A couple of comments .....
I think the pitch numbers are different for the Prince because
of the much wider effective chord & blade shape - which is reported
to change effective pitch with load & speed ....
It is my understanding that, for the Lycoming O-320,
Sensenich recommends:
74 x 58 cruise
74 x 56 all-purpose This is what I have on mine !
74 x 54 climb
I'm sorry to rain on your (and others) parades, but, in 18 years
of flying all variations of the Rebel, I've seen NO measureable
speed advantage from the spring gear ! I did see a very slight -
about 1 mph - speed DECREASE with the die spring gear, compared
to the bungee gear. This, I believe is because the bungees were
covered (simple leather), while the die springs are exposed because
of my desire to oil them every 3 or 4 flights ....
There is no doubt that the Rebel is draggy, and relatively
simple fairings can make a big difference - we gained over 10 mph
just by fairing the gear legs & filling gaps at the top of the gear,
ailerons, and wing roots, on our original Rebel. Steve Sloan has
achieved MUCH more, through careful attention to detail - he
even made fairings for the float attach fittings !
Fairings for the inner gear legs (simple Lexan wraps) can make
a big difference. There are considerable differences between
Rebels - even with the same engine, and similar configurations,
some are noticeably faster ... like 5+ mph ... The same
variations are found on production aircraft - I've flown Citabrias
that varied by 8 mph in cruise, with the same engine & prop !!
(Rigging differences ???? )
Harmon Kauffman & his son flew at my old gliding club, back in
the early '80's - his son almost bought my wife's glider at one point ....
You can look forward to many enjoyable years tweaking your
Rebel, once you get it flying ! :-)
--
......bobp
bobp@prosumers.ca
http://www.prosumers.ca/Ramble09
http://bpatterson.qhealthbeauty.com
http://apatterson2.qhealthzone.com
http://apatterson2.ordermygift.com
-------------------------------orig.-------------------------
On Tuesday 20 January 2009 23:01, Gary Gustafson wrote:
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
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