Propeller Mechanics and Dynamics Quiz

Week 1 Complete

• AVIA-1052 course confirmed as week 1 complete
• Contact Matt C for corrections/improvements

Week 1 of 1 Day 1

• AVIA-1052 course, week 1, day 1

Previously On AVIA-1052

• First class together
• Now in first class

Today On AVIA-1052

• Discussion of propellers
• General propeller stuff
• Aircraft propeller theory
• Basic propeller controls

Where are we?

• Powerplant (Vol. 2) 7-2 to 7-6
• Start at general
• Stop at propeller location
• FAA-H-8083-30A, Aviation Maintenance Technician Handbook-General
• FAA-H-8083-31A, Aviation Maintenance Technician Handbook-Airframe Volume 1
• FAA-H-8083-31A, Aviation Maintenance Technician Handbook-Airframe Volume 2
• FAA-H-8083-32A, Aviation Maintenance Technician Handbook-Powerplant Volume 1
• FAA-H-8083-32A, Aviation Maintenance Technician Handbook-Powerplant Volume 2

Propellers

• Unit that absorbs engine power output
• Propeller development stages
• First propellers were fabric-covered sticks
• Designed to force air rearward
• Advanced from simple designs to complex systems
• Variable-pitch, constant-speed feathering, and reversing systems
• Increase flying efficiency
• Different propeller types
• Two, four, and six-bladed propellers
• Propeller-driven aircraft speed limitations related to propeller rpm

Constant-speed systems

• Flyweight-equipped governor unit
• Controls pitch angle of blades to maintain constant engine speed

Forces acting on a rotating propeller

• Centrifugal force
• Blade weight is important for propeller design
• Excessive blade tip speed results in poor efficiency,fluttering, and vibration

Propeller speed limitations

• Aircraft speed limitations relate to the propeller rpm
• Turbofan engines used for higher speed aircraft

Propeller advantages

• Shorter takeoff and landing times
• Less expensive to maintain
• Increased efficiency from new blade materials and manufacturing techniques
• Widely used in turboprops and reciprocating engine installations
• Many smaller aircraft will continue to use propellers

Propeller systems (different types)

• Many have been designed for specific aircraft, speed, and type of operation

Propeller diagrams

• Basic nomenclature of the parts of a simple fixed-pitch, two-bladed wood propeller
• Aerodynamic cross-section of a blade

Aerodynamic cross-section of a blade

• Angle of attack
• Chord line
• Blade face
• Blade back (relative wind)

Aircraft Propeller Theory

• The aircraft propeller is composed of two or more blades.
• Each blade is a rotating wing.
• The blades create thrust (force) to move the aircraft through the air.
• The power needed to rotate propeller blades is furnished by the engine
• A central hub is used to attach the blades.

Basic Propeller Principles

• Mounted on a shaft
• Low-horsepower engines, which is an extension of the crankshaft
• High-horsepower engines, which is mounted on a propeller shaft
• The engine rotates the airfoils
• The propeller transforms the rotary power of the engine to thrust

Propeller Aerodynamic Process

• Airplane moving through the air
• Creates drag force opposing its forward motion
• There must be equal force in the opposite direction to prevent the aircraft from stopping.
• This force is called thrust
• Work done by thrust is equal to thrust times distance
• Power expended by thrust is equal to thrust times velocity

Propeller Efficiency

• Ratio of thrust horsepower to brake horsepower
• Ratio of the useful power output to the power input
• Generally, 50-87% depending on the amount of slip

Propeller Pitch

• Angle between chord/chord line of a blade section and the plane in which the propeller rotates
• Usually measured in degrees

Geometric pitch

• Distance a propeller advances in one revolution with no slippage
• Usually expressed in pitch inches
• Calculated using a formula

Effective pitch

• Distance a propeller advances in one revolution with slippage
• Difference between geometric pitch and effective pitch

Chord line of a propeller blade

• Determined similarly to an airfoil
• A propeller blade can be considered as an infinite number of thin blade elements
• Each element has an airfoil section whose chord is the propeller blade width
• Drawn along the face of the propeller blade to identify specific areas

Typical propeller blade

• Describes characteristics such as a twisted airfoil, differing segments, and location from the hub as station numbers

Forces acting on rotating propellers

• Centrifugal force pulling the propeller blades away from the hub
• Torque bending force causes propeller blade bending in the direction opposite of rotation.
• Thrust bending force caused by the thrust acting on the aircraft.

Excessive blade speed

• Poor blade efficiency
• Vibration
• Flutter

Propeller speed limitation

• Maximum speed of propeller-driven aircraft limited to about 400mph (644km/h)
• turbofan engines used for higher speed Aircraft

Propeller advantages

• Takes off and landing can be shorter, and less expensive to maintain
• New blade materials/ manufacturing techniques have increased efficiency.
• Propellers widely used in applications such as turboprops and reciprocating engine installations
• Many smaller aircraft will use propellers into the future

Propeller systems

• Many different types of propeller systems have been developed for specific aircraft, speed, and operation type

Propeller diagrams

• Basic nomenclature of the parts of a simple fixed-pitch, two-bladed wood propeller
• Aerodynamic cross-section of a blade (next slide)
• Includes terminology to describe areas shown (e.g., trailing edge, leading edge, chord line, hub)

Aerodynamic cross-section of a blade

• Includes terms like angle of attack, chord line, blade back, and blade face

Basic propeller controls and instruments (various videos and explanations available)

• Video resources available, see resources pages for links.

Propeller controls and instruments (various explanations available)

• Video resources available, see resources pages for links.

Propeller controls

• fixed and constant

Specific propeller control components

• Control Lever, etc

Propeller Governor

• Engine and propeller rpm-sensing device
• High pressure oil pump
• Directs oil under pressure or releases oil from the hydraulic cylinder.
• In constant-speed propeller system
• Changes in oil volume in the hydraulic cylinder changes blade angle.
• Maintains specific rpm via the cockpit propeller control.

Propeller Governor Mechanism

• Constant speed controls consist of a gear pump, a pilot valve and flyweights.
• Governor flyweights to control flow of oil through governor to and from the propeller.
• The position of the pilot valve regulates the quantity of oil.
• Relief valve regulates the governor oil pressure.

Propeller Governor Mechanism - ON-SPEED

• When operating at the set rpm. governor flyweights are balanced by the speeder spring.
• Pilot valve does not direct oil.

Propeller Governor Mechanism - OVERSPEED

• When engine speed is above the set rpm. the centrifugal force causes flyweights to move out.
• Pilot valve releases oil, increasing blade pitch, and slowing the engine.

Propeller Governor Mechanism - UNDERSPEED

• When engine speed is below the set rpm. The centrifugal force decreases, and the flyweights move inward, towards the speeder spring.
• The pilot valve is forced down, increasing oil flow, decreasing propeller pitch, increasing engine speed.

Feathering Propellers

• Used on multi-engine aircraft to reduce drag during one or more engine failure conditions
• Blade angle changed to approximately 90° to the line of flight.
• This greatly reduces drag.
• Propeller stops turning when blades are parallel to the airstream.

Feathering Propeller Mechanism

• Uses oil pressure to move blades to low pitch position during shutdown and to high pitch during operation.
• Latches lock the blade to the low pitch position during startup and shutdown.
• Most small feathering props use oil pressure.

Reverse-Pitch Propellers

• Used to change blade angle of propellers in flight.
• Allows the blade angle to be changed to a negative value.
• Useful to slow the aircraft down during landing.

Reverse-Pitch Propeller Operation

• When the aircraft lands, the propellers blades can be moved to negative pitch.
• Increased airframe drag slows the aircraft down.
• Allows fast maneuvering with little to no engine power change

Propeller Auxiliary Systems: Ice Control Systems

• Ice formation during flight causes loss in propeller efficiency.
• Ice formation also causes vibration and increases the weight on the blades.
• Fluid Anti-Ice Systems is used for ice prevention.

Propeller Auxiliary Systems: Fluid Anti-Ice System

• Fluid Anti-ice system consists of a tank with a pump to force fluid to each propeller and a control system.
• The system permits variation of the pumping rate to properly de-ice the propeller based on the ice conditions.

Propeller Auxiliary Systems: Fluid Types

• Isopropyl alcohol, phosphate composites
• High Availability and low cost
• Not widely used.

Propeller Auxiliary Systems: Feed Shoes

• Rubber strips that extend from the blade shank to the propeller radius.
• Installed on the blades' leading edges
• The airflow disperses the fluid to areas where ice does not develop

Propeller Auxiliary Systems: Slinger Rings

• Fluid transfer is transferred through nozzles from a stationary nozzle, to a U-shaped channel on the rear of the propeller assembly.

Propeller Auxiliary Systems: Electric Deicing Systems

• An electric propeller-icing control system consists of an electrical energy source, a resistance heating element mounted on the propeller spinner and blades.
• The system can be controlled internally or externally and the necessary wiring.
• The system has a master switch, toggle, selector switch, light, and timer to control the blade sequence when deicing.
• A cycling unit energizes the heating elements in sequence and a brush block transfers electricity to the slip ring.
• Control circuitry should reflect operation of timers and contain protective devices such as current limiters or circuit breakers.

Propeller Synchronization and Synchrophasing

• Synchronization systems are used on most multi-engine aircraft to provide a method to control and synchronize rpm
• Reduces vibrations and eliminates the unpleasant beat from unsynchronized propeller operation.

Autofeathering System

• Automatic propeller system
• Used for takeoff, approach, and landing
• Automatically feathers the propeller if there is a loss of engine power.

Propeller Governor

• Engine rpm-sensing device
• High-pressure oil pump
• Responds to engine speed changes by either directing or releasing oil to the propeller hydraulic cylinder to change blade angle to maintain specific rpm.

Wood Propeller Inspection

• Wood propellers should be inspected frequently to ensure airworthiness
• Inspect for defects such as cracks, dents, warpage, and glue failure
• Inspect the blade tips, leading edge, trailing edge, and grooves to identify any defects or damage.

Metal Propeller Inspection

• Metal propellers and blades are generally prone to fatigue failure resulting from the concentration of stresses.
• Inspect the full length of the leading and trailing edges, grooves, shoulders, dents, and scars with a magnifying glass to decide if the defects are scratches or cracks.
• Tachometer inspection accuracy must be verified. This is critical because inaccurate reading may cause damage to the propeller and engine.

Aluminum Propeller Inspection

• Carefully inspect aluminum propellers and blades for cracks and other flaws.
• Multiple deep nicks or gouges on the blade's leading edge may be cause for rejection.
• Use dye penetrant or fluorescent dye penetrant to confirm suspected cracks.
• Refer any unusual conditions or appearance to the manufacturer.

Composite Propeller Inspection

• Visual inspection for nicks, gouges, loose material, erosion, cracks, delaminations, and lightning strike.
• Check for delamination and debonds by tapping the blade or cuff with a metal coin.

Propeller Vibration

• Vibration can be caused by propeller imbalance, blade angle variations, or problems with the track.
• A propeller that vibrates at specific rpm ranges may indicate a problem with the engine-propeller match
• Possible issues to check for include the centrifugal force, improper settings or parts, and engine vibration.

Blade Tracking

• The process of determining the blade tips' position relative to each other.
• Blades should track each other closely, and any difference exceeding the manufacturer's tolerance may indicate a problem.

Checking and Adjusting Propeller Blade Angles

• Obtain the blade angle settings from the manufacturer's instructions and check the blade position at the specified station.
• Metal scribes or other sharp instruments should not be used to mark blade stations.

Universal and Digital Propeller Protractor

• The universal propeller protractor can be used to check blade angles on a balancing stand or the aircraft engine.

Propeller Balancing

• Propeller unbalance is a source of aircraft vibration that may result from static or dynamic imbalances.
• Static unbalance occurs when the center of gravity of the propeller does not coincide with the rotational axis.
• Dynamic unbalance is when the components such as blades or weights do not rotate in the same plane.

Static Balancing

• The standard method for checking propeller assembly balance.
• Involves several steps (e.g., checking blade angle, inserting bushings, and placing the propeller on specific locations on a specific balance check device) to determine if the propeller is properly balanced staticaly.

Dynamic Balancing

• Propellers are balanced by using special kits with vibration sensors and analyzers to calculate and adjust the weights.

Propeller Removal and Installation

• The spinner dome is removed
• The safety wires are removed if necessary
• The propeller mounting studs are unscrewed
• The propeller is then supported by a sling, and carefully removed from the mounting flange and place on a cart for transportation
• The appropriate procedures are followed for installation.

Setting the Propeller Governor

• The propeller governor regulates the maximum speed at which the engine runs.
• The governor's adjustable stop will move the propeller to the low-pitch position once the takeoff rpm is reached which increases engine load and maintains the maximum engine speed

Servicing Propellers 

• Cleaning: washing with suitable cleaning solvents or use of a noncorrosive soap solution
• Lubrication: propeller lubrication procedures are usually published in the manufacturer's instructions.
• Replenishing: if a propeller has been subjected to salt water, it should be washed until all traces of salt have been removed, then dried out, and coated with fresh oil or similar.

Cleaning Propeller Blades

• Propeller blades should be cleaned with a suitable cleaning solvent, brush, or cloth
• Any tool or substances that may scratch or mar the blade should be avoided.
• For wood propellers use warm water and mild soap.

Charging the Propeller Air Dome

• Examine the propeller, ensure it's in the start locks, and use the proper control
• Charge the cylinder with dry air, Nitrogen is preferred, using the proper temperature and pressure chart.

Propeller Lubrication

• Some propellers do not require lubrication. Proper lubrication procedures must be followed to avoid corrosion and ensure the smooth operation of moving parts.