25 Propellers Flashcards
Where along the blade is the blade angle measured from?
75% from hub to tip
How efficient are Propellers?
80 - 85% efficient
What is the Blade angle and what is the equation for it?
Blade angle is the angle between the plane of rotation and the chord line of the blade (measured at 75% of length)
- Blade angle = Helix angle + AOA
What is the AOA for a propeller blade?
Like on an aerofoil, AOA is the angle between the RAF and the chord line
What is the Helix angle of a propeller blade?
Helix angle is the angle between the plane of rotation and the RAF
What is the Geometric Pitch of a propeller blade and give its equation?
- Geometric Pitch is the distance a propeller blade could theoretically travel in one rotation
- Geometric Pitch = Effective Pitch + Slip
What is the Effective Pitch of a propeller blade?
Effective Pitch is the distance a blade actually travels in one rotation
What is the effect on AOA on a prop blade with increasing TAS and constant RPM?
AOA decreases with increasing TAS
Describe fine and coarse pitch angels of prop blades
- Fine pitch angles are small pitch angles
- Smallest (0 degrees) called fully fine
- Coarse pitch angles are larger pitch angles
- Largest (90 degrees) called feathered
What is the effect on AOA on a prop blade with increasing RPM but constant TAS?
AOA increases with increasing RPM
Which type of pitch angle would be used in the event of an engine failure to reduce drag?
Feathered pitch
What are the pros and cons for a fixed pitch propeller?
Pros:
- Simple design
- More reliable
Cons:
- Only effective at one TAS
Describe a constant speed propeller
Maintains a constant RPM and the AOA is mechanically controlled by a ‘governor’ to keep the blades at the optimum AOA
