15.3 Inlet Flashcards
What is total head pressure
Pressure of air when brought to rest in front of the wings and intakes
What is compression
Intakes increase in pressure within the intake at increasing forward speeds
What is recovery
Regain as much of the ram air velocity as possible and convert it into pressure at the face of the engine
What is intake momentum drag?
As forward speed increases, thrust decreases.
What is ram ratio
The ratio of the total pressure at the inlet to the compressor, to static pressure the entrance to the air intake
What is the intake or air inlet
The structure that directs the airflow to the engine, usually a divergent duct
What is the engine inlet
The point at which the airflow passes from the air inlet into the engine
What is subsonic
Any velocity where all the airflow around a body is below Mach 1
What is transonic
Typically velocity between 0.8 and 1.4
What is the name of the inlet as it gets gradually wider
Divergent
What happens to pressure as it moves through a divergent duct
Increases static pressure
What is the first component of a gas turbine engine
The inlet, it should be as straight and smooth as possible
For air to flow smoothly though a compressor what is the optimum speed range at the compressor inlet
Between 0.4 and 0.7
How are intakes designed to slow down the airflow?
By converting Kinetic energy into pressure energy
What is the most efficient subsonic inlet configuration
Pitot type quasi-circular diffuser
Why type of inlet lip must be avoided for high speed flight
Thicker or larger radius lips
What is restricted if a crosswind exceeds a certain velocity
Restricted from applying full power until they have reached a specified ground speed
What is the most practical transonic inlet
A pitot type design up to Mach 1.5
At transonic speeds what is the inlet designed to do
To keep the shock wave out
How does a transonic inlet keep the shockwave out
By using a normal shock diffuser to decelerate the supersonic airflow efficiently to the speed needed for the compressor
What is supercritical condition?
Should the engine require more air than the inlet can provide a pressure drop will occur in the inlet which mean it will swallow the shock wave.
Airflow hitting the compressor will still be supersonic which isn’t good for the compressor
Due to the physical effects of air flowing into and through the inlet, energy loss or ram recovery can be affected by what o
- Frictional losses due to fuselage air/skin friction
- Frictional losses due to the intake duct walls
- Turbulence losses due to structures or components in the intake
- Turbo props cause drag and turbulence losses due to the spinner and blade roots
- When the aircraft yaws, a loss of ram pressure will exist on one side
What can normal duct inefficiencies of 1% cause
Thrust loss of between 1% - 4%
Which intake designs are most efficient
- Pitot: 96-99% efficient
- Wing root: 87-95% efficient
- Side: 80-89% efficient
- turbo prop annular (dart): 74-82% efficient
What are the 3 types of shock waves associated with subsonic, transonic and supersonic flight
- Normal or perpendicular shock
- Oblique shock
- Bow wave shock
What happens in a normal shock wave
Occurs at low supersonic speeds and stands perpendicular to the airflow. Velocity abruptly drops from supersonic to subsonic and pressure increases rapidly
Not all Kinetic energy is turned into pressure, some is turned into heat
What happens in an oblique shock wave
Airflow is forced to change direction.
Airflow remains supersonic but reduced in value
This is utilised by aircraft designers
What happens in a bow shock wave
If conditions for an oblique shock wave no longer exist and speeds fall below supersonic it will instantaneously jump upstream and change to a normal shock wave with a considerable increase in flow losses
What is the biggest disadvantage in the normal shock diffuser
Abrupt loss of efficiency as the Mach number increases (in the region of 30% at Mach 2)
The oblique shock diffuser was invested to rectify this problem
What is the duct recovery point or (Critical condition)
Maximum airflow to the compressor to allow maximum thrust
What is intake or diffuser buzz
By expelling a normal shock wave forwards it becomes highly unstable, oscillating at high frequency between being swallowed and expelled.
The extreme pressure fluctuations can severely damage the engine and is known as defuser or inlet buzz
How did designers overcome the buzz effect
A variable geometry air intake under control of the (ADC) Air Data Computer is required
The computer will sense flow data such as dynamic pressure, static pressure and air temperature
Why are inlets allowed to produce a normal shock wave to slow down the air at low supersonic speeds and not high supersonic speeds
At low supersonic speeds the normal shock wave is not as aggressive as when speeds are higher and does a sufficient job of slowing down the air with out having huge temperature and pressure issues
What type of shock waves do the variable geometry inlets produce
Several oblique shock waves to gradually bring the speed down before a normal shock wave is appropriate
What is used on variable geometry air inlets to prevent intake buzz
A Bleed door
What is air bled off from the ramps of a variable geometry inlet used for
Engine bay cooling
What are the 2 negative effects that ice build up has on an engine inlet
- A disturbed airflow that reduces the performance of the engine and can lead to a compressor stall
- The engine sucks in pieces of ice. These pieces can damage fan blades or inlet vanes. This would result in the engine stopping completely
What is used to prevent ice build up on the inlet
Engines have a thermal anti ice system
- Bleed air
- Electric heating
What are the 4 main requirements for an anti icing system
- Reliable
- Ease of maintenance
- No excess weight penalty
- No appreciable loss of engine power
What are the anti icing methods for the spinner
- Bleed air
- Hot oil
- A rubber spike is placed on the tip of the spinner, this prevents ice forming
What are anti icing valves used for
Protect against over-pressurisation and limit the bleed air from the engine
The amber valve light inside the switch/light illuminates until the low pressure switch senses a duct pressure of more than what Pressure
5 PSI
What type of power is supplied to the electrical heater mats
Either AC or DC
What is ice prevention cycling time
Due to the high loads required for heating (typically 18 amps continuous and 29 amps cycling) the sensors allow a small amount of ice build up before getting rid of it instead of sapping the engine generators for continuous power
What is used for correct power usage indication
It is provided by an ammeter fed from a current transformer
What are the two typical ice protection cyclic sequence cycles
FAST: (ON/OFF) 2 mins OAT between -6 Celsius and +10
SLOW: (ON/OFF) 6 mins OAT below -6 Celsius
Functional testing of a complete anti icing system must be carried out when
- Periods specified in maintenance schedules
- when a system malfunction occurs
- when a new or overhauled heater mat has been installed
- After replacement of a component (e.g. Cyclic timer or heater element)
- After repairs to a heater mat
When insulation testing heater mats what values can it not fall below
2-4 mega ohms are typical
What is a telltale sign of a heater mat that has overheated
Discolouration
