DGCA Technical General: Gas Turbines — Principles & Construction Questions
271 exam-style Gas Turbines — Principles & Construction questions for the DGCA Technical Generalpaper (CPL & ATPL). The first 12 are free below with answers and explanations — sign up free to practise all of them with progress tracking.
- Q1
What makes the non-rigid fittings of compressor and turbine blades rigid when the engine is running.
- A Spring locks
- B Thrust and drag forces
- C Aerodynamic and Centrifugal force
- DTapered bead seats
Show answer
Correct answer: C. Aerodynamic and Centrifugal force
Compressor and turbine blades are deliberately fitted with a slight looseness in their fir-tree or dovetail roots so they can be assembled and to allow for thermal movement. Once the engine is running the enormous centrifugal loading, aided by aerodynamic gas loads, throws the blades firmly outward and seats the roots rigidly.
- Q2
Where does the air expand in an impulse turbine?
- ARotors.
- BStators.
- CRotors and stators.
- DIt does not expand.
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Correct answer: B. Stators.
This Explanation is Coming Soon
- Q3
The purpose of an air inlet is to provide a relatively____________supply of air to________ the of the ___________compressor.
- Aturbulent free,face,low pressure
- Bturbulent, face, low pressure
- C turbulent free,rear low pressure
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Correct answer: A. turbulent free,face,low pressure
The air inlet must deliver a smooth, turbulence-free and uniform supply of air to the face of the low pressure compressor. Turbulent or distorted flow at the compressor face degrades efficiency and can provoke blade stall and surge.
- Q4
In a centrifugal compressor
- AThe air enters the eye tangentially and leaves the periphery axially
- BThe air enters the eye tangentially and leaves the periphery axially
- C The air enters the eye radially and leaves the tip tangentially
- DThe air enters the impeller axially at the eye and leaves at the periphery tangentially.
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Correct answer: D. The air enters the impeller axially at the eye and leaves at the periphery tangentially.
In a centrifugal compressor the air is drawn into the centre, or eye, of the impeller in an axial direction, is then flung radially outward by the rotating vanes, and leaves the impeller tip tangentially at high velocity before the diffuser slows it to raise pressure.
- Q5
An axial flow compressor when compared to a centrifugal compressor
- ATakes in less air and is less prone to rupturing
- BTakes in more air and is more prone to rupturing
- C Takes in more air and is less prone to rupturing
- D Takes in less air and is more prone to rupturing
Show answer
Correct answer: B. Takes in more air and is more prone to rupturing
An axial-flow compressor passes a much greater mass of air than a centrifugal type of the same frontal area, giving higher thrust, but its many thin, highly stressed blades and discs make it more vulnerable to damage and rupture.
- Q6
The turbine blades in gas turbine engines?
- AAre mainly impulse-type at the tips and mainly reaction-type at the roots.
- BAre mainly impulse-type at the roots and reaction -type at the tips.
- CAre mainly impulse-type at all points.
- DAre mainly reaction-type at all points.
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Correct answer: B. Are mainly impulse-type at the roots and reaction -type at the tips.
This Explanation is Coming Soon
- Q7
429.When gas passes through a convergent duct
- AVelocity and temperature increases and their pressure decreases
- BVelocity increases and their temperature and pressure decreases
- CTheir velocity decreases and their temperature and pressure Increases
Show answer
Correct answer: B. Velocity increases and their temperature and pressure decreases
When subsonic gas flows through a convergent duct it accelerates; in accordance with Bernoulli's principle the gain in kinetic energy is paid for by a fall in static pressure, and the temperature falls with it.
- Q8
In a high by-pass engine with a 'pitot' Intake, with the engine running and the brakes on, what will P1 be in relation to PO?
- ASame
- BGreater
- C less
Show answer
Correct answer: C. less
With the aircraft stationary the engine must draw its own air into the intake; the air accelerates into the duct and its static pressure falls, so the compressor-face pressure P1 is lower than ambient P0.
- Q9
What happens to pressure, temperature and velocity of the air in the diffuser of a centrifugal compressor
- AVelocity increase, pressure and temperature decrease.
- BVelocity decrease, pressure and temperature increase.
- C Velocity, pressure and temperature increase.
- D Velocity, pressure and temperature decrease.
Show answer
Correct answer: B. Velocity decrease, pressure and temperature increase.
In the diffuser the high-velocity air leaving the impeller is progressively slowed as it passes through the divergent passages, converting kinetic energy into pressure. As velocity falls the pressure and temperature both rise.
- Q10
An engine having a ‘Free turbine’
- AThere is a mechanical connection between the power output shaft and the free turbine
- BThere is no mechanical connection between the power output shaft and the free turbine
- C There is a mechanical connection between the compressor and the propeller shaft
- D Air enters via compressor inlet on the turbine
Show answer
Correct answer: A. There is a mechanical connection between the power output shaft and the free turbine
In a free-turbine engine a separate power turbine drives the output (propeller or rotor) shaft through a mechanical connection, but it is not mechanically linked to the gas-generator compressor and its turbine. The power turbine is free to run at its own optimum speed, driven only by the gas stream.
- Q11
When air is bled from the compressor of a gas turbine engine?
- AEGT & EPR both decrease.
- BEGT & EPR both increase.
- CEGT decreases and EPR increases.
- DEGT increases and EPR decreases.
Show answer
Correct answer: D. EGT increases and EPR decreases.
This Explanation is Coming Soon
- Q12
The pressure rise across each stage of an axial flow compressor is:
- AGreater than that of a centrifugal compressor.
- B4:1
- CBetween 1.1 and 1.2 to one.
Show answer
Correct answer: C. Between 1.1 and 1.2 to one.
Each axial compressor stage produces only a small pressure rise, typically about 1.1 to 1.2 to one, which is why many stages are stacked in series; a single centrifugal stage, by contrast, can achieve around 4:1.
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