Velocimeters - Honeywell Vibrex 2000 User Manual

4.4.1
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
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Velocimeters

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Chapter 4 – Installation