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

Vance W Jaqua wrote:

This was in The EAA mag a bunch of years ago.  The thing that blew me away
was the
wide range of so called "pitch" which led me to believe that you cannot
believe
the stated number from most manufacturers. The only place that a constant
speed prop shines is in take off and slow climb, and the ability for
"overdrive"
operation at cruise (low rpm nigh manifold pressure)

Say again - what your strage MPG number means ?? - Vance

Constant speed props are metal and most of the rest of these props
are wood so there is a big advantage right there, Note the high efficiency
of the fixed pitch metal Senenich prop compared to the rest of the wood
props. Thin is better than thick.

My MPG number assumes the engines all have the same BSFC so fuel burn
is going to be directly proportional to HP. All I did is divide
cruise speed by HP.

Paul Lamar
 
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