Dynamic rollover critical conditions: The forces acting on a helicopter with counterclockwise rotor rotation, and the right skid on the ground. The critical rollover angle is 5°-8°. Once exceeded, main rotor thrust continues the roll, and recovery via cyclic control is impossible.[1]
A helicopter is susceptible to a rolling tendency,
called dynamic rollover, when close to the ground,
especially when taking off or landing.
For dynamic rollover to occur, some factor has to first
cause the helicopter to roll or pivot around a skid, or
landing gear wheel, until its critical rollover angle is
reached. Then, beyond this point, main rotor thrust continues
the roll and recovery is impossible. If the critical
rollover angle is exceeded, the helicopter rolls on its
side regardless of the cyclic control corrections made.[2][3]
Dynamic rollover begins when the helicopter starts to
pivot around its skid or wheel. This can occur for a
variety of reasons, including the failure to remove a
tiedown or skid securing device, or if the skid or wheel
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 the pilot does 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
the pilot does not use the proper corrective technique.
Once started, dynamic rollover cannot be stopped by
application of opposite cyclic control alone. For example, if
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.
^Helicopter Flying Handbook, FAA-H-8083-21A(PDF). U.S. Dept. of Transportation, FAA, Flight Standards Service. 2012. pp. 11-8–11-12, 11-17–11-20.
^Dynamic Rollover - Dynamic Flight.com
^Helicopter Dynamic Rollover Archived 2009-12-29 at the Wayback Machine - Naval Postgraduate School - School of Aviation Safety
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