Mast Bumping - Cont'd

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