Chapter 5 – General Operation
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5.8.3 Order Analysis
Order analysis is a way to observe vibrations harmonically
related to a primary frequency. For example, a vibrating string
(as on a violin or guitar) has a primary frequency most
associated with the sound and a host of harmonics.
These harmonics are mathematically related in frequency to the
primary tone and give the string its tone quality. The following
list provides important facts to keep in mind while working with
order analysis:
A rotor on a helicopter, for example, has a primary
frequency, the RPM, and its harmonics are determined by
the number of blades and multiples of the number of blades.
For example, a two-bladed rotor system turning 300 RPM
has a first order (fundamental) of 300 RPM, a second order
at 600 RPM, a fourth order at 1200 RPM, and so on.
Reciprocating four-cycle engines have harmonics in one-half
order increments, beginning with the one-half order (half the
engine speed) and continuing up through approximately
eight times the crank shaft speed.
Each engine type has a distinctive spectral signature, and
each different engine of a particular type has a unique
signature of its own.
Engine characteristics that determine spectral signature are
the evenness of combustion, intake losses, ignition timing,
internal component balance, the type of propeller installed,
and the condition of the internal dynamic counterweights (if
so equipped).
To view a list of the orders in a signature while using the Vibrex
unit, press the context key next to the <RPM PEAK IPS> line
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