Minimum rate of descent: This configuration
is desirable if you need to maneuver to a landing site that is close to
you. In this scenario, you have set your airspeed to the best rate of
descent, and now you are maintaining the rotor rpm in the green arc; you will
have the minimum rate of descent that you can possibly achieve in your current
configuration, and you may have more time to maneuver to a close landing
zone. Note that this is NOT
the best distance scenario! This will simply give you your best time in
the descent, but you will not be covering much ground. Note that while in
autorotation, your sink rate will vary between 1,300 and 1,800 feet-per-minute.
Important: In the event of an actual failure as well as
during practice, it is imperative to always set a stabilized autorotation
first, then adjust to the configuration you desire. There may not be time
to adjust as is usually true with low time pilots. Always set the optimal
airspeed and stabilize the rpm in the green arc then adjust from there if time
permits.
What you see is what you get - As a student or low time pilot who is building skills it is important to
understand that a stabilized situation is the primary objective. This
never changes, however the time it takes to become stabilized decreases as
experience increases. Once you have descended through 300-feet AGL, it is
to late to change what you have; it is now what-you-see is what-you-get.
During practice autorotations, at 300-feet AGL you must either be stabilized,
or you must smoothly abort the autorotation (smoothly increase throttle to set
top of the green rpm, then increase collective to go-around). In a real
life failure, at this point (300-feet AGL) you must focus on making the best of
what you have.
Why is the autorotation Vne different from the powered
flight Vne? In training aircraft there is usually no difference and you will not
likely be aware that in some aircraft there is a different Vne for
autorotation. Autorotation Vne is usually marked on the airspeed
indicator of a helicopter by a blue radial line. This exists on larger
helicopters which are usually faster than training aircraft. Take the
Robinson R44 for example; this aircraft has an autorotation Vne of 100 knots,
and is depicted by the blue radial line on the airspeed indicator. The
Bell 206 Jet Ranger is similar.
Remember that the up-flow of air through the main
rotor is what drives the rotor system while in autorotation. If your airspeed
is to high, you will not be able to achieve sufficient up-flow, and the rotor
rpm will decrease.
Advanced
Autorotation Techniques
Once a pilot becomes comfortable with autorotation,
and once he or she sees and understands that the helicopter does not fly any
different in autorotation than it does in powered flight, then these techniques
can be taught, used, and experimented with. I do not mean that you should
experiment with anything untested, or not demonstrated to you. It is
better that you have a qualified instructor sitting next to you who can keep
you within the margins of safety. Never let yourself become a test pilot
before your time!
Airspeed - A helicopter will autorotate at any airspeed (below
autorotation Vne). This means that it will autorotate just fine at zero
airspeed, however you will have a high rate of descent. This higher rate
of descent is acceptable provided that at the right time (above the H/V Curve),
you gain the airspeed necessary for a reasonable flare and/or touchdown.
This airspeed is best at or above 45-knots to make a safe and non-destructive
landing however the skill level of the pilot must be higher than that of a
student or private pilot at these lower airspeeds. The helicopter can
even be flown backwards in autorotation with no dangers other than the fact
that you must gain forward airspeed prior to touchdown. You must also
keep in mind that rotor rpm can decrease significantly especially with forward
cyclic inputs while manipulating the cyclic utilizing advanced autorotation
techniques.
Maneuvering - A helicopter is no less maneuverable in autorotation
than it is in powered flight. The only thing you must do, is maintain
rotor rpm within the acceptable range, and fly the helicopter to where you want
to go. It really is as simple as that. Never stop flying the
aircraft.
RPM - What controls rotor rpm during autorotation?
The collective right? What else? The collective directly controls
rotor rpm through the pitch of the rotor blades. The cyclic also controls
rotor rpm through disk loading. If you will maintain a straight-in
autorotation, the collective is the only means of rpm control. If you
will make any turns, depending upon how steep, you may have to manipulate the
collective to prevent an over-speed. This is because you have increased
the g-load on the rotor disk in the turn which increased rotor rpm. If
you input forward cyclic, you will decrease the g-load on the rotor, and rotor
rpm will decrease as well. Caution must be used when a forward cyclic
input is made due to the fact that a rotor rpm decrease will occur.
Does a little more altitude really make much
difference? Yes it does, and there is no argument against this
fact. Each 100-feet of altitude will add approximately 4-seconds to your
glide time and every second counts. This means that if you normally fly
at 500-feet AGL, and you increase your altitude by just 100-feet to 600-feet
AGL, you will increase your potential glide time by 20 percent. A 20
percent increase in glide time for 100-feet of increase in your altitude is a
BIG difference.
Extracto de http://helicopterflight.net/autorotation.htm