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is above the CG, the tail rotor thrust causes the helicopter
to roll rapidly to the right, If you attempt to stop the
right roll by applying full left cyclic before regaining
main rotor thrust, the rotor can exceed its flapping
limits and cause structural failure of the rotor shaft due
to mast bumping, or it may allow a blade to contact the
airframe. [Figure 11-9]
Since a low G condition could have disastrous results,
the best way to prevent it from happening is to avoid the
conditions where it might occur. This means avoiding
turbulence as much as possible. If you do encounter
turbulence, slow your forward airspeed and make small
control inputs. If turbulence becomes excessive,
consider making a precautionary landing. To help prevent
turbulence induced inputs, make sure your cyclic
arm is properly supported. One way to accomplish this
is to brace your arm against your leg. Even if you are
not in turbulent conditions, you should avoid abrupt
movement of the cyclic and collective.
If you do find yourself in a low G condition, which
can be recognized by a feeling of weightlessness
and an uncontrolled roll to the right, you should immediately
and smoothly apply aft cyclic. Do not attempt
to correct the rolling action with lateral cyclic. By
applying aft cyclic, you will load the rotor system,
which in turn produces thrust. Once thrust is restored,
left cyclic control becomes effective, and you can roll
the helicopter to a level attitude.
LOW ROTOR RPM AND BLADE STALL
As mentioned earlier, low rotor r.p.m. during an
autorotation might result in a less than successful
maneuver. However, if you let rotor r.p.m. decay to the
point where all the rotor blades stall, the result is usually
fatal, especially when it occurs at altitude. The
danger of low rotor r.p.m. and blade stall is greatest in
small helicopters with low blade inertia. It can occur
in a number of ways, such as simply rolling the throttle
the wrong way, pulling more collective pitch than
power available, or when operating at a high density
altitude.
When the rotor r.p.m. drops, the blades try to maintain
the same amount of lift by increasing pitch. As the
pitch increases, drag increases, which requires more
power to keep the blades turning at the proper r.p.m.
When power is no longer available to maintain r.p.m.,
and therefore lift, the helicopter begins to descend.
This changes the relative wind and further increases
the angle of attack. At some point the blades will stall
unless r.p.m. is restored. If all blades stall, it is almost
impossible to get smooth air flowing across the
blades.
Even though there is a safety factor built into most helicopters,
anytime your rotor r.p.m. falls below the green
arc, and you have power, simultaneously add throttle
and lower the collective. If you are in forward flight,
gently applying aft cyclic loads up the rotor system and
helps increase rotor r.p.m. If you are without power,
immediately lower the collective and apply aft cyclic.
RECOVERY FROM LOW ROTOR RPM
Under certain conditions of high weight, high temperature,
or high density altitude, you might get into a
situation where the r.p.m. is low even though you are
using maximum throttle. This is usually the result of
Figure 11-9. In a low G condition, improper corrective action
could lead to the main rotor hub contacting the rotor mast.
The contact with the mast becomes more violent with each
successive flapping motion. This, in turn, creates a greater
flapping displacement. The result could be a severely
damaged rotor mast, or the main rotor system could separate
from the helicopter.
11-11
the main rotor blades having an angle of attack that has
created so much drag that engine power is not sufficient
to maintain or attain normal operating r.p.m.
If you are in a low r.p.m. situation, the lifting power of
the main rotor blades can be greatly diminished. As soon
as you detect a low r.p.m. condition, immediately apply
additional throttle, if available, while slightly lowering
the collective. This reduces main rotor pitch and drag. As
the helicopter begins to settle, smoothly raise the collective
to stop the descent. At hovering altitude you may
have to repeat this technique several times to regain normal
operating r.p.m. This technique is sometimes called
“milking the collective.” When operating at altitude, the
collective may have to be lowered only once to regain
rotor speed. The amount the collective can be lowered
depends on altitude. When hovering near the surface,
make sure the helicopter does not contact the ground as
　
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