25 Propellers

Question 1

Where along the blade is the blade angle measured from?

Answer 1

75% from hub to tip

Question 2

How efficient are Propellers?

Answer 2

80 - 85% efficient

Question 3

What is the Blade angle and what is the equation for it?

Answer 3

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

Question 4

What is the AOA for a propeller blade?

Answer 4

Like on an aerofoil, AOA is the angle between the RAF and the chord line

Question 5

What is the Helix angle of a propeller blade?

Answer 5

Helix angle is the angle between the plane of rotation and the RAF

Question 6

What is the Geometric Pitch of a propeller blade and give its equation?

Answer 6

• Geometric Pitch is the distance a propeller blade could theoretically travel in one rotation
• Geometric Pitch = Effective Pitch + Slip

Question 7

What is the Effective Pitch of a propeller blade?

Answer 7

Effective Pitch is the distance a blade actually travels in one rotation

Question 8

What is the effect on AOA on a prop blade with increasing TAS and constant RPM?

Answer 8

AOA decreases with increasing TAS

Question 9

Describe fine and coarse pitch angels of prop blades

Answer 9

• 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

Question 10

What is the effect on AOA on a prop blade with increasing RPM but constant TAS?

Answer 10

AOA increases with increasing RPM

Question 11

Which type of pitch angle would be used in the event of an engine failure to reduce drag?

Answer 11

Feathered pitch

Question 12

What are the pros and cons for a fixed pitch propeller?

Answer 12

Pros:

• Simple design
• More reliable

Cons:
- Only effective at one TAS

Question 13

Describe a constant speed propeller

Answer 13

Maintains a constant RPM and the AOA is mechanically controlled by a ‘governor’ to keep the blades at the optimum AOA