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
cribe.wcx-----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
*--------------------------------------------
-----------------------------*
To unsubscribe from this list go to:
http://www.dcsol.com:81/public/code/html-subs
Archives located at
http://rebel:builder@www.dcsol.com:81/default.htm
To contact the list admin, e-mail mike.davis@dcsol.com
*------------------------------------------------------
-------------------*
*-------------------------------------------------------------------------*
To unsubscribe from this list go to:
http://www.dcsol.com:81/public/code/html-subscribe.wcx
Archives located at http://rebel:builder@www.dcsol.com:81/default.htm
To contact the list admin, e-mail mike.davis@dcsol.com
*-------------------------------------------------------------------------*
-----------------------------------------------------------------
List archives located at: https://mail.dcsol.com/login
username "rebel" password "builder"
Unsubscribe: rebel-builders-unsubscribe@dcsol.com
List administrator: mike.davis@dcsol.com
-----------------------------------------------------------------