Ramon Collado. wrote:
Hello Paul, When making a carbon fiber propeller, do you know the type of
fiber/resin to use and how many layer would it require for this job???
And when considering the right length for a prop, what would you suggest
???? My airplane will weight around 1800 gross with a 13B and probably a
Ross 3:1 prsu.....Thanks
Ramon e-mail rcollado@sprint.ca
The length is more a function of the landing gear length.
The longer the prop the more static thrust\efficiency more or less.
A peripheral port 13B with a 3:1 PSRU could run up to 9000 RPM
with a prop of around 65 inches in diameter and 3000 RPM.
The rule of thumb is keep the tip velocity below .8 mach.
500 MPH or so.
Use bid, high density foam and epoxy. Vary the angle of the plies.
The steel hub is available in the hardware store as a conduit coupling.
Machine the outside with shallow sine wave grooves.
The thing about carbon fiber is it has such an excellent strength
to weight ratio that it is OK to over design with it. In other
words it is hard to make the prop too heavy. To be conservative
and, as I recall each layer is about 0.010 thick, I would use at least
five layers on each side of the prop. Out near the ends it will
be solid carbon fiber.
If you are using my design of steel hub
you want to wrap one thin layer of glass over the hub before you add
the carbon fiber. This electrically isolates the steel from
the carbon fiber (a conductor). If you don't do this you might
get a weird corrosion effect.
If you are using my design put a plastic film over the hub
area after you finish laying up
one side and wrap rubber bands around the steel hub area
to force the carbon fiber to conform to the sine wave grooves.
The last layer should be glass fiber on both sides of the blade.
Remove the rubber bands and the film and lightly sand the area before
you lay up the other side. When you are done laying up the second
side wrap unidirectional strands of glass around the steel hub
to a thickness of about 1/8 of an inch. That should lock the
carbon fiber into the sine wave grooves of the steel hub.
Make up a test blade to get a feel for it. Workmanship
has a lot to do with the quality of the final product.
You can tensile test the blade by attaching a weight to the end
that is twenty to fifty times the weight of the blade.
Screw something into the steel hub and hang the blade and
weight vertically.
I forgot what the FAA calls out for this but I would follow their
recommendation to the letter. I think they test it by spinning it
at twice the expected RPM or something like that. Its in the FAR's.
You can also test the blade in bending but surprisingly that is
not all that important. Prop blades tend to "helicoptor" by
staying in the plane of rotation due to centrifugal force.
If this was not the case the IVO prop would never work.
The third test is one of torsion. Attach a lever to the end
of the prop and apply a torque to the blade. Increase the
torque, record it and measure the twist of the blade. This is
probably the most severe test you can apply to a prop
blade. If it fails (and it will eventually) note the point
of failure and beef that area up with more layers of
carbon fiber. It will probably fail this test along the
leading or trailing edge joint.
Compare that to an old scrap wood prop blade.
This is very experimental design. It has never been tested
on an airplane. Take a lot of pictures while building it
and please send them to me.
Paul Lamar
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