Question: Helicopters can autorotate back to earth in case of emergency. The FAA recommends a flight profile similar to the following procedure:

[![enter image description here](https://i.stack.imgur.com/jATGq.png)](https://i.stack.imgur.com/jATGq.png)

Notice that in section (2) there is forward speed: vertical autorotation is not recommended. Why is that?

Answer: 
There are three good reasons to avoid Vertical Autorotation.

First, Vertical Autorotation is way more likely to lead to a Vortex Ring condition where the air exiting the rotor disk gets recirculated back through the rotor disk. The recirculated air in the Vortex Ring then limits the amount of airflow/energy available for conversion to Rotor Disk rotational velocity which is critical to successfully completing that part of the Auto Rotation maneuver essential to survival (the flare up to touchdown). 

Second, the forward velocity increases the amount of energy available for conversion to rotor disk speed. 

Third, forward velocity allows landing shock to be shared between different harnesses and seat shock absorbers thus decreasing the load on any particular axis.
Rotor rpm control is by collective. No vortex ring in autorotation..since Air is flowing in wrong direction. Vertical autorotation is possible but risky since limited excess energy, greater energy with airspeed ,so you have more energy for the flare, allows margins for errors during flares. and you can pick your landing area fly to it and obviously see it in advance ..