Why is a helicopter's climb initially accelerated after an increase of the pitch angle?
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A
The larger pitch angle causes a decrease and a compression of the airflow until the climb rate corresponds to the induced velocity through the main rotor disc.
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B
The inertia of the air increases the effective angle of attack temporarily until an additional component of the climb rate reduces the effective angle of attack again.
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C
The initially high airflow increases the rotor thrust until the lower air density reduces the rotor thrust again.
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D
The conversion of energetic air into kinetic rotor energy decreases continuously with increasing density altitude.
When a helicopter increases its pitch angle, the angle of attack of the rotor blades temporarily increases, resulting in more lift and thus acceleration upwards. This is because the inertia of the moving air means the increased pitch angle takes a moment to balance out with the new airflow conditions. However, as the helicopter climbs, an additional upward velocity component forms, which alters the effective angle of attack back to a more stable state, reducing the initial acceleration. So, the initial acceleration is due to the lag in the airflow's response to the increased pitch angle, and it stabilizes once the climb rate affects the angles involved.
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Austro Control1
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