Powerfin Inc.
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Trimming and Sealing the Tips of a
Powerfin Propeller
APPLY THE FOLLOWING PROCEDURE AT YOUR OWN RISK
Powerfin Inc. propellers are formed with a foam core
surrounded by a “shell” of carbon fiber and glass fabric. When a blade is cut to
a shorter length by the customer, the foam core is exposed at the tip and needs
to be properly sealed to safely operate the propeller and provide the
durability of a blade produced at the factory. The result of failure to
properly seal the tip after trimming will be premature wear and the possibility
of delamination and catastrophic failure of the blade. For this reason Powerfin
Inc. strongly recommends that the factory performs these operations. It is
expected, however, that some individuals will attempt field repairs despite the
recommendations of the manufacturer. In the interest of safety, a guide for
this procedure is provided.
Please recognize that any modification of a Powerfin Inc.
propeller voids any warranty offered by either the manufacturer or any dealer
representing the manufacturer. Please note also that no dealer of Powerfin Inc.
propellers is authorized by the manufacturer to make any type of repair.
It is essential to understand how to balance a propeller
before beginning the procedure of trimming and sealing the blades. Chances are
good if these directions are followed closely, the propeller will remain in
balance. That result cannot, however, be guaranteed.
APPLY THE FOLLOWING PROCEDURE AT YOUR OWN RISK
Trimming the Blade to the Desired Length
The root of the blade has a molded lip, or flange which
retains it in the hub against the load of centrifugal force. The blade must be
jigged and trimmed in such a way as to ensure that the distance between this
molded flange and the tip is exactly the same on every blade.
The blades can be trimmed with a chop saw using a
composite blade or any other blade designed for cutting metal, or by using a
standard band saw with a metal cutting blade. A milling machine can also be
used that has a fairly high spindle rpm (3000 or higher). A small diameter, two
flute, carbide endmill should be used (3/16" or smaller).
With any method of cutting an attempt should be made to
minimize heat buildup. No coolant or lubricant of any kind may be used with any
cutting process.
When trimming the blades, remember to keep the cuttings.
They will be useful to practice on in the next operation.
Recessing the Foam in the Tips
Cutting the blade will expose the foam core which must be
sealed. Before epoxy is used to seal the tip the foam must be recessed no less
than 1/8" in order to provide the epoxy a solid edge with which to adhere.
Using a Dremel tool or other high speed rotary tool, rout out the foam until it
is recessed into the tip 1/8". A simple cutting bit for the Dremel can be
fabricated with a wire brad or nail, bending it over 90 degrees, and clipping
it to 1/8". The result is an "L" shaped piece of wire with the
long leg chucked up in the Dremel and the 1/8" leg rotating, doing the
cutting. With the Dremel angled at 90 degrees relative to the tip, a very
consistent depth of 1/8" can be achieved with a little practice. Safety
glasses are prudent with the use of any high speed tool, especially one with a
self-fabricated cutting bit.
The goal with this operation is to clear the foam away
from the inside of the carbon fiber shell, making the walls of the recess as
clean as possible, creating a consistently wide 1/8" edge, and providing a
freshly scuffed, clean surface for bonding. The reason you saved the tip
cuttings from the previous step is so you can practice recessing the foam with
the Dremel tool, perfecting your technique before you risk the attempt on the
blade itself.
Filling the Tips
The blades must be stabilized to the edge of a table top
somehow before filling the tips with resin. A good quality, room temperature
curing, two part, unfilled epoxy must be used. The epoxy must have low
viscosity and be able to easily pour. Fill the cavity in each tip until level
or slightly rounded. It is best to avoid the use of 5 minute epoxies for one
particular reason. As you fill the cavity, the foam may allow a bit of the
epoxy to seep into it. This can take more than 5 minutes, precluding the full
process from occurring before the epoxy hardens, resulting in a less than
optimal bond. On a separate point, if there is enough seepage of the epoxy into
the foam, the level of epoxy will drop, causing a bit of a depression. You
should be ready to fill the tip with more material if this occurs. Follow the
curing instructions provided by the manufacturer very carefully.
Dressing the Tips
When the epoxy is hard enough to sand, a small, palm
sized sanding block with 240 grit sandpaper can be used to dress the tips. The
only objective in this operation is to sand away rough edges without over
sanding. In this section it must be stated what NOT to do rather than what TO
do. Do NOT sand such that the epoxy at the tip is less than 1/8" when
finished. Do NOT attempt to create an exaggerate bevel, chamfer, or radius
around the edge. Do NOT attempt to radius the corners of the tip to any
significant degree.
Once the basic work is done, switch to 400
grit, then 600 grit sandpaper. Remember, smoother is better from both a
structural and aerodynamic standpoint.
Lastly, make sure to wait the entire recommended cure
cycle before attempting to use the propeller. As an example, a 1 oz. weight on
the end of a 36" string spun at 2500 rpm will multiply that weight by a
factor of more 8000, creating a centrifugal load of more than 500 lbs. The
small epoxy patch at the tip of the blade weighs only a few grams but this
example hopefully illustrates how easily the repair is ruined if the propeller
is run prematurely.
Powerfin Inc.
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