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AGK: Piston 2 (Diesel (aka Compression Ignition Eng.) 8 (No spark plugs,…
AGK: Piston 2
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11 Prop Systems
- Fixed pitch prop: neither eng nor prop at optimum RPM over large speed range
- +VE TORQUE = when eng driving the prop
- -VE Torque = when prop is driving the engine ∴ need ⬆️power
- Constant Speed Prop: Thrust⬆️ by ⬆️blade AoA rather than ⬆️RPM
- PCU(Pitch Control Unit) - uses operating pins/links to chg blade AoA
- Single Acting PCU
- Eng oil pumped into piston against spring+N2. Piston pushes causing blade to rotate
- In a TWIN (usual) spring pushes blade to fully coarse ∴ less drag is fail safe posn.
- In Single spring pushes blade to flight fine to aid in eng. restart
- Centrifugal Latch: only in ground fine posn. When RPM⬇️ for engine off, latches engages and holds prop fine ∴starter motor no need to fight feathered prop
- CSU:Constant Speed Unit
- Governor flyweights linked to speeder spring. When RPM⬆️, they push up against spring which pulls up pilot valve = PCU oil drains =⬇️press = coarsens prop = RPM⬇️
- When on speed, pistons creates hydraulic lock = blade angle constants
- When RPM⬇️, flyweights in = spring pushed down = pilot valve down =⬆️oil to PCU (press ⬆️) = push PCU spring back = prop finer as Blade angle⬇️
- UNFEATHERING via ACCUMULATOR charged via NRV in normal running. Move RPM lever to fine, accumulator discharged via solenoid valve to move prop to fine
- Dble Acting PCU
- Oil pumped to both sides of piston.
- Allows better ctrl of blade angle.
- CSU now chgs direction of flow rather than dump press.
- Electric motor used to feather/unfeather
- Turboprops
- More sophisticated sys, dedicated hydraulic oil supply to PCU.
- Pitch lock holds prop at crz angle if oil leak
- Autofeather
- If EF @ TO sudden torque drop = EF and is feathered. Blocking relay prevents both engs being feather. Time delay prevents gusts leading to autofeather
- Turboprop Power Lever
- Beta range: 4 ground mvmnt allows ground fine, reverse thrust
- Alpha range: lever moved via detent where it acts as constant speed prop. Low Pitch Stops prevent move out of this range w/o weight on wheels
- Condition lever: Posns = Fuel on(run) > fuel off(stop) -> Feather
- Cabin noise: Blades produce pulses. Multi props uncomfy pulsing when diff RPMs
- Synchronising = 1 eng = master, synchro unit makes small pitch chgs to syncho RPM of all other engs to master
- Synchrophasing = offsets to phase of each prop so they don't cross and "slap" air. Done via magnetic trim to governor after synchronising
- -ve Torque Signal
- Prevent windmilling @ hi speed & lo power.
- System will set PCU to give pulses of coarse pitchThis is continued til eng torque is ⬆️
7 Turbo/Super Charger
- As Alt⬆️ density ⬇️ ∴power output ⬇️
- T/Schargers pre-compress air b4 inlet manifold = more air into cylinder = ⬆️volumetric eff.
- TC uses exhaust to power turbine to power compressor = ⬆️thermal eff.
- Compressor ⬆️Temp & Ps in 2 divergent ducts = net vel. change = 0.
- MAP gauge: downstream of throttle and b4 inlet. Aneroid cap. = baseline for atmospheric ∴ MAP always relativ to it
- EX: GROUND BOOSTED sys = ⬆️ MAP > MSL press. = ⬇️TOR ⬆️Traffic load
- ALTITUDE BOOSTED = maintain MSL press.
- Waste Gate
- ⬆️power = ⬆️exhaust press = ⬆️TC speed etc...runaway cond. if not controlled ∴WGate prevents MAP run away
- Absolute Controller
- Controls WGate via press feed from inlet manifold to aneroid cap. ⬆️MAP = capsule compress = raise oil restrictor valve = ⬇️oil press. for single acting actuator with a spring. Spring overcomes ∴WGate = open = exhaust gas bypasses TC turbine = boost⬇️
- If boost is set and alt⬆️ this will =⬇️MAP = cap. expand etc...reverse happens and WGate closed = ⬆️boost
- Eng not on = no oil press = WG fully open (cos of spring)
- Critical Alt = WGate fully close, TC RPM max. Any ⬆️alt =⬇️TC rpm & BHP ⬇️
- Overboost relief valve prevents structural damage
- EX: What is posn of WG @ start of TOR = ALMOST FULY OPEN
- EX: Why does HPwr ⬇️ from TO to Critical Alt despite having constant power?
- **A: As WGate gets nearer exhaust back press ⬆️∴ volumetric efff. ⬇️ slightly
- Intercooler
- Cools pressurised inlet air to ⬇️detonation risk. 2° effect = charge density ⬆️
- SuperCharger
- Compressor is ENGINE DRIVEN via crankshaft∴no HOT exhaust used so can be used to compress AIR & FUEL
- Power output ⬆️ as climb cos volumetric eff⬆️ cos exhaust back pressure ⬇️
- @ TO could blowup eng @ full throttle ∴lever limited by dble acting actuator to full throttle height
- Intercooler: see TC
9 Eng Instruments
- EX: Rated Power = specified MAP & RPM permissible 4 safe continuous operation
- RPM Gauge: crankshaft RPM (drag cup or 3-phase tacho genny)
- Manifold Press Gge: manifold pressure in Hg In. Dble Bellows
- Oil Temp Gge: temp in return line. Wheatstone bridge
- Oil Press Gge: pressure just after the pressure filter. Bourbon tube (no electrical input)
- EX: Its important oil press registers within 30secs of eng start
- Cylin Head Temp Gge: temp of the hottest cylinder (usually rearmost). Thermocouple
- Exhaust Gas Temp Gge: temp in exhaust gas. Thermocouple. Used to adjust mixing ratio. 15:1 gives highest EGT
- Induction System Temp Gge: temp @ carburettor inlet
- Red line = max induction temp
- Yellow line = icing likely
- Boost Press Gge: manifold press in PSI relative to MSL
- Fuel Press Gge: monitors fuel pump output press on carburettor engines & manifold output press on injected engines, both being proportional to fuel flow. Bourbon tube
10 Propellors
- AoA = btwn RAF & Chordline 4°
- Blade Angle = btwn chord & plane of rotation. Measured @ 75% root to tip
- Blade Angle ⬇️ towards tip to maintain AoA (Blade twist)
- Variable Pitch props allow optimum AoA over speed range
- TAS ⬆️ AoA ⬆️
- RPM ⬆️ AoA ⬇️
- Blade Pitch Posns:
- Fully Coarse (Feathered)
- Fine = T/O. As TAS ⬆️ blade pitch made coarser (high blade angle)
- Alpha Range (i.e.Flight) = range of blade angle within which we will maintain a constant speed (RPM)
- Beta range (i.e. Ground = range of blade angles within which blade angle is scheduled (i.e. selected manually)
-Pitches = Feather->Coarse->Flight Fine->Ground Fine->Reverse
- Windmilling: Eng. Out = prop/air driving the engine = lots of drag. Feathering prevents
- Prop Eff. = 80% = Power In (Torque x RPM) c/f Power Out (TAS x Thrust)
- ATM: Aerodynamic Turning Moment: caused by CoP fwd of pivot pt. ⬆️ Blade Angle = tries to feather itself
- CTM: Centrifugal turning moment ⬇️ Mass of blade trying to align with plane of rotation. Blade angle (finer)
- At operating RPMs CTM>ATM. Counterweights ⬇️load of CTM on pitch control mechanism
- Think ATM (Clkwise), CTM(Anti-clkwise)
- Windmilling = both ATM & CTM are fining forces