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motion than lateral cyclic, because it reduces the main
rotor thrust (lift). A smooth, moderate collective reduction,
at a rate less than approximately full up to full down
in two seconds, is adequate to stop the rolling motion.
Take care, however, not to dump collective at too high a
rate, as this may cause a main rotor blade to strike the
fuselage. Additionally, if the helicopter is on a slope and
the roll starts to the upslope side, reducing collective too
fast may create a high roll rate in the opposite direction.
When the upslope skid/wheel hits the ground, the
dynamics of the motion can cause the helicopter to
bounce off the upslope skid/wheel, and the inertia can
cause the helicopter to roll about the downslope ground
contact point and over on its side. [Figure 11-8]
The collective should not be pulled suddenly to get airborne,
as a large and abrupt rolling moment in the
opposite direction could occur. Excessive application
of collective can result in the upslope skid rising sufficiently
to exceed lateral cyclic control limits. This
movement may be uncontrollable. If the helicopter
develops a roll rate with one skid/wheel on the ground,
the helicopter can roll over on its side.
PRECAUTIONS
The following lists several areas to help you avoid
dynamic rollover.
1. Always practice hovering autorotations into the
wind, but never when the wind is gusty or over
10 knots.
2. When hovering close to fences, sprinklers,
bushes, runway/taxi lights, or other obstacles that
could catch a skid, use extreme caution.
3. Always use a two-step liftoff. Pull in just enough
collective pitch control to be light on the skids
and feel for equilibrium, then gently lift the
helicopter into the air.
4. When practicing hovering maneuvers close to
the ground, make sure you hover high enough to
have adequate skid clearance with any obstacles,
especially when practicing sideways or
rearward flight.
5. When the wind is coming from the upslope direction,
less lateral cyclic control will be available.
6. Tailwind conditions should be avoided when
conducting slope operations.
7. When the left skid/wheel is upslope, less lateral
cyclic control is available due to the translating
tendency of the tail rotor. (This is true for
counter-rotating rotor systems)
8. If passengers or cargo are loaded or unloaded, the
lateral cyclic requirement changes.
9. If the helicopter utilizes interconnecting fuel lines
that allow fuel to automatically transfer from one
side of the helicopter to the other, the gravitational
flow of fuel to the downslope tank could change
the center of gravity, resulting in a different
amount of cyclic control application to obtain the
same lateral result.
10. Do not allow the cyclic limits to be reached. If the
cyclic control limit is reached, further lowering of
the collective may cause mast bumping. If this
occurs, return to a hover and select a landing point
with a lesser degree of slope.
11. During a takeoff from a slope, if the upslope
skid/wheel starts to leave the ground before the
downslope skid/wheel, smoothly and gently
Tail Rotor Thrust
Slope
Horizontal
Area of
Critical Rollover
Full Opposite Cyclic Limit
to Prevent Rolling Motion
F
Figure 11-8. Downslope rolling motion.
11-10
lower the collective and check to see if the
downslope skid/wheel is caught on something.
Under these conditions vertical ascent is the only
acceptable method of liftoff.
12. During flight operations on a floating platform, if
the platform is pitching/rolling while attempting to
land or takeoff, the result could be dynamic rollover.
LOW G CONDITIONS AND MAST
BUMPING
For cyclic control, small helicopters depend primarily
on tilting the main rotor thrust vector to produce
control moments about the aircraft center of gravity
(CG), causing the helicopter to roll or pitch in the
desired direction. Pushing the cyclic control forward
abruptly from either straight-and-level flight or after a
climb can put the helicopter into a low G (weightless)
flight condition. In forward flight, when a push-over is
performed, the angle of attack and thrust of the rotor is
reduced, causing a low G or weightless flight condition.
During the low G condition, the lateral cyclic has
little, if any, effect because the rotor thrust has been
reduced. Also, in a counter-clockwise rotor system (a
clockwise system would be the reverse), there is no
main rotor thrust component to the left to counteract
the tail rotor thrust to the right, and since the tail rotor
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