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Which fuselage shape reduces drag best?

  • A

    The best shape is flat at the end, so the airflow to separate smoothly.

  • B

    The best shape is rounded at the end, so the airflow to separate fast.

  • C

    The best shape is sharp at the end, so the airflow to separate fast.

  • D

    The best shape is sharp at the end, so the airflow to separate smoothly.

Interference drag is caused when the different local airflows around the helicopter collide, causing vortices or turbulent flow.

This sort of drag typically occurs at the conjunctions of horizontal, vertical, or diagonal surfaces. For example, at the back part of the fuselage, the attachment between skid and fuselage, tail boom and horizontal stabilizer, or unfaired main rotor head, where the airflow passing around the aircraft does not separate smoothly.

To reduce interference and drag, the back part of the fuselage must as sharp as possible and the joints between different parts of the helicopter should be streamlined or faired, allowing the airflow around the aircraft to separate as smoothly as possible.

For example, fairing the main rotor head and smoother fairings and fillets will ease the transition between components and thus reduce the interference drag.

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

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