Rotorcraft Flying Handbook, FAA-H-8083-21

contacts a fixed object while hovering sideward, or if
the gear is stuck in ice, soft asphalt, or mud. Dynamic
rollover may also occur if you do not use the proper
landing or takeoff technique or while performing slope
operations. Whatever the cause, if the gear or skid
becomes a pivot point, dynamic rollover is possible if
you do not use the proper corrective technique.
Once started, dynamic rollover cannot be stopped by
application of opposite cyclic control alone. For example,
the right skid contacts an object and becomes the
pivot point while the helicopter starts rolling to the
right. Even with full left cyclic applied, the main rotor
thrust vector and its moment follows the aircraft as it
continues rolling to the right. Quickly applying down
collective is the most effective way to stop dynamic
rollover from developing. Dynamic rollover can occur
in both skid and wheel equipped helicopters, and all
types of rotor systems.
CRITICAL CONDITIONS
Certain conditions reduce the critical rollover angle,
thus increasing the possibility for dynamic rollover and
reducing the chance for recovery. The rate of rolling
motion is also a consideration, because as the roll rate
increases, the critical rollover angle at which recovery
is still possible, is reduced. Other critical conditions
include operating at high gross weights with thrust (lift)
approximately equal to the weight.
Refer to figure 11-6. The following conditions are
most critical for helicopters with counter-clockwise
rotor rotation:
1. right side skid/wheel down, since translating tendency
adds to the rollover force.
2. right lateral center of gravity.
Tail Rotor Thrust
Main
Rotor
Thrust
Tip Path Plane Neutral Cyclic
Tip Path Plane Full Left Cyclic
Crosswind
3. crosswinds from the left.
4. left yaw inputs.
For helicopters with clockwise rotor rotation, the opposite
would be true.
CYCLIC TRIM
When maneuvering with one skid or wheel on the
ground, care must be taken to keep the helicopter cyclic
control properly trimmed. For example, if a slow takeoff
is attempted and the cyclic is not positioned and
trimmed to account for translating tendency, the critical
recovery angle may be exceeded in less than two seconds.
Control can be maintained if you maintain proper
cyclic position and trim, and not allow the helicopter’s
roll and pitch rates to become too great. You should fly
your helicopter into the air smoothly while keeping
movements of pitch, roll, and yaw small, and not allow
any untrimmed cyclic pressures.
NORMAL TAKEOFFS AND LANDINGS
Dynamic rollover is possible even during normal takeoffs
and landings on relative level ground, if one wheel
or skid is on the ground and thrust (lift) is approximately
equal to the weight of the helicopter. If the
takeoff or landing is not performed properly, a roll rate
could develop around the wheel or skid that is on the
ground. When taking off or landing, perform the
maneuver smoothly and trim the cyclic so that no pitch
or roll movement rates build up, especially the roll rate.
If the bank angle starts to increase to an angle of
approximately 5 to 8°, and full corrective cyclic does
not reduce the angle, the collective should be reduced
to diminish the unstable rolling condition.
SLOPE TAKEOFFS AND LANDINGS
During slope operations, excessive application of cyclic
control into the slope, together with excessive collective
pitch control, can result in the downslope skid rising
sufficiently to exceed lateral cyclic control limits, and an
upslope rolling motion can occur. [Figure 11-7]
Tail Rotor Thrust
Full Opposite Cyclic Limit
to Prevent Rolling Motion
Pivot Point
Bank Angle
Area of
Critical Rollover
Slope
Weight
Horizontal
Figure 11-6. Forces acting on a helicopter with right skid on
the ground.
Figure 11-7. Upslope rolling motion.
11-8