Aerodynamics of Propeller Systems

Week 1 Complete

• AVIA-1052 course, completed Week 1
• Course instructor is Matt C
• Contact Matt C for errors or improvements

Week 1 of 1 Day 1

• AVIA-1052 course, first class
• Topic is propellers

Today On AVIA-1052

• Topics for the day are propellers, general propeller stuff, aircraft propeller theory, and basic propeller controls

Where are we?

• Current topic is Powerplant (Volume 2) 7-2 to 7-6
• Starting point is 7-2
• Ending point is 7-6
• FAA handbooks used are H-8083-30A, H-8083-31A, and H-8083-32A

Propellers

• The unit that absorbs the engine's power output
• Propeller development evolved through many stages
• First propellers were fabric-covered sticks designed to force air rearward

Powerplant 7-2

• Propellers started as simple two-bladed wood propellers
• Advanced to complex turboprop systems
• Variable-pitch, constant-speed feathering, and reversing propeller systems allow for varying the engine RPM to maintain efficiency during different flight conditions.

Constant-speed systems

• Flyweight-equipped governor unit
• Controls the pitch angle of the blades
• Engine speed is kept constant

Propeller positions

• Feather, High pitch, Low pitch, Flight idle, Locks, Ground idle, Reverse

Common Propeller Types

• Efficiency charts show relationships between efficiency and speed for fixed-pitch and constant-speed propellers.

Propeller characteristics

• Most propellers are two-bladed
• Developed four and six-bladed propellers for high power and large diameters
• Propeller-driven aircraft are limited by revolutions per minute(RPM)

Forces on a propeller

• Centrifugal force that tends to pull the blades out of the hub at high RPM
• Blade weight is important in propeller design

Excessive blade tip speed

• High blade tip speed may result in poor blade efficiency, fluttering, and vibration.

Propeller Speed and Aircraft Speed

• Aircraft speed of propeller driven aircraft is limited by propeller speed
• Turbofan engines were used for higher aircraft speeds.

Propeller Advantages

• Propeller-driven aircraft have advantages in takeoff and landing and maintenance costs
• New blade materials and manufacturing techniques increase efficiency

Propeller systems

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

Diagrams of propeller parts

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

Aircraft Propeller Theory

• The aircraft propeller consists of two or more blades
• Each blade is a rotating wing
• Produces a thrust force to propel the airplane

Basic Propeller Principles

• Mounted on a shaft
• Low-horsepower engines are attached to the crankshaft directly
• High-horsepower engines are geared to the engine crankshaft

Propeller Aerodynamic Process

• Airplane moving through the air creates drag
• A force equal to the drag is required to maintain forward flight (thrust)
• Work done by thrust equals thrust multiplied by distance
• Power expended by thrust is equal to thrust multiplied by velocity
• Thrust horsepower is a measurement of power expended by thrust.
• Engine supplies brake horsepower through a rotating shaft to the propeller, which then converts it into thrust horsepower.
• Propeller must be designed so that the waste of power during conversion to thrust horsepower is minimized to ensure maximum efficiency

Propeller Efficiency

• Propeller efficiency is the ratio of thrust horsepower to brake horsepower and a machine's efficiency is the ratio of the useful power output to the power input
• The symbol for propeller efficiency is the Greek letter η(eta)
• Propeller efficiency ranges from 50%-87% depending on the amount of slippage

Geometric pitch

• Distance the propeller advances during one revolution in the absence of slippage
• Computed using a formula (GP= 2 x πR x tangent of blade angle at 75% station)

Effective pitch

• Distance the propeller advances in one revolution in flight conditions, adjusting for slippage
• Effective pitch = geometric pitch - slip

Chord line

• Imaginary line from the leading edge to the trailing edge of a propeller blade
• Defines the aerodynamic shape of the blade

Blade element

• Propeller blades as an airfoil section whose chord is the width of the propeller blade at a particular section

Blade terms

• Blade back
• Blade face
• Chord line
• Angle of attack
• Leading edge
• Trailing edge

Principal forces acting on a rotating propeller

• Centrifugal force tends to throw the blades outward.
• Blade weight is vital to propeller design

Excessive blade-tip speed

• Causes poor blade efficiency, fluttering, and vibration

Propeller System

• The factors that limit aircraft speed

Propeller Advantages

• Takeoff and landing can be shorter, and less costly to maintain.
• New blade materials increase efficiency
• Used in turboprops and reciprocating engine installations. propellers will remain in aircraft use in the future.

Many different types of propeller systems

• Developed for specific aircraft installations, speed, and type of operation

Diagrams of propeller parts

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

Basic propeller controls and instruments

• Details on fixed pitch propellers and constant speed propellers.
• Explanations of propeller control lever positions
• Instrument usage for measuring RPM and manifold pressure

Speed Sensing

• Tachometer generator measures rotational speed using an internally generated electrical signal
• Alternating current is used to display the reading on the tachometer.

Previous on AVIA-1052

• Some general prop stuff
• Aircraft propeller theory
• Basic propeller controls

Today on AVIA-1052

• Propeller location
• Types of propellers
• Fixed-pitch propeller
• Test club propeller
• Ground-adjustable propeller
• Controllable-pitch propeller
• Constant-speed propeller
• Feathering propeller
• Reverse-pitch propellers
• Governor

Where are we?

• Powerplant, Vol. 2
• 7-6-7-14
• Start at: Propeller Location
• Stop at: Metal Fixed-Pitch Propellers

Tractor Propeller

• Mounted at the upstream end of a drive shaft, in front of the supporting structure
• Common type of propeller in most aircraft
• Major advantage: less stress induced in the propeller due to the relatively undisturbed surrounding air

Pusher Propeller

• Typically mounted at the downstream end of a drive shaft, behind the supporting structure
• Commonly used in seaplanes and amphibious aircraft

Pusher Propeller Issues

• More prone to damage than tractor propellers
• Less clearance between the propeller and the ground in land planes compared to the propeller and water in a watercraft
• Rocks and debris may be drawn into the propeller
• Exposure to water spray during takeoff and landing can cause damage

Propeller Types

• Various types of propellers
• Simplest are fixed-pitch and ground-adjustable propellers
• Controllable-pitch and complex constant-speed systems are more complex

Fixed-Pitch Propeller

• Blade pitch is built into the propeller
• Cannot be changed after construction
• Typically made of wood or aluminum alloy
• Designed for best efficiency at a single RPM and speed

Fixed-Pitch Wood Propeller Uses

• Used in single-engine aircraft
• Advantages are less expensive and easier to operate
• Disadvantages are limited operating conditions

Test Club Propeller

• Designed to test and break in reciprocating engines
• Used to add the proper load to the engine during the break-in period
• May provide added cooling air flow

Ground-Adjustable Propeller

• Operates as a fixed-pitch propeller, but its pitch can be adjusted when not rotating by loosening/tightening the clamping mechanism
• Used infrequently due to its limitations

Controllable-Pitch Propeller

• Blade pitch can change while rotating
• Provides optimal performance across varying conditions
• Limited pitch positions compared to constant-speed propellers

Constant-Speed Propellers

• Maintains a constant RPM regardless of flight conditions through a governor which adjusts blade pitch.
• The governor uses flyweights that move to change oil pressures that adjust the pitch.
• Overspeed conditions automatically adjust the pitch angle to maintain an appropriate RPM

Constant-Speed Feathering Propeller

• Uses a governor to provide a constant RPM despite changes in load and speed.
• Has a mechanism to feather the blades at a 90-degree angle, reducing drag
• Propellers need constant oil pressure to prevent them from feathering during operation.

Propeller Governor

• Engine RPM-sensing device and high-pressure oil pump
• Responds to changes in engine RPM by directing oil under pressure to the propeller hydraulic cylinder, or releasing oil for decreased RPM

Governor Mechanism

• Constant-speed control consists of gear pump to increase engine oil pressure
• A pilot valve controlled by flyweights to adjust oil flow through the governor to and away from the propeller
• Relief valve regulates the operating oil pressures in the governor

Overspeed

• When the engine is operating above the set RPM, the flyweights move outward and raise the pilot valve to increase oil flow to increase propeller pitch and decrease RPM
• A decrease in oil flow to the propeller and reduce pitch angle may reduce the engine RPM.

Underspeed

• When the engine is operating below the set RPM flyweights tilt inward, allowing less oil pressure to slow the engine
• Decreases blade angle to lower the load on the engine increasing RPM

Propeller Governor Operation

• The governor maintains required balance between control forces through metering and draining the proper amount of oil from the propeller piston to regulate the propeller blades to maintain constant speed operation
• If the engine speed increases above a set level, the flyweights move outward, causing the pilot valve in the governor to release oil and increase blade pitch to reduce engine RPM
• if the engine speed decreases below the set level, the flyweights move inward, causing the pilot valve in the governor to meter additional oil flow and decrease blade pitch, thereby increasing engine RPM

Propeller Governors

• Engine speed is sensed by an appropriate device
• To counteract this (if increasing speed), the governor causes a mechanism to turn the blades to a higher angle
• This increases the load on the engine and decreases the RPM, preventing over-speed condition

Auto-feathering systems

• Automatic systems used to engage the feathering position in the blade when power is lost.

Propeller Inspection and Maintenance

• Inspect propellers regularly, noting blade and hub nicks, scratches, cracks, warpage, glue failure, and corrosion.
• Visual inspection using a magnifying glass, along with manufacturer's instructions and use of templates and protractors as needed
• Check for issues such as blade imbalance, out of track, and component fatigue to assess if balance can be restored or if replacement is necessary
• The tachometer must be used for checking to ensure the proper RPM for the aircraft

Wood Propeller Inspection

• Consistent inspection for cracks, dents, warpage, glue failure, delamination, charring in the area between the propeller and the flange

Metal Propeller Inspection

• Carefully inspect the propeller for flaws
• Check the full length of the leading edge (especially near the tip)
• Check the full length of the trailing edge
• Check the grooves, and shoulders on the shank
• Check for dents and scratches

Tachometer Inspection

• Accuracy needs to be verified at 100-hour intervals or on annual inspection, whichever occurs first

Aluminum Propeller Inspection

• Inspect aluminum propellers and blades for cracks/flaws.
• Transverse crack or flaw of any size cause for rejection.
• Multiple deep nicks and gouges on the leading edge/blade face cause for rejection.
• Use dye penetrant or fluorescent dye penetrant to confirm suspected cracks in the propeller.
• Refer unusual conditions to the manufacturer.

Composite Propeller Inspection

• Inspect for nicks, gouges, loose material, erosion, cracks, debonds, and lightning strikes.
• Tapping blades and cuffs to locate delaminations/debonds.
• Blades with cuffs have a different sound when tapped.
• If there’s any unusual sound or hollow or dead sound this may represent a possible issue that requires further inspection

Propeller Vibration

• Vibration can occur due to balance, angle or track problems
• Excessive vibration when the propeller rotates may not always stem from propeller issues but may come from engine imbalances.

Blade Tracking

• Track is the process of checking the blades' relative position to ensure they are properly aligned in a single plane.
• Blades must remain properly aligned within tolerances to prevent excessive vibration.

Checking and Adjusting Propeller Blade Angles

• Refer to manufacturer's instructions for blade angle checks if any blade angle issue is discovered.
• Use a bench-top protractor to perform the inspection with care.

Universal and Digital Propeller Protractor

• Used to check propeller blade angles.
• Newer models use digital inclinometers for their accuracy and ease of use.

Propeller Balancing

• Propeller unbalances may be either static or dynamic
• Static unbalance occurs when the center of gravity of the propeller or parts is not aligned with the rotation axis.
• Dynamic unbalances occur when the center of gravity of the propeller or parts does not lie in the same plane as the rotation axis

Static Balancing

• Used to balance propeller assemblies
• Procedure involves checking the blades at various angles.
• Checking of the blades at the vertical and horizontal angles to note if the assembly remains in place.
• If an unbalance is detected, weights may be added or removed to correct the issue.

Dynamic Balancing

• Balances the entire propeller assembly, including the blades, spinner, and all parts.
• Uses analyzer kits to measure vibrations and adjust weights
• Involves using a vibration sensor
• If issues are identified that stem from the engine, this is a reason to check other areas of the powerplant.

Propeller Removal and Installation

• Procedures for disassembly and inspection follow manufacturer's instructions
• Tighten mounting hardware evenly to prevent damage

Setting the Propeller Governor

• At the time of propeller, propeller governor or engine installation.
• Move the throttle to the correct takeoff position, note the resulting RPM, and recorded manifold pressure
• If RPM or manifold pressure is not as specified, use the governor to set the rpm
• This ensures that the engine will consistently perform at an optimal rpm for the prescribed conditions

Servicing Propellers

• Cleaning of propellers should involve appropriate cleaning solutions depending if they are metal or wood.
• Thoroughly flush propellers with fresh water if exposed to salt water or chemicals.

Propeller Lubrication

• Lubrication intervals are crucial due to corrosion
• Propeller lubrication procedures are based on manufacturer specifications.
• Remove the lubrication fitting and check.
• Tighten the fittings.
• Apply a proper lubricating solution to the necessary areas.

This quiz explores the functioning and mechanics of propeller governors, including the adjustment of blade angles in constant-speed systems. Test your knowledge on fundamental forces, optimal conditions, and the relationship between torque and thrust in propeller operations.