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Interesting for certain.
I have 88" of pitch which is much more than my
original prop which had 72" turning 5600 rpm using the 2.17.
There is no question that my acceleration on take off and rate of climb is
significantly improved with the slower turning 74x88" prop. The key (I
believe - not being a prop designer) is swinging the longest prop you
can which bites a bigger disk of air. My new prop started out with an 8"
larger diameter (now 6" larger) which I believe made the difference. The
older 68x72 prop would be tearing up the air 6000 (engine rpm) but,
it always felt to me like it was trashing the air, but just wasn't pushing
a whole lot backwards.
At 5200 rpm and 2.17 the old prop was turning 2400
rpm. At 6000 and 2.85 the new prop is only turning 2105 rpm - yet the new
prop is clearly out pulling the old prop. Again I think the key is the
length. Part of the blade may indeed be stalled with 88" of pitch but the
outer unstalled portion I suspect more than makes up for it. Since that
region of the prop is also further out from the disruptive influence of the
cowl, there is probably better efficiency provided your tip is below 80% sonic
(slower turning makes that more likely).
Also looking at the power difference at the two different
rpms, I calculate 160 at 5200 and 180HP at 6000, whether that is accurate
absolute power it shows a possible 12.5% increase. The disc area
carved out by the 74" prop is approx 18% more although its speed is approx 13%
less. So I think more area, more HP and more pitch appears to add up
in my case to considerably better low speed performance. The top speed
appears not to have changed, still a shade under 200 MPH around 197MPH for both
props.
So, don't have answer except I think the diameter of the
disc your prop carves out of the air has a very significant impact
(especially at lower airspeeds and lower prop rpm). I do recall one prop
designer said for maximum take off performance swing the biggest prop you have
clearance for.
Interestingly enough before I had the prop shortened, I
was a Tracy Crooks and was doing a run up to get some exhaust sound
readings. It was a cool morning and the engine was turning around
5800-6000. Tracy and I (as well as the sound meter) could hear the prop
blade stalling and unstalling (apparently as the blade rotate different
orientation with respect to the cowl and effect the airflow enough to cause it
to stall and that point and then recover). You could hear a distinct "wop!
wop! Wop!" sound as the prop stalled and unstalled.
In summary, I believe if you have two props of the same
diameter and one with a much larger pitch then static blade stall may be more of
a factor than if you take advantage of a longer bladed prop, increase its pitch
and slow its speed as we are able to do by changing gear boxes - of course, not
being a propeller designer, I could be all wrong about the above.
just my 0.02 worth
Ed
----- Original Message -----
Sent: Saturday, December 10, 2005 10:57
AM
Subject: [FlyRotary] Elippse
Propeller
Guys,
Here is an exchange I just had with Paul Lipps the designer
of the "Elippse"
prop blades.........Those of you that have heard about his blades in
"Contact"
mag. and "Sportsman Pilot" mag. may find this info of
interest.....FWIW
--
Kelly Troyer
Dyke
Delta/13B/RD1C/EC2
--------------
Forwarded Message: --------------
From: keltro@att.net (Kelly Troyer)
To: "Paul Lipps" Subject: Re: Elippse Propeller
Date: Sat, 10 Dec
2005 07:54:00 +0000
Paul,
Thanks for the info and suggestions......I have e-mailed Grand
Aero for info on
your blades with the numbers for my a/c , engine , etc....
Thanks again,
--
Kelly Troyer
Dyke
Delta/13B/RD1C/EC2
--------------
Original message from "Paul Lipps"
Hi, Kelly!
I don't make props, I only design
them. I have designed a prop blade for Grand Aero that will be used in a
three-blade, ground-adjustable hub. It's been designed for the RV-series
of planes, and will easily accomodate 150hp-200hp for cruise speeds in the
170 mph to 220 mph region. One thing I might caution you about, though.
You mention only turning 2100 rpm because of your gear ratio. Takeoff with
the prop adjusted for that low a cruise rpm would require a fairly high
blade angle. The blades would be stalled over a large portion during the
early stages of the takeoff run, and so would not give much initial
thrust, making for a longer takeoff. If possible, you might consider a
different gear ratio allowing you to set the blades at a lower angle,
giving much better static and takeoff thrust. The three-blade prop on my
Lancair turns 2230rpm static, 2440rpm in a climb at 115mph IAS, and 2800
rpm at 201 mph cruise at 10,000' dalt. This should give
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