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.

I divided the climb rate by ten and multiplied the max speed and the
climb rate together. That is the first added column on the right.

The second added column is a number proportional to MPG at 2600 prop
cruise RPM.
Cruise speed divided by HP.

What blew me away was the mediocre performance of the constant speed
prop in this regard. I suspect something is wrong with this data.
Either that or the CS blade twist was all wrong for 180 MPH or so.

Paul Lamar

This is a point Everet Hatch tried to make to more than a few people, at
least here in Oregon.  He gave at least one presentation to our EAA chapter
105 on engines and at the time his Superlite Rotary.  At the time 10 years
ago he had VanGrunsven convinced that the rotary would be the only feasible
auto conversion.  That was no small feat.  Oops, digressing.

Hatch was convinced that CS props were for most of us a waste of money and
too heavy.  "If you want to go faster just spin it faster" he would
say.  We tend think of prop rpm limits in Lycoming terms and fixed pitch
props pitched like they came on our grandfather's Cessnas.  That is with
enough pitch that you have to really try or be really stupid to over-speed
the engine.  We here don't have that handicap.

Mike McGee, RV-4 N996RV, O320-E2G, Hillsboro, OR
13B in gestation mode

 
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