16 / 20

During an autorotation with normal forward speed, compared with a vertical autorotation in still air, the Rate of Descent (ROD) is:

• A
Always lower.
• B
Almost equal.
• C
Dependent on the direction of rotation of the main rotor.
• D
Always greater.

Refer to figure.
Autorotative force in forward flight is produced in the same way as it is for a helicopter descending vertically in still air. The three regions of the rotor disc are displaced towards the retreating side if horizontal movement is added to the rate of decent airflow.

In autorotative state the downwash acts in the opposite direction from the rate of decent airflow. This effectively reduces the velocity of the rate of decent airflow so that the effective airflow is less than the actual rate of decent. The result is a modification in inflow angle.

There are three factors that determine if the rate of decent will increase or decrease. Both disc tilt and horizontal flow reduce the inflow angle and therefore angle of attack. On the contrary the forward movement reduces the downwash component resulting in an increase in effective rate of decent airflow, increasing the angle of attack.

At lower speeds the reduction in downwash component is larger than the influences of the disc tilt. However, at higher speeds the disc must be tilted more and more to overcome the rise in parasite drag, increasing the rate of decent.

In the end if you plot the rate of the decent against forward speeds, it will strongly resemble the power required curve.

The answer states the Rate of Descent is always lower with forward airspeed in autorotation, however at high speed the graph shows an increase of ROD reaching similar numbers as a vertical autorotation with zero wind conditions. Helicopter have a lower never exceedance speed (Vne) in autorotation than in forward powerd flight therefore, the ROD at speeds close to Vne in autorotation will most probably be lower than vertical autorotation in no wind conditions.

Your Notes (not visible to others)

This question has appeared on the real examination, you can find the related countries below.

• Spain
• Switzerland