LET’S GET STARTED
;e first thing to do is to check the fit of the prop
to the engine. ;e hole in the prop hub should be a
precise fit with little to no slop or play. If the hole is
too big, you can add a couple of layers of tape to the
prop shaft until the propeller fits nicely. If the hole
is too small, then you should use a prop reamer to
precisely enlarge the hole diameter. With the O.S.
. 25 FX engine used in this article, I used a stepped
metric reamer. Reamers are available at most hobby
shops as well as online.
When using a reamer, hold it square to the prop
hub and gently push it while twisting it counterclockwise between a quarter and a half turn at a time.
Once you have reamed the full length of the hole,
back it out while still twisting it in the same direction.
Now place it on the balancer. With the Du-Bro
prop balancer, there is mandrel with a solid stop and
a movable spring-loaded stop. All you have to do is
remove the stop and spring, slide the propeller onto
the mandrel, and replace the stop and spring along
with the washer and the piece of silicone tubing
that keeps the spring compressed against the stop.
But before we can balance the propeller, you need
to make sure the mandrel is level. You want to set
up the side supports so that the propeller can turn
360 degrees without touching the base. Adjust the
balancer’s height with the four adjustment screws
(two on each side) until the prop is free to rotate,
then measure the ends of the mandrel, making sure
they are the same height above your workbench. If
the mandrel is not level, it will be di;cult for you to
balance the prop precisely.
BY THE NUMBERS
. Place the prop and mandrel on the
balancer to see which prop blade drops to the
bottom. ;is is the heavy prop and you will have
to remove some material from the blade using the
sanding bar. You can remove some material directly
Before balancing your propeller, make
sure it fits precisely on the engine’s
prop shaft. If the hole in the prop
hub is too small, use the proper-size
reamer to enlarge the hole.
Before placing the propeller on the
balancer, make sure the mandrel
is level by measuring the distance
from the workbench to each end of
the mandrel. ;ey should be the
placed on the
Sand the heavy
blade, and check the
from the very end of the tip, but you should not
remove more than 1/16 inch. If this doesn’t bring
the prop into balance, more work needs
to be done.
Step 2 Flip over the prop and the
mandrel so that the prop is facing in the
opposite direction. See if the same prop
blade drops to the bottom. If it does,
then you need to remove material from
the front and back sides of the blade. It
doesn’t take very much: Make three or
four swipes with the sanding bar, clean
o; the sawdust, and replace the prop
on the balancer; keep doing this until
the propeller sits level on the balancer.
Once it does, flip it over again so that it
is facing the opposite direction and see if
it still sits level. If it doesn’t, then the hub
is slightly out of balance.