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.
There are many wrong answers in this thread but I want to address one that any rated helicopter pilot should be able to answer (which judging by the answers in this thread may be not as true as I’m hoping) and that is:

YOU CANNOT GET VORTEX RING STATE IN AN AUTOROTATION

The three things you need for VRS or “Settling with Power” (FAA definition) are:

* Rate of descent greater than 300fpm
* Speed less than ETL
* POWER APPLIED

Obviously if the engine has quit you cannot have power applied. 

That being said, you do still have energy stored in the helicopter in the form of potential energy (in altitude above ground level) and kinetic energy (in the helicopters airspeed). Kinetic energy increases exponentially with airspeed i.e. if you double your airspeed (ex. 30 to 60 kts) you triple the amount of kinetic energy you can use to maintain your rotor RPM in the flare (that you will need a lot of to arrest the rate of descent and forward airspeed prior to touching down) 

If you have no airspeed you have a higher rate of descent in the auto and less energy to arrest that rate of descent with prior to landing. 

Answers to these questions and more can be found in the Helicopter Flying Handbook which is free to download from the FAA website! 

-FAA RW Commercial Pilot and CFI-I