Propeller IMBALANCING

To All:

Recent posts have covered several aspects of propeller balancing,
largely for the purpose of convincing those who have never carved a
prop to give it a try. Posts have dealt on the benefits of using as
many laminations as PRACTICAL, so as to ensure adequate stiffness in
the finished prop. Maple laminae a quarter of an inch thick is the
goal to shoot for but few amateurs are lucky enough to have access to
such material. In fact, most amateurs are stuck with using whatever
is available from the local Borg -- typically birch in 3/4" thickness.

In making up the blank, recent messages have stressed the importance
of applying glue to both surfaces in quantity sufficient to result in
a massive squeeze-out. This offers visible evidence the particular
joint will have enough glue to eliminate any possibility of a 'dry' or
glue-deficient joint.

The next step will be explaining how the propeller's pattern is laid-
out upon the blank, including however many stations the builder
chooses to use. As with the number of laminae, when it comes to
stations, more is better. Thanks to the universal availability of
computers and the low cost of a homebuilder's time, the the use of ten
or more stations is encouraged and the next series of messages will
address this issue. The ubiquitous nature of computers makes it
practical to give the amateur builder nothing more than the equations
used to produce the airfoils of each station, allowing the computer to
do the required laying-out of the airfoils. In a similar vein, a
pattern will be provided for the laying-out of the propeller's
center. This pattern is to be glued to the blank, making it difficult
to err when it comes time to drill-out the hub recess, center hole and
bolt holes, after which the propeller's profile may be cut out as well
as the initial angles at each station.

The result of the above is a rough blank that requires only smoothing
to produce a usable propeller. The airfoil patterns printed out
during an earlier step are used to ensure each station has the proper
curvature.

However, I do not believe we have covered a particular balancing
problem in sufficient detail. That problem occurs when the profile of
the blank is cut-out inaccurately, leaving one blade longer than
another. The amount of the error is typically small and typically a
percentage of the average station width. That is, the more stations
you have, the better your chances of making a SMALL error.

The reason this is worthy of mention has to do with the performance of
the finished propeller, in that the shorter blade, while equal to the
longer in MASS and therefore is found to be in BALANCE, will NOT be
equal to the longer blade in terms of THRUST. This is because the
outer-most third of the propeller blade produces more than 50% of the
blade's total thrust. The upshot is that your beautifully finished
propeller will produce less thrust than it should, and the thrust it
does produce will be asymmetric leading to a host of problems the cure
of which calls for reducing the length of the longer blade so as to
match the other, altering the profile so as to match the other, and
then rebalancing.

Bottom Line: Be careful to keep the blade-length EQUAL throughout the
carving & finishing.

-R.S.Hoover