Chapter 7. Propeller Balancing
7.1. Minimising other sources of vibration
Propeller mass imbalance can be a major source of vibration. However, there
are other sources of vibration as well. To minimise the overall vibration level
and to make the dynamic balancing process more effective, all other sources
of vibration must be minimised before dynamic balancing is carried out.
Important
Unless the engine is running smoothly, there is little point in trying
to balance the propeller. Carburettor imbalance, dirty plugs, loose
engine mounts and general wear and tear are just some of the
reasons why the engine could be producing excess vibration.
Propellers with an adjustable blade pitch will produce a lot of vibration if all
the blades are not set to the same pitch. This is critical: if a blade's pitch differs
from its neighbours' by even a fraction of a degree, it will produce vibration
that appears to be caused by mass imbalance but cannot actually be removed
by mass balancing.
Important
Before attempting to dynamically balance a variable pitch propeller
(either ground adjustable or in-flight adjustable), confirm that the
blades' pitch are equal to within the tolerance specified by the
propeller's manufacturer (typically, 0.25°).
7.2. Propeller mass imbalance
When an object rotates around an axis, if the mass of the object is not
uniformly distributed around that axis, a force (the centripetal force) will be
generated and will cause vibration. As the magnitude of the force is
proportional to the square of the rotational velocity, at high RPMs (high
rotational velocity) even a small mass imbalance in a propeller will generate
an appreciable amount of force (and hence vibration). This vibration can be
measured by mounting a sensor on the engine as close to the propeller as
possible. Conventionally, the magnitude of a propeller's vibration is reported
as a peak velocity in units of Inches Per Second (IPS).
7.3. Static propeller balancing
A propeller can be statically balanced in the workshop using a static balancing
tool. This often involves suspending the propeller from its central axis. If the
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