Autorotation


Autorotation is a condition of helicopter flight during which the lifting rotor of a helicopter is being turned by the force of the relative wind with no power from the engine. It is a maneuver where the engine is disengaged from the main rotor system and the rotor blades are driven solely by the upward flow of air through the rotor.

The most common reason to perform an autorotation is engine failure. But autorotation is also performed during complete tail rotor failure as it produces virtually no torque, and a safe landing is possible.

At the instant of engine failure or tail rotor failure, the main rotor blades are producing lift and thrust from their AOA and velocity. By immediately lowering the collective pitch, the lift and drag of the helicopter are lowered and the helicopter begins to descend producing an upward flow of air through the rotor system immediately. This upward flow provides sufficient thrust to maintain rotor rpm throughout the descent.

Airspeed and rotor rpm are the primary ways to control the rate of descent in autorotation. Higher or lower airspeed is obtained with the cyclic pitch control just as in normal powered flight. The rate of descent is high at zero airspeeds and decreases to a minimum at approximately 50–60 knots (depending upon the particular helicopter), beyond which the rate of descent starts to increase again.


When landing from an autorotation, the only energy available to arrest the descent rate and ensure a soft landing is the kinetic energy obtained from the descent. Since the tail rotor is driven by the main rotor transmission during autorotation, the heading is maintained with the tail rotor as in normal flight.

In normal powered helicopter flight, the air is drawn into the main rotor system from above and exhausted downward. But during autorotation, air moves up into the rotor system from below as the aircraft descends

If the engine fails during a hover in still air and the collective pitch is reduced, the helicopter will accelerate downwards until such time as the angle of attack is producing a total reaction to give an autorotative force to maintain the required rotor rpm and a rotor thrust equal to the weight.

The pilot should use collective pitch control to manage rotor rpm. If rotor rpm builds too high during an autorotation, pilot should raise the collective sufficiently to decrease rpm back to the normal operating range, then reduce the collective to maintain proper rotor rpm. If the collective increase is held too long, the rotor rpm may decrease rapidly.  

During Vertical autorotation, the rotor disk is divided into three regions are the driven region, the driving region, and the stall region.

The rate of descent is the same for all sections. Rotational velocity will decrease from the tip towards the root. Comparing these sections, the inflow angle must therefore be progressively increasing




The following project link will redirect to the project page which can also be viewed on mobile. It explains the phenomenon of Autorotation step by step. The steps can be repeatedly viewed back and forth as per the will of the user.

Autorotation on Scratch

Click the Next Button to see and understand the different conditions that are going through the phases of autorotation.

The rotor blade is divided basically into 3 sections.

The Phases of Autorotation can be easily visualized using the next or previous buttons.













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