Paul Lamar wrote:

An aircraft engine is not like a car. The RPM range is less than
a 1000. Static to max speed.

Russell Kent replied:

For a traditional aviation engine this is true.  For a PSRU-equipped
engine, this may be less true.  1000 RPM (engine) on a 2.85:1 PSRU is
350 RPM (prop).  So 2,200 to 2,550.  That's a *big* part of the normal
operating range of normal aviation props, but that isn't *all* of it.
I'd think that 1,500 is a more reliable approximation, but the gist of
your argument is still sound.

Paul Lamar responded:

I run my C182 engine from 2000 RPM to 2400 red line. A 2.85:1
PSRU = 1140 RPM range. Rarely do I run it below 2000 RPM and that
is with a constant speed prop. Fixed pitch props with AC engines
are less than that.

Clarification requested: by "less than that" are you saying that fixed
pitch props have a smaller range, or that the lower bound (2000 RPM) is
even lower which would imply a greater range? I think you mean the
former, but it isn't clear.

Russell Kent

I think so. In my limited experience flying a 172 the range of RPM used is
less than that in my 182 as I recall. I flew a 172 all over the outback.

Paul Lamar

Hi Paul and Russell and All, I have a Stinson Voyager with a 165 HP Franklin
with fixed pitch prop. Static rpm 2350, climb 2450, cruise at max continous
2550(80% power). Max allowable rpm, 2800 for 165hp, is only achievable at 125
mph+ in a descent. So usable rpm range is only about 300.With the flexible
rpm of the rotary, I think a fixed pitch prop would be much more usable than
with a piston engine. Engine rpm range say 5500 to 8500 (RX8)with 2.85
reduction makes it about 2000 to 3000 prop rpm; right in the efficiency range
of 'off the shelf' props available today. FWIW, Dave McC

 
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