15.3 Inlet Flashcards
What’s the fundamental purpose of an engine inlet?
Designed to provide a relatively distortion-free airflow to the inlet of the compressor in the required quantity.
What is the purpose of Inlet Guide Vanes (IGVs)?
To straighten the airflow and direct it into the first stages of the compressor. (prevents comp. stall).
What is the inlet considered a structural part of?
The airframe, NOT the engine.
What do many turboprop and turboshaft engines use to protect their inlets from FOD?
Inlet Screens
What tends to happen to thrust as airspeed increases?
Thrust tends to decrease with increasing airpeed.
What is the definition of Ram Recovery?
The added pressure at the inlet of the engine increases the pressure and airflow to the engine
Another name for Ram Recovery?
Total Pressure Recovery.
What can cause a drop in air pressure in the inlet duct?
Friction against the duct walls, and bends in the duct system.
What factors dictate duct entrance design?
- The location of the engines on the airframe.
- Intended operation of aircraft airspeed.
- Intended operation of aircraft altitude.
- Intended operation of aircraft attitude.
What type of aircraft would you find a ‘Divided-Entrance Duct’ on?
High-speed, single or twin engine aircraft.
(Think F35 Lightning side intakes)
Why are the side intakes on a divided-entrance duct placed as far forward as possible?
To increase distance from compressor so that the bend angle of the ducting is a gradual as possible.
What assists straightening the airflow into side-scoop inlets?
Turning vanes.
What is the disadvantage of using side-scoop inlets?
A ram-air pressure imbalance can develop across the two ducts when peforming sudden manoeuvres. Can cause small power-loss.
At what speed must airflow through an engine always be?
Below Mach 1.
What happens to supersonic air as it passes through a converging duct?
Velocity decreases, pressure increases. (Opposite to subsonic air in Bernoulli’s principle).
What are the 3 methods for slowing down supersonic air as it enters an inlet?
- Variable geometry inlet.
- Variable airflow bypass arrangement.
- Using shockwaves in the airstream.
How does a Variable Geometry Inlet slow down supersonic air?
By introducing a movable restriction, such as a ramp or wedge, inside the duct.
How does a Variable Airflow Bypass Arrangement slow down supersonic air into the inlet?
Extracts part of the inlet airflow from the duct ahead of the engine.
How do shockwaves in the airstream slow down supersonic air into the inlet?
The shockwave results in the diffusion of the airflow, which, in turn, decreases the velocity of the airflow.
How are shockwaves in the airstream generated in the inlet?
A shockwave is created by a small restriction/obstruction which automatically protrudes into the duct at high speed.
What is the purpose of a Compressor Inlet Screen for turboprop/shaft engines?
Prevents FOD from entering and damaging the compressor.
Which type of Turboprop/shaft engine especially requires an Inlet Screen?
An axial compressor engine with aluminium blades.
What are the disadvantages of an Inlet Screen?
- Can cause a loss of inlet duct pressure.
- Can ice-up.
What can result in more damage, a failed inlet screen or having no screen at all?
A failed inlet screen can cause more damage than having no screen at all.
Why is a mechanically-rectractable inlet screen beneficial?
Can be withdrawn from the airstream after take-off or whenever icing conditions are encountered.
What are the disadvantages of a mechanically-rectractable inlet screen?
- prone to mechanical failure.
- add weight to the installation.
On what type of engine would an inlet screen not be required?
On turbofan engines with steel or titanium compressor (fan) blades.
Disadvantages outweigh the advantages.
Where are ‘Bellmouth Inlets’ used?
Fitted to the engine inlet during testing.
What does a Bellmouth inlet measure?
Intake temperature, pressure (total and static), and thrust.
Why are Bellmouth inlets used in testing?
They reduce aerodynamic losses to zero, so that the most accurate data can be obtained from testing. Can produce the TSFC and Flat Rating.
How does a turbofan engine protect its core from FOD damage?
The fan takes debris and throws it out and through the bypass duct area.
What are the main problems with ice formation at the inlet?
- Disturbs the airflow, reducing performance and may cause compressor stall.
- Pieces of ice can damage the fan blades and inlet vanes.
How can turboprops keep their inlet free of ice?
Turboprops use the warm oil from the engine oil reservoir to provide anti-icing, together with a minimum of additional hot air.
At what temperature in DRY AIR can ice start to form at the inlet?
5 degrees C in dry air.
At what temperature in MOIST AIR can ice start to form at the inlet?
7 degrees C in moist air.
What are the two types of ice protection ‘Thermal’ system?
- Bleed air.
- Electrical heating.
What type of engines use bleed air?
High-mass airflow engines.
Where is bleed air normally ‘bled’ from?
HP compressor, and passed through anti-icing regulator valve.
What parts of the engine is the bleed air supplied to?
- Intake lip.
- Spinner.
- Surface of inlet guide vanes.
What conditions (other than icing) do the anti-ice valves protect against?
- Overpressure.
- Engine power losses from bleeding too much air.
How are the anti-ice valves operated?
Electronically controlled, pnematically operated.
What type of valve can the anti-ice valve be described as?
Solenoid-controlled pressure regulating valve.
Where does bleed air that’s used to heat the intake end-up, after its use is completed?
It’s ejected overboard.
Where does bleed air that’s used to heat the spinner and IGVs end-up, after its use is completed?
Ejected OVB or returns to the LP compressor intake
What possible indications/cautions are there for the anti-ice valve? (5)
- Valve not in position selected.
- Low duct pressure.
- High duct pressure.
- Low duct temperature.
- High duct temperature.
Which of the 5 cautions/indications causes a system ‘lockout’ and require an ON-OFF reset?
- Low duct pressure.
- High duct pressure.
- Low duct temperature.
- High duct temperature.
(All of them apart from Valve Position).
What do all anti-valves have, in a failure situation?
A manual override.
What do FADEC systems provide for the anti-ice valves?
Auto control for flow and temperature, when anti-icing is switched on or detected.
What do turboprops/shafts normally use to anti-ice their inlets?
Electric-thermal heating. Cycles ON-OFF only when engine is running, to prevent the elements from burning out when there is no airflow.
What safety factor should be observed when testing thermal anti-icing systems on the ground?
No personnel should be in the test area during ground tests of thermal anti-ice systems.
