Aerofoil: Lift, Drag, Thrust & Blade Angle

• Lift is the aerodynamic force created by air flowing over an aerofoil.
• An aerofoil shape increases airflow velocity over its surface, reducing pressure and creating lift.
• Propellers use lift as their basis of blade element theory.
• Drag opposes thrust caused by airflow disruption over an aerofoil.
• Thrust is a forward-acting force reacting to the mass of air accelerated rearwards.
• Momentum is a moving body's motion quantity, measured by mass and velocity.
• Total reaction is the resultant of lift/drag and thrust/torque pairs, acting simultaneously on the blade.
• Increased rotational speed raises these forces equally until blade tips approach the speed of sound.

Blade Angle

• Blade angle is the angle between the chord line and the plane of rotation.
• Measured in degrees relative to a datum point.
• The datum point is typically 75% of the radius from the propeller hub's center.
• Aircraft Maintenance Manuals and Type Certificates specify these angles and positions.
• For example: Sensenich propeller M74DMS5-2-60 has a 74-inch diameter and a 60-inch pitch at the 75% station.

Angle of Attack

• Angle of attack is the angle between the chord line and relative wind/airflow.
• Best results are typically between 2° and 4°, allowing air compression and expansion for thrust.
• It combines the aircraft's True Air Speed (TAS) and the propeller's rotational speed (RPM).

Blade Twist

• Blade twist is a gradual twist from hub to tip.
• It ensures a consistent angle of attack (2° to 4°) along the blade length.
• An additional consideration is avoiding shockwaves caused by tips reaching the speed of sound.

Pitch

• Pitch is the distance a propeller moves forward in one revolution.
• This can vary with different blade angles on variable pitch propellers.

Propeller Slip

• Propeller slip is the difference between geometric and effective pitch.
• Geometric pitch is a calculated distance through a solid medium in one revolution.
• Effective pitch is the actual distance in air.
• Example: A propeller with a 50-inch geometric pitch moves only 35 inches in air, making it 70% efficient with 30% loss.
• Most propellers operate between 75% to 85% efficiency.

Propeller Fundamentals

• Aerodynamic forces
• Centrifugal forces
• Thrust forces
• Torque
• Relative airflow effect on blade angle of attack
• Vibration and resonance
• If a propeller is driven anti-clockwise, it generates torque, affecting the aircraft's structure.
• This propeller torque tends to roll the aircraft clockwise.
• Counter-clockwise propeller rotation causes a clockwise torque reaction.

Propeller Gyroscopic Effect

• A rotating propeller resists changes in its plane of rotation.
• During a turn, the plane of rotation changes at a 90° angle to the applied force.

Propeller Slipstream

• A rotating propeller imparts rotational motion in the same direction.
• It can negatively affect the aircraft's fin, potentially causing the aircraft to turn.
• Contra-rotating propellers eliminate torque, slipstream, and gyroscopic effects by spinning in opposite directions.
• It straightens the slipstream and improves control.

Forces Acting on (Rotating) Propellers

• Centrifugal force tends to throw blades away from the hub.
• The results can amount to many thousands of newtons.
• Centrifugal Twisting Moment (CTM) is a force that rotates blades toward a fine angle on variable pitch propellers.
• CTM is usually greater than ATM, used by manufacturers to alter blade angles from coarse to fine.

Aerodynamic Twisting Moment (ATM)

• ATM is the force increases blade angle to a coarser angle.
• Some designs use ATM to aid feathering.
• Both ATM and CTM (torsional stresses) are quadratically increased with RPM increases.

Bending Forces

• Torque bending force: Air resistance (drag) bends blades opposite the rotation direction.
• Thrust bending force: Thrust bends blades forward.

Force Coupling

• Coupling centrifugal force and thrust creates severe stresses that are greatest near the hub.
• The blade face experiences tension from centrifugal force and bending.
• Propellers need to withstand these stresses, which increase with rpm.
• A simple scratch or dent can have severe consequences.

Propeller Angle of Attack

• Propeller performance depends on rotational velocity (rpm) and aircraft velocity (TAS).
• Performance (thrust) of a fixed-pitch propeller varies with changes in rpm or TAS.

Altered Rotational/ Forward Velocity

• Increased rotational velocity (at constant forward velocity) increases the angle of attack.
• Increased forward velocity (at constant rotational velocity) decreases the angle of attack.
• This may lead to a negative angle of attack, turning the propeller into a brake.
• Changing either the rotational or aircraft velocity alters the propeller's angle of attack.
• Varying the propeller's angle of attack beyond its designed parameters reduces efficiency.

Blade Tip Speed vs. Efficiency

• Larger propellers allow engines to absorb more power but can lose efficiency.
• This is through tip vibration or flutter from shock waves approaching the speed of sound (approximately 1200 ft/s or 660 kt).
• Propeller tips must stay below the speed of sound while still absorbing available engine power.
• Can achieve this by increasing number of blades or altering blade shape and section.

Blade Vibration

• Blade vibration occurs due to aerodynamic and mechanical forces.
• Aerodynamic forces cause buffeting and vibration by bending blades forward.
• Mechanical vibrations from engine power pulses are more destructive.
• They cause standing wave patterns, leading to metal fatigue and structural failure and are most critical around 6 inches from blade tips.
• Most designs have eliminated detrimental effects of vibrational stress through careful design.
• Critical range is indicated on the tachometer by a red arc.
• Engine operation in the dangerous range should be limited to a brief passage from one rpm setting to another.