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To mitigate the hazards associated with rotor blade sailing and the potential for a tail boom collision in gusty winds for a two-blade main rotor helicopter, which actions should be taken into account?
  • A
    The start-up and shut-down should be carried out with the helicopter facing out of the wind.
  • B
    The start-up and shut-down should be carried out facing downwind.
  • C
    Flapping restrictors, if fitted, should be withdrawn before start-up or shut-down.
  • D
    Ensure that rotor RPM during start-up or shut-down are increased or decreased as slowly as possible.

Refer to figure.
Helicopter blade sailing refers to a phenomenon that can occur in helicopters when they are parked or positioned in windy conditions.
Specifically, high winds and gusts can make main rotor blades flap up and down more than usual during starting and stopping as the droop stops could be damaged, or a particularly flexible blade could hit the tail boom (two-bladed helicopters at low RRPM in gusting winds are especially vulnerable).

Dissymmetry of lift still applies, even at low speeds. If the helicopter is facing into the wind, flapback could make the rotor blade strike the tail boom, hence the use of the flap restrainers and droops stops above. Blades will reach their maximum height at the front of the disc.

However, the cyclic is less effective at low RPM, so other ways of minimizing the effect include:

  • Parking away from the downwind side of obstructions or the downwash or slipstream of other helicopters,

  • Keeping the collective down,

  • Accelerating and decelerating the blades as quickly as possible (rotor locked "high wind" start-up / earlier rotor brake, during engine shutdown, implementation according to helicopter’s limits).

  • Pointing the nose out of the wind so that the lowest deflection is away from the tail boom.

In addition, with clockwise-rotating blades, it is crucial to have the wind from the left side, and vice versa - the blades will then start to rise as they pass over the tail boom and the wind will lift the blades over it when slowing down or starting up (but they will be down at the front).

BE CAREFUL!!! Having the wind from the rear helps the pilot keep an eye on the low blade at the front, but it means landing downwind.

Another crucial point is that the blades will pass in and out of the stall at certain critical speeds (50-100 RPM).
Positioning the cyclic in the direction of the wind (and applying the rotor brake) will keep the pitch of the advancing blade to a minimum and help stop it lifting in the first place.


Note:
  • Blade Sailing is a particular problem for helicopters with semi-rigid rotor heads

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