Re: REFLECTOR: Propellers and prop extensions

["James F. Agnew" <Jim_Agnew@ibm.net>]

The problem applies to the Franklin as well when it is used with prop
extensions and a WOOD propeller.  This is an issue of delivering the
power to the propeller and the size of the propeller drive crush
plates.  Torsional elasticity enters into this problem.

As far as the IVO goes, I have yet to see any  test data on torsional
vibration testing, blade harmonics, etc. on any engine.  The fact that
an engine has 6 cylinders does not make anything OK.  Each different
4-6-8 cylinder (e.g., Continental) engine can have vastly different
torsional vibration characteristics due to differences in the elasticity
of the drive train.  Even the same engine with an upgraded drive
component (e.g., a crankshaft made of different alloy) could have
different characteristics.  Remember those yellow arcs on the tachs of
some planes with a particular type of prop?

Case in point, Caterpillar Tractor had a bunch of 200 ton dump trucks
that started breaking 36" in diameter drive axles when the owners
changed from the factory supplied tires to radial tires.  It was found
that the new tires changed the torsional vibration characteristics of
the axles and was causing 6 million ft. lb. torque pulses in the
direction opposite to the drive direction!!  Put the right engine prop
combination together and the same thing could happen.  Strangely, the
failure point may not be directly in the vibration producing component,
the Cat D-10 early on kept breaking the frame under the right motor
mount, the fix was to drill a hole in the left rear of the frame and
move the track tread closer together.  The moral of this is story is
look at anything vibrating (e.g., some instrument in your panel) and
find out why or dampen it to stop the vibration.

I posted this information as food for thought for those that are
planning to use wooden props and/or prop extensions.  However,  the
problem is the same with any propeller engine combination and can only
be proven by careful testing or computer simulation.

It is in the best interest of all of us to ask any prop/engine
manufacturer about their combination testing and recommendations.  If
there is no test data or a long history of satisfactory operation then
you are truly experimenting in your experimental.

Jim

"Brian K. Michalk" wrote:

> Excellent info.
>
> At least with the Franklin's six cylinders, this isn't as much of
> a problem.  I think this is why IVO/Franklin combinations seem
> to work.
>
> --
> Brian Michalk  <http://www.awpi.com/michalk>
> Life is what you make of it ... never wish you had done something.
> Aviator, experimental aircraft builder, motorcyclist, SCUBA diver
> musician, home-brewer, entrepenuer and SINGLE!

--
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// James F. Agnew
// Tampa, FL
// Velocity 173 FG Elite ( http://www.VelocityAircraft.com/ ) under
construction