M15.03 Inlet

Question 1

What is total head pressure?

Answer 1

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

Question 2

What is compression?

Answer 2

Increases in pressure within the intake at increasing forward speeds

Question 3

What is recovery?

Answer 3

To regain as much of the ram air velocity as possible and convert it into
pressure at the face of the engine. If all available ram air pressure is
converted, it is known as Total Pressure Recovery

Question 4

What is intake momentum drag?

Answer 4

As forward speed increases, thrust decreases. This is due to the momentum
of the air passing into the engine in relation to the forward speed of the
aircraft. The amount of thrust produced is proportional to the difference
between the inlet velocity of the airflow and the exit velocity of the exhaust.
Fn = Vo - V

Question 5

What is ram ratio?

Answer 5

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

Question 6

What is mach number?

Answer 6

It is defined as the speed of an object relative to a fluid medium, divided by
the speed of sound in the same medium. Mach 1 being the speed of sound

Question 7

What is the speed at which you are subsonic?

Answer 7

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

Question 8

What are the transonic speeds?

Answer 8

The velocity in which both subsonic and supersonic airflow conditions exist
around a body. Generally taken as being in the range of Mach 0.8 to Mach 1.4

Question 9

What are the supersonic range?

Answer 9

When all the airflow velocity around a body exceeds that of the speed of
sound

Question 10

Explain the intake shape?

Answer 10

The air intake has an intake nose and an inlet duct.
The air inlet duct gets wider. This shape is named divergent.
Using the Bernoulli Principle, this shape increases the static pressure of air
that is moving through the duct. This is an advantage for the engine

Question 11

What does the inlet also do?

Answer 11

The intake nose also helps to smooth the airflow.
This stops air disturbances from entering the inlet duct, which would reduce
engine efficiency.
Air disturbances can be caused by damage to the intake nose, ice build-up or
even by crosswinds during low speed aircraft operations.
As the aircraft moves through the air, the air enters the engine from the front.
This is because of the ram air effect at high airspeeds

Question 12

What occurs with an engine running on the ground?

Answer 12

If the engine is running but the aircraft is not moving, there is no ram air
effect.
In this situation, air is also sucked in from the side of the engine. This is very
dangerous if maintenance must be done to an engine which is running

Question 13

How does the inlet shape slow down the air?

Answer 13

For air to flow smoothly through a compressor, it’s velocity should be between
Mach 0.4 and 0.7 at the compressor inlet. Therefore, air intakes are designed
to decelerate the airflow: by converting kinetic energy into pressure energy
without any undue shock or losses over a wide range of aircraft speeds

Question 14

How must an inlet be designed?

Answer 14

It must be designed so that the ram velocity of the air
stream is slowly and smoothly decreasing, while ram pressure is slowly and
smoothly rising. The ideal compressor inlet pressure should be the same as
the total head pressure at the air inlet lip

Question 15

What is the most efficient inlet type known as?

Answer 15

The pitot type quasi-circular diffuser

Question 16

Explain nose suction

Answer 16

When total ram recovery has been achieved, the airflow approaching the inlet
will be faster than the compressor is capable of tolerating.
Because of this pressure increase in the inlet the engine benefits by
expending less mechanical energy for compression. The increased pressure
of the free-stream airflow is full forced to accelerate over the outside of the
inlet duct. This acceleration reduces ram drag in front of the inlet and causes
a drop in pressure, producing a suction effect acting in the direction of engine
thrust called ‘nose suction

Question 17

Explain the compromises of producing nose suction

Answer 17

To produce this effect, the inlet lip radius must be smaller (thinner) than that
required for low speed flight. This will degrade the performance at take off
and low speed flight by causing the air to separate at the interior part of the
inlet lip. As these two requirements conflict with each other, a compromise
towards safety must be made, even at the expense of reduced high speed
performance.

Question 18

Explain the pitot type inlet at transonic speeds

Answer 18

At transonic speeds the duct is designed to keep the shock wave out. This is
done by using a normal shock diffuser to decelerate the supersonic airflow
efficiently to the speed needed by the compressor.
The normal shock wave will produce a pressure and temperature rise with a
velocity decrease to subsonic before the air enters the duct. This is a subsonic
design behind a normal shock front

Question 19

How should a pitot inlet be designed for transonic operations?

Answer 19

Ensuring that the inlet lips are as thin as possible allows the angle of the shock
front to be small enough to keep wave drag to a minimum.
The cross section of the inlet must be sufficient for the maximum airflow
requirements of the engine, and will apply to only one particular Mach
number and altitude. This is known as the Critical condition

Question 20

How would the engine thrust being changed effect the the transonic inlet?

Answer 20

If for the same Mach number, the engine thrust setting is changed, the
pressure at the compressor inlet will also change, causing the shock-wave to
change position and type

Question 21

Explain what happens in he inlet if RPM demand is reduced in a transonic inlet

Answer 21

If rpm demand is reduced, the pressure in the inlet will rise allowing less air
in, the excess airflow being forced to flow outside the inlet. Because a normal
shock reduces the velocity to subsonic, the spilled air must merge with the
supersonic flow making it unstable and preventing the shock. This causes
the shock to become detached from the lip and move further upstream (Bow
Shock-wave) reducing the Mach number. When this occurs, it is known as the
sub-critical condition

Question 22

Explain what happens when airflow demand is greater than what the transonic inlet can cope with?

Answer 22

Should airflow demand become greater than the inlet can provide, this will
cause a pressure drop in the inlet causing the shock to be swallowed. This
will allow the airflow to enter the inlet at supersonic velocity. This change in
velocity will be inconsistent with the design of the duct, resulting in complex
shock-waves and turbulence due to flow separation at the duct walls causing
an unacceptable flow into the compressor. This is known as the super-critical
condition

Question 23

What are the intake losses?

Answer 23

1. Frictional losses due to fuselage air/skin friction.
2. Frictional losses at 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. A divided intake duct suffers from losses due to boundary layer problems.
   When the aircraft yaws, a loss of ram pressure will exist on one side.

Question 24

How much of a percentage of thrust loss is encountered with one 1% duct inefficiency?

Answer 24

4%

Question 25

What occurs if inlet airflow is inefficient?

Answer 25

if the airflow in the inlet is inefficient and then the turbine temperature increases. Therefore it is necessary to increase the propelling nozzle area to ensure the temperature remains within limits.

Question 26

What is the efficiency ratings of the various inlet designs?

Answer 26

* Pitot 96 - 99% efficient * Wing root 87 - 95% efficient * Side 80 - 89% efficient * Turbo Prop Annular (Dart) 74 - 82% efficient

Question 27

What are the three shockwaves encountered with supersonic flight?

Answer 27

1. Normal, or perpendicular shock 2. Oblique shock 3. Bow wave shock

Question 28

What is a normal shockwave?

Answer 28

The normal shock-wave occurs at low supersonic speed and stands perpendicular to the airflow. Velocity drops abruptly from supersonic to subsonic and the pressure rises rapidly, irrespective of the velocity of the supersonic flow upstream of the shock. The direction of flow across the shock-wave will remain constant. However, not all the kinetic energy of the airflow is converted into pressure energy, a large amount is turned into unusable heat

Question 29

What is an oblique shockwave?

Answer 29

If the airflow is forced to change direction, the shock wave that forms will be inclined at an oblique angle determined by the upstream velocity. This shockwave differs from a normal shock in that the changes across it are less severe in magnitude and the flow, although staying supersonic, reduces in value. Because the intensity of change is less than across a normal shock-wave, it is utilised by aircraft designers.

Question 30

What is a bow shockwave?

Answer 30

If the conditions for the oblique shock-wave no longer exist (velocity drops below a minimum supersonic Mach number, or the flow deviation exceeds a maximum angle) it will instantaneously jump upstream and change to a normal shock with a considerable increase in flow losses. The adjacent parts of the shock will bend in a down stream direction to form an oblique shock. The shock is then said to have detached

Question 31

Around mach 2 what is the % loss of efficiency?

Answer 31

Around 30%

Question 32

What is the duct recovery point or critical condition?

Answer 32

The configuration of the inlet requires either a cone or a wedge suitably located within the subsonic diffuser. The number of oblique shocks being determined by the number of times the airflow direction is forced to change. With this type of inlet, the angle of the oblique shock is determined by the free stream Mach number and the apex angle of the wedge or cone. 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 the maximum airflow to the compressor allowing the engine to produce maximum thrust. This point is known as the duct recovery point or critical condition

Question 33

When can icing occur?

Answer 33

Icing conditions can also occur on the ground during conditions of low visibility, low air temperatures, as well as conditions of high humidity in temperatures well above 0°C

Question 34

What pressure msut be sensed before a valve light will extinguish?

Answer 34

The amber valve light inside the switch/light illuminates until the low pressure switch senses a duct pressure of more than 5 psi

Question 35

What are the two condition that will cause a fault light to illuminate on the anti ice system?

Answer 35

This system is monitored by two pressure switches, found downstream of the valve, and one valve limit switch. The fault light illuminates when the wing anti-ice push-button is switched on, but the pressure does not reach 14 psi. It also comes on when the push-button is switched off but the valve is not fully closed. A special situation arises when the aircraft is on the ground. The wing anti-ice valves close automatically when the aircraft lands. The wing anti-ice push-button does not need to be switched to off at this moment. When the pressure decreases below 14 psi, there is not a fault light. This is because the ground sensing switch opens the circuit. The blue on-light stays on as long as the push-button is pressed

Question 36

What are the two types of electrical heating?

Answer 36

Continuously Heated Elements These prevent the formation of ice on the leading edge of the aircraft engine air intake lip and propeller spinners. These type of systems are anti-icing systems. Intermittently Heated Elements These type of elements, situated behind the continuously heated elements, during the heat OFF period allow an insulating layer of ice to form on the element. When the system cycles to heat ON the temperature increases rapidly (due to the layer of ice), causing the ice to melt sufficiently for it to break away with the slipstream. To ensure that the ice breaks away easily, these elements are divided into separate segments. These type of systems are de-icing systems.

Question 37

Explain electrical heating control

Answer 37

Due to the high loads required for heating (typically 18 amps continuous and 29 amps cycling), and to keep the size and weight of the engine generators to a minimum, control is needed to cycle power automatically to the elements, and to sense and protect against overload conditions. Cycling time ensures that the engine can accept the amount of ice that collects during the Heat Off period; yet ensures that there is adequate ice shedding during the Heat On period

Question 38

Explain electrical heating indication

Answer 38

Indication of correct power usage is provided by an ammeter fed from a current transformer. Time switch operation is indicated by a cycling light (blue/ green).