Engine Propulsion

8/8/2019 Engine Propulsion

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A propeller is usually a typical type offan that pass on power by changing rotary

movement into thrust for propulsion of a transport such as an aircraft, ship, or

submarine through a mattersuch as waterorair, by spinning two or more twisted

blades about a central shaft, in a way similar to spinning a screw through a solid.

The blades of a propeller perform as spinning wings, and create a force through

the applications of both Bernoulli's principle and Newton's third law, producing a

dissimilarity in pressure between the front and the back surfaces of the airfoil-

figured blades and by speeding up a mass of air backwards.

Description

Vineed .M. Thomas Engineering Propulsion 2
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These blades are constructed in the form of an airfoil like an aircraft wing. The

blades create lift when the engine spins the propeller blades. This lift is termed

thrust and shifts the aircraft frontward. Most airplanes have propellers that drag

the airplane through the air. These are termed tractor propellers. Some aircrafts

have propellers that push the aircraft. These are called pusher propellers.

Leading Edge of the airfoil is the cutting edge that slices into the air.

Blade Face is the surface of the propeller blade that corresponds to the lower

surface of an airfoil.

Thrust Face is the curved surface of the airfoil.

Blade Shank (Root) is the section of the blade nearest the hub.

Blade Tip is the outer end of the blade farthest from the hub.



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Thrust

Thrust is the force that moves the aircraft through the air. The thrust is produced

by the propulsion system of the aircraft. The propulsion system usually utilizes

Newtons 3rd law to produce thrust, propeller is one of the system which produces

thrust, the thrust produced by the propeller is used to move the aircraft through

the air. A propeller may consist of a minimum 2 airfoil shaped blades which are

attached to a hub, the hub serves a connection between the blades and the

engine shaft.

The blades of an aircraft are made in the shape of an airfoil like the wings of an

aircraft. When the wings of an aircraft move through the air it produces lift,

likewise when the blades of a propeller are rotated at a high rpm the also

produce lift, this lift is called thrust which pushes the aircraft forward. Most

propellers pull the aircraft through the air, this type of propeller is called a tractor

propeller.



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Torque

The torque of the propeller has a turning effect on the body of an aircraft, this is

dew to Newtons 3rd

law every action has an equal and opposite reaction thisimplies the propeller moves in one direction with a force while another force acts

on the aircraft body which rotates it in the opposite direction.

Propeller torque effect

When the aircraft is in flight this force causes the aircraft to roll. One of the ways

to correct this is to increase the lift on the wing which is being forced down. This

is a permanent factor in the aircraft, in other situation aileron provide further

adjustment.

While on the ground a turning moment along the vertical axis is induced by the

torque reaction. As one side of the aircraft is being forced down by the torque

reaction, more weight is being placed on landing gear on that side. This causes

friction on that tyre, causing a turning moment on that side.



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Blade angle of attack

Blade angle of attack is defined as the angle in between the chord of the element

and the relative wind. The greatest efficiency of the propeller is attained at an

angle of attack about 2 to 4 degrees.

Blade Forces acting on a propeller in flightThrust is the power on the propeller which is parallel to the route of advance and

creates bending stress in the propeller.

Centrifugal force is produced by the rotary motion of the propeller and it usually

throws the blade out from the center.

Torsion is a force in the blade produced by the forces of the air which usually

twist the blades in the direction of a lower blade angle.



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Propellers Efficiency

The Propellers Efficiency can be calculated in 2 ways, they are:

Thrust Horsepower (THP) / Brake Horsepower

Lift generated by the propeller / Drag of the Propeller

The job of the propeller is to convert power which is provided by the engine into

thrust. This is done by accelerating a large mass of air to a higher velocity. The

effectiveness with which a propeller performs this conversion is known as

propeller efficiency. As we know when the propeller blade moves through the air

it produces lift and drag, as the RPM of the propeller is increased the so does the

lift and drag, until it reaches a point where the drag produced is very high and the

efficiency of the propeller falls.



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Fixed Pitch Propeller

The propeller is prepared in one piece and is frequently made of wood or metal.

Wooden Propellers: A wooden propeller consist of a number of separate layers

of wood, but the most satisfactory are yellow birch, sugar maple, black cherry,

and black walnut. The lamination of wood will decrease the tendency for

propeller to deform.

Metal Propellers: Modern metal propellers are made-up from great power, heat-

resistance and aluminium alloy. Metal propellers are now broadly utilized in the

building of propellers for all type of airplane. The general look of the metal

propeller is same to the wooden propeller except that the parts are usually

thinner.



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Two Pitch Propellers

These propeller can be adjusted for two pitchs the purpose of this is to maintain

an optimal angle of attack on the propeller as the aircraft speed changes. These

types of propeller were used in the earlier aircrafts and the pitch was changed by

the pilot.

Constant Speed Propellers

A constant speed propeller is a type of propeller in which the blade pitch can be

change to make better use of the power supplied by the engine, the mechanism

may differ in different models but the purpose is to change the angle of attack of

the blades to vary the mass accelerated by the propeller.

Governor

A governor is a device that s used to measure the speed of an engine.



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Variable Pitch Propellers

The two kinds of variable-pitch propellers are the adjustable and controllable

pitch propellers.

The adjustable pitch propellers is one in which the pitch can be changed only by

a mechanic to serve a particular purpose-speed or power.

The controllable pitch propeller is one in which the pilot can change the pitch of

the propeller during the flight or by functioning the engine by means of a pitch

changing mechanism that can be hydraulically functioned.



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Wind Milling Propellers

In a fixed pitch propeller the velocity depends on the forward speed of the aircraft

and the RPM of the propeller. In case of an engine failure the pilot pitches the

aircraft down to maintain the forward airspeed which decreases the RPM of thepropellers, while this is happening the angle of attack must constantly be

decreasing. At some point the angle of attack will become negative and the lift

becomes negative and the propeller rotates by it self. This drives the engine as

an air pump resulting in an increase in the aerodynamic drag which is greater

then the drag produced by the stationery propeller. If the forward speed is

increased the wind milling will increase and vice versa. To stop wind milling the

forward airspeed should be reduced to near stall airspeed, this will cause the

engine air pump torque and friction to stop the rotation and decrease the

reversed lift.



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Reverse Pitch Propeller

A reversing pitch propeller can alter the pitch of the blades to a negative angle.

That is, the blade pitch is changed so that they have their leading edge pointing

slightly opposite to the direction of flight. This mode allows reverse thrust to be

developed by the propeller. In larger commuter and transport aircraft this feature

is often used to slow the aircraft rapidly after landing, but in sport aircraft it is

more usually used to enhance maneuvering on the ground. A popular application

is in seaplanes, where the ability to maneuver backwards, and sometimes to

reduce the thrust to nothing, is especially useful.



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Full Feathering

It is a constant speed propeller which has the ability to turn the edge to the wind

and thereby eliminate drag and wind milling in the event of engine failure. The

term Feathering refers to the operation of rotating the blades of the propeller to

the wind position to reduce drag. Therefore a Feathered blade is in an

approximate in-line-of-flight position, streamlined with the line of flight (turned the

blades to a very high pitch). Feathering is necessary when the engine fails or

when it is desirable to shutoff an engine in flight.

Propeller Synchronization

Propeller Synchronization is defined as the mechanism which automatically

synchronizes all propellers of a multiengine, propeller-driven airplane so that they

revolve at the same speed.

Propeller synchronization main function is to increase the comfort of passengers,

since its primary function is to decrease the beats created by the propellers that

are rotating at dissimilar speeds.

Some airplanes give a visual indication of the propeller synchronization in the

cockpit. Pilots can use these indications to resolve whether or not to take on the

propeller synchronization or to physically help them to synchronize propeller

speeds.

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Beta Control

A beta propeller is a propeller which allows the physical repositioning of the

propeller blade angle outside the normal low pitch stop. A beta control is used

frequently in taxiing, where thrust is physically controlled by adjusting the blade

angle with the power lever.

Propeller control

The speed sensitive governor of the propeller robotically controls the blade angle

as necessary, so that a constant RPM is sustained.

There are 3 things that control RPM of the engine during the process, these

aspects are power, airspeed and air density. For the RPM to be constant the

angle should differ frankly with the power and airspeed and inversely with air

density. The speed sensitive governor gives the means by which the propeller is

automatically fixed and power is transformed into thrust.

There are 3 forces, which are utilized to control the blade angle, these are:

1. Centrifugal twisting moment, centrifugal force which acts on the rotating

blades and usually shifts the blades into low pitch.

2. Oil at engine pressure, this supply the centrifugal twisting moment in the

direction of low pitch.

3. Propeller governor oil, this stabilizes the first two forces and shifts the blades

towards high pitch.



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Pitch change mechanism

Hydromantic propellers

This kind of pitch changing mechanism is a mechanical hydraulic scheme in

which hydraulic forces performing on the system are transformed to mechanical

forces which perform on the blade. The piston movement produces the propeller

to modify the pitch. The governor controls the hydraulic fluid.

Single acting propeller

In this system the governor directs fuel into the inboard side of the piston only.

This kind of propeller utilizes three kinds of forces at constant speed process.

The blades centrifugal force constantly tends to shift the blades in the direction

of low pitch, oil at engine pressure applied against the outboard side of the

propeller piston and this force to supplement the centrifugal twisting moment

toward the low pitch throughout the constant speed operation and oil from

governor pressure applied against the inboard side of the piston. The oil pressure

from governor was amplified from the engine oil contributor by the governor

pump and the force is monitored by metering the high pressure oil to or draining

it from the inboard side of the propeller piston which stabilises the centrifugal

twisting moment and oil at the engine pressure.



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Double acting propeller

The governor directs its output either side of the piston as the operating condition

required. Double acting propeller uses double acting governor. This type of

propeller, the governor pump output oil is directed by the governor to either side

of the propeller piston.

Over speed condition

When the engine speed increases above the RPM, oil supply is boosted in

pressure by the engine driven propeller governor, is directed against the inboard

side of the propeller piston. The piston and the attached rollers move outboard.

As the piston moves outboard, cam and rollers move the propeller blades toward

a higher angle, which intern decreases the RPM.

Under speed condition

When the engine speed drops below the RPM for which the governor is set.

Force at flyweight is decrease and permit speeder spring to lower pilot valve,

thereby open the oil passage allow the oil from inboard side of piston to drain

through the governor. As the oil from inboard side is drained , engine oil from

engine flows through the propeller shaft into the outboard piston end. With the

aid of blade centrifugal twisting moment, The engine oil from outboard moves the

piston inboard. The piston motion is transmitted through the cam and rollers .

Thus, the blades move to lower angle.



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Conclusion for Propellers

By studying different types of propeller and the factor affecting them, we have

come to an understanding that, they are an effective and economical

system to provide thrust for the aircraft and with the use of constant speed

propellers the prospects have been further increased, creating more room for

improvement. The only drawback in the propeller system is its inability to perform

at high speed, which limits its use on to slower aircrafts.



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Crank Case

The crankcase is a body that carries all the parts of the engine. It is the biggest

part of the engine which should be both strong and light.

Crankcase in a 2 stroke engine

In a two stroke engine the crank case is sealed and used as a combustion

chamber and pressurization chamber for the air and fuel mixture.

Throughout combustion the air go into the crankcase through the inlet valve, as

the piston shifts down opening the exhaust outlet vale and closing the air inlet

valve, at the similar time it permits the valve to allow air to enter into the

combustion chamber and as the cycle goes on the solidity of that air goes on.



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Crankcase for 4 stroke engines

In a four stroke engine the crankcase is potted from the air and fuel combination.

It mainly contains air and oil. The oil flow in the 4 stroke engine is cut off from the

air and fuel mixture in the cylinders, this accumulates the oil from combusting.

The oil is compressed by the means of an oil pump which is then transferred

through a filter to eradicate any sort of contamination. The oil is then sprayed into

the crankcase and the parts inside. It is designed in a way that there is minimum

connection between the oil and the crankshaft when it is sprayed in the

crankcase.



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Crankshaft

It is a device that converts the reciprocating motion of the pistons into rotary

motion. To perform this job the crankshaft has crankpins which are additional

bearing surfaces whose axis is offset from that of the crank, at the end of which

the connection rods are attached.

The crank shaft is usually connected to flywheel which reduces the pulsation

caused by the 4 stroke cycle. A vibration damper might also be installed on the

other end which reduces the torsion vibration.



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Sumps

The sump is a space below the engine which collects the oil and other

undesirable substances dropping from the engine. The sump acts as a reservoir

and the oil collected in it is used to lubricate the engine. This is done by the use

of a pump. Some times during a hard cornering oil in the sump surges which

starves the pump of oil, for this reason high performance racing engines are dry

sumped. In a dry sumped engine a secondary external reservoir is used to

lubricate the engine. Dry sump is used mainly in large diesel engines, racing cars

and aerobatic planes.



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Accessory Gearbox

It is a gearbox which is connected to an engine, which strengthens the engine

accessories and also to transmit the drive of the engine to the accessories such

as electric generators, de-icing apparatus, landing gear retracting mechanism

and etc.



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Cylinder & Piston Assemblies

A cylinder is one of the main part of a piston engine, it is where the piston travels.

A number of cylinders are arranged side by side the engine block.

These cylinders are mostly cast from aluminum and iron before other features

are added to it. Because the cylinders are subjected to high temperatures it also

needs a cooling system which keeps a check on its temperature.

The piston is a component of a reciprocating engine. It is located in a cylinder

and made gas-tight by piston rings. Its purpose is to transfer force from

expanding gas to the crankshaft through a piston rod. In a 2 stroke engine it acts

as a valve which covers and uncovers ports in the cylinder wall.



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Types of fuel used in piston engines;

1. Petroleum spirit

2. Petroleum diesel

3. Auto gas (liquefied petroleum gas)

4. Compressed natural gas

Ignition system

The ignition system depends on the fuel utilized. For example petrol is ignited by

a spark which is given by a timed spark plug and diesel is ignited by compression

heating.

Spark ignition system

Spark ignition system is used in an Otto cycle which uses petrol as fuel. In this

system the mixture of fuel and air is compressed to a certain pressure then a

precise time controlled spark plug produces an electrical spark which ignites the

mixture of compressed air and fuel.

Compression controlled ignition system

This type of system is mainly used in diesel engines, the air and fuel is

compressed to the point where the temperature rises to combustion temperature.

In this type of ignition the timing is controlled by the fuel injectors.

Ignition timing

The efficiency of an engine depends on the time when the ignition of the fuel

takes place. The Carnot heat cycle tells us that if the ignition takes place when

the temperature of the mixture of fuel and gas is high it will be able to provide

more work. So it is important that the ignition takes place at the right pressure

and position of the piston.



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Carburettors

The task of the carburetor is to mix the fuel and the air for an engine. It woks on

Bernoullis principle which states, the faster air moves, the lower its static

pressure and the higher its dynamic pressure. A carburetor basically consists of

an open pipe a throat through which the air passes into the inlet manifold of the

engine. The pipe is like a venture its diameter decreases in the middle and then

again increases, this increases speed of the airflow. Right below the venture is a

butterfly valve also called the throttle valve. This valve can be adjusted to restrict

the flow or increase it. This valve controls the airflow which in turn controls the

air/fuel mixture. This is a governing factor for the power produce and the speed

of the engine. The throttle is connected to a cable or the mechanical linkage by

rods and joints to the accelerator peddle in a car or the throttle lever in an

aircraft.



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Fuel injection

It is a device which is utilized to combine air with fuel in an internal combustion

engine. A fuel injection system is particularly made and standardized for various

kinds of fuel. The introduction of electronic fuel injectors has permitted us to

utilize same injectors for all kinds of fuel.

The main purposes for fuel injection:

Power output

Fuel efficiency

Reliability

The electrical fuel injection is monitored by a central unit described the engine

control unit. This component observes the different parameters such as engine

speed ambient temperatures and workload etc, and then computes the quantity

of fuel necessary to be inserted in the system. The injector is usually closed andopens to provide compressed fuel only when electricity is provided to the

solenoid coil, and when the fuel is no longer necessary the engine central unit

disconnects the electric supply from it thus ending the fuel provide.



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Superchargers

A supercharger is basically a compressor which compresses the air before it is

fed to the engine this increases the pressure ratio, power and also the efficiency.

The supercharger is driven by the crankshaft and takes up almost a third of its

power to run. An inter cooler is also installed to cool the air as compression

increases the temperature of the air. Supercharging also decreases the fuel

economy.



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Turbocharger

The purpose of the turbocharger is to increase the intake pressure of the engine

to increase the pressure ratio, power output and increase the efficiency. A

turbocharger consists of a turbine and a compressor connected together by a

shaft. The turbine receives the exhaust gases from the engine which rotates it

and in turn rotates the compressor, this compresses the ambient air and

increases the pressure and feeds the engine with this pressurized air, sometimes

the compressed air is passed through an intercooler before feeding to the engine

this decreases its temperature.



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Conclusion for a Piston Engines

The piston engines are one of the most successful forms of engines and are

available for a number of applications. Though the principle at which it workshavent changed but research and development with this engine has made it

more efficient, reliable and also economical. Its not just the engine that has been

improved other systems associated with it such as the fuel injection system and

the ignition system has seen significant changes in them.)



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Source of information

www.wikipedia.org Ref 1

http://www.thaitechnics.com/propeller/prop_intro.html Ref 2

http://www.aerospaceweb.org/question/dynamics/q0015a.shtml Ref 3

http://www.pilotfriend.com/training/flight_training/fxd_wing/props.ht

m

Ref 4

Aircraft Propulsion Systems Technology and Design Ref 5

Engineering Thermodynamics Ref 6

Aircraft Manuals Ref 7
http://www.wikipedia.org/http://www.thaitechnics.com/propeller/prop_intro.htmlhttp://www.aerospaceweb.org/question/dynamics/q0015a.shtmlhttp://www.pilotfriend.com/training/flight_training/fxd_wing/props.htmhttp://www.pilotfriend.com/training/flight_training/fxd_wing/props.htmhttp://www.wikipedia.org/http://www.thaitechnics.com/propeller/prop_intro.htmlhttp://www.aerospaceweb.org/question/dynamics/q0015a.shtmlhttp://www.pilotfriend.com/training/flight_training/fxd_wing/props.htmhttp://www.pilotfriend.com/training/flight_training/fxd_wing/props.htm

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Engine Propulsion