INLETS Flashcards
What is the total head pressure?
Pressure of air when brought to rest in front of the wings and intakes.
If all ram air pressure is converted into pressure at the face of the engine, what is it called?
Total Pressure Recovery
What is intake momentum drag?
As forward speed increases, thrust decreases.
What is ram ratio?
Ratio of total pressure at inlet to compressor, to static pressure at the entrance to the air intake
What duct is normally used for air intakes?
Divergent.
What is the advantage of a convergent duct for air intake?
Increases static pressure of air moving through the duct.
What could air disturbances at the inlet be caused by?
Damage to intake nose, ice build up or crosswinds at low speeds.
For a flow smooth of air through a compressor, what must the velocity be between at the compressor inlet?
Mach 0.4-0.7
Air intakes designed to convert kinetic energy to pressure energy
What can result in a compressor stall in a subsonic inlet?
Poor air pressure and velocity distribution.
What is the most efficient inlet configuration for subsonic flight?
Pitot type quasi-circular diffuser
For take off, what is the problem of the engine drawing in air from behind the intake lip?
Air suffers a severe directional change which can result in flow separation from extreme local acceleration, degrading engine performance.
How is flow separation reduced from air being drawn in from behind the intake lip?
Intake lip well rounding and thicker radius.
However thinner radius is needed for high speeds
What can be created for engines mounted underneath the wing?
A ground vortex swallowed by the inlet.
And low pressure core of the vortices can ingest FOD
What speeds are supersonic inlets suitable for?
Low supersonic speeds where the normal shockwave isn’t to strong.
(High supersonic unsuitable as normal shock too strong.)
What is the purpose of an oblique shock inlet?
Employs a series of weak oblique shocks to slow the airflow before the normal shock occurs.
Oblique shock inlets leads to the least waste of energy. What does this in turn lead to?
Highest pressure recovery.
What does the optimum shape of a supersonic inlet depend on?
Inlet flow direction with the MACH number.
At Mach 1 what position is a variable supersonic inlet at?
Fully open and aircraft flies with high angle of attack.
At flight speeds above Mach 1, what position is the variable supersonic inlet at?
Actuators slightly change position creating a single normal shock wave.
At high Mach numbers, what is the variable supersonic inlet at?
Actuators move panels to employ 3 oblique shock waves then a normal shock wave.
How do Variable geometry air inlet ducts work?
- Normal shock held In inlet by moving variable inlet ramp (ensures correct air for engine)
- high angles of attack and supersonic manoeuvres, a door opens bleed air preserves engine stall margin
How many shockwaves take place inside a variable geometry air inlet duct?
4
3 oblique, 1 normal
How is ramp position controlled on variable geometry air inlet ducts?
Automatically by the air data computer.
What does the bleed door prevent in variable geometry air inlet ducts?
Intake buzz
Bled air used for engine cooling
What negative effects does ice cause to engines?
- Disturbed airflow, which can lead to compressor stall
- engine sucking in ice
What system is used to prevent ice build up in engines?
Thermal anti icing system
Where are icy conditions possible?
- On the ground low visibility
- low air temperatures
- high humidity in temperatures above 0 degrees.
What are the system requirements for an anti-icing system?
- reliable
- ease of maintenance
- no excess weight
- no loss of engine power
What are the two types of anti-icing systems?
- electrical
- bleed air
What anti icing system will protect both the spinner and inlet lip from ice?
Bleed air
What methods can be adopted to prevent ice build up on the spinner?
- hot oil (scavenge oil from engine oil system pumped around)
- rubber spike (ice builds up, becomes unbalanced and flings off)
What stage of the engine is bleed air taken from?
HP compressor
- passed through pressure regulating BV to provide surface heating to inlet guide vains and inlet lip.
How do anti-icing valves prevent loss of engine power?
Prevent over pressure and limit bleed air off engine.
Where is the bleed air for anti icing directed after use?
- intake anti ice overboard via small grills
- nose cone/inlet guide vains back into LP compressor.
Anti-icing valves are electrically selected. What indications cause a latched relay to be tripped off forcing reset?
Low/high duct pressure
Low/high duct temperature
(Reset by selecting OFF then ON)
The amber valve light will illuminate until the pressure switch senses a pressure over what PSI?
5 PSI
Duct over-pressures are sent to ECAM-EICAS
When ground testing thermal anti-icing systems, why should caution be taken?
Air is very hot, no personnel in the area.
Where is electrical anti ice systems employed?
Turbo-prop engines
Heater mats either AC - DC
What does continuously heated elects concern?
Leading edge of inlet lip, propellers and spinners
Imminently heated elements concern what in the engine?
Parts behind continuously heated.
- OFF period allow insulation layer of ice
- ON period rapidly heats due to insulation
How are engine generators kept to minimum weight and size?
Control is automatic to the elements and to sense and protect against overload conditions.
What is cycle time?
The amount of ice that will build up during the OFF period and ice shredding during ON
What are the typical cyclic sequences?
FAST (ON/OFF) 2 Minutes OAT between minus 6 and plus 10 degrees
SLOW (ON/OFF) 6 Minutes OAT below minus 6
What determines which cyclic sequence is selected?
Outside air temp.
- Between 10 degrees + and -6 degrees = FAST
- Below -6 degrees = SLOW
What should heater mats be examined for?
Splits, wrinkling, discolouration and general condition.
How is corrosion avoided on heater mats?
Anti-corrosion strips renewed at signs of splitting or Adv. erosion.
What causes heater mats to be found tacky?
Contact with oils and solvents
Must be replaced
When is an insulation resistance check carried out on heater mats?
If there is a breakdown between heater mats insulation
What are the minimum typical in service values for heater mats?
2 to 4 mega-ohms
When checking insulation resistance, what needs to be taken into account?
The specification of the cement used for bonding as it has a bearing on the resistance value obtained.
Where should particular attention be paid to when functional testing anti-icing systems?
Limitation on supplied voltage to the engine intake elements, engine speeds and duration of tests.
What will be caused on the windward side of an intake in crosswinds?
Speeds which could exceed speed of sound and flow separation.
(Can restrict applying full power)
When is air no longer drawn from behind the intake lip?
When forward speed increases.
What is nose suction?
When total ram air recovery has been achieved, the airflow approaching the compressor is faster than is capable.
The increased pressure is forced to accelerate over the outside of the inlet duct.
Reduces ram drag at inlet and causes drop in pressure, producing a suction effect acting in direction of engine thrust.
What is needed to produce nose suction?
Nose intake lips thinner than those needed at low speed flight.
What is one method to compromise between needed thin and thicker lips at different speeds?
Increase intake cross sectional area to match airflow demands at high and low speeds.
(High speed performance is jeopardised, and to prevent flow separation the curvature of intake is increased, and if not done correctly sonic flow can occur)
At transonic speeds what is the engine duct designed to do?
Keep the shock wave out.
Using a normal shock diffuser
What does a normal shock wave cause?
Temperature and pressure increase, velocity decrease.
What is the critical condition for transonic profiles?
Ensuring the cross sectional area is large enough for maximum airflow requirements of the engine, and will apply to one particular Mach number and altitude.
What happens in RPM is reduced in a transonic inlet?
Pressure increase in the inlet allowing less air in, excess air forces to flow outside the inlet.
What is the sub-critical condition in a transonic inlet?
A shock becoming detached from the lip, creating a bow wave upstream and reducing the Mach number.
What is the super-critical condition?
If airflow demand becomes greater than inlet can provide, pressure drops causing a shock to be swallowed.
An unacceptable flow into the compressor.
What can energy loss or ram air recovery be affected by?
- Frictional losses from fuselage/air skin friction
- Frictional losses at intake duct walls
- Turbulence losses due to components or structures in intake
- Turbo props cause drag due to spinner and blade roots
- Divided intake causes flow separation
What will normal duct inefficiencies of 1% cause a thrust loss between?
1-4%
What happens if the airflow in the inlet is inefficient?
Turbine temperature increase.
Necessary to increase the propelling nozzle area to ensure temp remains in limits
What is the most efficient intake design?
Pitot (96-99% efficient)
What happens to the velocity of air after an oblique shock wave?
Remains supersonic, but reduces in value.
What happens to the velocity of air after a bow shockwave?
Velocity drops below minimum supersonic Mach number.
What is a disadvantage of a normal shock diffuser in supersonic inlets?
Abrupt loss of efficiency as Mach number increases
(Region of 30% at Mach 2)
Overcome by creating series of oblique shock waves.
In a supersonic inlet, what is the angle of the oblique shock wave determined by?
Free stream Mach number and apex angle of wedge or cone.
What is the duct recovery point (critical condition) for supersonic inlet?
Oblique shock wave in the inlet lip preventing spillage and providing intake with maximum airflow for maximum thrust.
What is a Diffuser Buzz?
Extreme pressure fluctuations which can severely damage the engine due to the sound produced.
How is the problem of a Diffuser Buzz rectified?
Using a variable geometry air intake under the control of an air data computer.
