Aircraft Propeller Mechanics

Week 1 Complete

• AVIA-1052 course completed for week 1
• Contact Matt C for any corrections or improvements needed

Week 1 of 1 Day 1

• AVIA-1052 course, first class
• First class overview
• Initial topic overview for the day
• Propeller overview

Previously On AVIA-1052

• Topics covered in the previous class
• General discussions on propellers
• Aircraft Propeller Theory
• Basic Propeller Controls

Today On AVIA-1052

• Focus areas for the current class
• Propeller Location
• Types of propellers
• Fixed-pitch propeller
• Test Club propeller
• Ground-adjustable propeller
• Controllable-pitch propeller
• Constant-speed propellers
• Feathering propellers
• Reverse-pitch propellers
• Propeller Governor
• Propellers used on General Aviation Aircraft

Where are we?

• Powerplant (Vol. 2) 7-6 to 7-14
• Start at
• Propeller Location
• Stop at
• Metal Fixed-Pitch Propellers
• FAA-H-8083-30A, General
• FAA-H-8083-31A, Airframe Volume 1
• FAA-H-8083-31A, Airframe Volume 2
• FAA-H-8083-32A, Powerplant Volume 1
• FAA-H-8083-32A, Powerplant Volume 2

Propellers

• The unit that absorbs the power output of the engine
• Development stages
• Fabric-covered sticks were the first
• Designed to force air in a rearward direction
• Propellers started as simple two-bladed wood propellers, evolved to complex turboprop systems
• Variable-pitch, constant-speed feathering, and reversing propeller system
• Constant-speed systems (flyweight-equipped governor unit)
• Control the pitch angle to maintain constant engine speed
• Parts of the propeller
• Normal movement with low and high pitch
• Feather is used when the engine stops, reduces drag, zero to negative pitch reverse pitch

Aircraft Propeller Theory

• Aircraft propeller consists of two or more blades
• Each blade is similar to a rotating wing
• Produce forces and create thrust
• The power to rotate blades comes from engine
• Central hub to which blades are attached

Basic Propeller Principles

• Mounted on a shaft
• Low-horsepower engines (extension of crankshaft)
• High-horsepower engines (mounted on a propeller shaft geared to crankshaft)
• Engine rotates airfoils of blades high speeds
• Propeller transforms rotary power into thrust

Propeller Aerodynamic Process

• An airplane creates drag force
• There must be an equal and opposite force (thrust)
• Work done by thrust equals thrust times distance
• Power expended by thrust = thrust x velocity
• Thrust horsepower if the power is measured in horsepower units
• Engine supplies brake horsepower through a rotating shaft
• Propeller converts brake horsepower into thrust horsepower
• Some power is wasted in the conversion
• Propeller efficiency is the ratio of thrust horsepower to brake horsepower (Greek letter n or eta)
• Propeller efficiency = 50-87%, depends on slip
• Pitch is different than blade angle
• Geometric pitch: advance per one revolution (no slip) calculated by a formula
• Effective pitch — considers slip (difference between geometric and effective pitch equals slip)
• Chord line of the propeller blade is determined in a similar manner to an airfoil
• Blade back (one side is cambered or curved); blade face (other side is flat)
• Chord line, imaginary line through the blade (from leading edge to trailing edge)
• Leading edge, thick edge of the blade the air as the propeller rotates

Principal forces acting on a rotating Propelller

• Centrifugal force, tends to pull blades out of hub at high rpm
• Blade weight important to propeller design
• Excessive blade tip speed can lead to poor blade efficiency which leads to fluttering and vibration.

Propeller blade tip speed

• Rotating the propeller too fast may result in poor blade efficiency
• Flutter and vibration from excessive blade tip speed

Most Propellers

• Two-bladed
• Four- and six-bladed propeller types are developed due to increases in power output
• Propeller-driven aircraft speed limited by rpm

Forces Acting on Rotating Propellers

• Centrifugal force pulling blades outward.
• Torque bends the blades in the opposite direction to rotation.
• Thrust bends the blades forward as the aircraft pulls through the air.
• Aerodynamic twisting force that rotates the blades into high pitch when the plane is in takeoff and climb mode.
• Centrifugal twisting force, tends to force the blades toward a low blade angle.

Propeller Types

• Fixed pitch
• Ground adjustable
• Controllable pitch
• Constant speed

Propeller Governor

• Engine rpm-sensing device and high-pressure oil pump
• Responds to changes in engine rpm by directing oil to or releasing oil from the propeller hydraulic cylinder
• In a constant-speed propeller system, the change in oil volume in the hydraulic cylinder maintains blade angle.
• The two main instruments used: engine tachometer; manifold pressure gauge
• Parts of the propeller governor
• Fundamentals forces used to control blade angle
• Centrifugal twisting moment
• Aerodynamic twisting force
• Forces on the rotating blades are not equal in strength, this creates a balance

Governor Mechanism

• Constant-speed valve control consists of a gear pump, which increases engine oil pressure, and a pilot valve
• The pilot valve is controlled by flyweights in the governor to control the oil flow to and away from propeller.
• Governors regulate quantity of oil by controlling the position of the pilot valve (relative to metering port).
• Relief valve system, regulates operating oil pressures in the governor.

Propeller Governor Operation

• Underspeed conditions
• Overspeed conditions
• On-speed conditions

Feathering Propellers

• Used in multi-engine aircraft to reduce drag during single-engine operation or failure.
• By rotating the blade angle parallel to the line of flight.
• The propeller ceases operation.
• Windmilling is reduced or stopped
• Methods involve oil pressure to move blades to a low pitch, and flyweights in the propeller

Reverse-Pitch Propeller

• Change blade angle for negative value.
• Provides an opposite thrust to normal forward direction.
• Used for braking effect on the ground.
• Loss of forward motion during engine failure.

Propeller Auxiliary Systems

• Ice control systems
• Anti icing fluid system (tank, pumps, forces, control systems)
• Fluid types (isopropyl alcohol, phosphate compounds)
• Feed shoes (narrow strip of rubber extending)
• Slinger ring (transfers fluid to blades)

Ice Control Systems

• Prevents the buildup of ice on blades
• Ice formation causes: distortions in blade section, causes loss of propeller efficiency collecting asymmetrically, produces propeller imbalance, destructive vibration which increases weight of blades

Deicing Systems

• Electric propeller
• Electrical energy source
• Resistance heating element
• Mounted internally on propeller spinner and blades
• Necessary wiring for the deicing system

Propeller Removal, and Installation

• To remove and install propellers, follow the manufacturer guidelines to properly support the propeller.
• Ensure correct identification of the propeller parts

Propeller Overhaul

• Checks for and corrects blade, spinner, and hub defects

Propeller Balancing

• Static balancing: when the center of gravity of the propeller and its parts don't coincide with the axis of rotation
• Dynamic balancing: when the CG of component parts of the propeller and accessories do not lie in the same plane of rotation

Propeller Vibration

• Caused by several problems
• Imbalance in the blades, incorrect blade angle, or tracking problems
• The intensity of the vibration can vary with the rpm.
• Propeller hub checks are helpful if suspected of unbalance issues

Blade Tracking

• The path/position of propeller blades when rotating at the same plane of rotation
• The difference in track at like points is checked for issues by the manufacturer's tolerance specs.

Checking and Adjusting Propeller Blade Angles

• Follow the guidelines from the appropriate manufacturer.
• Using measuring инструменты to pinpoint blade issues
• Protractor tool used

Composite Propeller Inspection

• Look for nicks, gouges, loose material, erosion, cracks, and debonds.
• A metal coin is used to tap composite blades for sound and determine if delamination or debonding is likely