Module 15 – Gas Turbine Engines (DCAM Part-66)

Comprehensive Q&A flashcards covering fundamentals, performance, inlets, compressors, combustion, turbines, exhaust, noise and thrust-reverse systems for DCAM Part-66 Module 15 Gas Turbine Engines.

What principle discovered by Hero and demonstrated with steam power underpins modern turbine engines?

The reaction principle (action–reaction)

Which Egyptian mathematician invented the aeolipile, an early reaction device?

Hero of Alexandria

What early Chinese invention (circa 1200 AD) applied the reaction principle with black powder?

The solid-fuel rocket

Who patented the first turbo-jet aircraft engine in 1930?

Sir Frank Whittle

State Newton’s First Law of Motion.

A body at rest remains at rest and a body in motion continues in uniform straight-line motion unless acted upon by an external force.

Write Newton’s Second Law as an equation.

F = m a (Force equals mass times acceleration)

What does Newton’s Third Law state?

For every action there is an equal and opposite reaction.

Define potential energy in the context of gas turbine engines.

Energy stored due to position, state or pressure (e.g., compressed air in a bottle)

Define kinetic energy in the context of gas turbine engines.

Energy possessed by a mass due to its motion (e.g., high-velocity gas stream)

Name the thermodynamic cycle on which a gas turbine operates.

The Brayton Cycle (constant-pressure cycle)

List the four continuous processes of the Brayton Cycle.

Compression, constant-pressure heat addition, expansion, constant-pressure heat rejection

Give the SI unit of work.

The Joule (J)

What is shaft horsepower (shp)?

The brake horsepower delivered to the propeller or rotor shaft of a turboprop/turboshaft engine.

Explain ‘specific fuel consumption’ (SFC) for a turbo-jet.

Fuel flow (lb/hr) divided by engine thrust (lb); units lb/hr/lb thrust.

What is engine pressure ratio (EPR)?

Ratio of turbine discharge total pressure (Pt7) to compressor inlet total pressure (Pt2); used to set thrust.

Define bypass ratio.

Mass flow of air that bypasses the core divided by the mass flow passing through the core (always expressed x : 1 where 1 is core flow).

Which intake design is ideal for sub-sonic pitot inlets?

A smooth divergent duct acting as a diffuser

What phenomenon begins to recover thrust lost to momentum drag at ≈160 mph (Mach 0.1)?

Ram pressure recovery (ram effect)

Name two main types of gas-turbine compressors.

Centrifugal (radial-flow) and axial-flow

What is the typical pressure ratio of a single-stage centrifugal compressor?

About 4 : 1

Why are axial compressors built with multiple stages?

Each stage adds only a small pressure rise (~1.2 : 1); many stages are needed for high overall ratios.

Give four basic sections of a gas turbine engine.

Inlet, compressor, combustion section, turbine, exhaust (plus accessory gearbox)

Describe ‘compressor stall’.

Local airflow separation on rotor blades due to excessive angle of attack, leading to loss of compression.

What is compressor ‘surge’?

Total breakdown of flow through the compressor causing violent reversal of airflow and large pressure fluctuations.

State two methods used to control compressor airflow at low rpm.

Bleed valves and variable inlet/stator guide vanes (VIGV/VSV)

What is the main purpose of turbine nozzle guide vanes?

Convert gas pressure to velocity and direct the flow onto turbine blades at the optimum angle.

How is a modern first-stage turbine blade cooled?

Hollow blade with internal air passages and small exit holes providing impingement, convection and film cooling.

Explain ‘creep’ in turbine blades.

Permanent time-dependent elongation of metal under high temperature and centrifugal stress.

Which alloy family is commonly used for turbine discs?

High-nickel alloys such as Inconel

What is the common blade-to-disc attachment for turbine rotors?

Fir-tree (serrated) root

Name the three main combustor types used in aero-engines.

Multiple can (can), can-annular, annular (including reverse-flow annular)

State the chemically correct (stoichiometric) air–fuel ratio for kerosene.

15 : 1 by mass

What proportion of total compressor air typically enters the primary combustion zone?

≈25 %

Why are duplex fuel nozzles used?

Provide good atomisation over a wide flow range via separate primary and main spray passages.

Function of a combustor drain valve.

Allows unburned fuel to drain after shutdown or false start, preventing hot-section fires.

List two advantages of a high-bypass turbofan over a low-bypass engine.

Higher propulsive efficiency/lower SFC and reduced exhaust noise.

What is active clearance control (ACC) in turbines?

System that cools or heats turbine casing to maintain optimum blade tip clearance.

How does a convergent nozzle create thrust?

Accelerates subsonic exhaust flow to near-sonic velocity converting pressure energy to kinetic energy.

Why is a convergent-divergent (C-D) nozzle required on supersonic aircraft?

To expand choked flow beyond Mach 1 and regain pressure-to-velocity conversion for maximum thrust.

Define ‘choked nozzle’.

Condition where gas reaches Mach 1 at the throat; mass flow becomes constant and further pressure increases produce pressure thrust.

Name two exhaust noise-reduction features on modern engines.

Mixing lobe (chevron) nozzles and acoustic liner panels in nacelles

Purpose of a thrust reverser.

Redirect engine exhaust or fan flow forward to produce reverse thrust and shorten landing roll.

Which reverser type employs blocker doors and cascades on the fan duct?

Cold-stream (fan) cascade reverser

State three safety interlocks that prevent in-flight thrust-reverser deployment.

Weight-on-wheels (squat) switch, throttle lever at idle detent, system hydraulic/pneumatic locks

What does SFC typically do as altitude increases up to ~36,000 ft?

Decreases (improves) owing to lower inlet temperature and improved engine efficiency.

Explain ‘flat rating’ of a gas-turbine engine.

Limiting maximum take-off thrust to a lower fixed value, allowing that rated thrust to be maintained up to a specified ambient temperature.

Which engine efficiency measures the conversion of fuel heat to kinetic energy in the jet stream?

Thermal efficiency

Give the formula for propulsive efficiency (ηₚ).

ηₚ = 2V / (Vj + V) where V = aircraft speed, Vj = jet velocity

What is a typical bypass ratio for a high-bypass civil turbofan?

Greater than 3.5 : 1 (often 5 : 1 or above)

Describe the function of a plenum chamber in a double-entry centrifugal compressor.

Delivers uniform, low-velocity air to the rear face of the impeller.

Why are compressor blade tips sometimes fitted with ‘squealer’ tips?

To minimise tip clearance losses by cutting their own track in abradable casing liners.

What is the main cause of low-speed compressor stall?

Excessive angle of attack on front rotor blades due to reduced axial airflow at low rpm.

Which engine component provides the highest pressure in the cycle?

Diffuser (compressor outlet)

Name two common turbine blade crystal technologies that resist creep.

Directionally solidified (DS) and single-crystal (SC) blades

State two key roles of the inlet guide vanes (IGV) on an axial compressor.

Pre-swirl the air in the rotor direction and control mass-flow angle at low rpm (variable IGV)

What is the typical temperature drop per turbine stage in a turbo-jet?

≈200 °C

Function of the exhaust cone support struts besides structural support.

Straighten residual swirl in the exhaust flow and house service lines to aft bearings.

What pressure ratio across the jetpipe will choke a convergent nozzle?

Approximately 1.85 : 1 (jetpipe pressure to ambient)

Define ‘resultant thrust’.

Forward-acting thrust forces minus rearward-acting forces through the engine; equals gross static thrust.

Why are turbine discs sometimes drilled with large radial holes?

To allow blade-root cooling air and reduce disc weight while maintaining strength.

What design feature of high-bypass engine fan blades improves FOD resistance?

Wide-chord, hollow, low-aspect-ratio titanium blades

Explain ‘compressor cascade effect’.

High-pressure side of one aerofoil pumps air into low-pressure side of the next, maintaining flow through successive stages.

Name the three noise metrics typically regulated for civil aircraft.

Take-off (fly-over), sideline, and approach noise levels

How does a bellmouth intake benefit a ground-test engine?

Provides minimal pressure loss and uniform flow for calibration; fitted with mesh for FOD safety.

What is the purpose of the anti-icing hot-air system on engine inlets?

Prevents ice formation on cowl, IGV and probe surfaces by heating them with compressor bleed air.

Give two advantages of a two-spool compressor over a single-spool.

Higher overall pressure ratio with fewer stages and better starting/acceleration characteristics due to independent spool speeds.

Why is oil used to heat some inlet components (hot-oil anti-ice)?

Utilises engine scavenge oil heat to prevent ice while saving bleed-air fuel penalty.

Where is Pt7 measured?

At the turbine discharge total pressure probe (station 7)

What does an amber REV UNLOCK light (or equivalent) indicate?

Thrust reverser has unlocked and begun to translate/deploy but is not yet fully deployed.