Prop Pitch Question

[Eugene Wendland]

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

[James Lloyd]

Eugene if it was me,I would just go for the 2700 or so in the summer
air,in level flight,wide open trottle.You will have a climb/cruise prop
then.Jim

[Kevin Horton]

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