"Morgans" wrote in message
...

"Bill Daniels" bildan@comcast-dot-net wrote

Suppose the prop shaft is to be just long enough for the gear belt pulley
and the neccessary bearings - say 10 inches. But the engine flywheel
pulley is to be 4 - 6 feet below the prop shaft. The idea is to use a
very large multi-blade carbon fiber prop turning 800 - 1000 RPM driven by
a 4 cyl Soob turning at best power RPM. The idea is to get best thrust
in the 0 - 60 knot range. The airframe configuration is a prop over
tail boom pusher - an ultralight on steroids. (BTW, I'm not looking for
a long engine life under these conditions. I'll treat the Soob as a
throwaway power plant.)

Strange, but I am considering a system, very much like that. Flying
boat, with the engine in the fuselage, instead of on a pylon.

I'm thinking there isn't too much torsional vibration concern with very
short shafts, high reving engine and a stiff carbon fiber prop. The prop
will be seeing 6 - 7 power pulses per rev from the high reving Soob.

On that, I would agree. I believe you will have a new problem, though.

I believe that you will have a problem with the belt vibrating, like a
guitar string. At some speeds, it will get to the same resonance of the
belt, and slap like the devil. I have a 20" band saw that does this every
time, as it slows down, right before it stops.

One way to deal with this is to make the string (belt) shorter, with some
idler pullies. At least that is my plan. It would be a good idea to
make the distances between the plies a little different, or it will have a
strong tendency to vibrate as a complete string, at a higher fundamental
(lowest) frequency.

It is the same (in a way) as muffler design. The volume in the different
chambers is a little different, so when the frequency is resonating in one
chamber, it will not be resonating in the other chamber, thus more
frequencies are muffled.
--
Jim in NC

A low RPM high thrust prop on a high thrust line would be ideal for a
seaplane.

Actually this is not too different than some motorgliders with the engine
buried in the fuselage and the prop on a retractable pylon. The tooth belts
have only one or two idler pulleys. I figgured on at least two idlers to
maintain belt tension and to damp belt vibrations.

I want the engine in an external conformal pod below the fuselage for
accessability and to locate it below the pilot for survivability reasons.
I'd want the whole engine/cooling system in this pod so it could be removed
as a unit. The prop and drive belt would remain with the airframe.

Bill D