The Murphy Builders Site

-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On
Behalf Of Mike Kimball
Sent: Tuesday, May 20, 2003 10:28 AM
To: murphy-rebel@dcsol.com
Subject: RE: I picked a Prop and PSRU!


Unfortunately, I can't get more diameter than 84" from
IVO-PROP. And I can turn it slower than 2700 rpm. I can do
that by coarsening up the pitch or reducing power. Of
course, those options for turning slower are only good for
cruise since I want max power at takeoff. Perhaps the
reference to "efficiency" refers to cruise for a plane like
the Super Rebel. If that is the case, my ability to change
the prop pitch in-flight addresses that. However, I am by no
means proficient with this stuff and welcome any input. I'm
about to spend lots of money so if anyone thinks I'm paddling
up the wrong creek, don't worry about hurtin' my feelings.
By all means, give me your opinions! Thanks everyone.

Mike Kimball
SR #044

-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com]On
Behalf Of Walter Klatt
Sent: Monday, May 19, 2003 5:34 PM
To: murphy-rebel@dcsol.com
Subject: RE: I picked a Prop and PSRU!


I have also seen those prop tip speed calculator sites,
but they seem to go against what little that I have
learned about prop efficiency. I thought a slower
turning prop was more efficient. I am sure there is a
limit. I know fast airplanes do OK with smaller
diameters and higher RPM's but always thought for MAM
type planes, more diameter and slower prop RPM was
better. Hopefully your prop manufacturer concurs with
your choice.

Walter

-----Original Message-----
From: mike.davis@dcsol.com
[mailto:mike.davis@dcsol.com]On Behalf Of
Mike Kimball
Sent: Monday, May 19, 2003 6:28 PM
To: murphy-rebel@dcsol.com
Subject: I picked a Prop and PSRU!


Yay, I have finally made a decision! I have
decided to go ahead with the
IVO-PROP magnum 84" three bladed prop using
their electric pitch control and
electronic governor for constant speed. I
have also decided to fork out the
big bucks and go with the Firewall Forward
CAM 500 helical gear drive PSRU.
The CAM 500 will also provide a place to
mount my vacuum pump and solves
that little problem. The only thing I was
struggling with was deciding on a
1.756:1 ratio versus 2.118:1. Using the
handy prop tip speed calculator on
the web I have decided to go with the
1.756:1. It looks like I need to turn
an 84" prop about 2700 rpm to get the 0.88
mach speed (at 60 deg F) at the
tip that I need for maximum thrust. That
puts my engine at a max takeoff rpm
of 4741 rpm. That's well below the
capability of my engine but gives me the
conservative approach for reliability that I
want while still producing good
torque. I do not want to turn my engine at
5718 rpm which is what the
2.118:1 would give me, even though the
engine would be able to take it. If
I decide to fly when it's colder than 20 deg
F it will get noisy because
I'll be over 0.92 mach, but I can just back
off the power and let that dense
air lift me up. The thing that made me
decide to get into the 0.88-0.92
mach range at max power instead of being
more conservative down around 0.80
mach was the sound I hear whenever I see a
float-plane take off. There must
be a reason why float-plane operators like
those props that close to mach.
I'm inclined to believe what the prop tip
speed calculator website tells me.
If you want maximum thrust (and I do) you
need to be between 0.88 and 0.92
mach.

Mike Kimball
SR #044




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