Module 17A: Aircraft Propellers Overview

Module 17A: Propellers Overview

• This EASA Part-66 Module 17A document covers Aircraft Propellers.
• It is intended for use in training for the B1 Licence Category.
• It covers fundamentals, construction, pitch control synchronising, ice protection, storage and preservation.

Propeller Glossary

• Accumulator: A device aiding propeller unfeathering.
• Aerodynamic twisting moment: Operational force increasing propeller blade angle.
• Angle of Attack: Angle between propeller blade chord line and relative wind.
• Anti-icing system: Prevents ice formation on propeller blades.
• Automatic Propeller: Changes blade angles in response to operational forces, not cockpit control.
• Back: Curved side of a propeller airfoil section seen from the front.
• Blade: One arm of a propeller from the hub to the tip.
• Blade angle: Angle between blade section chord line and plane of propeller.
• Blade index number: Maximum blade angle on a Hamilton-Standard counterweight.
• Blade paddle: Tool used to turn blades in the hub.
• Blade Root: Portion of a blade nearest the hub.
• Blade station: Distance from the propeller hub center, measured in inches.
• Boots: Ice elimination components on the leading edge of blades.
• Boss: Center portion of a fixed-pitch propeller.
• Brush block: Component of a de-icing/reversing system, holding brushes transferring electrical power to the slip ring.
• Centrifugal Force: Force on a propeller throwing blades outward fromthe center.
• Centrifugal twisting moment: Force decreasing propeller blade angle.
• Chord Line: Imaginary line from the leading edge to the trailing edge of a blade airfoil section.
• Comparison Unit: Compares master and slave engine signals in synchronization system.
• Cone: Centers the propeller on crankshaft in a splined-shaft installation.
• Constant-speed system: Uses a governor to adjust prop blade angle and maintain a selected RPM.
• Controllable-pitch propeller: Pitch changeable in flight by pilot control.
• Critical range: RPM range with destructive harmonic vibrations.
• De-icing system: An ice elimination system which allows ice to form and then breaks it loose in cycles.
• Dome assembly: The pitch-changing mechanism of a Hydromatic propeller.
• Effective pitch: Distance an aircraft moves forward in one propeller revolution.
• Face: Flat or thrust side of a propeller blade.
• Feather: Rotating propeller blades ~90° to eliminate drag of a windmilling propeller.
• Fixed-pitch propeller: Blade angles cannot be changed, used on light aircrafts.
• Flanged shaft: Crankshaft where prop mounting surface is a flat plate, 90 degrees to the shaft centerline.
• Frequency generator: Engine RPM signal generator for synchronization systems.
• Geometric pitch: Theoretical distance an aircraft moves forward in one propeller revolution.
• Governor: Propeller control device in constant-speed system.
• Go no-go gauge: A gauge used to measure wear between the splines of a splined crankshaft.
• Ground-adjustable propeller: Adjustable on the ground to change blade angles.
• Hub: Central propeller portion fitted to crankshaft, carrying blades.
• Hydromatic®: Trade name for one type of Hamilton-Standard hydraulically operated propellers.
• Integral oil control assembly: Self-contained unit used on some transport aircraft.
• Leading edge: The forward edge of a propeller blade.
• Overhaul facility: FAA-approved facility for major propeller overhauls and repairs.
• Pitch: Same as geometric pitch; interchangeable with blade angle.
• Pitch distribution: Twist in a propeller blade along its length.
• Pitch lock: Mechanism preventing overspeeding if the governor fails on transport aircraft.
• Plane of rotation: Plane in which the propeller rotates (90 degrees to crankshaft centerline).
• Propeller: A device for converting engine horsepower into usable thrust.
• Propeller disc: Disc-shaped area in which the propeller rotates..
• Propeller repair station: Same as overhaul facility.
• Propeller track: Arc described by a propeller blade as it rotates.
• Pulse Generator: Unit generating RPM and blade position signal in a synchrophasing system.
• Radial Clearance: Distance from propeller disc edge to a nearby object, perpendicular to crankshaft centerline.
• Reversing: Rotation of propeller blades to negative angle to produce braking/reverse thrust.
• Safetying: Installation of safety device like safety wire or cotter pin.
• Selector Valve: Propeller control unit in a two-position prop system.
• Shank: Thickened blade portion near propeller center.
• Shoe: Same as boot.
• Shoulder: Flanged area on butt of a propeller blade retaining blades in the hub.
• Slinger Ring: Fluid distribution unit on the rear of a propeller hub using an anti-icing system.
• Slip: Difference between geometric and effective pitch.
• Snap ring: Component aiding propeller removal in splined/tapered shaft installation.
• Spider: Central component in controllable pitch props that mounts on crankshaft and has arms on which blades are installed.
• Splined shaft: Crankshaft extension with splines to prevent prop rotation on the shaft.
• Static RPM: Max RPM obtained at full throttle on the ground in no-wind.
• Synchronization System: System keeping all engines at the same RPM.
• Synchrophasing system: Allows pilot to adjust blade relative position as they rotate.
• Tachometer-generator: The RPM-sensing unit of some synchro systems.
• Tapered shaft: Crankshaft with tapered propeller surface, acting like a cone seating surface.
• Thrust Bending Force: Operational force bending propeller blades forward.
• Tip: Blade portion farthest from the hub.
• Torque bending force: Operational force bending propeller blades opposite rotation direction.
• Two-position propeller: Propeller with a capability to change between two blade angles.

Module 17.1 - Fundamentals

• Aircraft require a device to convert engine power into thrust.
• Early aircraft used propellers for thrust.
• Airfoils generate lift to pull aircraft forward.
• Aluminum alloy propellers came into wide usage as aircraft designs improved and propellers used thinner airfoil sections with greater strength.
• The two-position propeller and the constant speed propeller system came with the acceptance of changing propeller blade angle in flight .
• Composite materials, new airfoils, and multi-blade configurations continue to improve propeller design.

Propulsive Force

• A propeller converts engine power into propulsive force.
• A rotating propeller imparts rearward motion and reaction with forward force on the propeller blades.
• Propellers move a larger air mass at a relatively slow speed, unlike gas turbines.
• Thrust = Mass (Vo - V₁); where Vo = Propeller wake velocity, V₁ = Aircraft Velocity

Propeller Terms

• A propeller is a rotating airfoil with two or more blades attached to a central hub, mounted on the engine crankshaft.

• Propellers convert engine power to thrust.

• Propeller components include a leading edge, trailing edge, tip, shank, face, and back.

• Blade Angle: angle between propeller plane of rotation, and chord line of propeller airfoil.

• Blade station: reference position on the blade, specified distance from the center of hub

• Pitch is the distance a propeller section moves forward in one revolution

• Pitch distribution is the gradual twist in the propeller blade from shank to tip

Effective Pitch, Geometric Pitch, and Slip

• A specific blade station is selected to specify the pitch of a blade due to the angle varying along the length.
• Some propeller manufacturers express pitch in inches at 75% of the radius.
• Geometric pitch is the distance an element moves forward in one revolution along helical spiral.
• Effective pitch is the actual distance propeller advances through air in one revolution
• Effective pitch is affected by forward velocity and air density.
• Propeller slip refers to the difference between geometric pitch and effective pitch
• An effective pitch is 35 inches and the efficiency is 70% which can also be expressed as pitch of 50 inches in theory should move the aircraft 50 inches in 1 revolution but forward movement was actually 35 inches per revolution.

Angle of Attack

• Thrust produced by a propeller is like lift produced by a wing, determined by the blade's angle of attack.
• Angle of attack is the acute angle between propeller blade chord line and relative wind
• Angle of attack relates to blade pitch angle; it is not a fixed.
• Angle of attack varies with forward speed and propeller RPM.
• Angle of attack is the same as blade pitch angle when there is no forward speed of the aircraft.
• Angle of attack becomes much less than blade pitch angle as an aircraft moves forward.

Propeller Configuration

• The type of propellers include four main configurations:
  • Pusher
  • Tractor
  • Contra-Rotating
  • Counter-Rotating
• Small two-bladed propellers are often paired with small piston engines.
• Three, four and five bladed propellers are used for high-powered piston/gas turbine engines.
• Pusher propellers push, rather than pull,the airframe.
• Tractor propellers pull the airframe, typically fitted forward of the mainplane.
• Contra-rotating utilizes two propeller units on one shaft, driven by the same engine with the opposite rotation.
  • An advantage of contra-rotating is a lower undercarriage configuration which can be used or higher RPMs from the engine used because of reduced tip speed.
• Inefficient propellers with more than six blades can combat the problem of using contra-rotating.
• A rear propeller tips will not be affected by air vortices coming from the front propeller tips and tend to have a smaller diameter compared to the front propeller.
• Counter-rotating propeller produces a strong turning moment or torque on the airframe with a large rotating mass on multi-engined aircraft.
  • One would have the port engine propeller rotate clockwise and the starboard engine propeller anti-clockwise, balancing the torque effects with counter-rotating propellers

Propeller Solidity

• Propeller solidity describes the ability of a propeller to absorb power from the engine.

• A C130 propeller requires high solidity, while a Cessna 150 is less.

• 'The surface area of the area divided by the surface area of the propeller disc is to be described as propeller solidity

• Solidity may be increased by:

  • Increasing number of blades (but if limited by hub strength, contra-rotating becomes an option)
  • Increasing chord of the 'paddle' of the blades (C130s)
  • Increasing length of the tips of the blades

Propeller Clearances

• Ground Clearance: clearance between the propeller tip and the ground when in normal flying attitude.
• Aircraft with a tail wheel configuration would be in the takeoff position when ground clearance is measured
• Multi-engined aircraft is the clearance between the side of the fuselage and the propeller with respect to Fuselage Clearance

Right and Left Handed Propellers

• Right-handed propellers rotate clockwise when viewed from aft (looking forward).
• Left-handed propellers rotate counter-clockwise when viewed from aft (looking forward).

The Blade Element

• Aerodynamics of the propeller can be understood by considering motion of a propeller blade element (section).
• A propeller blade section, known as an airfoil can be learned with the same methodology using aerofoil its aerodynamics

Forces On Blade Element

• An aerodynamic develops required force on blade element
• Must be set at a small positive angle of attack relative to resultant airflow.
• Helix Angle plus the angle of attack = blade angle (blade pitch).
• A blade element advances through space prescribing a helix.
• Geometric Pitch = distance a helix moves in 1 revolution if the blade were 100% efficient.
• Effective pitch is a forward distance caused by tip losses

Variation of Propeller Efficiency with Speed

• Angle of attack changes if RPM is constant; a fixed pitch propeller is traveling at different speeds

• Decreasing angles of attack and their effect on a constant speed increase

• There will be only one speed at which the blade is operating at most efficient angle of attack (4–6°) and where the propeller efficiency will be a maximum

• Effects of zero efficiency:

  • High forward speed = angle of attack is close to zero lift incidence with thrust reducing to zero
  • Low speeds = the thrust increases as angle of attack increases, blades won't stalled
  • Zero forward speed = not efficient.

Windmilling

• A condition known as windmilling; occurs in variable pitch propellers. As the Propeller sufferes a loss of positive torque.

  • the pitch will fine off in an attempt to maintain the governed RPM selected at the time.

• Relative Airflow (RAF) will impinge on the front surface of the blade section / cause drag and will drive the engine rather than resist rotation; drag and negative torque is also caused by the windmilling propeller that the engine will also cause.

• Small negative attack causes total reaction as windmilling occurs. The total Reaction is a total of two forces; Thrust, Acting as the reverser, and the torque that assists the rotation

  • Can cause the prop speeding up and powerplant damage. Note: small positive blade angle occurs as a small position of a windmill

Feathering

• feathering would cause drag in a windmilling propeller following engine failure is caused by an overspeeding or by a cival speeding
  • to secure and possibly prevent engine fire the aggregate effect of the blade sections produces zero torque, propeller stops
  • feathered location is located 90° angle to the plane of rotation with a zero lift in a wind

Reverse Thrust

• Propellor gets turn thru the pitch-stop to about neg 20° / power applied, reverse thrust obtained

Forces Acting on the Propeller

• The forces that act on the propeller:
  • Bending
  • Torque Bending
  • Aerodynamic Twisting Moment (ATM)
  • Centrifugal Force (CTM)
• Centrifugal puts the greatest stress because centrifugal force tries to pull blade of the hub by several thousand times
• Torque bending tries to ben blade back over direction oppisite direction of rotation
• Aerodynamic tries to twist blades over a higher angle

Handling Effects - Single Engine Aircraft

- Airframe inclined upwards ( to direction ) due to angle of attack of aircraft for asymmetric factors
- Propeller blades have greater angle / greater upward moving angle for downward blades	and it develops a thrust for downwards moving effect

Yawing moments occur due to thrust on two sides.

• Aircraft tends to roll at left for all right hand props b/c of torque reactions
  • Newton 3rd law of motion, or every action there is an equal opposite reaction

Thrust and Power Development

• Output power of piston engine dpeneds on density.
  • On stroke the piston moved to clinder. Inlet valves open to the fuels and air mixture and prepped to the cab into Cylinder
  • Atmospheric conditions at sea level press = 17lbf/in^2 cylinder Turboprops; gas turbine, majority of jet is a power-Free connected directly to reduction , propeller to turbine assmebly

Integral Oil Control Assembly

• The governor for fuel is known as Power
• Lever in turboprops

Vibrational Forces and Resonance

• Forces are there when propers is produced and aerondynamic = mech forces exist.
• Tip cause by transonics

Module 17.2 Propeller Construction

Propeller Blade Materials

• Most propellers are made of wood, aluminum alloy, steel, or composite materials.
• The thick, rounded portion of the propeller blade near the hub is the shank.
  • Shank provides strength; the blade butt fits in propeller’s hub.
• Blade tip is generally the last 6 inches of a blade.
  • A Blade Cuff is a structure affixed on the round shank transforms round shank → aero foil.
  • The Cuffs primarily increase flow of cool air to the engine nacelle for that aero foil. Cuffs are used within that assembly or using bonding materials that holds it

Blade Stations & Nomenclature

• Blade analysis divides blades into segments with station numbers measured inches / center → propeller hub assembly.
• Blade stations are reference points used for maintenance/damage assessment and angle checks.

Wood Propellers

• Since the Wright Flyer's original design from 1903 wood propellers are cost effective and still popular on amateur designs.
• Wood's light weight/ strength & economy well suited to this application. Constructred with laminated hardwood. Metal tipping to leading edge.
• Woods such as mahogany or walnut can be utilized.
  • Birch is the most often used wood for its laminated segments, typically with each segment up to ¾ inch thick.
  • A fabric covering is cemented to the outer blade, with a metal tip made of Monel metal, or Stainless steel
    • Secured to edge with countersunk wood screw.
  • The Screw heads must be soldered to prevent loosening, with solder filed to smooth surface