14.82 Forces Acting on a Helicopter Rotor

14.82.2 For a nil-wind hovering condition identify the following vectors:

(a) rotational flow;

• The airflow caused by the rotation of the rotor blades
• Acts parallel to the plane of rotation (tip path plane)
• Direction is opposite to the blade’s motion relative to the air

Summary:
Rotational flow is the horizontal airflow across the blade due to rotation, lying in the plane of rotation.

(b) induced flow;

• The airflow downward through the rotor disc produced by rotor lift in hover
• Acts perpendicular to the tip path plane (downwards)

Summary:
Induced flow is the downward airflow through the rotor disc generated by lift in hovering flight.

(c) relative airflow.

• The resultant airflow acting on the rotor blade
• Formed by the combination of:
  • Rotational flow (horizontal)
  • Induced flow (downward)
• Acts at an angle to the plane of rotation

Summary:
Relative airflow is the combined effect of rotational and induced flow, determining the blade’s aerodynamic behaviour.

14.82.4 For a nil-wind hovering condition identify:

(a) pitch angle;

• The angle between the blade chord line and the plane of rotation (tip path plane)