# KR> Prop pitch

```Owen said,

"He had it pitched for high speed cruise.  It was a pretty efficient
combo but the high pitch seemed to be loading up the engine making  quite
hot."```
```
I'm wondering if Owen means it was pitched to allow the engine to turn up
at a very high RPM, thus causing the plane to go faster, thus "high speed
cruise"?  If so, then the resulting heat - the hot engine - would be the
result of internal friction - parts turning against each other (friction)
inside the engine.  The faster the engine turns, the more friction is
generated.  More friction = more heat.   If this is what Owen means, the
prop was set to a flat pitch.

Owen's second sentence refers to "loading up the engine" however.  This
would imply the prop was set to a sufficienty coarse pitch to limit the
engine's ability to turn at its optimum RPM.  In this case one could say
the engine was "over propped" - the coarse pitch "loading up the engine".
This would not cause the engine to get hot since at the lower RPM
friction would be low.  With such limited RPM however, the plane would
not be capable of "high speed cruise.".

Perhaps Owen would clarify which it was.  What do you mean Owen with the
phrase "loading up the engine"?  Your two sentences seem to contradict
each other in meaning.  This is a very interesting subject with lots of
variables and I'd like to comment, but to do so I'd need to presume what
you meant and I'm a lousy mind reader.

****************

Regarding my own plane, when I first got it I had a heck of a time
keeping oil temps down and eventually learned I had a glitch in my
electrical system that caused the gauge to read high.  In the process of
figuring things out however, and in the process of finding a prop to do
just what I wanted - these two objectives running in parallel - I was
enormously impressed with how easier it was to keep my oil temperature
down by going to a coarser prop, one that allowed me to run at WOT at
cruise altitudes (anything above 8 thousand feet) at the cruise RPM I
wanted ( ±3100).  Why 3100?  Initially it was to keep my tip drag at an
ideal speed but the immense improvement I got from reduced oil
temperature as a result of lower engine RPM's was a very welcome
secondary benefit.

With my GP 2180 w/Ellison EF-2 and a Sterba 52 x 56, my engine operates
right on the edge.  That is to say, when starting the take-off roll the
engine is slightly overpropped.  I have to pull it back slightly since
the engine cannot accept that last little bit of throttle.  Once underway
however, once the prop is unloaded and I am in the climb phase and
especially once in the cruise phase, the engine can accept full throttle.
At the higher altitudes I normally curise at on cross-country flights,
the air is less dense and I've found a prop that turns 3100 WOT at 5
thousand feet will turn 3200 WOT at 12 thousand feet while using less
fuel.  I attribute this to the air being less dense, thus less drag.
It's the same phenomenon that allows the plane to go faster for a given
amount of thrust, the higher it goes.  For my plane there is an optimum
altitude where power and drag are at their respective optimum points.
It's 12-13K for my KR-1½.

One advantage of using the Lycomings and Continentals over the VW is
their rated power RPM of 2700 is much lower than the VW's rated power RPM
of 3500.  Although props are longer for the certified engines, RPM is so
much lower that mach tip speed limitations are of no consequence as far
as I know.

Mike
KSEE

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