I don't know how I missed this back in 1990 but it was republished
in the RV Aviator book.

Something I never thought about was if you have a climb prop
on your airplane your engine is probably generating more
HP at top speed. Interesting. The rotary, just like a direct drive
aircraft engine, generates more HP in direct proportion to RPM.
This is because, like a direct drive aircraft engine, it is not
breathing limited. Most large displacement piston engines start
to become breathing limited about 5,000 RPM. Smaller piston engines
and the rotary to a lesser extent become breathing limited around
7,000 to 8,000. The main reason for this is the piston engine poppet

valve.

A peripheral port rotary will keep generating more power right up to
the point the rotor bearing fails due to centrifugal force overcoming
oil pressure in the bearing. I digress too much. Getting back to
fixed pitch props.

Any engine that is running past it's torque  peak is operating
breathing limited. Breathing limited is not a shut off like
an rpm limiter . but the area where the torque tapers off as the
flow resistance of the inlet systen no longer fills the cylinder
completely (ie volumetric efficiency is falling off. Wankels
are not immune to the laws of physics, and generally have
max torque points below max rated HP - Vance

peak

True but the rotary defies the laws of piston engine physics by not having
the beloved poppet valves to get in the way :) The point is the torque peak on
a rotary is always a lot higher in the RPM scale as there are no
poppet valves to get in the way. Here is a power curve for a 160 cubic
inch displacement Mazda 4 rotor engine. Rotary engine volumetric efficiencies 
run 120% on average due to dynamic tuning effects. You should
know that Vance as you have driven my 237 HP 80 cubic inch RX8 :)
How else would you get that much power out of such a small
(12" x 12" x 18") normally aspirated engine?

What I meant by breathing limited is the HP curve starts to curve 
downward, flatten out and then continue to curve downward.
Lets call it severe breathing limited if you wish.

Paul Lamar
 
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