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reduce harmonic vibrations; this method
removes much of the vibration buildup.
Mounting high-level vibration parts such as
drive shafting on shock-absorbent mounts.
Installing vibration absorbers in high-level
vibration areas of the airframe.
Lateral
Lateral vibrations are evident in side-to-side swinging
rhythms. An out-of-balance rotor blade causes
this type of vibration. Lateral vibrations in helicopter
rotor systems are quite common.
Vertical
Vertical vibrations are evident in up-and-down movement
that produces a thumping effect. An out-oftrack
rotor blade causes this type vibration.
High-Frequency
High-frequency vibrations are evident in buzzing and
a numbing effect on the feet and fingers of crew
members. High-frequency vibrations are caused by
an out-of-balance condition or a high-speed, moving
part that has been torqued incorrectly. The balancing
of high-speed parts is very important. Any buildup
of dirt, grease, or fluid on or inside such a part (drive
shafting for example) causes a high-frequency vibration.
This type vibration is more dangerous than a lateral
or vertical one because it causes crystallization of
metal, which weakens it. This vibration must be corrected
before the equipment can be operated.
Ground Resonance
Ground resonance is the most dangerous and
destructive of the vibrations discussed here. Ground
resonance can destroy a helicopter in a matter of
seconds. It is present in helicopters with articulated
rotor heads. Ground resonance occurs while the
helicopter is on the ground with rotors turning it will
not happen in flight. Ground resonance results when
unbalanced forces in the rotor system cause the
helicopter to rock on the landing gear at or near its
natural frequency. Correcting this problem is difficult
because the natural frequency of the helicopter
changes as lift is applied to the rotors. With all parts
working properly, the design of the helicopter landing
gear, shock struts, and rotor blade lag dampeners
will prevent the resonance building up to dangerous
levels. Improper adjustment of the landing gear
shock struts, incorrect tire pressure, and defective
rotor blade lag dampeners may cause ground
resonance. The quickest way to remove ground
resonance is to hover the helicopter clear of the
ground.
NONDESTRUCTIVE INSPECTION
Nondestructive inspection (NDI) methods determine
integrity, composition, physical/electrical/thermal
properties, and dimensions without causing a change
in any of these characteristics in the item being inspected.
NDI includes —
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•
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Liquid penetrant methods.
Magnetic particle methods.
Electromagnetic methods.
Ultrasonic methods.
Penetrating radiation.
Harmonic bond testing.
NDI in the hands of a trained and experienced technician
is capable of detecting flaws or defects with a
high degree of accuracy and reliability. Maintenance
engineering personnel should know the capabilities
of each method. Equally important, they should
recognize the limitations of each method. NDI is not
a panacea for inspection ills – it is merely a means of
extending the human senses. No NDI method should
ever be considered conclusive. A defect indicated by
one method must be confirmed by some other
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FM 1-514
method to be reliable. Further, NDI equipment is example, ultrasonic inspection equipment is fully
highly sensitive and capable of detecting discon- capable of detecting normal grain boundaries in
tinuities and anomalies which may be of no conse- some cast alloys. Inspection criteria must be
quence to the particular service a component is used designed to overlook these “normal” returns and to
for. Limits for acceptance and rejection are thus as discriminate in favor of those discontinuities that will
much apart of an inspection as the method itself. For affect the component in service.
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FM 1-514
CHAPTER 2
FUNDAMENTALS OF ROTORS
Of all airfoils the rotor blade on a helicopter is
unique. Like most airfoils it provides lift, but it also
provides thrust and directional control. The rotor
system produces the lift, thrust, and directional control
needed for helicopter flight.
ROTOR SYSTEM
The rotor system includes a rotor head, rotor blades,
and control systems that drive and control the pitch
angles of the blade. The rotor head is the main
assembly of the rotor system; it contains the rotor
hub, blade attachment fittings, and blade controlling
　
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