All this really means is that if the pilot acts correctly, the forces in the rotor will balance
and keep the rotor at a constant RPM.
TECHNIQUE
Now that we all understand why the rotor turns during an autorotation, let's move on to
different techniques for aircraft control.
The first step is to identify the need to enter autorotation. There are several indications
available to the pilot that the rotor is no longer being driven by the engine. The pilot may
experience a yaw in the aircraft with a change in engine noise a decrease in rotor RPM
plus some other failed instruments. Many a pilot has been fooled by getting only an
instrument failure and misdiagnosing that as an engine failure. Always verify this
emergency with several indications.
After the pilot enters the autorotation by lowering the collective, he must think about
selecting a landing area, rates of descent, and of course aircraft control. As the pilot
approaches the ground, he/she now must think of slowing down. Normally about 100 feet
or so, (depending on the aircraft, density altitude, gross weight) the pilot applies aft cyclic
to begin a deceleration. This will slow the rate of descent and forward airspeed. As the
helicopter settles (hopefully vertically) the pilot applies collective to increase the pitch in
the rotor blade. This creates an increased lift vector so termination can be like a normal
landing. The trade off is that both of these control movements will dramatically effect
rotor RPM. You only have one chance to use the controls like this so you have to make it
count.
Airspeed! Watch your airspeed! I can not over-emphasize the importance of
airspeed. Remember, we have to put this helicopter down on the ground in a small area
and calmly but completely exit the aircraft (we want our bodily fluid loss to exit the
appropriate orifice at the appropriate time). We want to be successful at landing this
aircraft with no further damage to the people in it, the aircraft, or anything on the ground.
The optimum airspeed varies with the type of helicopter. If the pilot does not have
enough airspeed then he might descend faster than he wants. If the pilot has too much
airspeed, then the pilot will again descend faster than he wants. Each type of helicopter
has what is known as a minimum rate of descent airspeed and a maximum
glide airspeed. The minimum rate of descent airspeed is exactly what it sounds like.
It's the airspeed at which the helicopter descends the slowest. The problem with this
airspeed is that the aircraft is going to have a rather steep rate of descent. This can be
beneficial if the landing area is close or is very small or you need more time to think about
what landing area you will select. Many people teach to automatically adjust to this
airspeed in the event of an engine failure. I consider this ignorance. The technical
difficulty for completing the autorotation at minimum rate of descent is very high. The
tolerance for error is small. In addition getting lost airspeed back is very difficult. For an
inexperienced pilot, walking the line at maintaining that airspeed can be difficult. And,
when you finally get to the deceleration part of the autorotation, decel and collective
application are almost simultaneous.
The preferred method is to maintain situational awareness and adjust your airspeed as
necessary to arrive at your destination. (Sometimes easier said than done.) Never give up
too much airspeed because getting it back can be costly. It is much easier to shorten your
path using maneuvering than trying to increase airspeed to extend your glide.
Maneuvering brings up another point to remember about autorotation and rotor RPM.
Turns can create too much rotor RPM. This also depends on the type of rotor system
being used. Remember the driving and driven region? Well G forces and turns may cause
these regions to adjust so the Driving region is larger. More lift forward of the axis of
rotation causes the rotor speed to increase. If this happens, a slight increase in collective
is necessary.
Just remember there is always a trade off when it comes to using all this inertia in the form
of rotor RPM and altitude loss. Attempt to stay calm, control your aircraft, look for a
place to land, watch out for wires, and do what you've been taught. Sounds simple
enough anyway.
Next Month: Can A Helicopter Fly at the North Pole?
You can send your feedback and input to Chuck at chuckm@aero.com
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