Coincidently 6000 RPM  is where the rotor bearing loads are a
minimum so that is a good cruise RPM. Bearing loads are higher
both above 6000 RPM and below 6000 RPM.

Could you explain this to a non-engineer?

R Berglund

Rotor bearing loads come from two sources.

A. Combustion pressures acting on the face of the rotor causing an
inward
force on the bearing.

B. Since the 9 pound rotor is rotating around the center of the
e-shaft there is a centrifugal outward force acting on the rotor
bearing.

At 6000 RPM the two forces cancel each other.
Below 6000 RPM the combustion forces dominate.
Above 6000 RPM the centrifugal force dominates.
At about 10,000 RPM the centrifugal force is so high
it is the limiting factor on the functioning of the
rotor bearing.

All this assumes a normally aspirated engine with a 9 pound
cast iron rotor. If you use a lighter titanium rotor this
RPM point rises as the centrifugal loads are reduced.

If you turbo charge the engine, increasing the combustion pressure
on the rotor, this  RPM point also rises but don't suddenly reduce
the boost or you will still fail the bearing at RPM's over 10,000.

This is the key to the extraordinary amount of power one
can obtain with boost levels over 100 inches of Hg in a Mazda
Wankel.


Paul Lamar

Can you give a approximate rpm for a twin tubo 20B to give some idae of
the reduction necessary for good criuse.

It is the same number regardless of the number of rotors.
The RPM point goes up as you boost the engine.
For practical reasons I would not cruise much over 9000 RPM.

There is another BSFC factor involved and that is engine friction
increases causing it to hurt the BSFC. Low friction ceramic apex seals reduce
engine friction so the best BSFC number with out ceramic apex seals
would be slightly higher than the Le Mans engine which used ceramic
apex seals. Also ceramic seals are impervios to cracking if the turbo engine
detonates for some strange reason. The same thing cannot be said about
iron apex seals.


Paul Lamar