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HELICOPTER AERODYNAMICS WORKBOOK
CHAPTER 5
from a pullup. Each of these maneuvers has in common an application of forward cyclic and/or
a reduction of collective pitch which unloads thrust from the rotor head. Absence of main rotor
thrust makes lateral cyclic control inputs ineffective.
In normal flight, the rotor head is loaded and all forces are in balance. If abrupt forward
cyclic is applied, the main rotor is unloaded, significantly reducing thrust. The aircraft rolls
right, due to the thrust of the tail rotor, which produces a rolling moment above the longitudinal
axis of the helicopter. To counter this right roll, the pilot may apply left cyclic, causing
excessive lateral flapping and mast bumping.
How should the pilot recover from this situation? Smoothly apply aft cyclic to restore thrust
on the rotor head, then center the cyclic laterally. The pilot can resume normal inputs to bring
the aircraft to a level flight attitude.
Mast bumping can result from incorrect pilot reaction to engine failure. Let's begin with a
helicopter flying in normal cruise. The rotor disk and fuselage are tilted slightly forward.
Viewed from the rear, the rotor disk is tilted slightly toward the left to counter the right tail rotor
thrust. The aircraft roll axis is located slightly below the tail rotor thrust axis. All forces are
balanced.
As the engine fails, rotor rpm, altitude, and airspeed will start to decay. Because the engine
is no longer driving the main rotor, torque is diminishing. The tail rotor thrust produces a left
yaw and right roll. The left yaw exposes the right side of the fuselage, aggravating the yaw.
The pilot sees a new aircraft attitude--nose down and left yaw. The aircraft appears to be in a
roll to the right. Normal pilot reaction is to apply right pedal and left aft cyclic. The cyclic input
tilts the rotor disk left and aft, creating larger flapping angles and possible mast bumping. The
problem is the pilot reacted to the roll and not the engine failure. The correct response is to
lower collective to maintain Nr and right pedal to return the aircraft to balanced flight, then
maneuver the aircraft to a landing zone.
Another possible cause of mast bumping is tail rotor failure in forward flight. At the instant
of failure, antitorque thrust goes to zero, and the aircraft yaws right. The aircraft rolls left, due to
the left tilt of the main rotor system which counteracted the right thrust of the tail rotor above the
roll axis.
The pilot sees an abrupt right yaw and left roll and counters with right/aft cyclic and left
pedal. These inputs tilt the rotor disk toward the fuselage, dramatically increasing blade
flapping. Mast bumping becomes a strong possibility. Correct pilot reaction for this failure is
immediate reduction in power to reduce torque. This will reduce the yaw and allow time to
correct for the roll tendency.
The last possible causes of mast bumping we will look at are slope landings and takeoffs.
When a helicopter rests on a slope, the mast is perpendicular to the slope, while the rotor disk
remains parallel to "level" ground. Cyclic control stops, static stops, or mast bumping limits the
FLIGHT PHENOMENA 5-11


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