Autorotation

Question 1

Describe autorotation.

Answer 1

In powered flight the rotor drag is overcome with engine power but when the engine fails or is deliberately disengaged from the system another force must be used to maintain the voter RPM.

This is achieved by allowing the helicopter to descend by lowering the collective lever fully so that the resultant airflow strikes the blades in such a manner that the airflow itself provides the driving force.

Question 2

How do the driving and dragging forces change across the blade in autorotation and why?

Answer 2

The ROD flow is the same across the blade but the rotational velocity is greater at the tip than the root.

Because of washout the pitch angle and angle of attack is greatest at the root.

Towards the tip the Total Reaction lies behind the axis of rotation (dragging / decelerating)

Towards the root the Total Reaction lies ahead of the axis of rotation (driving / autorotative / accelerating).

Question 3

How is NR control achieved?

Answer 3

Movement of the collective balances the driving and dragging portions to maintain NR.

Question 4

What is the avoid curve?

Answer 4

The height and speed combination from which in the event of a total engine failure a safe auto rotative landing is unlikely to be achieved.

Question 5

What are the four flare effects?

Answer 5

Thrust reversal (speed reduces)

Increase in total rotor thrust (aircraft climbs)

Nose pitches up (CoG moves forward)

Increase in Rotor RPM (conservation of angular momentum and reduced rotor drag)