Vance and Emily Jaqua wrote:

In your thoughts about output shaft
size have you considred the design point
 comparison to a NASCAR front hub - surely the
loads are higher and the the RPM pretty close.
I would be pretty sure that altho they are
far from stock, I doubt that they are 2 inch ?
Vance

They only need to last two hours :-)

The loads are many. In a loop the gyro loads are like a
overhung belt or chain drive. Each time the shaft rotates
the stress is reversed on the top and bottom surface of the
shaft. Turbulence can have somewhat the same effect.
Classic case of the type of oscillating load that causes 
fatigue failure.

Another is due to the torque pulses from the engine.
One cannot completely rely on the rubber bushings or 
springs in a clutch disk to smooth these torque pulses
out completely. Making the shaft small in the hopes the
torsional resonant system will be below the forcing 
frequency is very optimistic indeed as one cannot always
predict what prop will be used. Obviously there is a lower
limit to the shaft diameter. Nobody in their right mind
would make it only 3/4 of inch in diameter for example.
The attached chart from Taylor's books on the RR Merlin system 
illustrates this can of worm quite well.

A third force is the one of thrust. In a pusher it might even
help shaft life. 

The fourth is the steady average torque on the shaft.

A fifth is an imbalance in the thrust from each blade
causing a steady bending moment on one point of the shaft. 

Have I forgot anything?

The combination of all these loads is mind boggling.

IMHO it is good to make it big, stiff and overkill and let the
rubber bushings do all the torsional vibrating.
 
Paul Lamar
 
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