I'll probably put my foot in something here but (here goes) this doesn't
surprise me.
With the std. Bing carb, other things being equal the Jabiru 3300 develops
max HP at around 2800 RPM +/-. As all props "unload" with increased airspeed
and, if the props being compared are more or less appropriate for the
engine, the more finely pitched prop will be more unloaded ( at a higher
cruise speed than will a coarse prop. To put it another way, the more coarse
cruise prop will still be pulling (with its max thrust) at higher airspeed
AND slower RPM, because the latter (2800 RPM) also yields greater HP.
(Notice the difference in WOT: the cruise prop WOT was 300 RPM less than the
climb prop means it was more fully loaded, developing more thrust, even at
slower RPM, when at cruise+ airspeed.) Another way to say it is at cruise+
airspeeds, a finer pitched prop's thrust-per-RPM will max out at a lower
speed.
Of course, the airspeed vs. RPM vs. prop pitch is not a linear set of
tradeoffs, with each one pivoting around the others in perfect correlation,
in large part because drag increases with the square of airspeed, but also
because of the sweet spot where max HP occurs.
In short, the cruise prop developed more thrust at 2800 RPM (max 120 HP)
than the climb prop developed at 3300 RPM (l~100 HP). Of course, as we all
know, different results occur in the lower speed ranges and different loads,
i.e., climb.
That makes sense to me... I think.
On Tue, Oct 21, 2008 at 8:29 PM, Walter Klatt <
Walter.Klatt@shaw.ca> wrote:
Ron, why would the coarser prop at 2900 rpm do 130 mph while the finer one
do only 125 mph at 3200 rpm? That engine should be producing more hp at the
higher rpm, which should translate into more speed, not less. With my
Rebel,
I have always got more speed out of it if I could pitch my prop for maximum
rpm at WOT.
Walter
-----Original Message-----
From:
mike.davis@dcsol.com [mailto:
mike.davis@dcsol.com] On Behalf Of Ron
Shannon
Sent: October 21, 2008 7:19 PM
To:
rebel-builders@dcsol.com
Subject: Re: [rebel-builders] Jabiru engines
Alright thrill seekers... although the hubbub has settled down of late
[Yea!] I can't resist passing on this contribution to The Great Jabiru
Debate [Ed.: See message archives, this subject line. Thou shalt not
repeat.
<g>]
Some months ago I made my first contact with a real Jabiru 3300-powered
Rebel in the wild. Dan Dyet has one of the earlier mechanical valve lifter
models on 730R. He reported at first he had problems with cooling, which he
acknowledged have been addressed in newer models. (He apparently did not
use
a Jabiru nose bowl.) However, he included the following performance data:
---
---
I am using the std Bing carb without the economy tuning kit. Haven't had
any
issues with it at all, good carb.
Engine runs great and starts like a charm (with preheat during winter).
...
I have one of the original models,mechanical lifters.
I started out with the GT wood prop then switched over to a Sensenich
ground
adjustable carbon fiber.
GT prop at 64" dia and 51" pitch is too coarse for the Rebel airframe.
GT prop on climb out at 70 mph gave me 2700 rpm 700 fpm solo, cruise 2750
rpm at 120 mph, WOT 2900 rpm 130 mph.
Sensenich prop 64" at a pitch setting of 2.5 gave me 2900 rpm at 70mph 1000
fpm solo climb out, cruise 2750 rpm at 110 mph, WOT 3200 rpm 125 mph. With
this prop I use up less than 200 ft of runway before I'm in the air and
flying.
---[end quote]---
He didn't mention the takeoff gross he was carrying for these results, but
IMHO, with either of those props, the numbers are not too shabby for 120
HP.
The newer hydraulic lifter models have redesigned cylinder cooling fins,
redesigned baffle ducts, better oil pump, better carb tuning, they specify
a
more efficient Positech cooler and reportedly have a bit more demonstrated
HP than the early models. The total width of the 3300 is barely 24" which,
with a properly fitted nose bowl/cowl, makes for a pretty small drag
profile
up front.
Ron
254R
http://n254mr.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
-----------------------------------------------------------------
-----------------------------------------------------------------
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
-----------------------------------------------------------------
-----------------------------------------------------------------
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
-----------------------------------------------------------------
