Yeovil college

BTEC HNC/D AEROSPACE ENGINEERING

Unit 15 Aircraft Propulsion Technology Legacy & NQF

William James Beattie

27/12/2012

Table of Contents1 Investigate propulsion engine performance 2

1.1 Apply Newton’s and gas laws to gas turbine and piston engine cycles. 2

3 Investigate engine performance characteristics 4

3.2 Describe material limitations for each module in Outcome two 4

4 Evaluate Power Plant Installation …………………………………………………………………………………………… 10

4.3 Investigate a piston engine powered fixed wing aircraft to determine the following in the way that the power plant is installed: ……………………………………………………………………………………………10

4.4 Describe engine monitoring and ground operating procedures …………………………………………14

Bibliography ………………………………………………………………………………………………………………………………….17

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YEOVIL COLLEGE

DEPARTMENT OF ENGINEERING

BTEC HNC/D AEROSPACE ENGINEERING

Unit 15 Aircraft Propulsion Technology Legacy & NQF

1 Investigate propulsion engine performance

1.1 Apply Newton’s and gas laws to gas turbine and piston engine cycles.

Q1

If the mass of air through a propeller is 1000Kg/s, the aircraft’s velocity is 100m/s and the slipstream velocity is 120 m/s. Calculate the thrust.

A1

Thrust = Mass airflow X (slipstream velocity-aircraft velocity).

Thrust = 1000 x (120-100) =20000N

Q2

A turbo fan engine has a bypass ratio of 1:5. If the mass of air entering the engine is 500Kg/s, the velocity of the aircraft is 120m/s, the bypass velocity is 180 m/s and the jet velocity is 220m/s. Calculate the engine thrust.

A2

Thrust= Mass flow rate through bypass x velocity increase + mass flow rate through core engine x velocity.

Thrust = 100 x (180 – 120) + 400 x (220 – 120)

= 100 x 60 + 400 x 100

=6,000 + 40,000

= 46,000N Thrust

Q3

A petrol engine working on the Otto cycle has a compression ratio of 9:1, and at the beginning of compression the temperature is 32° C. After heat energy supply at constant volume, the temperature is 1700° C. The index of compression and expansion is 1.4.

Calculate:

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a) temperature at the end of compression

b) temperature at the end of expansion

c) Air standard efficiency of the cycle.

A3

a)

T 1 = 32 °C = 305 KV 2 = 9V 1 = 1

γ = 1 . 4

T 2

T 1

=(V 1

V 2)( γ−1)

T 2=T 1(V 2

V 1)( γ−1 )

T 2=305 (91 )1 . 4−1

T 2=734 .5K

b)

T 3=1700° C+273=1973KV 3=1V 4=9

γ=1 . 4

T 4

T 3

=(V 3

V 4)γ−1

T 4

1973=(19 )

1. 4−1

T 4=1973×0 . 42¿828 . 7K

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c)

Air Standard efficiency η

η=1-(T4-T1 )(T 3−T 2 )

η=1−(828 . 7−305 )(1973−734 .5 )

=1−523 .71238 . 5

=0.577=57 .7 % air standard efficiency

Q4

The temperature of gas entering a turbine is 750° C. If the gas leaves the turbine at a temperature of 500° C and the mass flow rate of the gas is 3.5 Kg/s, calculate the power developed by the turbine. Take Cp of the gas to be 980 J/KgK.

A4

W = ˙m.cp (T1 – T2) = 3.5 × 980 × (750 – 500)

= 857 500 J = 857.5 kJ

3 Investigate engine performance characteristics

3.2 Describe material limitations for each module in Outcome two

Q5 Investigate and determine material limitations regarding power rating; centripetal forces; temperatures.

Within a standard gas turbine engine there are four basic modules (fig.1);

1. Intake/Fan System

2. Compressor

3. Combustion chamber/combustor

4. Turbine

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Fig 1 – A Gas Turbine Engine.

Although you can class the turbine engine as having four main parts there are many more associated parts to the engine which are paramount to its operation.

To answer this question I have researched the Rolls-Royce Trent 1000 turbo-fan jet engine (Fig 2).

Figure 2. Rolls-Royce Trent 1000 Turbo-fan jet engine.

Intake/Fan System

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The intake of any engine is fundamental to its successful operation. An intake is designed to channel and deliver subsonic smooth airflow at an even pressure and velocity to the Axial compressor to ensure that the engine, for which it has be designed, operates correctly within its design parameters. The majority of intakes are made of a simple light alloy assembly that will continue to hold its shape no matter what outside force are applied to it or temperatures that it is subjected to. It will have been constructed using countersunk fasteners to ensure that the airflow delivery is not disturbed or restricted.

The most obvious part of any large aircraft engine is its fan (Fig 3). At high power the fan can suck in up to a squash courts worth of air every second. Around 90% of this air is routed straight out of the back of the engine; this is known as bypass air and provides around 75% of the engines thrust. In order to make the most efficient engine the fan needs to be as large and as light as possible. Design features such as hollow blades made of titanium are used to achieve this. The hollow blades are made from a single piece of titanium alloy, about 10kg in mass, 100cm high and 40cm wide. This alloy is called Titanium 6/4. It contains mostly titanium but also small amounts of aluminium, iron, oxygen and vanadium. However the lightness of the fan needs to be balanced with its strength and when you consider that each fan blade experiences 100 tons of centrifugal load, an OAT operating range of 60°C to -50°C and a centripetal force at the blades base of around 900KN you can see why. Not only is the fan one of the most safety critical parts of the engine it is also the most exposed to the environments, with fan blade tips travelling at 1000mph, 40% faster than the speed of sound, they have to be designed to withstand impact damage from objects such as large birds which create a colossal amount of force on impact. Composite materials are being developed that will make fan blades even lighter, stronger and with greater impact resistance.

Physical Data:

Name: Titanium 6/4 Density: 4.5g/cm3 Specific Heat: 0.5263 J/g°C

Electrical resistivity: 0.000178 ohm-cm Melting Point: 1604-1660°C

Thermal Conductivity: 3.0 W/mK Young’s Modulus: 110GPa Tensile Strength: 1000MPa

Figure 3- A Trent 1000 Fan System.

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Compressor

The compressor is basically an air pump that sucks in air and raises the pressure, temperature and density of it. There are two main types of compressor; the centrifugal and the axial flow compressors (Fig 4). The centrifugal compressor is made up of a rotating impellor, a fixed diffuser and a manifold, which collects and turns the compressed air. To increase the pressure ratio designers fit two single faced compressors in a row. The benefits of these compressors are they provide a large pressure ratio per stage (5:1) and low manufacturing costs. The axial compressors are used in most modern turbo engines from medium to high thrust aircraft, they can deliver high mass flow rates together with large pressure ratios.

Axial Compressor Figure 4 Centrifugal Compressor

The compressors deliver high pressure air to the combustor (Fig 5). The more the air is compressed the power can be extracted inside the turbines. Some of this compressed air is used for secondary tasks such as cooling hot components. The challenge is to maximise the compression ratio and efficiency without increasing the weight and complexity of the engine.

10% of the air from the fan system is routed into the engines compressors in its central core; there it is mainly used to generate the power to drive the fan. The cold air is compressed by up to 50 times its original size; simply the act of compressing the air can raise the temperatures to over 700°C.

The compressor is split into two parts the IPC (Intermediate Pressure Compressor) and the HPC (High Pressure Compressor), at the start of the IPC the temperatures are relatively low, around 150°C. When the air leaves the HPC the temperatures are now around 700°C, because of this compressor blades need to have a high strength to cope with the fluctuating stresses that occur when compressing air at 10,000 revolutions per minute. In addition they need to be corrosion resistant at high temperature, resistant to deformation and have a low density so they are as light as possible. Nickel based alloys are used for compressor blades as they exhibit all of the quality previously mentioned as well as being able having a very high melting point of around 1340°C and a tensile strength of 1441MPa.

Physical data:

Name: Nickel based alloy Density: 8.2g/cm3 Specific Heat: 0.424 J/g°C

Electrical Resistivity: 0.00011 ohm-cm Melting Point:1260-1340°C

Thermal Conductivity: 11.4 W/mK Young’s Modulus: 207GPa Tensile Strength: 1441MPa

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Figure 5 – Trent 1000 Compressor Blades

Combustion Chamber

The combustor burns fuel with air fed from the compressors (Fig 6). The combustor must generate a large amount of energy in order to drive the turbines; the challenge is to generate the maximum amount of heat from the smallest amount of fuel with the lowest emissions.

The combustor must be designed to slow the air around the fuel nozzles to help combustion take place. The flame must also remain alight at all times even during great storms when there may be more water than fuel in the combustor.

The combustion chamber is made from a partially Yttria-stabilised Zirconia with an advanced ceramic thermo-barrier coating. After combustion the temperature of the air can be up to 2100°C, this is nearly half the temperature of the surface of the sun; to stop the engine from melting cooler compressor air is routed to cool the walls of the combustion chamber to around 1400°C which would still be too hot for the metals used themselves so the advanced ceramic coating is added.

Physical Data:

Name: Partially Yttria Stabilised Zirconia. Density: 5.1g/cm3 Specific Heat: 0.6 J/g°C

Electrical Resistivity: 0 ohm-cm (ceramic) Melting Point: 2700-2850°C

Thermal Conductivity: 0.7W/mK Young’s modulus: 20GPa

Tensile Strength: 25MPa

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Figure 6 – Trent 1000 Combustor.

Turbine

The final stage of a jet engine is called the turbine; it is a series of bladed discs that act like a windmill gaining energy from the hit gases leaving the combustor.

This hot gas expands and cools as it goes through the turbines and finally exits through the propelling nozzle (Fig 7). It is this expansion that forces the turbines with enough energy to drive both the compressors and the fan. The conditions in the turbine are extreme, each blade spins at 10,000 revolutions per minute subjecting them to 157KN of centrifugal load and is extracting 560kW of power from the hot gas which is equivalent to a small family car.

In addition the temperatures are around 1600°C; this is beyond the melting point of even the most advanced materials. To overcome this complex cooling techniques are used, each blade is coated with a thin film of air from the compressor. It is important that the materials used to make the blades can withstand the arduous environment they work in and are not prone to creep. Creep is where metals at high temperature deform or change shape and can even break as they age.

Turbine blades are made from a single crystal of a Nickel based super alloy which increases their strength before being coated with an advanced ceramic thermo-barrier to aid in cooling; although nickel is denser than titanium it can withstand much higher temperature and is less prone to creep. In fact if you use titanium above 600°C in an environment where it can rub against other metals it will catch fire.

Physical data:

Name: Nickel Super Alloy Density: 8.4g/cm3 Melting point: 1350°C

Young’s Modulus: 110GPa Tensile Strength: 1000MPa

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Figure 7 - Trent 1000 hot gas leaving turbine.

Future Jet Engine Design.

Following research into future jet engine developments the following future developments have been noted1;

New fan concepts with the emphasis on two types: counter-rotating and lightweight fans. New booster technologies for different operational requirements; low and high speed,

associated aerodynamic technologies, new lightweight materials and associated coating and noise reduction design.

Polymer composites and corresponding structural design and manufacturing techniques are studied in parallel with advances in metallic materials and manufacturing processes.

Shaft torque density capabilities through the development of metal matrix composites and multi metallic shafts.

Low pressure turbine weight savings through ultra-lift aerofoil design, ultra high stage loading, lightweight materials and design solutions.

Technologies for light weight and low drag installation of high bypass ratio engines related to nozzle, nacelle and thrust reverser.

Outcome 4.3 Investigate a piston engine powered fixed wing aircraft to determine the following in the way that the power plant is installed:

The piston engine powered fixed wing aircraft I have chosen to investigate is the Grob G115 (Tutor) which is fitted with a Textron Lycoming AE-360-B piston engine. The Grob G 115 is an advanced general aviation fixed-wing aircraft, primarily used for flight training. It is built in Germany by Grob Aircraft (Grob Aerospace before January 2009).The E variant with a 3-blade variable pitch propeller is in Royal Air Force as an elementary flying trainer2.

The engine firewall

The firewall is an important component in an aircraft. Essentially, it is a fire-resistant bulkhead that separates the engine compartment from the cockpit area. This special bulkhead must be constructed so that no hazardous quantity of liquid, gas or flame can pass through it. In theory should a fire occur

1 http://publications.lib.chalmers.se/records/fulltext/local_140736.pdf2 http://en.wikipedia.org/wiki/Grob_G_115

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in the engine compartment, the firewall would protect the aircraft’s occupants from the flames long enough for an emergency landing to be carried out.

In addition to providing protection, the firewall has other useful functions. It provides a convenient surface on which to mount accessories and other essential units that are normally located in the engine compartment. And, since sooner or later it may be necessary to remove the engine or to replace other parts, it makes a junction for the disconnection and removal of engine control linkages, fuel lines, and various electrical and ignition wires (Figure 8).

Figure 8 – Engine Firewall/Bulkhead.

The position and purpose of the cowlings

A cowling is the covering of a vehicle's engine, most often found on automobiles and aircraft3.

The purpose of a cowling on a Grob G115 is:

for drag reduction

for engine cooling by directing airflow

for decorative purposes

The engine cowlings are positioned over the top of the engine from just behind the propeller to the engine fire wall (See Figure 9).

3 http://en.wikipedia.org/wiki/Cowling

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Figure 9 – Engine Cowlings location.

The use of acoustic absorbent panel

On the Grob G115 the acoustic absorbent panel is made from a polyimide foam encased in a thermal insulation (which is designed to prevent contamination and moisture retention, and to improve the acoustic performance of the materials) and is fitted to the cockpit firewall bulkhead, this is because absorption materials are almost always used in conjunction with a barrier of some type, since their porous construction permits noise to pass through relatively unaffected.

An absorber, when backed by a barrier, reduces the energy in a sound wave by converting the mechanical motion of the air particles into low-grade heat. This action prevents a build-up of sound in enclosed spaces and reduces the strength of reflected noise4.

The design of the mounts to overcome torque and vibration

The engine mount connects the engine to the airframe or fuselage. Other important secondary features are to distribute the weight of the engine, while spreading the torque and vibration generated by the engine. The mount has to achieve this not only on the ground but under g-forces from turbulence and other pilot induced load factors (Figure 10).

Figure 10 – Engine mounts maintenance illustration of a Grob 115

4 http://www.earaircraft.com/img/uploads/lit/news_2.pdf

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The engine is not bolted directly onto the mount; this would result in vibrations being transmitted from the engine to the airframe and control surfaces which over time would cause stress fractures and fatigue damage. Instead, rubber shock mounts of varying strength and thicknesses are used. This dampens the vibration and movement, giving a much smoother flight and running engine. One of the main advantages of using rubber bushing on a relatively simple aircraft like the Grob is that they require little to no lubrication reducing the maintenance impact on the operator.

The operational controls required and how they are connected from the cockpit to the power plant

As you can see from Figure 11 there are only a few controls on this aircraft that pass from the cockpit through the firewall to the engine.

Figure 11 – Engine Controls through the Engine Firewall.

Master Switch - Most often actually two separate switches, the Battery Master and the Alternator Master. The Battery Master activates a relay (sometimes called the battery contactor) which connects the battery to the aircraft's main electrical bus. The alternator master activates the alternator by applying power to the alternator field circuit. These two switches provide electrical power to all the systems in the aircraft.

Throttle - Sets the desired power level. The throttle controls the mass flow-rate of air (in fuel-injected engines) or air/fuel mixture (in carburetted engines) delivered to the cylinders.

Mixture Control - Sets the amount of fuel added to the intake airflow. At higher altitudes the air pressure (and therefore the oxygen level) declines so the fuel volume must also be reduced to give the correct air/fuel mixture. This process is known as "leaning". Both the Throttle and Mixture Controls are connected to the engine via Teleflex cables that are attached to control arms on the engine that turn linear movement into rotation movement. This also has the added benefits of easy range adjustment (secondary stops) and that the engine can be removed/installed without removing the entire control system just a bolts, nut and some washers.

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Tachometer - A gauge to indicate engine speed in revolutions per minute (RPM) or percentage of maximum.

Manifold Pressure (MP) Gauge - Indicates the absolute pressure in the intake manifold.

Oil Temperature Gauge - Indicates the engine oil temperature.

Oil Pressure Gauge - Indicates the supply pressure of the engine lubricant.

Outcome 4.4

Determine the ground and air tests required to prove a piston engine installation form normal operation after an engine change. You should consider all the required test and checks to be carried out during the first ground run prior to flight, on the flight test and follow up action that will be required after set flying hours. You must explain the reason for each of the tests and checks. It would be best if you were to use an engine to which you have access to the maintenance manual and with which you are familiar.

The following is taken from the Textron Lycoming Reciprocating Engine Break-In and Oil Consumption Limits Servicing Instruction (Annex A) – I will explain pertinent tests/checks in Italics below those sections.

GROUND TEST.1. Face the aircraft into the wind.

2. Start the engine and observe the oil pressure gauge. If adequate pressure is not indicated within 30 seconds, shut the engine down and determine the cause. Operate the engine at 1000 RPM until the oil temperature has stabilized or reached 140°F. After warm- up, the oil pressure should not be less than the minimum specified in the applicable operator’s manual.This is a general leak check carried out during the ground test, if the oil pressure does not indicate it is likely due to a faulty sender unit but may also be a sign that an oil pipe is damaged/not connected and further investigations must be carried out.

3. Check magneto drop-off as described in the latest revision of Service Instruction No. 1132.

4. Continue operation at 1000/1200 RPM for 15 minutes. Insure that cylinder head temperature, oil temperature and oil pressure are within the limits specified in the operator’s manual. Shut the engine down and allow it to cool if necessary to complete this portion of the test. If any malfunction is noted, determine the cause and make the necessary correction before continuing the test.This is a test of the engine at cruising speed and tests the cooling efficiency of both the engine oil system and the engine cooling system (like a radiator on a car). It is also another oil leak check but this time at working pressure so that any small defects (pin prick holes) will mist oil out under pressure as well as checking the oil pump.

5. Start the engine again and monitor oil pressure. Increase engine speed to 1500 RPM for a 5 minute period. Cycle propeller pitch and perform feathering check as applicable per airframe manufacturer’s recommendation.As Above.

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6. Run engine to full- static airframe recommended power for a period of no more than 10 seconds.As Above.7. After operating the engine at full power, allow it to cool down moderately. Check idle mixture adjustment prior to shut down.The idle mixture check ensures that the correct fuel/air mixture (usually around 1/15) is maintain when the engine is hot from operating.8. Inspect the engine for oil leaks.

9. Remove the oil suction screen and the oil pressure screen or oil filter to determine any contamination. If no contamination is evident, the aircraft is ready for flight testing.This check ensure that the internal parts of the engine are not breaking down or wearing on each other – if this was occurring small amounts of metal fillings would be found in the oil and oil filter (see figure 12).

NOTECompile a log of all pertinent data accumulated during both the ground testing and flight testing.

Having read the Flight Test the purpose is to ensure that the correct temperatures and pressures can be sustained for a normal flight however this also acts as another leak/cooling check of the engine to ensure that the vibrations/forces felt during a normal flight do not adversely affect the new/reconditioned engine.

C. FLIGHT TEST.WARNINGENGINE TEST CLUBS MUST BE REPLACED WITH APPROVED FLIGHT PROPELLERS BEFORE FLYING AIRCRAFT.1. Start the engine and perform a normal pre-flight run- up in accordance with the engine operator’s manual.

2. Take off at airframe recommended take off power, while monitoring RPM, fuel flow, oil pressure, oil temperature and cylinder head temperatures.

3. As soon as possible, reduce to climb power specified in operator’s manual. Assume a shallow climb angle to a suitable cruise altitude. Adjust mixture per pilot’s operating handbook (POH).

4. After establishing cruise altitude, reduce power to approximately 75% and continue flight for 2 hours. For the second hour, alternate power settings between 65% and 75% power per operator’s manual.This activity also helps to seat the piston rings in a newly overhauled engine ensuring a smooth and efficient engine (if this was not carried out prior to ‘release to service’ the engine would smoke very badly, this is caused by oil leaking past the piston rings and being ignited during the combustion stroke which would cause greater heat and eventual damage to the piston head and casing as well as increased oil consumption) – See Note Below NOTEIf the engine is normally aspirated (non-turbocharged), it will be necessary to cruise at the lower altitudes to obtain the required cruise power levels. Density altitude in excess of 8,000 feet (5,000 feet is recommended) will not allow the engine to develop sufficient cruise power for a good break- in.

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5. Increase engine power to maximum airframe recommendations and maintain for 30 minutes, provided engine and aircraft are performing within operating manual specifications.

CAUTIONAVOID LOW-MANIFOLD PRESSURE DURING HIGH ENGINE SPEEDS (UNDER 15” HG.) AND RAPID CHANGES IN ENGINE SPEEDS WITH ENGINES THAT HAVE DYNAMIC COUNTERWEIGHT ASSEMBLIES. THESE CONDITIONS CAN DAMAGE THE COUNTERWEIGHTS, ROLLERS OR BUSHINGS, THEREBY CAUSING DETUNING.6. Descend at low cruise power while closely monitoring the engine instruments. Avoid long descents at low manifold pressure. Do not reduce altitude too rapidly or the engine temperature may drop too quickly.

CAUTIONAVOID ANY CLOSED THROTTLE DESCENTS. CLOSED THROTTLE OPERATIONDURING DESCENTS WILL CAUSE RING FLUTTER CAUSING DAMAGE TO THE CYLINDERS AND RINGS.

7. After landing and shutdown, check for leaks at fuel and oil fittings and at engine and accessory parting surfaces. Compute fuel and oil consumption and compare the limits given in operator’s manual. If consumption exceeds figures shown in manual, determine the cause before releasing the aircraft for service.See above.8. Remove oil suction screen and oil pressure screen or oil filter to check again for contamination.Again this is to check that not internal break down of components has occurred.NOTETo seat the piston rings in a newly overhauled engine, cruise the aircraft at 65% to 75% power for the first 50 hours, or until oil consumption stabilizes.

Figure 12 – An example of a contaminated Engine Oil Magnetic Plug

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Bibliography

http://www.zenithair.com/stolch801/engine.html

http://en.wikipedia.org/wiki/Grob_G_115

http://en.wikipedia.org/wiki/Cowling

http://www.earaircraft.com/img/uploads/lit/news_2.pdf

www.roll-royce.co.uk

http://www.zenithair.com/design/engine-install.gif

http://en.wikipedia.org/wiki/Aircraft_engine_controls

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