct.
Question 47-36 : One of the functions of a gas turbine engine subsonic intake is to ?
Decrease airflow velocity.
Question 47-37 : One of the advantages of multiple spool jet engine designs is that ?
A smaller air starter driving only a single spool can be used.
.the air starter motor is possibly the most popular starting system presently in use. it is light, simple to use and very economical utilizing low pressure air. the air starter motor is fastened to the accessory gearbox of the engine. the sources of air available for engine start, in order of preference they are the aircraft apu. the ground power unit. a cross bleed start, where air from an already started engine is used..air from one of these sources is fed through an electrically controlled start valve to the air inlet to rotate the turbine rotor and is then exhausted..on a multiple spool jet engine, a small air starter motor can drive a single spool, which, when started, permits to start the other spools.
Question 47-38 : Which of these statements about compressor stall are correct or incorrect..i. a compressor stall is most likely to occur in a compressor designed for a high pressure ratio operating at high rpm...ii. a compressor stall is most likely to occur when the air flow stagnates in the front stages of the ?
I is incorrect, ii is incorrect.
Like the stall of a wing, compressor stall is caused by a breakdown of airflow..if your wing is suitable for high speed, and you fly at low speed and large angle of attack, the stall is most likely to occur...a compressor stall is most likely to occur when the air flow stagnates in the rear stages of the compressor. a compressor blade is hitting the oncoming airflow at an angle of attack that is determined by a combination of the rotational speed and the incoming airflow..when the incoming speed is low the angle of attack will be high. when the angle of attack gets too high the blade stalls. so when the axial flow in a compressor backs up the blades stall pressure increase in the rear stages this slows down the flow..you can prevent this by dumping some late stage air to atmosphere via bleed valves.
Question 47-39 : After a successful start of a high bypass turbofan engine, the air starter ?
Is disengaged by the coupling between engine n2 spool and starter by the use of centrifugal forces.
Air starting is used on most commercial and some military jet engines. it has many advantages over other starting systems, and is comparatively light, simple and economical to operate. 10. an air starter motor transmits power through a reduction gear and clutch to the starter output shaft which is connected to the engine...the starter turbine is rotated by air taken from an external ground supply, an auxiliary power unit a.p.u. or as a cross feed from a running engine. the air supply to the starter is controlled by an elec trically operated control and pressure reducing valve that is opened when an engine start is selected and is automatically closed at a predetermined starter speed. the clutch also automatically disengages as the engine accelerates up to idling rpm and the rotation of the starter ceases.
Question 47-40 : Both gas turbine and piston engines use a cycle made up of induction, compression, combustion and exhaust phases. which of these statements about a gas turbine are correct or incorrect.i. the process in a gas turbine engine is continuous..ii. theoretically the combustion occurs at constant volume. ?
I is correct, ii is incorrect.
.piston engine the combustion process is intermittent, governed by the rythm of the back and forth movements of the piston in a cylinder. the combustion occurs at constant volume in the cylinder...turbine engine the combustion process is continuous and occurs at constant pressure.
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