Aircraft Engines Flashcards
What is the 4 stroke engine 5 event cycle?
Induction, Compression, Power, Exhaust
(Ignition)
SUCK, SQUEEZE, BANG, BLOW !
What is a carburettor?
The CARBURETOR is a device which provides a
suitable mixture of fuel and air to the cylinder
for ignition.
Air flowing through the carburetor draws fuel
from a reservoir through a ‘VENTURI’ tube.
1: The air accelerates through the Venturi
2: There is a drop in air pressure; (Benoulli’s principle)
3: This causes fuel to be drawn through the main jet
where it mixes with air in the Venturi.
The fuel/air mixture is then fed to each cylinder in turn.
What are the problems with the Venturi?
-
Prone to Carb Icing
- Pressure drop = temp drop
fuel burn - latent heat from air reduces temp further = Double Cooling.
So more water vapour = greater icing
How to avoid carb icing?
Fly above/below where moisture present;
Carb heating;
Fuel injection carburettors
What is Feathering?
Adjustment of Variable Pitch Propeller by rotating it parallel to airflow
(reduce drag) if Engine fails
What are the advantages of Piston Engined propeller a/c? A:
- Simpler design = reduced manufacture & maintenance costs;
- More efficient at low speed and low altitude
- Faster initial acceleration than jet engines
- Immediate reaction to pilot input
- Propwash over wings generate lift at low speeds = reduced t/o run & aids controllability
- Air cooled
DISADVANTADGES of piston engine and propellers
Power output limited
Propeller performance limited
- Propellor blade Tips can become supersonic and this decreases their aerodynamic efficiency and noise increases.
What is Blade pitch?
Blade pitch is defined as the angle made by the chord of the blade and its plane of rotation.
When the angle is great, the propeller is said to have a COARSE pitch.
What is coarse pitch? And when is it used?
For speed at cruising altitude, a COARSE pitch is best.
What is Fine pitch? And when is it used?
Used for Take off and climb
What is standard pitch?
Prioritising efficiency over a range of power settings.
Usually fixed props
What is variable pitch propellor?
Enables the pilot to change the pitch of the propeller to suit differing requirements
(Constant speed propellor)
What can you do with variable pitch propellor?
Can be rotated to be parallel to airflow to reduce drag if the engine fails, or is deliberately shut down. This is known as FEATHERING
Reverse pitch angle
This reverses the direction of thrust and it is used to reduce the landing run and to
save wear on tyres and brakes.
What are the Main differences in thrust between a jet and a propellor?
A jet engine accelerates a small volume of air by a large amount
A propeller accelerates a large volume of air by a small amount
What is Newton’s second law?
Force (F) = Mass (M) x Acceleration (A)
What are the principles of jet propulsion?
Intake - Compression - Combustion - Turbine - Exhaust
Suck, Squeeze, Bang, Blow
Air is sucked in and compressed between 3 – 40 times its original pressure.
The basic parts of the jet engine are:
AIR INTAKE
COMPRESSOR STAGE/S
COMBUSTION CHAMBER/S
TURBINE STAGE/S – drives the compressor
EXHAUST
Types of jet engines
Turbojet
Low Bypass Turbofan
High Bypass Turbofan
Turboprop
Ramjet
Scramjet
Turboshaft (Helicopters)
Two types of jet engine compressor?
Centrifugal Flow Compressor - 90’ to the airflow
Axial flow compressor – same direction as airflow
- needs more compression stages
What is a Turbo bypass engine?
Bypass engines use a fan to draw air both through, and around a core.
Developed in order to increase fuel efficiency
and to reduce noise,
Advantages of turbofan engines?
- Efficient –power derived from 2 sources, core & fan;
- Significantly more power than piston engines (80 000 lbs thrust);
- Higher speed achievable;
- Afterburner can be used for additional thrust.
DISADVANTADGES of turbofan engines?
- Complex design & manufacture results in high cost.
- Response to pilot input slow – ‘spool up’ time required to generate power.
What is a turbo prop engine?
A turboprop engine is a jet engine, which drives a propeller.
The turbine at the back is turned by the hot gases and this turns a shaft that drives the propeller via a gearbox
Benefit of a turbo prop?
Compared with a turbojet engine, the
turboprop has better propulsion
efficiency at flight speeds up to around
500 MPH.
ADVANTADGES of turbo props
- Propeller has a higher efficiency than jet exhaust
In dense air, i.e. lower levels, a - Can operate from shorter runways than jets;
- Propeller can be ‘feathered’ to minimize drag in
the event of engine failure, - Mechanical reliability improves due to relatively few
moving parts; - Light weight.
DISADVANTADGES of Turbo prop
- Propellers lose efficiency at high altitudes;
- Vibration and noise levels can cause passenger
discomfort; - Lower cruising levels can be susceptible to icing
and turbulence; - Lower cruising speeds can cause sequencing
problems for ATC when mixed with jet
operations.
