Propellers Flashcards
Define the helix angle of a propeller
The resultant helical path generated as a result of a circular motion (prop spinning) with forward velocity (TAS). The helix angle is the angle between the plane of rotation, and the relative airflow (RAF).
What two variables influence helix angle and what is their relationship?
- TAS - Directly proportional; and
- RPM - Inversely proportional.
How does the helix angle change with radius?
The helix angle is smallest at the tips and becomes larger as you move toward the root of the blade.
To maintain a constant optimum AoA for all sections of the prop, the blade is twisted to increase blade angle from:
a) Tip towards the root
b) Root towards the tip.
a) Tip Towards Root
NOTE: Full feathered (chord aligned with airflow) is considered a blade angle of 90º.
As you can see in the image if blade angle remained constant it would not produce the optimum AoA (red line is chord of the top aerofoil)
Define PITCH (in terms of linear dimensions)
Pitch: Distance travelled in one revolution (screw)
Define Geometric pitch
The distance travelled in one revolution if the blade had travelled along its chord line. In a fixed pitch prop this value is fixed by design.
Define effective pitch or Advance per revolution.
Actual distance travelled by blade in one revolution. It is a variable distance dependant on RPM and TAS. Effective pitch or APR is zero before rolling for takeoff, and increases with increasing TAS.
Define Slip
Slip = Geometric pitch - effective pitch.
Slip is required to give a positive AoA on the propeller blade section.
What is the difference between Geometric Pitch and Effective pitch known as?
Slip
Small blade angles will correspond to shorter pitch (in metres), this is termed as?
a) Slip;
b) Effective Pitch;
c) Fine Pitch; or
d) Coarse Pitch
c) Fine pitch, required at low speeds
Large Blade angles will correspond to longer pitch, termed as:
a) Fine Pitch;
b) Coarse Pitch
c) Geometric Pitch; or
d) Helix angle
b) Coarse pitch, required at high speeds
Which is more efficient in terms of propeller efficiency?
a) Fixed Pitch Propeller; or
b) Variable Pitch Propeller
b) Variable pitch propeller
What is the equation for propeller efficiency (n)?
n = Output / Input
= THP/BHP
= [(Thrust x Velocity) / (2πNRPMQTorque)] %
What is an advantage of the constant speed propeller?
It is efficient over a wide range of forward speeds (and RPMs)
How does a constant speed propeller remain efficient?
It retains an efficient AoA over a wide forward speed range by altering blade angle automatically.
What are the two load forces that act upon the propeller?
- Thrust Bending Loads; and
- Centrifugal Axial Tensile Loads.
What is the aerodynamic twisting moment?
Because the TR acts ahead of the axis of rotation for the prop at the centre of pressure, a moment is produced which, as AoA increases, tends to try and feather the prop.
What is the Centrifugal Twisting Moment (CTM)?
The CTM is a moment produced by the rotation of the propeller and acts to align the blade with the plane of rotation (0º AoA). This moment opposes the ATM and is of far greater magnitude.
*Tries to increase pitch to full fine
How can you increase power absorption of a propeller?
- Increase number of blades (Contra-rotating propellers);
- Increase the chord of each blade (AP-3C);
- Increase propeller diameter.
Yaw due to slipstream effect:
a) Increases with airspeed;
b) decreases with airspeed;
c) remains constant
b) It decreases due to the slipstream impacting the tail at a lesser angle, therefore imparting less force.
