Helicopters can autorotate back to earth in case of emergency. The FAA recommends a flight profile similar to the following procedure:
Notice that in section (2) there is forward speed: vertical autorotation is not recommended. Why is that?
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When the engine fails in a single engine helicopter it can autorotate back to earth: it gives up potential energy from reducing height to keep the rotor turning, which then continues to provide lift. Autorotation can take place straight downwards in vertical flight, but the general recommendation is to maintain forward speed, like a fixed wing gliding: the rate of descent is much lower if there is forward speed.
We associate helicopters with hovering, but this is a strenuous effort. Even a bit of forward velocity generates translational lift, which makes it much easier for the rotor to generate lift:
The effect of forward velocity on auto rotational rate of descent is depicted in a graph in this book:
Going back to vertical autorotation: the vertical upward airstream needs to go through the rotor fast enough to reach the windmilling state, so that lift is created by decelerating the air passing through it. If we compute the lift component per area of a vertically autorotating rotor, it is comparable to a $C_D$ value of 1.1 to 1.2 referenced to the rotor area. According to this source:
So in vertical descent, the auto-rotating rotor is almost as good as a parachute of the same area - it’s just that the parachute size is a bit small for the typical payload of a helicopter, resulting in vertical descent speeds of between 3,600 to 6,200 ft/min. By way of comparison:
Two factors make autorotation much more survivable though:
Note that in a vertical descend there is no pullback of the cyclic, only increase of collective - plus the rate of descent is higher. A higher speed and fewer means to brake, that is not recommended!
A side note
The windmilling rotor does not turn the other way! The incoming air from underneath tilts the lift vector forward, like in a glider. Part of the rotor disk is driven by the aeroforces into the same direction as it was when still under power.
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.
Forgetting THEORY... a vertical autorotation IS possible and can be accomplished safely. To answer this question put to me by my Co-pilot that day in Vietnam, I took our H-Model Huey up to 5,000' and confirmed hover. I lowered the collective and chopped the throttle to split the needles and monitored rotor RPM to ensure no over-speed while keeping the cyclic centered for true vertical. I used collective to stabilize our sink rate. Once satisfied that the maneuver was safe, I recovered at about 1,000' by pushing the cyclic forward and recovering power.
We experienced no issues with vortex ring theory or any indication that this maneuver was dangerous. Had I needed to actually put it down, I would have initiated a forward cyclic aspect in order to perform a standard flare and set-down once below that 1,000' I chose. In Army flight school in the 1960's, we practiced every possible aspect of auto-rotations ALWAYS to the ground with NO power recovery at 50' nonsense. I personally believe it's irresponsible and insane to not train a helicopter pilot for full ground contact autos. What? Is God going to suddenly show up and help in that last 50' or so?
SIMS are NOT an acceptable substitute for actual practice. We did hovering autos, run-on autos, 180° and 360° autos, full-speed deceleration autos, but my all-time favorite was called "Spot Autos." The IP picked a spot on the runway - usually one of the numbers and I initiated an auto from pattern altitude and speed on the runway heading noting my steep glide angle through the chin bubble.
Keeping collective down and using only cyclic, I flared the chopper to an almost dead stop - closely monitoring rotor rpm and pulling collective when needed to prevent over-speed. Cyclic was pushed forward to start the wonderfully fast descent again - oh wee - we could get at least two and sometimes three flares on the way down - the last one was to a stop right on the number chosen and pull what was in essence a hovering auto to land.
We always completed all our autos to the ground with all of these maneuvers and they were truly FUN to do! Our IP's defined expertise and competence and did an excellent job of instilling those same characteristics into all of us, knowing that where we were headed we would need every training advantage we could handle. My two cents (1,955 combat helicopter flight hours in Vietnam slicks and guns).
You have to consider the power required by the helicopter in flight, represented by the red curve in the graph.
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.
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.
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:
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