M15.03 Inlet Flashcards
What is total head pressure?
Pressure of the air when brought to rest in front of the wings and intakes
What is compression?
Increases in pressure within the intake at increasing forward speeds
What is recovery?
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
What is intake momentum drag?
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
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 mach number?
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
What is the speed at which you are subsonic?
Any velocity where all the airflow around a body is below Mach 1
What are the transonic speeds?
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
What are the supersonic range?
When all the airflow velocity around a body exceeds that of the speed of
sound
Explain the intake shape?
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
What does the inlet also do?
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
What occurs with an engine running on the ground?
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
How does the inlet shape slow down the air?
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
How must an inlet be designed?
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
What is the most efficient inlet type known as?
The pitot type quasi-circular diffuser
Explain nose suction
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
Explain the compromises of producing nose suction
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.
Explain the pitot type inlet at transonic speeds
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
How should a pitot inlet be designed for transonic operations?
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
How would the engine thrust being changed effect the the transonic inlet?
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
Explain what happens in he inlet if RPM demand is reduced in a transonic inlet
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
Explain what happens when airflow demand is greater than what the transonic inlet can cope with?
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
What are the intake losses?
- Frictional losses due to fuselage air/skin friction.
- Frictional losses at 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. - 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.
How much of a percentage of thrust loss is encountered with one 1% duct inefficiency?
4%
What occurs if inlet airflow is inefficient?
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.
What is the efficiency ratings of the various inlet designs?
- Pitot 96 - 99% efficient
- Wing root 87 - 95% efficient
- Side 80 - 89% efficient
- Turbo Prop Annular (Dart) 74 - 82% efficient
What are the three shockwaves encountered with supersonic flight?
- Normal, or perpendicular shock
- Oblique shock
- Bow wave shock
What is a normal shockwave?
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
What is an oblique shockwave?
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.
What is a bow shockwave?
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
Around mach 2 what is the % loss of efficiency?
Around 30%
What is the duct recovery point or critical condition?
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
When can icing occur?
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
What pressure msut be sensed before a valve light will extinguish?
The amber valve light inside the switch/light illuminates until the low pressure
switch senses a duct pressure of more than 5 psi
What are the two condition that will cause a fault light to illuminate on the anti ice system?
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
What are the two types of electrical heating?
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.
Explain electrical heating control
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
Explain electrical heating indication
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).
