This is not directly related to rotary engines, except that it's
         connected to one.



         I just received my prop from Catto, it's a beaut, but I have a
    problem.
         The backing plate, where the prop contacts the prop hub, is
aluminum. My
         spinner and it's rear bulkhead is made from carbon fiber. Aluminum
in
         direct contact with carbon fiber is a no-no. I was thinking of
applying
         RTV between the bulkhead and prop just before I install it and
torque
         the bolts down. This would theoretically insulate the aluminum
backing
         plate on the prop from the carbon fiber spinner bulkhead, but I
worry
         that it would mess up the friction between the prop and the spinner
         bulhead. Any suggestions about how to approach the problem of
insulating
         the these two components from each other in such a way as to not
do
         something bone- headed?



         Brian Trubee


         The usually way is use a thin layer of fiber glass on the carbon
fiber to
         electrically insulate
         the aluminum to the carbon fiber interface. I also think titanium
does
    more or
         less
         the same thing. Paint the bolts with a thin layer of epoxy paint.

         The spinner itself has very little torque on it so friction is not
a
    problem
         on the outer spinner bulkhead.

         It is a problem if the aft spinner bulkhead is between the prop and
the
    PSRU
         flange.
         In that case I would make an aluminum spinner bulkhead or contact
Catto
    for
         their
         recommendations.

         --
         Paul Lamar ...No rotor no motor.

    Ah, very good about having an aluminum rear spinner bulkhead, BUT, the
    spinner itself is carbon fiber. I think the best solution is to bond a
    thin layer of fibergalss to each side of the bulkhead spinner . Please,
    if anyone has inputs, please speak up.



    Thanks,

    Brian Trubee


    So don't believe me :). Look it up.

    The problem with that is the compressive strength of the epoxy composite
is not
    as high as aluminum and your prop may slip back and fourth. The pulsing
torque
    is transmitted by the friction between the parts and not the bolts in
shear.
    Wood props actually char under these kinds of loads.

    The compressive strength perpendicular to the fiber direction is
essentially

    that of the epoxy resin.

    Paul Lamar ...No rotor no motor.



 Yes, that was one of the concerns- that there is friction at the
 interface between the bulkhead spinner, prop and hub. So, how about this
 solution- an aluminum rear spinner bulkhead, with some sort of
 insulating material (RTV?) between the bulkhead and the spinner? Or, a
 layer of glass on the inside of the spinner where it meets the spinner
 bulkhead?



 Or conversely, does anyone want to purchase a carbon fiber spinner (16
 inch diameter by 19 long, parabolic in cross section) for $250 and I go
 with a spun aluminum spinner? I have heard of one person down in
 Portland who does aluminum spinning.



 Brian Trubee

 The friction is essential to driving the prop with out slipping back and
 fourth each time the engine pulses. Of course the rotary is nowhere near as
bad
 as a four cylinder Lycoming. The Lycoming tries to back up twice per rev.

 I think RTV is the wrong thing to do.


 Paul Lamar

(At least for some applications) I recall Brian Alley (composite guru,
pro Lancair builder) says that anodizing provides adequate galvanic
protection for certain applications..  Is the aluminum anodized?
Probably not..  I'd add one layup of thin fiberglass to the side of
the spinner backing plate that contacts the prop's drive side.  It
doesn't take much to keep the carbon fibers from making direct contact
with the aluminum.

Get some of the very light weight, fine weave fiberglass cloth from
the hobby store.  It's relatively easy to work with.  And some slow
cure resin/hardner.  Have you worked with fiberglass much?

I imagine that your setup uses drive lugs in the output flange?  I
think the dependence friction to transmit the torque loads is lower
than when just mating the prop to a flat flange.  The diameter of the
drive lugs can be much larger than the bolts, and so can handle the
shear loads much better.

RTV would not be my choice.  I suspect it would eventually cause you
headaches with prop tracking - especially after being removed and
installed a few times - the RTV will wad-up.  It's not structural
(soft).


Regards,

Matt-


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