Vertical Autorotation

In truly vertical autorotative descent with no horizontal incoming airflow ROD will settle to about 2000ft per min. The disc would look like this from above if you could see what the various regions do in a vertical autorotation. The pilot may need to use directional flight (it doesn't have to be forward) to get to a chosen landing site.

Forward Autorotation

In directional flight the way ROD airflow meets the disc has been changed. ROD airflow is now a mixture of vertical and horizontal incoming airflow.

In forward flight (for instance) inflow angles are changed. This causes the autorotative force to be moved toward the retreating side of the disc. ROD will be reduced (up to a point) when directional flight begins. ROD is now less than 2000ft/min.

Aurorotation Vectors

Descent flow or up-wash from beneath the plane of rotation replaces the induced flow produced in normal powered flight. This descent flow provides a relative airflow capable of driving the blades and replacing engine torque with autorotative force.

During autorotation not all sections of the blade are producing a driving force. This is because the direction of the Total Reaction vector changes relative to the axis of rotation as the inflow angle and angle of attack changes from blade tip to root.

 

1 Drag Area - The Total Reaction vector is positioned behind the axis of rotation, decelerating the blade. This is due to its Rotor Drag component acting opposite to the direction of rotation.

2 Equilibrium - The Total Reaction vector is acting along the axis of rotation, resulting in no Rotor Drag. This section of the blade is in equilibrium.

3 Autorotative Force - The Total Reaction vector is in front of the axis of rotation. The component of Total Reaction acting in the plane of rotation no longer causes drag but acts to accelerate the blade. This component is known as the autorotative force.

4 Stalled Area - The blade is stalled, resulting in drag.