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Question 48-1 : 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 front stages of the ? [ Result LAPL ]
I is correct, 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 48-2 : The following problems may occur when starting a twin spool turbofan engine ?
No n1 rotation, although n2 may accelerate normally.
.during engine start, the compressor is very inefficient. if the engine experiences more than the usual difficulty accelerating due to such problems as early starter cut out, fuel mis scheduling, or strong tailwinds , the engine may spend a considerable time at very low rpm sub idle. normal engine cooling flows will not be effective during sub idle operation, and turbine temperatures may appear relatively high. this is known as a hot start.
Question 48-3 : 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 ?
I is incorrect, ii is correct.
.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 48-4 : The most unfavourable conditions for adequate inlet air flow of a turbofan engine are ?
Maximum take off thrust, zero forward speed and strong crosswind conditions.
Question 48-5 : If a twin spool gas turbine engine is controlled by a conventional n2 control system, rupture of the lp shaft may result in ?
Overspeed of the lp turbine.
In a twin spool engine the lp turbine is connected to the lp compressor, the hp turbine is connected to the hp compressor. the lp turbine drives the lp compressor, if a rupture occurs, it may result in an overspeed of the lp turbine.
Question 48-6 : The rotor blades of an axial compressor of a gas turbine engine are often mounted on a disk or drum using a 'loose fit' because this ?
Limits damage to the blades due to vibration.
Question 48-7 : For a turbine engine, the self sustaining speed relates to the minimum rpm at which the engine ?
Will continue to operate without any starter motor assistance.
Question 48-8 : The primary function of the diffuser, located between the compressor and the combustion chamber, of a gas turbine engine is to ?
Decrease the flow velocity.
Before combustion can proceed, the air velocity must be greatly reduced..air leaves the compressor through exit guide vanes, which convert the radial component of the air flow out of the compressor to straight line flow. the air then enters the diffuser section of the engine, which is a very divergent duct. the primary function of the diffuser structure is aerodynamic. the divergent duct shape converts most of the air's velocity pi into static pressure ps. as a result, the highest static pressure and lowest velocity in the entire engine is at the point of diffuser discharge and combustor inlet. other aerodynamic design considerations that are important in the diffuser section arise from the need for a short flow path, uniform flow distribution, and low drag loss.
Question 48-9 : A fadec on a gas turbine engine ?
Is single fault tolerant.
Single input data failure will not affect the normal operation a 'full authority digital engine control' is single fault tolerant..a fadec is an engine control system in which the primary functions use digital electronics and the electronic engine control eec unit has full range authority over the engine power or thrust.
Question 48-10 : When a turbo prop aeroplane decelerates during the intial part of the landing roll, a constant speed propeller within the beta range ?
Produces negative thrust.
Question 48-11 : Most high bypass unmixed turbofan engines are equipped with a cold exhaust reverser fan reverser only in order to ?
Save costs and mass at the expense of a reduction in available reverse thrust.
High bypass turbofan engines are equipped with a cold exhaust reverser only in order to save costs and mass..it is not impossible to reverse the hot exhaust gas, but it is complex. the gain in reverse thrust will be less than 20% for an increase in maintenance costs and a higher inflight consumption due to weight of the system... typical high bypass thrust reversers. 1052
Question 48-12 : A booster or lp compressor in a twin spool turbofan ?
Rotates at the same rotor speed as the fan.
If the engine is a twin spool turbofan, it is highly likely that there will be a hp compressor, lp compressor, and a fan. the lp comp and the fan will be driven from the same turbine, on the same shaft, and will therefore be at the same speed... 948
Question 48-13 : The function of the swirl vanes round the fuel nozzles is to..1 increase air pressure at the fuel nozzles..2 reduce the average axial flow speed in order to stabilise the flame front..3 generate a swirl of incoming air to enhance mixing of the fuel with air...the combination that regroups all of ?
2, 3.
Question 48-14 : The take off of a jet aeroplane may be performed with 'reduced thrust' in order to ?
Reduce maintenance costs.
Question 48-15 : In an ignition system during a normal start of a gas turbine engine, igniter activation ?
Should commence prior to fuel entering the combustion chamber.
Question 48-16 : A gas turbine engine is run at ground idle for a period of time prior to shutdown to ?
Prevent seizure of the rotor blades in their seals.
This allows to the gas generator and the power turbine discs more time to cool than if the engines were immediately shut down. the danger is that the rotor will cool and will shrink more rapidly than the blades, and they could be seized in their seals.
Question 48-17 : The purpose of variable by pass valves in a high bypass turbofan engine between the lp compressor and the hp compressor hpc is to ?
Prevent fan stall and lp compressor stall during engine start and low rotor speeds.
A bypass valve is another name for a bleed valve. it can also be called a blow off valve, anti stall valve, or an acceleration valve.
Question 48-18 : If a gas turbine engine fuel/oil heat exchanger is located downstream of the hp fuel pump, internal leakage of the heat exchanger will cause the oil level to ?
Rise.
The oil in the fuel/oil heat exchanger located in the tanks is always at a higher pressure than the fuel, but the question states located downstream of the hp fuel pump , so it is a secondary fuel/oil heat exchanger in order to avoid fuel icing in the fuel regulation system... in this heat exchanger you will have high pressure fuel and relatively low pressure oil so any internal leak will transfer fuel to the oil system, raising the indicated oil level.
Question 48-19 : The compressor pressure ratio of an axial flow compressor is primarily determined by the ?
Number of stages.
Question 48-20 : In a theoretical gas turbine cycle, combustion takes place at constant... ?
Pressure.
Question 48-21 : When a gas turbine engine accelerates from idle to take off thrust and the compressor rpm is too high in comparison with the air inlet velocity, this may result in ?
A compressor stall.
Like the stall of a wing, compressor stall is caused by a breakdown of airflow..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 48-22 : Which of the following parameters remains constant in a divergent subsonic gas turbine engine intake ?
Total temperature.
Question 48-23 : The most favourable conditions to apply the flexible take off procedure are.1 high take off mass.2 low take off mass.3 high outside temperature.4 low outside temperature.5 high atmospheric pressure.6 low atmospheric pressure.the combination grouping all the correct answers is ?
2, 4, 5.
Question 48-24 : Assuming the jet pipe is not choked, the equation to calculate jet engine thrust t with mass flow m, airspeed vv and jet velocity vj is ?
T = m vj vv
Question 48-25 : Given. p0 = static ambient pressure.t0 = static ambient temperature.m = air mass flow.vv = true air speed.vj = jet efflux exhaust velocity.pj = jet efflux exhaust static pressure.tj = jet efflux exhaust static temperature.a = exhaust cross sectional area.the thrust t of a jet engine is ?
T = m vj vv + a pj p0.
Thrust = air mass flow. jet exhaust velocity true air speed + exhaust cross sectional area. jet exhaust static pressure static ambient pressure..thrust is equal to the change in momentum of the air entering and leaving it is measured by the acceleration of the air through the engine.
Question 48-26 : The axial compressor of a gas turbine engine usually has more stages than its driving turbine because ?
The power output of a turbine stage is higher than the power consumption of a compressor stage.
Question 48-27 : On a fadec gas turbine engine during normal flight, the power for the fadec's sensors comes from ?
The fadec power source s on the engine.
The fadec has its own independent power supply generator , which is separate from the aircraft generator to ensure that the fadec system has electrical power as long as the engine is running, regardless of any problems or faults with the aircraft system.
Question 48-28 : In a gas turbine combustion chamber ?
Secondary air is required for cooling of the inner casing.
The combustion system's purpose is to provide a smooth stream of uniformly heated gas, with the least loss of pressure and the maximum release of heat.. 1047.the combustion chamber is made from two tubes. one is the flame tube, which is inside the air casing, between the two is an air gap, through which any air that does not go into the combustion chamber passes secondary air..air enters the combustion chamber at around 500 ft/sec, and is diffused to around 80 ft/sec. even at this speed, the slow flame at 1/2 ft/sec would be blown away, so a region of low pressure is created round it to keep it alright.
Question 48-29 : If there is a single input data failure on a fadec gas turbine engine ?
The engine continues to operate normally.
Question 48-30 : Concerning the fuel system high pressure shut off valve hpsov of a gas turbine engine ?
Ignition is activated before fuel is delivered by the hpsov to the fuel nozzles.
Question 48-31 : Derated or reduced take offs save engine life primarily due to ?
Reduced low cycle fatigue damage.
Low cycle fatigue is where the stress is high enough for deformation to occur. this deformation will occur in the turbine since it is most sensitive to high temperatures.
Question 48-32 : The diagram shown in annex represents a twin spool turbofan. positions before and after air intake are located at numbers. 1054 ?
1 and 2.
Question 48-33 : What is the average compression ratio of a centrifugal compressor ?
1 4.
Img938. axial compressor.. 915. radial compressor..a centrifugal compressor will provide a compression ratio of 5 1 per stage maximum , which is relatively high compared to an axial compressor that achieves a ratio of only 1.2 1 per stage.
Question 48-34 : If engine compressor air is bled off for engine anti icing or a similar system, the turbine temperature ?
Will rise.
Gorny21 .the ratio of the tertiary airflow around the combustion flame tube that cools the turbine decreases, thus less cooling flow as the warm air is diverted for anti icing so the egt will increase as a result.
Question 48-35 : Increasing the frequency of variations in thrust setting during normal operation is detrimental to the life of hp turbine blades because ?
Low cycle fatigue damage is more severe.
It is commonly known that a metal subjected to repetitive or flunctuating stress will fail at a stress much lower than its ultimate strength. failures occuring under cyclic loading are generally termed 'fatigue'..the endurance of the gas turbine engine to high temperature is particularly marked by the creep resistance of hp turbine blade alloy..in turbine engines, low cycle fatigue is generally caused by repeated loading cycles caused by operation i.e. starting or takeoff conditions. if a turbine blade goes through one takeoff max power setting, it may be fine. if it is subjected to 8000 of them, it may fail due to fatigue.
Question 48-36 : An axial compressor, when operating below its design speed, has ?
A tendency to stall in the front stages.
Img938. axial compressor.. 915. radial compressor..the stall of a blade occurs in the same way as a wing stalls, and if the angle of attack exceeds a set value the blade/wing will stall..the first stage of blades on a compressor are rotor blades. when it turns, air is push rearwards increasing the velocity onto the blades behind, but also the pressure. if the horizontal flow is low it results in a large angle of attack, as this is most likely at the front of the compressor, it will tend to stall at the front.
Question 48-37 : The compressor blades in a rotating gas turbine engine are held in position by ?
The mountings and the resultant of aerodynamic and centrifugal forces.
Question 48-38 : Some gas turbine engines have multiple stages in the hp and lp turbines ?
Because one stage does not generate sufficient shaft power.
Question 48-39 : In which part of a gas turbine engine are high temperature requirements most critical for the materials used ?
The combustion chamber and front part of the hp turbine.
Usually titanium alloy is the material used for the combustion chamber and high pressure turbine.
Question 48-40 : A twin spool engine with an inlet air mass flow of 500 kg/s and a hp compressor mass flow of 250 kg/s has a bypass ratio of ?
1.
Img939..bypass ratio = bypass mass flow / hp compressor mass flow..500 250 = 250..250 / 250 = 1
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