AME Gas Turbine Engine Theory 5 — Questions and Answers
Question 1: What happens to gas velocity and pressure as subsonic gas flows through a convergent exhaust nozzle?
- Both increase
- Velocity decreases, pressure increases
- Velocity increases, pressure decreases (Correct answer)
- Both decrease
Correct answer: Velocity increases, pressure decreases
Per Bernoulli's principle, subsonic flow in a convergent duct accelerates while static pressure drops.
Question 2: A choked exhaust nozzle produces which type of additional thrust?
- Bypass thrust
- Pressure thrust (Correct answer)
- Ram thrust
- Reverse thrust
Correct answer: Pressure thrust
When the nozzle is choked, exit pressure exceeds ambient, and the pressure difference across the exit area adds pressure thrust.
Question 3: Using F = M x (V2 - V1), what happens to net thrust as aircraft forward speed increases, assuming constant exhaust velocity and mass flow?
- Net thrust decreases (Correct answer)
- Net thrust increases
- Net thrust becomes negative
- Net thrust remains unchanged
Correct answer: Net thrust decreases
Higher forward speed increases inlet velocity V1, reducing the velocity change and thus net thrust.
Question 4: What is the purpose of a convergent-divergent exhaust nozzle on supersonic aircraft?
- Slow exhaust gases to subsonic speed
- Cool the exhaust before it exits
- Reduce engine noise only
- Allow exhaust gases to expand to supersonic velocity for added thrust (Correct answer)
Correct answer: Allow exhaust gases to expand to supersonic velocity for added thrust
Flow reaches Mach 1 at the throat and accelerates supersonically in the divergent section.
Question 5: Which engine parameter is generally used as the primary thrust-setting indication on many turbofan engines?
- EPR or N1 (Correct answer)
- Oil pressure
- Fuel temperature
- Exhaust gas temperature
Correct answer: EPR or N1
EPR (exhaust pressure ratio) or fan speed N1 directly relates to thrust and is used for power setting.
Question 6: Thrust specific fuel consumption (TSFC) is defined as which of the following?
- Thrust divided by fuel flow
- Fuel flow per hour divided by thrust (Correct answer)
- Fuel weight per mile flown
- Fuel flow per engine RPM
Correct answer: Fuel flow per hour divided by thrust
TSFC is pounds of fuel burned per hour for each pound of thrust produced.
Question 7: How does a cascade-type thrust reverser on a high-bypass turbofan produce reverse thrust?
- By shutting off fuel to the core
- By blocking the fan duct and directing bypass air forward through cascade vanes (Correct answer)
- By closing the core exhaust nozzle completely
- By reversing the fan rotation
Correct answer: By blocking the fan duct and directing bypass air forward through cascade vanes
Blocker doors obstruct the bypass duct and cascade vanes turn the air forward.
AME Gas Turbine Engine Theory 5
What happens to gas velocity and pressure as subsonic gas flows through a convergent exhaust nozzle?