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: 
During autorotation, forward speed is used to control rotor head speed. In auto, the engine is disconnected (in the case of tail rotor failure) or just plain dead, so it can't be used to control rotor speed. 

In a vertical descent, the rotor head will eventually overspeed from the air rushing past the blades, and the rotor blades will depart from the aircraft. Not good.

The air hitting the blades from forward flight is used to induce drag on the rotor blades, slowing them down. The faster the helicopter moves forward, the greater the drag. 

So the pilot, in auto, uses forward airspeed to control rotor speed. If the rotor head speed builds up too much, nose down a bit to gain airspeed and slow the rotor down. If the rotor head starts slowing down, then back off the forward velocity to let the rotor head pick up speed.

Forward flight also aids in maneuvering the helicopter when choosing a touchdown point, but mainly, it is used to keep the rotor speed in the green. 

Every helicopter has an ideal autorotation forward flight velocity, that should keep rotor head speed in the green. In a Bell JetRanger, it's around 60 kts I believe, though this is a general guideline, not an absolute. 

Theoretically, one could increase blade pitch to an overspending head to slow it down, but forward flight is a more precise way of doing this, and avoids the risk of inadvertently slowing the head down too much.
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 ..