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: 
You have to consider the power required by the helicopter in flight, represented by the red curve in the graph.

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

The blue line is the power provided by the rotor in autorotation. As you can see at zero airspeed the power requiered is much bigger than at 50, 60 or 70 kts. Therefore, you power deficit will be also much bigger at zero airspeed, resulting in an increased rate of descent. 

Another advantage given by speed is the ability to flare (steps 3-4 in the flight profile recommended by FAA). During the flare you use the kinetic energy provided by forward speed to reduce your descent. With zero airspeed you completely lose this posibility, the only chance of reducing rate of descent remaining will be increasing the collective (and most likely it will not be enough to prevent a very hard landing).

A third reason would be that for the pilot it's easier to judge height above the ground if there is some forward speed, and also the helicopter is more stable directionally. 

Bottom line, the preferred airspeed range for autorotation is the one shaded green in the graph, between minimum-power-speed and best-range-speed. There are situations where the yellow range is used (confined area, some procedures for night autorotation), but this area doesn't extend below 30 kts, anyway.
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 ..