HELICOPTER AERODYNAMICS WORKBOOK
Application of collective pitch is more effective than lateral cyclic in controlling the rolling
motion because it changes main rotor thrust. A smooth, moderate collective reduction may be
the most effective way to stop a rolling motion. Collective must not be reduced so fast as to
cause the rotor blades to flap excessively and impact the fuselage or ground. Also, an excessive
collective reduction rate may create a high roll rate in the opposite direction.
A sudden increase of collective pitch in an attempt to become airborne may be ineffective in
stopping dynamic rollover. If the skid acting as a pivot point, does not break free of the ground
as collective is increased, the rollover tendency will become more likely. If the skid does break
free, a rolling motion in the opposite direction may occur as the mechanical axis attempts to
align itself with the virtual axis.
When performing maneuvers with one skid in contact with the ground, like slope takeoffs
and landings, care must be taken to keep the helicopter trimmed laterally. Control can be
maintained if the pilot does not allow lateral roll rates to accelerate, and if the pilot keeps the
bank angle from exceeding the critical rollover angle. The pilot must fly the aircraft into the air
smoothly with gradual changes and corrections in pitch, roll, and yaw.
The mechanical design of the semi-rigid rotor system dictates downward flapping of the
blades must have some physical limit. Mast bumping is the result of excessive rotor flapping.
Each rotor system design has a maximum flapping angle. If flapping exceeds the design value,
the static stop will contact the mast. It is the violent contact between the static stop and the mast
during flight that causes mast damage or separation. This contact must be avoided at all costs.
Mast bumping is directly related to how much the blade system flaps. In straight and level
flight, blade flapping is minimal, perhaps 2° under usual flight conditions. Flapping angles
increase slightly with high forward speeds, at low rotor rpm, at high-density altitudes at high
gross weights, and when encountering turbulence. Maneuvering the aircraft in a sideslip or
during low-speed flight at extreme CG positions can induce larger flapping angles.
The causes of mast bumping can be divided into most influential and less influential causes.
The most influential causes of mast bumping are as follows:
1. Low G maneuvers.
2. Rapid, large cyclic motion (especially forward).
3. Flight near longitudinal/lateral CG limits.
4. High-slope landings.
Less influential causes include sideward/rearward flight, sideslip, and blade stall.
Excessive flapping is most probable when pilots allow the aircraft to approach low G
conditions. Common maneuvers leading to low G flight include crossing a ridgeline during
high- speed terrain flight, masking and unmasking, acquiring or staying on a target, and recovery
5-10 FLIGHT PHENOMENA