Engines Flashcards
EXPLAIN Bernoulli’s Equation
-states that as any incomp. (subsonic) fluid passes through a convergent opening, its velocity increases and pressure decreases
DESCRIBE the behavior of airflow in a nozzle
- the shape of the opening increases the airflow’s velocity and decreases the airflow’s pressure
- convergent for subsonic
DESCRIBE the behavior of airflow in a diffuser
- pressure is increased and velocity is decreased
- divergent for subsonic
DESCRIBE the Brayton Cycle
-four simultaneous events: intake, compression, combustion, and exhaust
DESCRIBE a gas generator
- produces high energy airflow necessary for thrust
- minimally include: compressor, combustion chamber, and turbine
- on turboprop/shaft/fan, will include fan, properller, , or rotor blades
DESCRIBE how airflow properties change through each section of a gas turbine engine
I C D B T E
Temp / / _ / \ \
Pres / / / \ \ \
Velo \ / \ / / /
DESCRIBE engine thrust
-a combination of velocity of exhaust gas and inlet velocity
DESCRIBE the effects of airflow properties on thrust in a gas turbine engine
- density^: Th^
- Temp^: less Th, vary as much as 20%
- pressure^: ^Th
- altitude^: less Th, faster in isothermal
- AS^: less Th
EXPLAIN ram effect in a gas turbine engine
-thrust decrease with increase in airspeed offset by compressibility of air near supersonic which actually causes thrust neutral or increase
DESCRIBE the cockpit thrust measuring devices
- for gas turbine: Engine Pressure Ratio (EPR) gauge, it measures the ratio between inlet and exhaust pressures
- for prop or rotor driven: torquemeter, indicates shaft HP available
- tach: indication of engine speed
DESCRIBE inlet ducts
- designed to provide proper amt of hi pressure, turbulence free air to compressor
- broad range of operating cond.
- diffuser
- single or divided entrance
DESCRIBE compressors
- supply enough air to satisfy req. of combustion section
- increases pressure of air and directs it to the burners
- supply compressor bleed air to operate various comp.
- not closed system
- increase V without decrease in P thanks to drive shaft energy input
DESCRIBE the burner section of a gas turbine engine
- contains comb. chamber and provides means for proper fuel air mixing
- 25% primary air for F/A mix
- 75% secondary air flows around for cooling and flame control
DESCRIBE combustion chambers
- three types: can, annular, and can-annular
- can: older, centrifugal compressor engines, stronger, durable, poor use of space, P loss, uneven heat distro.
- annular: uniform heat distro., good use of space, better F/A mixing, cannot be removed easily, structural problems
- can-annular: in large hi perf. aircraft, combines easy manintenance with excellent thermodynamics of each
DESCRIBE the turbine section of a gas turbine engine
- drives the compressor and the accesories
- designed to increase airflow velocity
- 75% of energy used to spin turbine
- 25% used for thrust
DESCRIBE the phenomenon of creep in a gas turbine engine
- rotor blades undergo elongation
- cumulative process
- deformed blades not efficient and may fail catastrophically
DESCRIBE the exhaust section of a gas turbine engine
- direct flow of hot gases rearward to cause a high exit velocity and prevent turb.
- exhaust outer duct, exhaust inner core, and three or four raidal hollow struts
- fixed convergent type for subsonic
- variable C-D type for supersonic
DESCRIBE the afterburner section of a gas turbine engine
- aka thrust augmentation
- used in turbojets and turbofans
- increase maximum Thrust available by 50% or more
- huge fuel usage
- consists of spray bars, flame holders, screech liner, and variable exhaust nozzle
DESCRIBE the angle of attack of compressor blades
- angle btwn RW and rotor blade’s chordline
- can stall compressor if rotor blades spinning at wrong speed for inlet airapeed
DESCRIBE a compressor stall
- still an airfoil, spinning rotors encounter different AOA depending on speed
- anything that decreases inlet airflow or increases compressor RPM could stall
DESCRIBE four mechanical malfunctions that can lead to a compressor stall
1) variable inlet guide vane and stator vane: failure to change AOA
2) fuel control unit (FCU): wrong amounts of fuel can cause back-flow or flame out
3) foreign object damage: deformation of blade changes aero prop.
4) variable exhaust nozzle: if fails to open, extra back pressure could cause failure
DESCRIBE appropriate actions a pilot can take regarding compressor stalls
- avoid erratic/abrupt PCL changes esp. at high AOA or low AS
- fly min. AS and avoid abrupt changes in AC attitude
- avoid flt thru svr wx and turb
DESCRIBE four engine design features that can be incorporated into a gas turbine engine
design to minimize the potential for a compressor stal
- variable inlet guide vane and stator vane: changes AOA at low engine spd
- dual/twin/split-spool compressor: front rotor turns slower
- bleed valves: increase airflow in the front of the compressor @ low RPM
- variable exhaust nozzle: unload pressure during afterburner operation
DESCRIBE a turbojet engine
- simplest gas turbine engine
- inlet and exhaust section + basic gas generator
- thrust from highly accelerating small mass of air
- all air thru inlet goes thru GG
DESCRIBE the characteristics of a turbojet engine
- lightest specific weight
- higher and faster
- low propulsive efficiency at fwd speeds
- high TSFC at low alt and AS
- long T/O roll
DESCRIBE a turbofan engine
- cross btween TJ and TP
- gas from GG plus ducted fan
- fan accelerates farily large mass of air around GG (bypass.ducted air)
- bypass air = 30-60% of thrust
- better thrust/fuel than turbojet
DESCRIBE the characteristics of a turbofan engine
- higher Th at low AS
- lower TSFC
- shorter T/O distance
- noise reduction
- higher specific weight
- larger frontal area
- inefficient at higher alts.
DEFINE thrust specific fuel consumption
amount of fuel required to produce one pound of thrust
COMPARE the thrust specific fuel consumption of turbojet engines
- turbojet worse than turbofan
- high at low AS and altitude
COMPARE the thrust specific fuel consumption of turbofan engines
- better than turbojet
- higher bypass ratio better
- high at low AS and altitude
COMPARE the propulsive efficiency of airplane engines
- turboprop best at low AS but drops off rapidly after 350 mph
- turbofan best at middle-hi speeds
- turbojet best at hi speeds
DESCRIBE the effect of bypass ratio on turbofan engine performance
- hi BPR most efficient
- airliner/cargo higher
- fighter lower for smaller frontal area and better high speed performance
DESCRIBE a turboprop engine, to include the propeller assembly
- GG plus reduction gearbox and prop
- best qualities of GG with PE of propeller
- prop provides majority of thrust by imparting small accel. to large mass of air
DESCRIBE the operation of the reduction gear box of a turboprop engine
- basically one speed tranny’-prevents prop from reaching supersonic speeds
- converts high rmp and low torque of GG to low rpm and high torque for prop
- in T-6B 30000 to 2000 RPM
COMPARE the propulsive efficiency of airplane engines
as TP climbs to higher alt. mass of air being accel. by prop decreases due to decreased density
DESCRIBE the torquemeter assembly of a turboprop engine
- set of shafts located btwn the GG and the RGB
- used on some TP engined to transmit and measure power output from GG to RGB
- displays twisting on instrument panel in in-lbs of torque or SHP
DESCRIBE operations of the propeller of a turboprop engine
- TP accelerates large mass of air with prop to moderate speed
- prop. maintains constant 100% RPM
- changes blade pitch to maintain RPM
- alpha range: PCL btween idle and full, FCU works with prop governor to ensure constant RPM
- beta range: ground ops, idle to max reverse, PCL mechanically connected to pitch change assembly as well as FCU
DESCRIBE a turboshaft engine
- turboprop but spins helicopter rotor etc vice prop
- used to drive ships, tanks, trains, etc.
- GG + free/power turbine
DESCRIBE the operation of the free/power turbine of a turboshaft engine
- mechanically indepenedent from GG
- exhaust from GG turbine drive power turbine
- power turbine connected to main transmission thru coaxial main drive shaft
EXPLAIN how Pascal’s Law governs the forces and pressures associated with a confined
liquid
pressure applied to a confined liquid is transmitted equally in all directions without loss of pressure and acts with equal force on equal surfaces
EXPLAIN the relationship between linear displacement and the change in force between
the input and output pistons of a closed hydraulic system
- linear displacement is exchanged for change in force
- a 10 inch input displacement with 10 lbs acting over 2in^2 can push 200 lbs i inch with a 20in^2 surface area
DESCRIBE a basic aircraft hydraulic system
- main purpose is to multiply force
- power system: fluid supply, pump, selector valves, etc.
- near 3000 psi
- 2 systems, one for flight controls and one for utility systems + flight controls
DESCRIBE AC/DC electrical systems, to include their relative advantages
- AC reverses directions, used in homes
- DC flows in one direction, used in cars, boats, small electronic devices
- Aircraft usually use AC
- AC uses less current, smaller a/c wiring, less weight, lighter, reliable, simple
DESCRIBE a basic aircraft electrical system
engine powers accesory gearbox=>constant speed drive=>AC generator/alternator which powers the buses
engine powers accesory gearbox=>dc starter/generator=>buses and battery
four buses:
- essential bus: flight safety (att. gyro)
- primary bus: mission (radar)
- monitor/seconday bus: convenience circuits (lighting)
- starter bus: routes power to start a/c engines
DEFINE volatility
measurement of a liquid’s ability to convert to a vaporous state
DEFINE flashpoint
lowest temp. of a combustible substance that would ignite with a momentary application of a flame
DESCRIBE how temperature affects flashpoint
???
STATE the characteristics of common military aviation fuels
- JP4: previously most common, highly volatile, low FP (-35dgFP), easier starting, slower acceleration, lower operating temps, hi vapor lock tend., and shorter range
- JP5: Navy, MC, and CG primary fuel, thermally stable, high heat content, low volatility, high FP (140dgFP), safe on ships
- JP8: better fuel handling and operation safety, lower FP than JP5 (100dgF), AF uses this
DESCRIBE a basic aircraft fuel system
- must provide clean, vapor-free fuel at proper pressure and flow rates in all conditions
- fuel tank holds it, boost pump pumps it, emerg. shutoff valve, pressure gauge, fuel filter, engine driven pump, FCU controls flow, flow gauge, cooler/heat xchanger, manifolds, P&D valves, afterburner
DESCRIBE rated thrust
- maximum value of thrust based on TIT/ITT (turbine inlet temp)
- normal rated (NRT): trust at max cont. TIT with no time limitation (cruising)
- military rated thrust (MRT): thrust at max TIT for limited (30 mins) time
- combat rated (CRT): thrust with afterburner in operation
DESCRIBE the functions of lubricants
-primary: reduce friction caused by metal-to-metal contact
DESCRIBE the characteristics of synthetic lubricants
- don’t mix oils from different manuf. or used in same engine
- not compatible with mineral or petrol based oils and must never be mixed
- lower tendency to leave coking deposits, stronger chemical stability at high temps
- very corrosive
- short shelf life
DEFINE viscosity
- property of a fluid that resists the force tending to cause the fluid to flow
- oil gets thinner when hot
DESCRIBE a basic aircraft lubrication system
- wet pump: when limited supply of oil and cooling needed, limited to sump capa., hard to cool oil, not adaptable to extended unusual attitudes (inversion)
- dry sump: oil in tank located in airframe or on engine, larger supply, better temp regulation, streamlined, self contained, pressure and scavenger subsystem
DESCRIBE the types of accessories used on aircraft
- two types: air driven or mechanical
- air: A/C, cockpit pressurization, engine anti-ice, heating units
- mech: tach, hydraulic pumps, generators, alternators, etc.
DESCRIBE how accessories are driven
- air driven: compressor discharge air at hi P and T is bled from engine through ports or valves at intervals along comp. case and at end of the diffuser
- mechanical: geared drive taken directly from the main shaft connecting the turbine to the comp.
DEFINE interstage bleed air
-air ducted overboard to prevent comp. stall during certain periods of low thrust operation
DESCRIBE the starting sequence for a gas turbine engine
- starter accelerates compressor
- sufficient air established thru engine
- ingition activated
- fuel added
- starter continues until idle RPM attained
DESCRIBE abnormal starts of a gas turbine engine
- hot start: exceeding max allow. temp
- hung start: temp. in turb. section continues to rise and comp. rpm stabilizes below normal
- false start: comp. rpm stabilizes below normal and turb. temp. reamins within limits
- wet start: F/A mix does not light initially but has capa. to eventually ignite
DESCRIBE a DC electric starter
- most common
- mechanically connected to comp. and mounted on either engine acc. gear box or front frame of engine
- battery, APU, or external electrical source may power it
DESCRIBE an air turbine starter
- aka pneumatic starters
- most common on large gas turnines
- small geared air turbine motor attached to the engine
- air is directed to the air turbine which accelerates the compressor
- air supplied by ground cart or APU
DESCRIBE a basic aircraft ignition system
- hi energy capacitor type ignition normally used
- high voltage and hot spark
- works at even high alts.
- minimal fouling
- two ignitor plugs (spark ignitors)
- spark only needed for engine starting
