Aircraft Propulsion Principles

Module 17 - Propeller

• This module covers the maintenance training organization.
• The focus is on propellers for aircraft.
• The content includes propeller design, construction, and various maintenance topics.

Blade Element Theory

• Propellers generate thrust by accelerating air.
• Early propellers (Wright brothers) were two-bladed.
• Modern propellers are found on single-engine to large multi-engine aircraft.
• Turboprop engines use propellers in conjunction with turbines.
• Each blade is a rotating wing that generates lift.

Propeller Geometry

• Propeller design has evolved with increases in engine power.
• Geometric features of the blades determine characteristics.
• Basic geometrical elements:
  • Blade root and tip
  • Blade face and blade back
  • Blade angle and pitch distribution
  • Helix angle
  • Angle of attack of the blade
  • Twist

Propeller Components

• Descriptions and diagrams of propeller components like hub assembly, shank, hub bore, tip, leading edge, trailing edge, root and hub
• Blade airfoil diagrams illustrating leading edge, chord line, blade back, and blade face

Propeller Pitch

• Propeller pitch is a crucial aerodynamic feature.
• Geometric pitch is the distance between consecutive points on a cylinder after a rotation.
• Pitch and blade angle are proportional.
• Effective pitch is the actual distance the propeller advances in one revolution through the air.

Propeller Slip

• Propeller slip is the difference between the geometric pitch and effective pitch.
• This difference is due to inefficiencies in the propeller's ability to move air forward at the theoretical geometric pitch rate.

Propeller Thrust

• The basic function of a propeller is to convert combustion power into propulsive power.
• The propeller accelerates air in the opposite direction of the aircraft's motion to create a forward thrust.

Propeller Efficiency

• Propulsive efficiency (η) is the ratio between propulsive power and engine power.
• Efficiency varies from 50% to 85%, depending on the amount of slip.
• Maximum efficiency is near cruising airspeed.

Rankine Theory

• Blade element theory allows for calculation of the overall force on a propeller.
• The method divides the blade into infintesimal parts for force calculation.
• The blade element theory's limitation is that it disregards turbulent flow interference and overestimates efficiency.

Blade Element Theory Detail

• Important factors of blade element force:
  • Air density
  • Relative wind
  • Aerodynamic chord
  • Distance between the hub and section
  • Aerodynamic coefficient (related to attack angle)

Propeller Equations

• Thrust (T) and torque (Q) calculations:
  • T = pN²D⁴CT
  • Q = pN²D⁵CQ
  • Where:
    • p = density of air
    • N = number of rotations per second
    • D = propeller diameter
    • CT = coefficient of thrust
    • CQ = coefficient of torque

Blade Angle of Attack

• Low blade angles are suitable for takeoff and climb.
• High blade angles are best for high-speed cruise and high-altitude flight.
• Reverse thrust is possible by rotating blades to a negative angle.

Operating Regimes

• Propeller operating regimes vary with advance ratio, forward speed, thrust, and torque.
• Brake propeller: ground-based, engine running, aircraft pulled backward.
• Fixed-point propeller: zero forward speed, on the ground.
• Tractor propeller: normal operation, positive efficiency, decreasing efficiency with forward speed.
• Other cases: zero-thrust, braking, auto-rotating, and windmill.

Propeller Ice Protection

• Anti-icing systems prevent ice formation.
• De-icing systems remove existing ice.
• Anti-icing systems use fluids (e.g., isopropyl alcohol) to prevent condensation.
• A typical anti-icing system has a reservoir, pump, and slinger rings.

Propeller Maintenance

• Propeller balancing restores static and dynamic balance.
• Methods use bearings, oil leaks, radiator damage, and engine accessory failures.
• Balancing is done by adding/removing mass.
• Routine checks are crucial.

Types of Propellers

• Types of propellers:
  • Tractor propellers
  • Pusher propellers

Propeller Storage and Preservation

• Preservation procedures prevent corrosion and maintain functionality.
• These involve proper storage methods, protective coatings, and moisture control.

Other Notes

• Includes definitions for various types of propeller damage.
• Includes a table of materials with codes for cleaning and use.
• Explains propeller installation methods using different types of shafts.
• Discusses aspects of installation, particularly bottoming (front or back).