Testcell instrumentation usually includes temperature and
pressure gages to monitor engine performance. The engine is run in
the test cell with the same demands placed upon it as if it were
installed in an aircraft. The performance of any engine is
considerably influenced by changes in ambient temperature and
pressure, because of the way these conditions affect the weight of
the air entering the engine. To compare the performance of similar
engines on different days, under different atmospheric conditions, a
given engine's performance must be corrected to the standard day
condition of 29.92 inches of mercury and 59 F.
During the initial run after assembly, or after extensive
maintenance or overhaul, engine statistics are recorded on a test
log. This log sheet remains with the engine historical records until
such time as another data sheet is completed.
During testing, any problem that would limit the engine's
maximum speed, is corrected. In addition, oil temperature, bearing
scavengeoil temperature, seal leakage, and oil consumption must be
within established limits. These tests are usually performed under
other than standard day conditions, and data will then be computed to
a standard day rating by using the charts and tables in the engine
manual. This new information is entered on the engine test log sheet
as shown in figure 3.2 and becomes a permanent part of the engine
records.
3.3.
VIBRATION EQUIPMENT
Highfrequency vibrations must be detected and eliminated
because they can cause mechanical failure and extensive engine
damage. This paragraph discusses the cause of vibrations and the
equipment to analyze vibrations. The main source of vibration in the
gas turbine engine is the imbalance of rotating parts. Imbalance is
caused by an uneven distribution of weight and is measured in inch
grams or inchounces. An inchgram is one gram of unbalanced weight
concentrated one inch from the center of a rotating part. When an
unbalanced part is rotated, a force is generated. This force is a
product of the amount of imbalance and rotating speed.
To analyze this force, a vibration transducer is used; this is
a miniature generator. When attached to a vibrating object it
generates an electrical signal that is proportional to the force
being analyzed. The signal is sent to a meter that amplifies the
signal so it can be conveniently read. The meter has four input
channels that independently accommodate a signal from a transducer.
The meter,
69
Previous Page
