Term
| Explain Bernoulli's Equation |
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Definition
| As any incompressible fluid passes through a convergent opening its velocity increases and pressure decreases. Conversely, as a subsonic fluid passes through a divergent opening the velocity will decrease and pressure will increase. |
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Term
| Describe the behavior of airflow in a nozzle |
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Definition
| A nozzle increases velocity and decreases pressure of air. For subsonic flow, nozzles are convergent. For supersonic, nozzles are divergent. |
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Term
| Describe the behavior of airflow in a diffuser |
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Definition
| A diffuser increases pressure and decreases velocity. For subsonic airflow, diffusers are divergent. For supersonic, diffusers are convergent. |
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Term
| Describe the Brayton Cycle |
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Definition
Four cycles happening simultaneously, in a turbine-powered combustion engine.
Intake, compression, combustion, exhaust. |
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Term
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Definition
| Consists of a compressor, which decreases velocity and increases pressure of the air, a combustion chamber which mixes fuel with the high pressure air and explodes, creating thermal energy and velocity to turn a turbine, which in turn also turns the compressor. Exhaust gas or another turbine can be used to create thrust. |
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Term
| Describe how airflow properties change through each section of a gas turbine engine |
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Definition
In the compressor, pressure increases steeply but velocity also somewhat increases. It enters the diffuser, increasing more in pressure and decreasing in velocity.
In the burner/combustion chamber, pressure decreases slightly and velocity and temperature rise sharply. It gets pushed through stators and smoothed out in order to turn the turbine, increasing in velocity but decreasing in temp and pressure. |
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Term
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Definition
The result of many pressure, temperature and velocity changes as airflow passes through an engine.
Gross thrust - due solely from the velocity of the exhaust gases. Ignores velocity of the air at the inlet, stationary engine on the ground. Standard day conditions
Net thrust - correct for inlet airflow velocity. Tn = m * (Vfinal - Vinitial) / T
**Net thrust and gross thrust are only equal when inlet airflow velocity = 0 in standard day conditions** |
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Term
| Describe the effects of airflow properties on thrust in a GTE |
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Definition
Air density - directly proportional.
Air temperature - inversely proportional
Air pressure - directly proportional
Altitude - inversely proportional
Airspeed - inversely proportional (difference between inlet and outlet, acceleration imparted) |
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Term
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Definition
More and more air is being rammed into the inlet at higher speeds, increasing both mass and pressure of the air.
Compressibility of airflow increases as the speed approaches the speed of sound. Mass due to ram effect increases at an increasing rate.
Even though density is low, for high-performance aircraft, ram effect allows for high performance. |
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Term
| Describe the cockpit thrust measuring instruments |
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Definition
Engine Pressure Ratio (EPR) - used on Jets and Fans; engines that rely solely on exhaust gases for thrust. Indicates pressure ratio between inlet and exhaust airflow
Torque meter - prop or shaft engines. Indicates shaft HP to create torque. Works because these AC barely use exhaust gases and mainly use the shaft HP for thrust
Tachometer - Indication of engine speed. Energy being produced by engine, like in a car. Calibrated in *Percent RPM* |
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Term
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Definition
Inlet duct works as a diffuser, to decrease velocity and increase pressure.
Subsonic engines - Divergent shape.
Supersonic engines - Convergent shape in front, divergent behind once air is slowed close to or below supersonic speeds through the initial supersonic diffuser.
Single entrance and divided. Former is good because short and direct; no loss of velocity or turbulence from going around corners. Downsides? can't be as big as divided and has to be low increasing risk of FOD |
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Term
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Definition
Primary function - supply enough air to satisfy the requirements of the combustion section.
Increases pressure and directs it to burners.
Secondary function - supply compressor bleed air to operate components throughout engine/AC.
Centrifugal - air flows through impeller, accelerates air to the diffuser. diffuser increases pressure and decreases velocity. High ratio of compression, but can't be used for multiple stages. Also needs large frontal area.
Axial - Two components: rotor blades then stator vanes. Turbine turns rotor blades, increasing both pressure and velocity. Efficient and effective. Extremely susceptible to FOD.
Dual spool - LO HI HI LO. low-pressure compressor then high-pressure compressor. Former spins slowly to compress, latter spins faster. Produces vacuum to suck air through. High peak efficiency, but high power, susceptible to compressor stall and only efficient for narrow bands of speed.
Axial-centrifugal - combines benefits of both. Also has guide vanes at the inlet and reduces drag by swirling in engine motion, and exit guide vanes to straighten it for diffuser |
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Term
| Describe the burner section of a gas turbine engine |
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Definition
Contains combustion chamber and provides means to mix fuel and air at proper ratio. Delivers combustion gases to turbine at proper temperature (not too hot).
- Minimize pressure decrease
- keep high combustion efficiency
- keep flame from blowing out
- finish burning before gases entering turbine
25% air through compressor is used for combustion, 75% is secondary; cools and controls flame |
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Term
| Describe combustion chambers |
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Definition
can - older and cheaper. Consists of fuel nozzle, burner liner and casing. Poor use of space, pressure loss and uneven heat distribution. Failure of one causes non-uniform temperature at turbine
annular - continuous circular inner and outer shroud around compressor drive shaft. Uniform heat distribution, but not easily replaced/fixable. Good for smaller engines
Can-annular - structural strength can for mixing and burning in front, then hot gases pass to annular area to mix evenly. Efficient but expensive |
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Term
| Describe the turbine section of a gas turbine engine |
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Definition
Stators then rotors. Drives compressor and accessories. Increases airflow velocity
Stators prepare airflow in direction of turbine wheel
Rotors convert heat energy into mechanical energy.
Approximately 75% of pressure energy is converted. Remaining 25 is used for thrust. Single or dual spool.
Turbine is highest stress part of engine.
Turbine blades attached by fir tree method. |
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Term
| Describe the phenomenon of creep in a gas turbine engine |
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Definition
| Blades undergo elongation as they are heated. Cumulative process, and excessive temperatures over long time periods can result in permanent deformation. |
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Term
| Describe the exhaust section of a gas turbine engine |
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Definition
Directs flow of hot gases rearward to cause a high exit velocity while preventing turbulence. Directly behind turbine
Two types of nozzles: C and C-D. C is for subsonic. C-D is for supersonic; rearward flares outward at high speeds to provide divergent nozzle action |
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Term
| Describe the afterburner section of a gas turbine engine |
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Definition
Increase engine output by 50% or more. Expense of fuel consumption (300%).
Secondary air from burner used here, also bypass air from turbofans.
Spray bars spray the fuel, flame holders produce turbulent eddies to mix the air and fuel, screech liner reduces pressure fluctuations and vibrations as a shock absorber and variable exhaust nozzle or turkey feathers allows proper exhaust speed/shape of nozzle |
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Term
| Describe the AoA of compressor blades |
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Definition
| angle between relative wind the compressor blades experience and the chord line of compressor blades. can be changed by increasing RPM, pitching nose of aircraft or increasing speed of relative wind |
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Term
| Describe compressor stalls |
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Definition
airflow over an airfoil breaks away causing the airfoil to lose lift due to excessive AoA. usually result of airflow distortions or mechanical malfunctions.
ITT will rise (interstage turbine temperature), mild pulsation or vibration and loud bangs.
Airflow distortion caused by:
abrupt change in attitude
encountering turbulence
deficiency of air velocity or volume due to atmospheric conditions
rapid throttle change |
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Term
| Describe four mechanical malfunctions that can lead to a compressor stall |
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Definition
Variable inlet guide vanes and stator vanes - failure to change these can cause too little or too much airflow at low engine speed
fuel control unit - too little or too much fuel can cause either too hot (flameout) or too cold (high pressure backup)
FOD - deformation of blade changes aero properties
Variable exhaust nozzles - failure to change can cause back pressure to propagate to compressor |
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Term
| Describe appropriate actions a pilot can take regarding compressor stalls |
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Definition
Reduce attitude of aircraft, reducing inlet angle of attack
Slow PCL slightly to allow air to continue through. Be safe about takeoff and landing though. shut down if necessary |
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Term
| Describe for engine design features that can be incorporated into a gas turbine engine designed to minimize potential for compressor stall |
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Definition
Variable inlet guide vane and stator vane - maintain velocity of air within acceptable limits at low airflow; change AoA at low airspeed
Dual/split spool compressors - front turbine turns at slower rate, reducing chance of stall
Bleed valves - vent near middle/rear of compressor to increase airflow in the front at low RPM
variable exhaust nozzle - unload back pressure at high/afterburner operation |
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Term
| Describe a turbojet engine |
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Definition
Engine that creates thrust purely from exhaust gas. Has gas generator, inlet, exhaust.
Inefficient at low airspeeds. At high speeds, more efficient (100% if reaching exhaust velocity speed)
- Lightest specific weight and higher/faster than any other
- long takeoff roll required |
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Term
| Describe characteristics of a turbojet engine |
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Definition
Inlet and exhaust section slapped onto gas generator
Derives thrust by accelerating small mass of air through engine. 75% of heat energy drives compressor and accessories, other 25 used for exhaust thrust |
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Term
| Describe a turbofan engine |
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Definition
Cross between jet and prop. Thrust of exhaust combine with thrust from mass of air accelerated by fan on the front. Air that goes around called bypass/ducted.
Bypass produces 30-60% thrust, gas generator produces 70-40% of thrust
Fan either driven by turbine or by a free power turbine behind turbine section. 75% of energy from burner section turns the compressor. remaining 25 for thrust; part of that is mechanical energy for fans. |
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Term
| Describe characteristics of a turbofan engine |
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Definition
Increased efficiency over jet by converting more of fuel energy into pressure energy; small energy to turn turbine and fan
Bypass ratio: amount that bypasses vs goes through generator. High = fuel efficient, low ratio = performance.
Bypass air can also be used for afterburner with exhaust
- high thrust low airspeed, lower TSFC, shorter takeoff and quieter
- higher specific weight, larger frontal area and inefficient at higher altitudes |
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Term
| Define thrust specific fuel consumption |
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Definition
| Amount of fuel required to produce one pound of thrust |
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Term
| Compare thrust specific fuel consumption of turbojet engines |
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Definition
| Highest TSFC of any engine |
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Term
| Compare thrust specific fuel consumption of turbofan engines |
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Definition
| Lower than Jet, still higher than prop/shaft |
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Term
| Compare propulsive efficiency of airplane engines |
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Definition
| Jet most efficient at high altitudes/airspeeds. Prop most efficient at low speeds and low altitudes. |
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Term
| Describe effect of bypass ratio on turbofan engine performance |
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Definition
| High bypass ratio, more efficient (easier to turn fan and accelerate air than burn and accelerate that way) |
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Term
| Describe a turboprop engine to include prop assembly |
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Definition
Prop: blade, hub and dome/pitch change assembly
reduction gearbox driven by gas generator. Torque shaft inside and connected to GG and RG; reference shaft just connected to gas generator, measures torque and thus HP. Inlet, gas generator, then either prop connected to turbine or a free power turbine behind. For the 25% primary air, 90% of energy turns propeller and accelerates air; only the 10% is for exhaust gas |
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Term
| Describe the operation of the reduction gear box of a turboprop engine |
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Definition
| Converts high rpm of engine to high torque for prop. connected to torque shaft. |
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Term
| Compare propulsive efficiency of airplane engines |
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Definition
| More efficient than jet or fan. Heavier though. Complicated. |
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Term
| Describe torque meter assembly of a turboprop engine |
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Definition
| Located between gas generator and propeller. Reference shaft only connected to gas generator, measures torque of TS which is connected to both and applies twisting motion. |
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Term
| Describe operations of the propeller of a turboprop engine |
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Definition
| Accelerates a large mass of air to moderate speed. Changes in fuel flow directly affect power changes. Maintains propeller at constant 100% RPM and increases torque with increased engine performance |
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Term
| Describe a turboshaft engine |
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Definition
| GG and FPT. all of primary air (25%) used for shaft HP. negligible exhaust |
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Term
| Describe the operation of the free/power turbine of a turboshaft engine |
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Definition
| Independent mechanically from GG. exhaust gases from turbine drive power turbine. coaxial main drive shaft hooked up to main transmission. |
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