15.03 AIR INLET Flashcards
1
Q
What is meant by the Total Head Pressure?
A
The pressure of the air when brought to rest in front of the wings and intakes
2
Q
What is meant by compression?
A
The increase in pressure within the intake at increasing forward speeds
3
Q
What is meant by recovery?
A
To regain as much of the ram air velocity as possible and convert it into pressure at the face of th engine.
4
Q
What is meant by total pressure recovery?
A
If all ram air oressure from the recovery is converted, it is known as total oressure recovery
5
Q
What is meant by the intake momentum drag?
A
When the forward speeds increase, thrust decreases due to the momentum of the air passing into the engine in relation to the forward speed of the aircraft.
6
Q
What is the amount of thrust produced directly proportional to?
A
The difference between the inlet velocity of the airflow and the exit velocity of the exhaust
7
Q
What is meant by the Ram Ratio?
A
The ratio of the total pressure at the inlet to the compressor, to static pressure the entrace of the air intake
8
Q
What is meant by the Air Inlet?
A
The structure that directs the airflow to the engine
9
Q
What type of duct is an inlet normally?
A
Divergent
10
Q
What is meant by the Engine inlet?
A
The point at which the airlow passes from the air inlet to the engine
11
Q
Define Mach number
A
The speed of an object relative to a fluid medium, divided by the speed of sound in the same medium
12
Q
What is the speed of sound?
A
Mach 1
13
Q
Define subsonic
A
Any velocity where all the airflow around a body is below Mach 1
14
Q
Define transonic
A
The velocity in which both subsonic and supersonic airflow exists around a body
15
Q
What is the Mach number for the transonic region?
A
Between 0.8nto 1.4 Mach
16
Q
Define supersonic
A
When all the airflow around a body exceeds the speed of sound
17
Q
What principle does the air inlet duct use?
A
Bernoulli’s principle
18
Q
What should the air inlet ducts be?
A
As straight and smooth as possible
19
Q
The length, shale and llacement of the ducts should be determined by?
A
The location of the engines fitted to the aircraft
20
Q
What must the intake always ensure?
A
Airflow is being delivered to the compressor so that the engine can generate optimum thrust
21
Q
What do the shapes of the inlet depend on? (2)
A
Aircraft design speeds amd engine arrangements
22
Q
What should the airflow velocity be in order to operate smoothly through the compressor?
A
Between 0.4 and 0.7 Mach at the compressor inlet
23
Q
What are air inlets designed to do to airflow?
A
Decelerate the airflow but increase the pressure withiut any undue shock ot losses over a wide range of aircraft speeds
24
Q
What happens in the inlet duct when the engine is running but the aircraft is stationary?
A
Maximum thrust prior to take off
25
The subsonic duct must be shaped to do what?
Deliver air to the front of the compressor with an even pressure distribution
26
What can poor air pressure and velocity lead to?
Compressor stall
27
What should the ideal compressor inlet pressure be the same as?
The total head pressure at the air inlet lip
28
What is the most efficient inlet configuration for subsonic flight?
Pitot quasi-circular diffuser
29
How can you reduce the risk of flow seoaration at the inlet?
Inlet lip is well rounded, therefore has a thicker radius
30
At high speeds, what does the duct require in relation to flow separation?
Thinner lips to maintain performance
31
When engines are mounted under the wing, what can the intake swallow?
Ground vortex which can further degrade the engines performance
32
What will occur when total ram recovery is obtained?
Airflow approaching inlet will be faster than what the compressor is capable of tolerating
33
What is a pitot type intake design only practical for?
Relatively low supersonic Mach numbers up to Mach 1.5
34
What is the duct designed to do at transonic speeds?
Keep the shockwave out
35
How does the duct keep the shockwave out during transonic speeds?
By use if a normal shock diffuser which decelerates supersonic airflow efficiently to the speed needed by the compressor
36
What will the normal shock wave produce?
Pressureand temperature increase with a velocity decrease to subsonic before it enters the duct
37
What does a thinner inlet lip allow?
Allows the angle of the shock front to be small enough to keep the wave drag to a minimum
38
What is meant by Critical Condition? (Transonic ducts)
When the inlet has the sufficient cross section for maximum airflow requirements and applies to only one Mach number and altitude
39
What is meant by the term sub critical?
When the rpm is reduced and causes the normal chock waves to reduce to subsonic and merge with the supersonic shock waves. This causes the shock wave to detach from the lip and reduce the Maxh number
40
What is meant by the term super critical?
When the airflow demand is more than what the inlet can provide, supersonic airflow is swallowed and leads to turbulence then an unacceptable flow to the compressor
41
What can energy loss or ram recover be affected by due to the physical effects of airflowing into and through the inlet? (4)
- frictional losses due to fuselage air/skin friction
- frictional losses at the intake duct walls
- turbulence losses due to structures of components in the intake
- turboprops cause drag and turbulence losses due to the spinner and blade roots
- divided intake duct suffers from losses due to boundary layer problems
42
What percentage of thrust loss will normal duct inefficiencies of 1% cause?
Between 1% to 4%
43
What are the types of inlet designs and their intske efficiencies? (4)
- pitot 96%-99% efficient
- wing root 87%-95% efficient
- side 80%-89% efficient
- turbo prop annular (dart) 74%-82% efficient
44
What are the 3 different types of shock waves associated with the speed ranges?
Normal
Oblique
Bow wave
45
Describe a normal shockwave (5)
- Occurs at low supersonic speeds
- Perpendicular to airflow
- Velocity drops from supersonic to subsonic
- Creates more pressure
- Large amount of unusable heat energy produced
46
Describe an oblique shockwave (2)
- Occurs when the airflow is forced to change directions
| - from supersonic to less supersonic
47
Describe a Bow shockwave
- When oblique shockwaves no longer exist and velocity drops below min supersonic mach number, change into a normal shockwave going upstream
- considerable flow losses
- adjacent parts of the shock bend in a downstream direction forming an oblique shock
- shockwave is then said to be detached
48
What is a disadvantage of a normal shock diffuser?
Has an abrupt loss of efficienct as the Mach number increases
49
How can you overcome a drop in efficiency caused by a normal shock diffuser?
Using an oblique diffuser
50
What is an oblique diffuser designed to do?
To reduce the supersonic flow progressively using a series of oblique shock waves before terminating in the normal shockwave
51
What would the configuration of the inlet require in a supersonic duct?
Either a cone if a wedge suitably located within the subsonic diffuser
52
What determines the number of oblique shocks in an oblique diffuser?
The number of times the airflow direction is forced to change
53
Describe critical condition in a supersonic duct
When the duct reaches its design velocity, the normal shock will be in the diffuser throat and the oblique shock will have contacted the inlet lip preventing spillage and permitting max airflow to the compressor allowing the engine to produce max thrust 6
54
What is the buzz effect?
Extreme pressure fluctuations due to sound being produced
55
How can you overcome the buzz effect?
A variable geometry intake under the control of an air data computer
56
What will an air data computer do?
Sense flow data such as dynamic pressure, static pressure and air temperature
57
What must happen to the airflow before entering the compressor?
The airflow must be slowed to subsonic velocity with the least possible energy wasted
58
What does a normal shock diffuser inlet do?
Employs a single normal shock wave at the inlet to slow the air to subsonic velocity
59
What type of inlet is a normal shock diffuser suitable for?
Low supersonic soeeds where the normal shockwave is not too strong
60
Why is a normal shock diffuser nor suitable for high supersonic speeds?
If the normal shockwave is too strong, it causes a great reduction in the total pressure recovered by the inlet
61
What does a single oblique shock diffuser do?
Employs an external oblique shock wave to slow the supersonic airflow before the normal shock wave occurs
62
What is a more complicated variation of the single oblique shock inlet?
Multiple oblique shock inlet
63
What does a multiple oblique shock inlet do?
Employs a series of very weak oblique shock waves to gradually slow the supersonic airflow before the normal shock occurs
64
What does the optimum shape of supersonic inlets vary with? (2)
Inlet flow direction
| Mach number
65
How do you gain the highest efficiency and stability of operation?
At different speeds, the geometry of the inlet must be the correct angles
66
What are variable supersonic inlets equipped with?
Actuator operated panels
67
When is the engine inlet fully open and flying with a high angle of attack when using variable supersonic inlets?
When the flight speeds just below Mach 1
68
What happens with the actuator operated panels when the flight speeds just above Mach 1?
The actuators operate change the position of the panels slightly and thr inlet employs a single normal shockwave
69
What happens to the actuator operated panels when the flight speeds are muchhigher than Mach 1?
Actuators operate the panels so that they employ thee oblique shock waves and then a normal shockwave
70
Where is the normal shock held in a variable geometry air inlet duct?
Held in the inlet by a moving variable ramp to ensure rhe correct airfloe for engine demand
71
In transonic flight, what will form on the variable ramp?
One normal shock wave perpendicular to the airfloe entering the inlet
72
In supersonic flight, what shock waves will form?
3 oblique shock waves and one weak normal shock
73
What 2 negative effects does ive have on the engine inlet?
- a disturbed air flow that reduces the performance and can lead to compressor stall
- engine sucks in a piece of ice which can damage the fan blades or inlet vanes
74
What do all jet engines have to prevent ice build-up on the engine inlet?
Themak anti-ice system
75
Icing conditions can also occur on the cround during what? (3)
Low visibility
Low air temperature
High humidity in temperatures well above 0 deg C
76
What will stipulate the limits of operation with ice?
Maintenance Manuals
77
What are the requirements for an anti-ice system? (4)
- reliable
- ease of maintenance
- no weight excess penalty
- no appreciable loss if engine power
78
What are the most common types of engine ice protection systems known as?
Thermal systems
79
What can the thermal systems be sub-divided into? (2)
- bleed air
| - electrical heating
80
What other methods besides bleed cpair can be used to protect the spinner from ice build up? (2)
Hot oil from the scavenge oil system can be pumped around the spinner
Rubber spike forming the apex of the spinner, allowing small amounts of ice to accumulate on the spike but is flung off before ice can build up to damaging proportions
81
When is bleed air generally used on engines for ice protection?
Used in engines with high mass airflows
82
Where is the hot bleed air taken from and where is it passed through?
From the HP compressor and is passed through a pressure regulating bleed valve
83
What does the bleed air valve provide surface heating to? (3)
Inlet guide vanes
Spinner
Intake lips
84
How can the bleed valves be operated? (2)
Automatically via an ice detection system
| Manually from the flight deck
85
Where does the used bleed air go to?
Air used for aircraft untake anti-icing is ejected overboard via small grills
Air used for the nose cone and inlet guide vanes can be directed overboard or back into the LP compressor intake
86
How are the anti-icing valves normally selected?
Electrically selected, pressure operated
87
Anti-icing valve indications are given for what? (5)
- valve not in selected position
- low duct pressure
- high duct pressure
- low duct temperature
- high duct temperature
88
When will the amber valve light inside the engine intake thermal anti-icing switch stop illuminating?
Until low pressure switch senses a duct pressure of more than 5psi
89
Where does the engine anti-ice system take its hot air from?
The pneumatic system or directly from the engine
90
What area of the engine intake does the hot air heat up?
The leading edge
91
What are electrical ice protection systems normally employed on?
Turbo prop engined aircraft
92
What are the electrical supplies to heater mats normally?
Either AC or DC
93
What is the definition of continuously heated elements?
Prevent the formation of ice on the leading edge of the aircraft engine air intake lip and propeller spinners
94
What type of system are continuously heated elements?
Anti-icing systems
95
Describe what intermittently heated elements are
Heat is turned off for a period of time to allow an insulatingnlayer of ice to form before being heated again to cause the ice to break away easily
96
What type of system are intermittently heated elements?
De-icing
97
Why is control needed to cycle power automatically to the elements?
To protect against overload conditions due to the heavy loads required for heating and to keep thensapize and weight of the engine generators to a minimum
98
What is cycling time?
The time the engine can accept the amount of ice that collects during the heat off period and then the time for the ice shredding during the heat on periods
99
What are the 2 typical cycles for the Ice Protection sequence?
Fast
| Slow
100
What is the typical sequence for a fast cycle?
(ON|OFF) 2 minutes
OAT between -6deg C and 10 deg C
101
What is the sequence like for a Slow Cycle?
(ON|OFF) 6 Minutes
OAT below -6 deg C
102
What is the selected speed for the cyclic time switch dependent on?
The outside air temperature (OAT)
103
How does cyclic time switch indicate the timer is rotating?
The cycling light glows dim and bright
104
What prevents corrosion on heater mats?
Anti-erosion strips renewed as soon as there are signs of splitting
105
When must continuity and heater resistance checks be carried out?
At the prescribed inspection periods and following any heater mat repairs
106
What are the minimum resistance value limits for any heater elements?
2 - 4 mega-ohms
107
When checking the insulation resistance, what must be taken into account?
The specification of the cement used for bonding the elements to the intake
108
When must functional tests of a complete antixicing/de-icing system be carried out? (5)
- periods specified in the approved maintenance schedules
- when a system malfunction occurs
- when new or overhauled heater mats have been installed
- after replacement of a component
- after repairs to anheater mat
109
What should particular attention be paid to during any functional tests?
Limitation on supply voltages to the engine air intake elements, engine speeds and duration of tests during ground running
