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What is the main source of interference drag from the option below?

  • A

    BERP tip main rotor blades.

  • B

    Conventional tail rotor.

  • C

    Mast and unfaired main rotor head.

  • D

    Fenestron tail rotor.

Refer to figures.
Interference drag is caused when the different local airflows around the helicopter collide into each other causing vortices or turbulent flow.

This sort of drag typically occurs at the conjunctions of horizontal, vertical or diagonal surfaces. For example, the attachment between skid and fuselage or tail boom and horizontal stabiliser or mast and unfaired main rotor head.

To reduce the interference, drag, the joints between different parts of helicopter should be streamlined or faired.

The interference drag added to the skin friction drag and the form (pressure) drag make up the parasite drag.

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