ch 7 powerplant notes

chp7.json

{
  "Chapter 7: Propellers": "<h2>General Principles of Propellers</h2><p><strong>Purpose:</strong> Propellers absorb engine power output and convert it into thrust to move the aircraft through the air.</p><ul><li><strong>Limitations:</strong> Propeller-driven aircraft speed is limited to approximately 400 mph due to blade tip speed limitations. Turbofan engines are used for higher speeds.</li><li><strong>Advantages of Propeller-Driven Aircraft:</strong> Shorter and less expensive takeoff and landing, improved efficiency with new blade materials and manufacturing techniques. Smaller aircraft will continue to use them.</li><li><strong>Components:</strong> Blades and a central hub. Blades are essentially rotating wings.</li><li><strong>Blade Shank:</strong> Thick, rounded portion near the hub for strength.</li><li><strong>Blade Butt (Base/Root):</strong> End of the blade that fits into the hub.</li><li><strong>Blade Tip:</strong> Farthest part from the hub, generally the last 6 inches.</li><li><strong>Blade Back:</strong> Cambered or curved side, similar to the upper surface of a wing.</li><li><strong>Blade Face:</strong> Flat side of the propeller blade.</li><li><strong>Chordline:</strong> Imaginary line through the blade from leading edge to trailing edge.</li><li><strong>Leading Edge:</strong> Thick edge that meets the air during rotation.</li><li><strong>Propeller Efficiency ($\eta$):</strong> Ratio of thrust horsepower to brake horsepower, typically 50% to 87%.</li><li><strong>Pitch vs. Blade Angle:</strong> Often used interchangeably, but blade angle is the angle between the blade's chordline and the plane of rotation.</li><li><strong>Propeller Slip:</strong> Difference between geometric pitch (theoretical advance per revolution with no slippage) and effective pitch (actual advance).</li><li><strong>Formula:</strong> Geometric pitch - Effective pitch = Slip.</li><li><strong>Geometric pitch formula:</strong> $GP = 2 \times \pi \times R \times \text{tangent of blade angle at 75 percent station}$.</li><li><strong>Angle of Attack (AOA):</strong> Angle at which relative wind strikes the propeller blade. Creates dynamic pressure and thrust. Most efficient AOA is small, 2&deg; to 4&deg; positive.</li><li><strong>Thrust:</strong> Force equal to the mass of air handled multiplied by (slipstream velocity - aircraft velocity). Approximately 80% of the torque absorbed by the propeller.</li></ul><h3>Blade Angle Control:</h3><ul><li>Increasing blade angle increases AOA, requiring more horsepower to turn the propeller at a given rpm, thus slowing the propeller.</li><li>Decreasing blade angle speeds up the propeller.</li><li>This allows engine rpm to be controlled.</li></ul><h2>2. Forces Acting on a Propeller</h2><ul><li><strong>Centrifugal Force:</strong> Tends to pull rotating blades away from the hub; most dominant force. Also a component (centrifugal twisting moment) that moves blades to low pitch.</li><li><strong>Torque Bending Force:</strong> Air resistance tends to bend blades opposite to rotation.</li><li><strong>Thrust Bending Force:</strong> Thrust load tends to bend blades forward.</li><li><strong>Aerodynamic Twisting Force:</strong> Tends to turn blades to a high blade angle.</li><li><strong>Centrifugal Twisting Force:</strong> Tends to force blades toward a low blade angle.</li><li><strong>Propeller Stiffness:</strong> Must be rigid enough to prevent fluttering (vibration from blade ends twisting) which weakens the blade.</li></ul><h2>3. Propeller Types and Control Systems</h2><h3>3.1. Propeller Controls &amp; Instruments</h3><ul><li><strong>Fixed Pitch Propellers:</strong> No controls or adjustments in flight.</li></ul><h4>Constant-Speed Propeller Controls:</h4><ul><li>Propeller control lever (often in the center pedestal): &quot;Increase rpm&quot; (full forward) and &quot;decrease rpm&quot; (pulled aft). Directly connected to the governor speeder spring.</li><li>Can feather the propeller in some aircraft by moving to full decrease rpm.</li><li><strong>Instruments:</strong> Engine tachometer (for rpm) and manifold pressure gauge (adjusted by throttle).</li></ul><h3>3.2. Propeller Location</h3><ul><li><strong>Tractor Propeller:</strong> Mounted on the upstream end of a drive shaft, in front of the supporting structure. Most common type. Experiences lower stresses as it rotates in undisturbed air.</li><li><strong>Pusher Propeller:</strong> Mounted on the downstream end of a drive shaft, behind the supporting structure. More susceptible to damage from ground debris or water spray. Often mounted above and behind wings to prevent damage.</li></ul><h3>3.3. Types of Propellers</h3><h3>3.3.1. Fixed-Pitch Propeller</h3><ul><li>Blade pitch/angle is built-in and cannot be changed after manufacture.</li><li>Typically one-piece, made of wood or aluminum alloy.</li><li>Designed for best efficiency at one rotational and forward speed.</li><li>Used on low-power, low-speed, low-range, or low-altitude aircraft.</li><li><strong>Advantages:</strong> Less expensive, simple operation, no pilot control inputs in flight.</li></ul><h4>Wooden Fixed-Pitch Propellers:</h4><ul><li>Constructed from layers of selected hardwoods (e.g., birch, mahogany, cherry, black walnut, oak) glued together.</li><li>Fabric covering and metal tipping (terneplate, Monel metal, brass, stainless steel) on leading edge and tip for protection.</li><li>Small drain holes in tipping to release moisture.</li><li>Protective coating (water-repellent clear varnish) to prevent rapid moisture content change.</li><li>Hubs connect to crankshafts (splined, tapered, or flanged).</li><li>Attached using retaining nuts, locknuts, cotter pins, and sometimes front/rear cones.</li></ul><h4>Metal Fixed-Pitch Propellers:</h4><ul><li>Similar appearance to wood, but thinner sections.</li><li>Manufactured as one-piece anodized aluminum alloy.</li><li><strong>Advantages over wood:</strong> lighter, lower maintenance, more efficient cooling, pitch can be changed (within limits) by twisting at a repair station.</li><li>Identified by markings on the hub (serial number, model number, FAA type certificate, production certificate, reconditioning count).</li></ul><h3>3.3.2. Test Club Propeller</h3><ul><li>Used to test and break in reciprocating engines.</li><li>Designed to provide correct load and extra cooling airflow during testing.</li></ul><h3>3.3.3. Ground-Adjustable Propeller</h3><ul><li>Operates as a fixed-pitch propeller.</li><li>Pitch/blade angle can only be changed when the propeller is not turning, by loosening a clamping mechanism.</li><li>Not common on modern aircraft.</li></ul><h3>3.3.4. Controllable-Pitch Propeller</h3><ul><li>Permits change of blade pitch while rotating.</li><li>Pilot must directly change the blade angle; it does not change automatically.</li><li>Number of pitch positions may be limited (e.g., two-position) or continuously adjustable.</li><li>Not as common today, superseded by constant-speed types with governors.</li></ul><h3>3.3.5. Constant-Speed Propellers</h3><ul><li>Uses a governor to automatically adjust propeller pitch to maintain a constant engine rpm.</li><li>If engine speed increases (overspeed), the governor increases blade angle to increase load and slow rpm.</li><li>If engine speed decreases (underspeed), the governor decreases blade angle to reduce load and increase rpm.</li><li><strong>Pitch-Changing Mechanism:</strong> Typically hydraulic (oil pressure) using a piston-and-cylinder arrangement, converting linear motion to rotary motion to change blade angle.</li><li>Oil pressure usually boosted by an integral governor pump (approx. 300 psi) for quicker changes.</li><li><strong>Opposing Forces:</strong> Governor oil pressure moves blades to low pitch (high rpm). Flyweights, springs, compressed air, and aerodynamic twisting moment move blades to high pitch (low rpm).</li></ul><h4>Feathering Propellers:</h4><ul><li>Used on multi-engine aircraft to reduce drag in case of engine failure.</li><li>Changes pitch to approximately 90&deg; (blades parallel to line of flight) to stop rotation and minimize drag/windmilling.</li><li>Blades are held in feather by aerodynamic forces.</li><li>Feathering spring assists flyweights to increase pitch.</li><li>Feathering occurs if governor oil pressure drops to zero (e.g., engine runs out of oil or part breaks).</li><li><strong>Latches:</strong> Lock propeller in low pitch position during shutdown to prevent feathering and excessive load on engine during start-up. Held off seat by centrifugal force in flight.</li></ul><h4>Unfeathering:</h4><ul><li>Starting the engine to allow governor to pump oil back to reduce pitch.</li><li><strong>Accumulator system:</strong> Stores air-oil charge to quickly move blades from feather to lower angle for windmilling and engine start.</li><li><strong>Unfeathering pump:</strong> Provides pressure to quickly force propeller back to low pitch using engine oil.</li></ul><h4>Reverse-Pitch Propellers:</h4><ul><li>Controllable propellers where blade angles can change to a negative value.</li><li>Produces thrust opposite to forward direction, used for aerodynamic braking and reducing ground roll after landing. Mostly used on turboprops.</li></ul><h3>3.4. Propeller Governor</h3><ul><li>Engine rpm-sensing device and high-pressure oil pump.</li><li>Directs oil to/from hydraulic cylinder to change blade angle and maintain rpm.</li><li>Pilot sets rpm via flight deck control, which adjusts speeder spring tension.</li><li><strong>Components:</strong> Gear pump (boosts oil pressure), pilot valve (controlled by flyweights), relief valve (regulates operating oil pressure), speeder spring (opposes flyweights).</li><li><strong>Underspeed Condition:</strong> Flyweights tilt inward (not enough centrifugal force to overcome speeder spring). Pilot valve moves down, metering oil to decrease propeller pitch and increase engine rpm.</li><li><strong>Overspeed Condition:</strong> Centrifugal force on flyweights is greater than speeder spring force. Flyweights tilt outward and raise pilot valve. Pilot valve meters oil flow to increase propeller pitch and lower engine rpm.</li><li><strong>On-Speed Condition:</strong> Centrifugal force on flyweights balances speeder spring force. Pilot valve is stationary, no oil flows to/from propeller, and blade angle does not change.</li></ul><h2>4. Propeller Auxiliary Systems</h2><h3>4.1. Ice Control Systems</h3><p>Ice formation causes distorted airfoil, loss of efficiency, unbalance, destructive vibration, and increased blade weight.</p><h4>Anti-Icing Systems (Fluid):</h4><ul><li>Tank, pump, control system to vary pumping rate.</li><li>Fluid transferred via slinger ring to feed shoes/boots on blade leading edge.</li><li>Feed shoes have channels for fluid to flow to blade tip by centrifugal force.</li><li><strong>Fluids:</strong> Isopropyl alcohol, phosphate compounds.</li><li><strong>Disadvantages:</strong> Adds weight, limited anti-ice time, not used on modern aircraft.</li></ul><h4>Deicing Systems (Electrical):</h4><ul><li>Electrical energy source, resistance heating elements (internal/external on spinner/blades), controls, wiring.</li><li>Power transferred via slip rings and brushes.</li><li>Pilot controls with on-off switches, sometimes selector for light/heavy icing.</li><li>Timer/cycling unit determines sequence and duration of power to heating elements (boots).</li><li>Deice boot attached to blade leading edge.</li><li><strong>Purpose:</strong> Melt ice after formation but before excessive accumulation; proper control prevents runback.</li><li>Ammeters/loadmeters monitor current; system only used when propellers are rotating and for short test periods.</li></ul><h3>4.2. Propeller Synchronization &amp; Synchrophasing</h3><ul><li><strong>Synchronization:</strong> Controls and synchronizes engine rpm on multi-engine aircraft to reduce vibration and eliminate beat.</li><li><strong>Synchrophasing:</strong> Maintains a preset angular relationship between master and slave propellers to reduce cabin noise.</li><li>Electronic system matching engine rpm and setting blade phase.</li><li>Magnetic pickups provide rpm signals to control box; command signal sent to trimming coil on governor of slow engine.</li></ul><h3>4.3. Autofeathering System</h3><ul><li>Used during takeoff, approach, and landing to automatically feather a propeller if power is lost from an engine.</li><li>Solenoid valve dumps oil pressure from propeller cylinder when two torque switches sense low torque.</li><li>&quot;Test-off-arm&quot; switch to arm system.</li><li>Energizes holding coil (pulls in feather button) if thrust drops to preset value.</li><li>NTS (Negative Torque System) device can also initiate feathering by sensing negative torque (propeller driving engine).</li></ul><h2>5. Propeller Inspection &amp; Maintenance</h2><h3>5.1. General Inspection Points</h3><ul><li>Regular inspections are crucial; follow manufacturer's specified intervals.</li><li>Visual inspection of blades, hubs, controls, accessories for security, safety, condition.</li><li>Check for excessive oil/grease, weld/braze failures, nicks, scratches, flaws (use magnifying glass).</li><li>Check spinner/dome attaching screws for tightness, and lubrication levels.</li><li>If propeller is involved in an accident or ground strike, disassemble and inspect per manufacturer's manual.</li><li>Comply with Airworthiness Directives (ADs) and Service Bulletins (SBs).</li><li>Record all work in propeller logbook.</li></ul><h3>5.2. Specific Propeller Material Inspections</h3><h4>Wood Propeller Inspection:</h4><ul><li>Inspect for cracks, dents, warpage, glue failure, delamination, finish defects, charring from loose mounting bolts.</li><li>Check metal sleeve for cracks; tightness of lag screws.</li><li>Inspect metal cap, leading edge strip, and surrounding areas for looseness, separation of soldered joints, loose screws/rivets, breaks, cracks, erosion, corrosion.</li><li>Check for separation between metal leading edge and cap (discoloration, loose rivets).</li><li>Ensure moisture drain holes are open.</li><li>Fine line in fabric/plastic may indicate wood crack.</li></ul><h4>Metal Propeller Inspection (Steel &amp; Aluminum):</h4><ul><li>Susceptible to fatigue failure from nicks, cuts, scratches. Prompt repair is necessary.</li><li><strong>Steel blades:</strong> Visual, fluorescent penetrant, or magnetic particle inspection. Examine leading/trailing edges, shank grooves/shoulders, dents/scars with magnifying glass.</li><li><strong>Aluminum blades:</strong> Inspect for cracks and other flaws. Transverse cracks cause rejection; no repairs to shanks of adjustable-pitch blades. Multiple deep nicks/gouges on leading edge/face cause rejection. Use dye penetrant to confirm suspected cracks.</li><li><strong>Tachometer Accuracy:</strong> Inaccurate tachometers can lead to damaging high stresses. Verify accuracy at 100-hour or annual inspection intervals (within &plusmn;10 rpm recommended).</li></ul><h4>Composite Propeller Inspection:</h4><ul><li>Visually inspect for nicks, gouges, loose material, erosion, cracks, debonds, lightning strike.</li><li>Coin tap test for delaminations and debonds (hollow or dead sound indicates problem).</li><li>NDT techniques like phased array and ultrasound are available for detailed inspections.</li><li>Minor repairs can be performed by certificated mechanics; major repairs by certificated propeller repair stations.</li></ul><h3>5.3. Propeller Vibration &amp; Balancing</h3><h4>Causes of Propeller Vibration:</h4><ul><li>Imbalance (static, dynamic), improper blade angle settings, blades not tracking.</li></ul><h4>Troubleshooting Vibration:</h4><ul><li>If vibration occurs only at one rpm or limited range, likely an engine-propeller match problem, not a propeller problem.</li><li>Temporarily replace propeller with a known airworthy one for testing.</li><li>Spinner wobble indicates inadequate shimming, or cracked/deformed spinner.</li><li>If tracking and low-pitch blade angle are correct, it's likely static or dynamic unbalance.</li></ul><h4>Blade Tracking:</h4><p>Determining relative positions of blade tips (must rotate in same plane).</p><ul><li><strong>Tolerance:</strong> Must not exceed manufacturer's specification (e.g., within 1/10 inch from opposite blade's track).</li><li><strong>Method:</strong> Chock aircraft, remove spark plugs, position one blade down, place solid object/pointer, rotate slowly to check other blades.</li></ul><h4>Propeller Static Imbalance:</h4><p>CG of propeller does not coincide with axis of rotation.</p><h4>Static Balancing Procedure (Knife-Edge Test Stand):</h4><ul><li>Propeller must be free to rotate in a still environment.</li><li>For two-bladed props: Check vertical and horizontal positions, then reversed vertical.</li><li>For three-bladed props: Check with each blade in a downward vertical position.</li><li>All blades must be at the same blade angle.</li><li>Propeller remains at any position if properly balanced.</li><li><strong>Corrections:</strong> Adding/removing permanent fixed weights at manufacturer-specified locations.</li></ul><h4>Dynamic Unbalance:</h4><p>CG of similar propeller elements do not follow in the same plane of rotation. Can be caused by mass or aerodynamic imbalance.</p><h4>Dynamic Balancing Procedure (Analyzer Kit):</h4><ul><li>Reduces vibration and noise to cabin, reduces damage to other components.</li><li>Only improves vibration from mass unbalance of externally rotating components.</li><li>Requires clean blades and spinner dome (no grease leakage).</li><li>Vibration sensor attached to engine, analyzer unit calculates weight/location of balance weights.</li><li>Run engine at low cruise rpm; analyzer calculates needed weights.</li><li>Repeated runs may be needed.</li><li>Balance weights should not exceed manufacturer's limits.</li><li>Hartzell recommends balancing to 0.2 ips or less.</li><li>Reflective tape used for rpm reading should be removed after balancing.</li><li>Record balance weight details in logbook.</li></ul><h3>5.4. Propeller Removal &amp; Installation</h3><ul><li>Always use current manufacturer's information.</li><li><strong>Removal:</strong> Remove spinner dome, cut safety wire, support propeller with sling, unscrew mounting bolts/nuts, remove propeller carefully. Mark hub/flange for reinstallation if dynamically balanced.</li><li><strong>Installation:</strong> Clean engine/propeller flanges, install O-ring, align dowel studs, install on engine flange.</li><li>Install mounting nuts (dry) with spacers, torque to specifications, and safety wire.</li><li>Mounting hardware must be clean and dry to prevent excessive preload.</li><li>Tighten nuts evenly to avoid hub damage.</li></ul><h3>5.5. Servicing Propellers</h3><p>Includes cleaning, lubricating, and replenishing operating lubrication supplies.</p><h4>Cleaning:</h4><ul><li><strong>Aluminum/steel:</strong> Wash with suitable cleaning solvent (not acid/caustic), use brush/cloth. Avoid power buffers, steel wool, steel brushes that can scratch. Coat with clean engine oil after cleaning.</li><li><strong>Wooden:</strong> Use warm water and mild soap. Flush with fresh water if exposed to salt water.</li><li><strong>Grease/oil removal:</strong> Stoddard solvent on cloth, then wash with noncorrosive soap solution and rinse.</li></ul><h4>Charging Propeller Air Dome:</h4><ul><li>Position on start locks.</li><li>Charge cylinder with dry air or nitrogen (nitrogen preferred).</li><li>Use temperature/pressure chart to find correct charge pressure.</li></ul><h4>Propeller Lubrication:</h4><ul><li>Proper procedures and oil/grease specifications are in manufacturer's instructions.</li><li>Follow greasing schedule to prevent internal corrosion and ensure proper lubrication.</li><li>Water can get into blade bearing assembly.</li><li>Corrosion from dissimilar metals is a concern; overhaul periods are important for internal inspection.</li><li><strong>Example:</strong> Lubricate every 100 hours or 12 calendar months, or 6 months in adverse conditions. New/overhauled props lubricated after 1-2 hours due to grease redistribution.</li></ul><h3>5.6. Propeller Overhaul</h3><ul><li>Accomplished at maximum hours or calendar time limit.</li><li>Tracks components, researches ADs/SBs, preliminary inspection.</li><li>Discard most threaded fasteners; use specialized tools.</li><li>Dimensional inspection of worn components.</li><li>Aluminum parts anodized, steel parts cadmium plated for corrosion protection.</li><li><strong>Hub Inspection:</strong> Strip paint/anodize (nonferrous), use liquid penetrant inspection (LPI) for cracks. Some hubs eddy-current inspected.</li><li><strong>Steel Parts Inspection:</strong> Magnetic particle inspection (MPI) for flaws.</li><li><strong>Blade Overhaul:</strong> Precise measurement of width, thickness, face alignment, angles, length. Surface grinding, repitching, sometimes cold straightening (within limits).</li><li><strong>Reassembly:</strong> Recheck part numbers, lubricate/install per manual. Check high/low-pitch blade angles and for leaks by cycling with air pressure. Static balance. Safety wire hardware.</li><li>Major repairs/overhauls done by appropriately rated repair stations, manufacturers, or air carriers. Minor repairs by certified mechanics or under supervision.</li></ul><h2>6. Troubleshooting Propellers</h2><ul><li>Always refer to the correct manual.</li></ul><h3>Hunting &amp; Surging:</h3><ul><li><strong>Hunting:</strong> Cyclic variation in engine speed above/below desired.</li><li><strong>Surging:</strong> Large increase/decrease in engine speed, then returns.</li><li><strong>Check:</strong> Governor, fuel control, synchrophaser/synchronizer.</li></ul><h3>Engine Speed Varies with Flight Attitude (Airspeed):</h3><ul><li>Small variances are normal.</li><li>Possible causes (nonfeathering propeller, increasing speed descending): Governor not increasing oil volume, excessive transfer bearing leak, excessive friction in blade bearings/pitch changing mechanism.</li></ul><h3>Failure to Feather or Feathers Slowly:</h3><ul><li><strong>Check:</strong> Air charge (if lost/low), propeller/governor control linkage function/rigging, governor drain function, propeller for misadjustment or internal corrosion (blade bearings, pitch change mechanism).</li></ul><h2>7. Turboprop Engines &amp; Propeller Control Systems</h2><ul><li>Turboprop engines used for many aircraft, from small to large commuters.</li><li><strong>Turboprop powerplant:</strong> Propeller, reduction-gear assembly, and turbine engine.</li><li><strong>Turboprop vs. Turbofan:</strong> Turboprop produces thrust indirectly by furnishing torque to a propeller.</li><li>Fuel control and propeller governor are coordinated.</li></ul><h3>Control Systems:</h3><ul><li><strong>Flight (Alpha Range):</strong> Propeller blade angle and fuel flow governed automatically by a predetermined schedule. Propeller maintains constant engine speed (100% rated speed). Power changes by varying fuel flow, which increases turbine inlet temperature and torque, causing propeller to increase blade angle to maintain rpm and add thrust.</li><li><strong>Ground Operation (Beta Range):</strong> Below &quot;flight idle&quot; power lever position. Propeller blade angle controlled by power lever position, not governor.</li><li>Moving power lever below start position reverses pitch for reverse thrust and rapid deceleration after landing.</li></ul><ul><li><strong>Reduction Gear Assembly:</strong> Reduces high engine rpm to acceptable propeller rpm (prevents exceeding speed of sound at tip). Often planetary gear reduction.</li><li><strong>Propeller Brake:</strong> Prevents windmilling when feathered in flight, decreases time to stop after shutdown.</li><li><strong>Turbopropeller Assembly Subassemblies:</strong> Barrel, dome, low-pitch stop, overspeed governor, pitch control unit, auxiliary pump, feather/unfeather valves, torque motor, spinner, deice timer, beta feedback, propeller electronic control.</li><li>Modern turboprops use dual FADEC for engine and propeller control.</li><li><strong>Propeller Control (Large Turboprops):</strong> Dual acting, meaning hydraulic pressure is used to both increase and decrease propeller blade angle.</li></ul><h3>7.1. Pratt &amp; Whitney PT6 Hartzell Propeller System</h3><ul><li>Constant-speed, feathering, reversing propeller system using a single-acting governor.</li><li>Oil pressure to servo piston pushes it forward, compressing feather spring, and moving blades to low pitch (increases rpm).</li><li>Decreased oil pressure: Return spring and flyweights force oil out, changing blade pitch to high pitch (decreases rpm).</li><li>Governor oil supplied from engine, boosted by gear pump.</li></ul><h4>Governing Mode (On-Speed):</h4><p>Flyweight force equals speeder spring tension.</p><ul><li><strong>Increased engine power/speed (overspeed):</strong> Flyweights move out, control valve moves up, restricts oil flow to propeller dome. Feathering spring increases propeller pitch to maintain selected speed.</li><li><strong>Reduced power (underspeed):</strong> Flyweights underspeed, control valve moves down, more oil to propeller dome, lower pitch to control propeller speed.</li></ul><h4>Beta Mode (Low Power/Ground):</h4><ul><li>At low power, governor flyweights don't compress speeder spring. Control valve moves down, high pressure oil pushes dome forward, moving blades to low pitch.</li><li>Further movement pulls beta rod/slip ring forward, activating beta valve to stop oil supply, preventing blade angle from going lower than Primary Blade Angle (PBA).</li><li>In beta mode, propeller speed is controlled by limiting engine power, not changing blade angle.</li></ul><h4>Propeller Overspeed Governor:</h4><ul><li>Houses flyweights connected to a control valve, driven by beveled gear on propeller shaft.</li><li>If propeller speed exceeds limit, flyweights lift control valve, bleeding servo oil to increase blade angle, slowing propeller.</li><li>Can be tested at lower speeds by activating speed reset solenoid.</li><li>Second solenoid valve (on twin installations) works with autofeather system to quickly feather blades during engine malfunction.</li></ul><h3>7.2. Hamilton Standard Hydromatic Propellers</h3><ul><li>Used on older cargo aircraft, and some larger turboprops.</li><li><strong>Double-acting governor:</strong> Uses oil pressure on both sides of the propeller piston.</li><li>No flyweights in the pitch-changing mechanism; completely enclosed.</li><li>Oil pressure and centrifugal twisting moment used to turn blades to a lower angle.</li><li>Governor oil (boosted engine oil) acts on inboard side of piston to move blades to higher pitch.</li><li><strong>Main advantages:</strong> Large blade angle range, feathering, and reversing features.</li><li><strong>Components:</strong> Hub assembly, dome assembly, distributor valve assembly (or engine-shaft-extension assembly), anti-icing assembly.</li></ul><h4>Feathering Operation:</h4><ul><li>Pilot depresses feathering push-button switch, energizing feathering motor pump unit.</li><li>Auxiliary oil from pump shifts governor transfer valve, disconnecting governor and opening propeller oil line to auxiliary oil.</li><li>Oil flows to inboard piston end, moving piston outboard, which feathers blades.</li><li>High-angle stop ring prevents further movement in feathered position.</li><li>Electric cutout switch opens when set pressure reached, de-energizing pump and holding coil. Blades remain feathered as forces balance.</li></ul><h4>Unfeathering Operation:</h4><ul><li>Pilot depresses and holds feathering switch.</li><li>Motor-pump unit starts, supplying high-pressure oil to governor transfer valve.</li><li>Auxiliary oil shifts governor transfer valve, disconnecting governor and admitting auxiliary oil to distributor valve.</li><li>Piston moves inboard, unfeathering blades. Windmilling assists.</li><li>Operator shuts off pump at approx. 1,000 rpm, re-connecting governor to propeller.</li><li><strong>Governor Setting:</strong> Adjustable stop limits max rpm. Reset governor until takeoff rpm is obtained.</li></ul>"
}