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The Pilot of a helicopter in autorotation pulls the cyclic control back, leading to speed reduction and:

  • A
    Lift increasing, coning angle increasing and rotor rpm decreasing.
  • B
    Rate of descent decreasing, coning angle increasing and rotor rpm increasing.
  • C
    Lift increasing, coning angle decreasing and rotor rpm decreasing.
  • D
    Lift, coning angle and rotor rpm remains unchanged.

Refer to figure.
When an aft cyclic input is applied during autorotation, the effect will indeed be a decrease in airspeed and rate of descent. Here's why:

Aft Cyclic Input: The cyclic control in a helicopter is used to tilt the rotor disc and control the helicopter's movement in different directions. "Aft cyclic" refers to pulling the cyclic stick or control toward the pilot, which tilts the rotor disc slightly backward. This is shown as point 3 in the figure, known as flaring. when flaring the airflow direction is changing to a more vertical direction from below the rotor disc increasing the rotor RPM and increasing Lift due to higher angle of attack.

Effect on Rotor Disk Angle: Applying aft cyclic increases the pitch angle of the rotor blades as they pass through the rear part of the rotor disc. This action increases the angle of attack of the rotor blades on the retreating side and effectively decreases the angle of attack on the advancing side.

Decrease in Descent Rate: Increasing the pitch angle on the retreating side of the rotor disc leads to a temporary increase in lift production on that side. This additional lift counters the asymmetrical lift distribution that occurs during autorotation, where the retreating side has less lift due to lower airspeed. The result is a reduction in the rate of descent.

Decrease in Airspeed: As the rate of descent decreases due to the increased lift on the retreating side, the helicopter's forward airspeed also decreases. This is because some of the upward airflow through the rotor disc is now being redirected upward more vertically leading to higher angle of attack thus, more lift is generated resulting in a reduction in overall airspeed and rate of descent.

Increased Coning Angle: The Coriolis effect comes into play here. As the pilot pulls the cyclic back, the rotor blades not only change their angle of attack but also experience a flapping motion. Imagine the blades as they flap up and down like wings. This flapping is more pronounced at the tips of the rotor blades. This motion causes the rotor disc to tilt, and the tips of the blades move higher and lower as they spin around the rotor hub. This increase in the angle between the rotor blades and the plane of rotation is what we call the "coning angle."

It's important to note that while applying aft cyclic can help decrease the rate of descent and airspeed during autorotation, it needs to be done judiciously. Excessive aft cyclic could initially lead to a very high rotor RPM and slowing down to much will then lead to a loss of rotor RPM and an increased risk of entering a dangerous low rotor RPM state. Proper management of cyclic inputs, along with collective and pedal inputs, is crucial for maintaining control and ensuring a safe autorotation landing.

With the above we can derive that rate of descent will decrease, coning angle will increase leading to an increase of rotor RPM due to coriolis effect.

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