50 Chadwick-Helmuth Company, Inc.
Chapter 4 - Installation
4.4.1 Velocimeters
In general, the user may use guidelines given in The Smooth Propeller to
determine correct Velocimeter placement with the Vibrex 2000 or
Vibrex 2000 Plus system. In addition, the user may use the following
general guidelines for making sure the Velocimeters are in the correct
locations:
• Mount the balance Velocimeter as near to the plane of rotation of the
propeller as practically possible.
• The check Velocimeter is mounted at the rear of the engine. A rear
Velocimeter is mounted when the mass of the crank shaft and
propeller extend across some longitudinal distance, such as are found
in most direct-drive reciprocating engines.
• It is not necessary to mount a check Velocimeter on geared engines or
radial engines. The purpose of the check Velocimeter is to ensure the
imbalance comes from differences in the mass of the propeller and
not the crank shaft. Failure to monitor the rear of the engine and
attempting to balance may deteriorate the apparent vibration rather
than decrease it, even though the level at the front of the engine goes
down.
• Normally, for a direct-drive engine, the front and rear of the engine
provide measurements of similar amounts of imbalance, but at
opposite clock angles (if both Velocimeters point the same direction).
As the front of the engine becomes displaced upward by the
imbalance the rear of the engine shows a downward displacement,
and so on.
• When an imbalance happens solely because of propeller mass
differences, as the front inches-per-second (IPS) vibration level goes
down, the rear IPS also becomes reduced to acceptable levels.
• If the balance IPS level goes down and the rear IPS remains the same
or is reduced, the effect of balancing is still correct for the engine. If
the balance IPS level goes down but the rear IPS increases, the
propeller does not need to be balanced. The cause of the imbalance is
probably the crank shaft or an asymmetric aerodynamic loading on
the propeller.
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