High-altitude autorotations?

In article , Bill
McClain wrote:

And this got me wondering: Does anybody test to see how high up you
can successfully autorotate from? Is there an actual record for this?

Sounds like a self correcting problem. If you are too high to autorotate,
you will very soon be much lower.

Ummm, yeah, I guess so, but...seriously, is it even possible to try
and keep the RPMs up by diving and turning...I guess WITH the
direction of rotor spin...trying to maintain as much inertia in the
mast and blades before they lose so much torque as to be unable to
provide any lift to pull out of the dive and try to flare close to the
ground? I'm pretty much talking through my hat speculating like this;
I don't really know all that much about helicopters (other than that
I'm not all that keen on riding in one).

Believe it or not, the problem is just the opposite.

In a high altitude autorotation the rotor tends to overspeed if you
don't keep an eye on it and a small application of collective pitch is
necessary from time to time to keep it within limits. Turning and
diving are unnecessary.

When I was instructing at Fort Rucker many moons ago we would take
students to 10,000 MSL in a UH-1H and let them play with the
autorotative characteristics. The airspeed for minimum rate of descent
in the UH-1 is 63 knots indicated while the maximum glide distance is
attained at 98 knots. From 10,000 feet the student has lots of time to
vary the airspeed and get a feel for different rates of descent before
a power recovery is required.

We did touchdown autorations every day in the training cycle, but were
limited to six per student per day because they are so intense to a
student that any training benefit beyond that is negligible. On days
when I had three students, I would do eighteen touchdown autorotations
from 1000' to a concrete runway and not think a thing about it.
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http://travel.howstuffworks.com/helicopter.htm

What is autorotation?

Autorotation is a condition where the main rotor is allowed to spin
faster than the engine driving it. How is that achieved? It is actually
quite simple.
All helicopters are fitted with a free wheeling unit between the engine
and the main rotor, usually in the transmission. This free wheeling
unit can come in different forms but one of the most popular is the
sprag clutch. The free wheeling unit will allow the engine to drive the
rotors but not allow the rotors to turn the engine. When the engine/s
fail the main rotor will still have a considerable amount of inertia
and will still want to turn under its own force and through the
aerodynamic force of the air through which it is flying. The free
wheeling unit is designed in such a way to allow the main rotor to now
rotate of its own free will regardless of engine speed. This principle
is the same reason that if you are in your car and you push your clutch
in, or put it into neutral while the car is still moving, the car will
coast along under it's own force. This occurs regardless of what you do
to the accelerator pedal.


Controlled Descent ?

The next question you are probably asking yourself is: "Does the pilot
retain control of the helicopter?" The answer is yes. The pilot will
still have complete control of his descent and his flight controls. The
majority of helicopters are designed with a hydraulic pump mounted on
the main transmission. As the rotor will still be turning the
transmission, the pilot will still have hydraulically assisted flight
controls. The pilot will be able to control his descent speed and main
rotor RPM with his collective control stick. He will be able to control
his main rotor RPM by increasing the collective pitch, which will
increase drag on the rotor blades and thereby slow the main rotor. If
he needs to increase his rotor RPM, he can decrease his collective
pitch therefore decreasing drag.
The pilot will usually be able to find a suitable area for a safe
landing by normal manipulation of his cyclic control stick and his
directional, or tail rotor pedals.
Larger helicopters will usually have a generator mounted on the
transmission that will still provide electrical power for flight and
communication systems.


What happens to Torque Effect ?

Torque effect is the aircraft's tendency to rotate in the opposite
direction to the main rotor due to Newton's third law "Every action has
an equal and opposite reaction". This is the reason why we need a tail
rotor or some other form of anti-torque control. The question at hand
is what happens to torque effect during autorotation? Well torque
effect is directly proportional to the amount of force driving the main
rotor, so when when the engine fails the amount of force driving the
main rotor instantaneously decreases and therefore the torque effect
decreases. This being the case the fuselage of the helicopter will tend
to rotate due to the sudden lack of torque effect. The pilot will
therefore have to immediately manipulate his directional pedals to
overcome this problem and retain control of his aircraft.

Conclusion

So in conclusion if your helicopter's engine/s should fail it is not
just possible, but quite easy for the pilot to retain control and land
safely and gently. This is the reason I believe that helicopters are
far safer and more fun to fly in than fixed wing aircraft. A fixed wing
aircraft will always need forward speed to safely land, with or without
an engine operating. A helicopter can be made to land with zero forward
speed whether the engine is operating or not.