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86 Cards in this Set
- Front
- Back
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Define a Newton (What is a dyne) |
The force required to accelerate a mass of 1 kg 1metre per second per second (1 newton = 100,000) |
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What is the difference between velocity and speed? |
Velocity is a vector quantity and speed is scalar |
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Define work and power |
Work is the product of force and distance Power is the rate that work is done, work done / time taken |
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Define energy and potential energy |
Energy is the capacity to do work and potential energy is the stored energy of a body |
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What is kinetic energy? |
The energy possessed by an object, 1/2MV^2 |
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What is the difference between boyle’a law and Charles’ laws? |
Boyle’s law uses a constant temperature and Charles’ law uses constant pressure |
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What is the combined gas law / ideal gas law? |
(P1v1)/t1 = (p2v2)/t2 Aka the characteristic gas equation |
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State Newton’s three laws |
1. A mass will remain at a constant velocity unless acted upon by an external force 2. F=ma 3. To every action there’s Is an equal and opposite reaction |
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What are the two constituents of thrust? |
Mass of air and acceleration imparted to the air T=ma |
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Describe a turbo jet engine |
Best at high altitude, no by-pass, reaction engine |
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Describe a low / medium by-pass ,turbofan engine |
Two or three shafts, by-pass, quieter than turbojets and more fuel efficient |
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What are the by-pass ratios of low, medium and high? |
Low: below 2:1 Medium: 2:1 - 4:1 High: over 5:1 |
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A gas turbine is essentially a “?” |
Heat engine |
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What are the three major parts of a turbo jet engine and what cycle describes the thrust production? |
Compressor combustion chamber and turbine The Brayton cycle |
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Describe the Beaton cycle |
Induction compression ignition and exhaust |
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What is the continuity equation? |
Mass flow = density * area * velocity It’s true for any steady flow system |
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How does velocity vary with volume of a duct? |
Velocity varies inversely with changes in duct area |
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What makes up the total energy of a system? |
Static pressure: The pressure that would be felt by a body moving at the same velocity as the medium it is submerged in Dynamic pressure: The extra pressure created by the movement of the medium 1/2M*V^2 Total energy = static + dynamic |
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How do you measure total energy? |
Called total head pressure or pitot pressure, ram in the fluid flow parallel to flow |
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What is diffuser action? |
The flare of a divergent duct must retain smooth airflow so the angle must be only up to a certain value |
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What are the two main types of gas turbine engines? |
Reaction: jet reaction, internal reaction Power: mechanical output for another device |
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What are the two constituents of thrust? |
Mass of air and acceleration imparted to the air T=ma |
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Describe a turbo jet engine |
Best at high altitude, no by-pass, reaction engine |
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Describe a low / medium by-pass ,turbofan engine |
Two or three shafts, by-pass, quieter than turbojets and more fuel efficient |
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What are the by-pass ratios of low, medium and high? |
Low: below 2:1 Medium: 2:1 - 4:1 High: over 5:1 |
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Describe a high by-pass engine |
Fan is geared to be slower than the shaft, very fuel efficient quite and powerful, not appropriate for high speed flight |
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Describe a turbo prop engine |
No by-pass and use the turbine to run a prop, one or two shaft, direct drive or a separate shaft that drives the prop through a gear box |
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Describe a turbo shaft engine |
Mostly used in helicopters, constant speed, more efficient |
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State the key engine stations |
0 air before intake 3 compressor discharge (highest pressure) 7 just before the propelling nozzle |
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What is the equation for calculating thrust? |
T=(A*P)+WvJ/g A:area of flow section sq in P: Pressure W: mass flow Vj: velocity of flow G: 9.81 |
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What is the equation for ISA corrections? |
ISA corrected reading ISA standard atmosphere = Actual reading Actual atmosphere |
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For one pound of thrust that a turbo prop provides how many hp does it produce? |
3.5 - 5 hp |
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A gas turbine is essentially a “?” |
Heat engine |
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What is ram ratio? |
Total air pressure to static at intake |
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What is SFC |
Specific fuel consumption, pounds of fuel used per hp/Ibs of thrust per hour |
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What effect does an aircraft being “hot or High” ? |
To maintain a constant rpm less fuel is introduced as air density is lower as is mass flow |
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What is thermal/internal efficiency? |
The efficiency of converting fuel to kinetic energy |
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What are the three most important factors affecting thermal efficiency? |
Turbine inlet temperature, compression ratio and component efficiency |
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What is the equation for propulsive efficiency? |
Work done on aircraft / energy imparted to the engine airflow |
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What is the relationship between overall efficiency and airspeed? |
The greater the airspeed the more efficient the aircraft is, propulsive efficiency increases more than thermal efficiency decreases |
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What type of engine is the most efficient at low speeds? |
Turbo prop |
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How are turbojet and turbofan aircraft rated? |
By the number of pounds of thrust they are designed to produce for take off, max continuous, max climb and max cruise ratings |
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What is meant by flat rating? |
The throttle level used being less than the maximum to preform better on a hot day and more economically without over working the engine |
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What are the three major parts of a turbo jet engine and what cycle describes the thrust production? |
Compressor combustion chamber and turbine The Brayton cycle |
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What is ram recovery? |
The use of a divergent diffuser on the inlet to convert the air into ram pressure |
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What effects the degree of ram compression? |
Frictional losses of wetted surfaces and intake duct walls, turbulence losses and aircraft speed |
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Why is ram pressure important? |
As the ratio multiplied by the compressor pressure ratio gives a larger value |
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What is the issue with divided air intake ducts? |
Suffer in yaw and turn as a loss of ram pressure occurs on one side |
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What is significant about super sonic flight intakes? |
In subsonic flow a divergent duct is needed and at supersonic speeds a convergent/divergent duct is needed |
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What should the speed of the air entering the compressor be? |
0.5 Mach |
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What are the efficiency’s of the different air intakes? |
Turbojet: pitot 99% Wing root 87-95% Side 80-89% Turbo prop: Annular 74-82% 10% less if air is reversed on intake |
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Describe the three types of intake anti-iceing |
Hot air: taken from the hp compressor Electrical: spray and heater mats, switch on at 6 degrees or lower. Heater mats are made from nickel chrome foil and are woven in continuous filament glass yarn, insulation made from polytetrafluoroethylene(PTFE) Oil is used to supplement other systems |
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What are the different types of compressors? |
Axial: single, double and triple spool Centrifugal: single and double entry and single entry shrouded |
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How does a centrifugal compressor work? |
Air is forced from eye to tip and through the diffuser, kinetic energy is converted into pressure energy |
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Describe the Beaton cycle |
Induction compression ignition and exhaust |
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What affects the final mass air flow after the compressor? |
Pressure ratio (5:1 from a single and 8:1 from a double), operating rpm and the diameter of the impeller |
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Advantages and disadvantages of an axial flow compressor? |
High pressure ratio Low specific fuel consumption More capacity for development Greater axial thrust 9:1 over all and 1.25:1 per stage
Complex and expensive Critical to stall and surge |
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What is the difference between the order of stators and rotors of a compressor and a turbine? |
In a compressor it is stator rotor and in a turbine it’s rotor stator. The compressor blades have a divergent duct between them increasing pressure and due to the rotating rotors an increase in velocity |
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What is a IGV? |
Inlet Guide Vane, used to present the airflow onto the first stage rotor blades at a suitable angle |
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How is axial velocity maintained throughout the compressor? |
Use of convergent duct and longer blades at the low pressure regions and shorter blades at the higher pressure regions |
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How are stators secured in the compressor casing? |
Stator vane retaining rings, they are positively locked and some are variable |
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What are the three types of compressor blade attachment? |
Solid, fir tree and dovetail root |
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What is an integral blade and disk assembly called? |
A BLISK |
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What us surge and what causes it? |
Reversal of airflow in the compressor, proceeded by stalling the compressor blades. Air flows from the combustion chamber forward and once at the stalled blades causes violent oscillations |
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What is the design point of a compressor? |
The point is maximum efficiency, optimum angle of the blades |
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What is the continuity equation? |
Mass flow = density * area * velocity It’s true for any steady flow system |
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What is the relationship between blade angle and position in the compressor? |
The angle of attack at the rear of the compressor will be less than in the lower pressure front section |
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What effect do the bleed valves on the compressor have and when are they used? |
They are used at slower engine speeds to bleed off air to reduce mass airflow and prevent an unstable flow pattern when bleed valves are open the inlet guide vanes are partially closed |
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What is the lift produced by an aerofoil dependant on? |
Shape (area and smoothness ( Speed of airflow Angle of the blade |
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At what speed does an axial flow compressor operate? |
8000-10000 rpm |
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What causes compressor stall? |
Disturbance of smooth airflow (damage or dirt) (aircraft intake) High combustion chamber pressure caused by over fuelling during engine acceleration |
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Why are the blades of the compressor twisted? |
To get the optimum angle of stack for each part of the blade as the tip is travelling faster |
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How does a compressor bleed valve operate? |
Automatically in response to signals of compressor rpm |
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What is the purpose of the VIGV’s? |
They impart a swirl on the airflow before it enters the first stage of the rotor. The greater the degree of swirl the smaller the AOA of the blades |
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Advantages and disadvantages of a centrifugal compressor? |
Simplicity, cheaper, lighter and less prone to damage by Fod Not critical to Serge or stall Will tolerate icy conditions
Maximum pressure ratios of 4:1 and 5:1 Capacity limited by tip speed Severe directional changes of gas flow which leads to friction High specific fuel consumption
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Why is it important to maintain balance in the compressor? |
Increase passenger comfort, reduce wear the noise levels and also to increase the life of the engine between overhauls |
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How does velocity vary with volume of a duct? |
Velocity varies inversely with changes in duct area |
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What are the three main forms of loads on bearings? |
Thrust loads due to engine doing work Journal loads due to the dead weight of the engine parts Unbalanced loads |
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What causes unbalance? |
Eccentricity: a parts centreline not aligning with the axis of rotation Wall thickness: holes not in the centre of objects Blade distribution: unequal arrangement of a set of blades Unsymmetrical features Distortion: caused by stress or unequal thermal growth Fits and clearances: allowing relative movement causing shift of axis of rotation Swash: out of squareness, incorrect tightening of bolts |
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What are the three types of unbalance? And how are they fixed? |
Static: adding mass to the light side of the rotor Couple: rotors space that 180° from each other, when the two unbalances are equal but still cause vibration Dynamic: when the unbalanced masses are on equal or at another angle other than 180° |
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How do you sense vibrations in engines? |
To the use of velocity transducers or a prize-electric accelerometer, The vibration analyser determines what parts cause what frequency of vibration |
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What is it important to remember when replacing the fan blades? |
Ensuring that the bolts and spinner go back in the same position afterwards in some engines blades are changed in pairs |
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What makes up the total energy of a system? |
Static pressure: The pressure that would be felt by a body moving at the same velocity as the medium it is submerged in Dynamic pressure: The extra pressure created by the movement of the medium 1/2M*V^2 Total energy = static + dynamic |
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How do you measure total energy? |
Called total head pressure or pitot pressure, ram in the fluid flow parallel to flow |
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What is diffuser action? |
The flare of a divergent duct must retain smooth airflow so the angle must be only up to a certain value |
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What are the two main types of gas turbine engines? |
Reaction: jet reaction, internal reaction Power: mechanical output for another device |
