Propeller Dynamics Flashcards
What is the plane of rotation?
Perpendicular to the propeller shaft.
Why is the propeller blade twisted?
To keep the relative airflow constant along the length of the blade as the speed of prop at the tip is greater at the top.
What is the propeller blade angle?
The blade angle is the angle between the chord line of the propeller blade and the plane of rotation.
How far along the blade is the measurement of blade angle?
75%
What is the helix angle?
Angle between the plane of rotation and the relative airflow.
What happens to the helix angle with increasing forward speed?
Helix angle decreases
What is the blade AOA?
- Angle between the blade chord line and the relative airflow.
- Blade angle minus the helix angle.
What occurs to the angle of attack as forward speed increases?
Angle of attack will decrease
What is the propeller pitch?
- The distance the propeller would move forward in one revolution if the helix angle were equal to the blade angle.
- Angle between the chord and the plane of rotation
What is prop feathering?
When the blade angle is set to a zero lift angle, minimizing drag and stopping prop rotation.
What is reverse thrust?
When the blade is rotated in a reverse angle such that the prop produces reverse thrust.
What are some design methods to absorb greater engine power?
1) Increase the blade AOA
2) increase the blade length
3) Increase propeller RPM
4) Increase the camber of the prop blades
What is the preferred form of power absorption?
- Propeller solidity.
- the ratio of the blade frontal area to the area of the propeller circumference
What can be done to increase the blade solidity?
- Increase the chord of the propeller
- Increase the number of blades
What is the helical path?
The spiral path of the prop during flight.
What is the geometric pitch ?
- The theoretical distance the prop would move forward in one relocation if the AOA is zero degrees.
What is the effective pitch?
The actual distance the blade travels in one revolution due to its angle of attack.
What is the difference between the geometric and effective pitch?
The propeller slip
Why are the main forces that act on the propeller? Draw a diagram
- Total Aerodynamic force
- Drag
- Thrust
- Engine torque
What is the thrust bending force?
Lift force being greater further from the hub which results in a prop bending force.
How far along the chord is the thrust a maximum?
60-80%
What are the two main twisting forces on the propeller?
1) Aerodynamic twisting moment
2) Centrifugal twisting moment
What causes the aerodynamic twisting moment?
- The centre of pressure being in front of the pitch change axis, causing the total reaction to produce a moment.
- tends to increase pitch.
What is the centrifugal twisting moment?
Moment cause by centrifugal force that tends to fin the blade pitch angle.
