What are the 3 difference between a car’s piston engine and an a/c’s piston engine?
1) most are Air cooled
(saves weight and safer)
2) Duel ignition (2 independent magnetos and spark plugs)
3) Fuel/Air mix. (Manual control of fuel/Air ratio for high/low alt operations)
What are the 4 strokes of the engine?
What are the 5 events for the piston engine?
Intake Compression Ignition Power Exhaust
Where is the piston at the end of the intake stroke?
At bottom dead centre (BDC)
Describe the induction phase.
Piston movies downwards, intake valve opens. Descending piston creates vacuum and draws in air-fuel mix
Describe the compression stroke
Both valves are closed, piston rises and compressed mixture.
When does the power stroke begin?
Before the piston reaches the top.
What happen in the ignition phase?
The spark plug ignites the mixture which forces the piston down turning the crank shaft.
What happens in the exhaust stroke?
Exhaust valve opens and the burned gassed escape , clearing the cylinder.
What is the carburettor?
A device which provides a suitable mixture of fuel to air to the cylinder for ignition
As Air passes through the venturi section, what 3 things happen?
1) Air accelerates
2) Air pressure drops
3) Causes fuel to be drawn through main jet where it mixes with air in the Venturi
Where is the fuel/Air mix fed to from the carburettor?
To each cylinder of the engine
Where the air accelerates the pressure drops and accompanied by an instant __________________
Air temperature drop
What is released when petrol changes from a liquid to a vapour?
Latent heat of Evaporation . This energy goes into the metal of the carburettor, cooling the air further.
The refrigeration effect.
What is a problem with the Venturi type of carburettors?
They are prone to icing.
How much may Air temperature drop by in the carburettor?
As much as 20 degrees
What happens if the water vapour in the carburettor turns to ice?
It may form around the fuel inlet, and throttle valve/Venturi tube chocking the engines air supply. Worst case stopping the engine.
What are the 3 precautions a pilot should take when flying in conditions likely to cause ice?
1) Avoid areas where icing is likely
2) Turn on carburettor heating system
3) During descents apply engine power frequently (keeps engine warm)
What conditions are likely to cause ice for a carburettor?
Flying through cloud
What is Pitch?
The distance the propeller travels per turn.
What does a propeller provide?
Thrust (lift forwards)
What is blade pitch?
The angle made by the chord of the propeller blade and it’s plane of rotation
When the pitch angle is great it is said to be a what pitch?
When pitch angle is small it is said to be a
What is a coarse pitch used for?
What is fine pitch used for?
What is standard pitch?
Compromise between coarse and fine
What is a variable pitch propeller?
Allows the pitch of the propeller to be altered by the pilot.
What is reverse pitch?
A reverse pitch angle that generates reverse thrust.
What propeller system creates reverse pitch?
Certain constant speed propellers
Extra blades allow the power of the engine to be absorbed more efficiently. True or False?
What are the advantages of a piston engine?
- more efficient at low speeds
- Faster initial acceleration than jet
- Propwash over wings
- immediate reaction to pilot input
What are the disadvantages to a propeller engine?
- limited to supersonic blade tip speed
- not as fast as jets
Jet engines accelerates a small volume of air by a large amount.
True or False?
Mass x acceleration
What are the basic parts of a jet engine?
1 Air intake 2 Compression stage 3 Combustion Chamber 4 Turbine Stage 5 Exhaust
Describe briefly how a turbojet works
Air taken in
Compressed 3-12 times original pressure
Fuel added to air and burned in combustion chamber (1,100-1,300 F)
Hot air passes through turbine, driving compressor
Pressure discharged will twice the amount as air pressure.
Produces high velocity stream of gas creating thrust
List some types of jet engines
7 in total
Turbo jet (1st type) Low bypass turbofan High bypass turbofan Turboprop Ram jet Scram Jet Turbo shaft ( helicopter)
What are the two types of compressors?
Describe a centrifugal flow compressor.
Uses an impeller to accelerate air and a diffuser to increase pressure. air leaves compressor at 90 degrees to flight direction. Must be turned back to combustion chamber.
Describe an Axial flow compressor
Employs alternate rows of rotating blades to accelerate air. And stationary vanes (Stator) to diffuse air until pressure rise is obtained.
Advantages of centrifugal flow compressor are:
Large frontal area
Easy to make
What are the advantages of the axial flow compressors?
Longer and thinner
Needs more stages to increase pressure
Why was the turbo fan bypass engine built?
To increase efficiency and reduce noise by surrounding jet with a stream of slower moving air, bypassing the central core of the engine
What are the advantages of a high bypass ratio?
- more efficient
- after burner (reignite gasses, more thrust)
- further for less at high alt
What are the disadvantages of high bypass ratio?
- High cost
- longer to acceleration that piston engine
What is a turboprop engine?
A jet engine attached to propellers.
How does a turboprop work?
Turbine at the back is turned by the hot gases, this turns a shaft that drives the propeller via a gear box.
What is needed to prevent the speed of turboprop propeller blades reaching supersonic levels?
Reduction gearing ratio of between 5:1 to 20:1
What basic parts does a turboprop consist of?
Turboprop has the best propulsion efficiency below 500mph compared to turbojet. True or False?
What are the advantages of a turboprop engine?
- higher efficiency than jet at low levels
- operate into shorter runways
- propeller can be feathered to minimise drag
- mechanically reliable
What are the disadvantages of a turboprop?
Propeller loses efficiency at high altitude
High vibration levels (pax discomfort)
Enroute weather can cause problems
Older ones are slow
Pax discomfort due cruising level.