I have several questions concerning this discussion.
First could someone explain the reason for the big pitch
difference between that of a Prince prop (46) and a
Sensenich prop (54 or 56). There are differences in the cord
of each prop at various circumferences but the difference
should not be that great.
Second, why does Sensenich recommend a 74DM()S8-0-58 as a
standard prop and a 74DM()S8-0-56 as a climb prop for a
Murphy Rebel? This seems to be in disagreement to what Walter
is saying and has experienced.
Third, I would like to know the configuration of Walter's
and his two buddies' planes. Were they all on bungees, or coil
spring gear like that supplied by Wayne O'Shea, or did they
have spring gear? The reason is that drag enters into the
picture and can effect the performance of an aircraft
considerably. For example I calculate the drag of the
various gear configurations at 120 mph as: bungee gear at
99 lbs, O'Shea coil spring gear at 93 lbs, and the spring
gear at 27 lbs. So for a plane that has about 375 pounds of
total drag at 120 mph, this is a significant difference. These
calculations assume no fairings or wheel pants on any of the
configurations.
The speed difference between spring gear and the bungee or
coil spring gear then would be between 5% and 7.5% for
equally powered Rebels based solely on drag.
Lastly, does not the prop pitch have to vary with the speed of
the aircraft to maximize prop efficiency? According to my
calculations a 6% increase in speed would call for a 2" increase
in pitch.
I may be all wet but my calculations are based on 'NACA
Report 485' and 'The Cost of your Airplane's Parasite Drag'
by Harmen Koeffeman.
-----Original Message-----
From:
mike.davis@dcsol.com [mailto:
mike.davis@dcsol.com] On
Behalf Of
Rebecca Brownell
Sent: Monday, January 19, 2009 9:04 PM
To:
rebel-builders@dcsol.com
Subject: Re: [rebel-builders] Prop selection
Thanks all for the responces.I spoke to "Lonnie" at prince and
he
suggested a 74/46.Hoping that will do for floats.
Thanks
Jason 817R
On 15-Jan-09, at 2:08 PM, WALTER KLATT wrote:
When selecting a prop for the Rebel amphib, it is better to
have a
finer pitch prop. Locally here, I have compared performance
with
several Rebels now with different props, and the biggest
mistake
hindering performance has always been too much pitch. You
might as
well install a smaller engine, if you don't want to use the
power of
a 320. On initial take-off or static, you should be tuning at
least
2500 rpm, and WOT straight and level at least 2700.
For example, I had a Sensenich 74 X 54 when I had the 320. I
flew
with a couple of other guys that had 74 X 56 on otherwise
identical
planes, and they had significantly less performance on take-
off,
climb and top speed. They then changed their pitch to what I
had,
and their performance improved dramatically.
What I found when comparing different props, is that 2 inches
pitch
equates to 200+ fpm climb. Same holds true with my 360. I
tried both
a 76 X 54 and a 76 X 56, and there is a big difference
in
performance between these two props.
And a finer prop does not mean more fuel consumption at
cruise
speed. I have a fuel monitor on mine, and found that gph
(measured
to one decimal place) was the same when going the same
airspeed,
even though your engine had to turn higher rpm.
Top speed was also dramatically improved if you could get
it
spinning over 2700 rpm. My amphib 320 topped out at 2720 rpm
and 125
mph with the 54 pitch. The guys with the 56 had trouble
hitting 120.
Also, have been told by Bart at Aerosport not to worry about
over
revving these engines a little, if that might be a concern. So
if
you want to get the most performance out of your amphib, don't
be
afraid of a finer pitch prop.
JMHO
Walter
----- Original Message -----
From: Rebecca Brownell <
brownell@candlelight.ca>
Date: Thursday, January 15, 2009 9:46 am
Subject: Re: [rebel-builders] Prop selection
To:
rebel-builders@dcsol.com
Hi steve thanks for the reply,Iwas thinking 74/47(with my
vast
experience,not!)I just needed some idea of what I needed
before
calling them.Is Lonnie a tech guy at prince?
Thanks
Jason
On 15-Jan-09, at 12:05 PM, steve whitenect wrote:
which
would
perfect
bit
second
on the
shimming
elevation at
prop. Steve W #637R
the
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