On Sat, 24 Apr 2004 22:21:56 -0400, Eugene Wendland wrote:

Hi All

I fly a Pazmany Pl-2 (see link below for pictures) that had a power
upgrade from an 125 h.p. O-290 to a 160 h.p. O-320. I fly behind a
McCauley Metal prop. After the engine change, I had the prop repitched
from a 7063 to a 7067. Top speed increased and my initial climb rate is
about 1400 f.p.m. I recently changed the plugs and magneto leads and saw
another 100 r.p.m increase in straight and level flight to 2900 r.p.m.
I'd like to increase the pitch of the prop some more. A basic rule of
thumb is a one inch increase in prop pitch will reduce the r.p.m. by about
80. I figure a 3 inch pitch change should drop the r.p.m to around the
2700 mark.

A fellow in the next hangar flies a Thorpe T-18 with a severely pitched
wooden prop. He told me that he can only achieve a top r.p.m of about
2450 r.p.m. behind this prop.

My question is this - Is this common to pitch a prop so that full r.p.m.
(2700) can't be achieved? His plane has shorter wings and is much sleeker
than mine (same engine), and about 30 m.p.h. faster in top speed. What
kind of top r.p.m. are all those RV-6/7/8/9 flyers achieving with their
big pitched props?

Any advice would be helpful. I'm sitting on the fence. Go with a minimum
pitch increase to bring the r.p.m. to around 2700 or go with a bigger
pitch increase for a real airspeed increase.

Blue Skies

Eugene
http://home.cogeco.ca/~pl2/paz3.html

If you want the absolute fastest speed, without exceeding Lycoming's
redline, then repitch the prop to give 2700 rpm at full throttle at
whatever altitude you care about. Increasing the pitch more than that
will decrease the rpm, and thus decrease the power produced, so it should
result in slower speeds.

But, if you aren't prepared to run at 2700 rpm, and you would be pulling
the throttle back to reduce to some lower value (lets use 2500 for the
sake of argument), then it might be worthwhile increasing the pitch even
more so you can run full throttle at 2500. This would give
you more speed than if you have it pitched for full throttle at 2700, and
then pull the throttle back to run at 2500 with part throttle. But this
would likely have a big effect on take-off and climb performance, so you
need to pick your poison.

As far as I can tell from discussion, most RVers pick a pitch that lets
them run around 2700 at full throttle at their typical cruise altitude.
But they then pull the throttle back a bit when they cruise.

--
Kevin Horton RV-8 (finishing kit)
Ottawa, Canada
http://go.phpwebhosting.com/~khorton/rv8/
e-mail: khorton02(_at_)rogers(_dot_)com