SIFT P1

Airflow and Lift in Helicopter Rotor Systems

Introduction to Helicopter Lift

  • Focus of Discussion: Generation of lift by a helicopter's rotor system.

Key Concepts

Relative Wind

  • Definition: The wind that is parallel to the direction of movement of an airfoil but acts in the opposite direction.

    • Example:

    • If an airfoil moves forward, relative wind acts against it in the opposite direction.

    • For an airfoil moving in any direction, relative wind is always opposite the direction of movement.

Rotational Relative Wind

  • Definition: Extension of the relative wind concept considering the rotor's rotation.

  • Behavior in Helicopters:

    • Helicopters have a rotating rotor system, as opposed to a single airfoil moving in a straight line.

    • Configuration: The rotor rotates around the mast, with blades rotating counterclockwise when viewed from above.

    • Characteristics: As the blades rotate, the relative wind continuously changes direction across the rotor system.

Induced Flow

  • Definition: The downward movement of air induced by the rotor system's operation.

  • Proportionality:

    • Induced flow magnitude is proportional to the velocity at the rotor tips; higher velocities at the tips result in greater downward airflow.

    • The induced flow increases as velocity increases outwardly from the mast.

    • Induced flow decreases toward the center of the rotor, becoming negligible at the mast (center).

Resultant Relative Wind

  • Definition: The vector sum of rotational relative wind and induced flow.

  • Diagrammatic Representation:

    • Create an airfoil illustration with the following components:

    • Rotational Relative Wind Vector: Represented at any point in the rotor's rotation.

    • Induced Flow Vector: Depicted as a downward vector from the rotor system.

    • Chord Line: A line drawn from the leading edge to the trailing edge of the airfoil.

    • Calculation of Resultant Relative Wind:

    • Resultant relative wind is obtained by summing the two vectors (rotational relative wind and induced flow) graphically, usually through a parallelogram method to visualize the resultant vector.

    • This resultant vector is shifted such that its tail aligns with the leading edge of the airfoil to analyze its impact further.

Angle of Attack (AoA)

  • Stipulation: The angle formed between the chord line of the airfoil and the resultant relative wind vector.

  • Lift Generation:

    • AoA directly correlates with lift production. As AoA increases, lift also increases up to a certain limit.

    • Importance of maintaining an optimal AoA for maximum lift production in practical applications.

Effects of Induced Flow on Lift

  • Manipulation of vectors discussed:

    • If induced flow increases while keeping the rotational relative wind constant:

    • The resultant relative wind vector becomes steeper, leading to a smaller angle of attack.

    • This results in decreased lift for the same conditions of rotational relative wind.

    • Conclusion: Understanding the relationship between induced flow and angle of attack is crucial for helicopter performance and safety, especially in future lessons.