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Module 15 Richard > 15.3 Inlet > Flashcards

15.3 Inlet Flashcards

1
Q

What is total head pressure

A

Pressure of air when brought to rest in front of the wings and intakes

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2
Q

What is compression

A

Intakes increase in pressure within the intake at increasing forward speeds

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3
Q

What is recovery

A

Regain as much of the ram air velocity as possible and convert it into pressure at the face of the engine

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4
Q

What is intake momentum drag?

A

As forward speed increases, thrust decreases.

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5
Q

What is ram ratio

A

The ratio of the total pressure at the inlet to the compressor, to static pressure the entrance to the air intake

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6
Q

What is the intake or air inlet

A

The structure that directs the airflow to the engine, usually a divergent duct

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7
Q

What is the engine inlet

A

The point at which the airflow passes from the air inlet into the engine

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8
Q

What is subsonic

A

Any velocity where all the airflow around a body is below Mach 1

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9
Q

What is transonic

A

Typically velocity between 0.8 and 1.4

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10
Q

What is the name of the inlet as it gets gradually wider

A

Divergent

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11
Q

What happens to pressure as it moves through a divergent duct

A

Increases static pressure

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12
Q

What is the first component of a gas turbine engine

A

The inlet, it should be as straight and smooth as possible

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13
Q

For air to flow smoothly though a compressor what is the optimum speed range at the compressor inlet

A

Between 0.4 and 0.7

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14
Q

How are intakes designed to slow down the airflow?

A

By converting Kinetic energy into pressure energy

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15
Q

What is the most efficient subsonic inlet configuration

A

Pitot type quasi-circular diffuser

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16
Q

Why type of inlet lip must be avoided for high speed flight

A

Thicker or larger radius lips

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17
Q

What is restricted if a crosswind exceeds a certain velocity

A

Restricted from applying full power until they have reached a specified ground speed

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18
Q

What is the most practical transonic inlet

A

A pitot type design up to Mach 1.5

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19
Q

At transonic speeds what is the inlet designed to do

A

To keep the shock wave out

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20
Q

How does a transonic inlet keep the shockwave out

A

By using a normal shock diffuser to decelerate the supersonic airflow efficiently to the speed needed for the compressor

21
Q

What is supercritical condition?

A

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

22
Q

Due to the physical effects of air flowing into and through the inlet, energy loss or ram recovery can be affected by what o

A
  1. Frictional losses due to fuselage air/skin friction
  2. Frictional losses due to the intake duct walls
  3. Turbulence losses due to structures or components in the intake
  4. Turbo props cause drag and turbulence losses due to the spinner and blade roots
  5. When the aircraft yaws, a loss of ram pressure will exist on one side
23
Q

What can normal duct inefficiencies of 1% cause

A

Thrust loss of between 1% - 4%

24
Q

Which intake designs are most efficient

A
  • Pitot: 96-99% efficient
  • Wing root: 87-95% efficient
  • Side: 80-89% efficient
  • turbo prop annular (dart): 74-82% efficient
25
Q

What are the 3 types of shock waves associated with subsonic, transonic and supersonic flight

A
  • Normal or perpendicular shock
  • Oblique shock
  • Bow wave shock
26
Q

What happens in a normal shock wave

A

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

27
Q

What happens in an oblique shock wave

A

Airflow is forced to change direction.

Airflow remains supersonic but reduced in value

This is utilised by aircraft designers

28
Q

What happens in a bow shock wave

A

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

29
Q

What is the biggest disadvantage in the normal shock diffuser

A

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

30
Q

What is the duct recovery point or (Critical condition)

A

Maximum airflow to the compressor to allow maximum thrust

31
Q

What is intake or diffuser buzz

A

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

32
Q

How did designers overcome the buzz effect

A

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

33
Q

Why are inlets allowed to produce a normal shock wave to slow down the air at low supersonic speeds and not high supersonic speeds

A

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

34
Q

What type of shock waves do the variable geometry inlets produce

A

Several oblique shock waves to gradually bring the speed down before a normal shock wave is appropriate

35
Q

What is used on variable geometry air inlets to prevent intake buzz

A

A Bleed door

36
Q

What is air bled off from the ramps of a variable geometry inlet used for

A

Engine bay cooling

37
Q

What are the 2 negative effects that ice build up has on an engine inlet

A
  • 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
38
Q

What is used to prevent ice build up on the inlet

A

Engines have a thermal anti ice system

  • Bleed air
  • Electric heating
39
Q

What are the 4 main requirements for an anti icing system

A
  • Reliable
  • Ease of maintenance
  • No excess weight penalty
  • No appreciable loss of engine power
40
Q

What are the anti icing methods for the spinner

A
  • Bleed air
  • Hot oil
  • A rubber spike is placed on the tip of the spinner, this prevents ice forming
41
Q

What are anti icing valves used for

A

Protect against over-pressurisation and limit the bleed air from the engine

42
Q

The amber valve light inside the switch/light illuminates until the low pressure switch senses a duct pressure of more than what Pressure

A

5 PSI

43
Q

What type of power is supplied to the electrical heater mats

A

Either AC or DC

44
Q

What is ice prevention cycling time

A

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

45
Q

What is used for correct power usage indication

A

It is provided by an ammeter fed from a current transformer

46
Q

What are the two typical ice protection cyclic sequence cycles

A

FAST: (ON/OFF) 2 mins OAT between -6 Celsius and +10

SLOW: (ON/OFF) 6 mins OAT below -6 Celsius

47
Q

Functional testing of a complete anti icing system must be carried out when

A
  • 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
48
Q

When insulation testing heater mats what values can it not fall below

A

2-4 mega ohms are typical

49
Q

What is a telltale sign of a heater mat that has overheated

A

Discolouration

Module 15 Richard (18 decks)

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