Hey, Ron, looking forward to seeing you fly your Jab Rebel. Are you going to
be at Arlington again this year?
Theoretically, the Jab sounds good, light weight, smooth, and if you can
harness as many of those 120 ponies as possible on take-off, you could have
a great performer for your money (and simplicity as you mention). I think
your choice of propeller will be very important. What you have to watch is
that this engine is used in many faster airplanes. With those, they need a
prop with a lower static rpm so they don't over rev for top speed. Instead
you want a prop that gives the highest static rpm, and still reaches only 32
or 3300 rpm WOT straight and level at Rebel speeds.
That's what I found with my last 150 hp 0320 and the metal Sensenich. Many
with this engine only get 2400 or 2450 static. But this prop allowed 2500 on
take-off but would only hit 2720 flat out, so it was very well matched to
the plane and engine. And according to the 0320 power charts, it produced
140 hp at 2500 rpm full throttle, which means I didn't lose much given the
max of 150 at 2700.
I tried to choose a similar set-up with my new 180 hp engine, with a prop
size that should give me 2500 on take-off. And it's power charts are even
better, with 170 hp at 2500 with a max of 180 at 2700.
If you can get 108 hp at 2750, and find a prop that can efficiently transfer
that to thrust, you should at least be equal to the 912S on take-off and be
better at cruise. Do they use Warp props on the Jabs? At least they are
ground adjustable and you can play around with the static rpm.
Anyway, look forward to seeing you again this summer, if you can make it.
Walter
-----Original Message-----
From:
mike.davis@dcsol.com [mailto:
mike.davis@dcsol.com] On Behalf Of Ron
Shannon
Sent: March 10, 2008 1:22 PM
To:
rebel-builders@dcsol.com
Subject: Re: [rebel-builders] Jabiru engines
No problem, Ben. As apparently the only Jabiru powered plane builder on the
list, it's hard to stay mum when others are touting only the advantages of
the Rotax, in a completely unbalanced way. As the thread began, I had hoped
to avoid regurgitating the archives on this... but some people don't get
around to reading those, so... the bell rings and we go. :-)
Surely max torque at "X" RPM is only one desirable spec in a Rebel/bush
plane. Others may have different opinions, but if I am 300 miles from the
nearest airport, I'd rather be servicing a simple air-cooled Jabiru, which
in my case will also have a very simple, manual mixture carb. (Yes, I'll
have to duck again, but... have you seen the exploded parts view of the
constant depression Bing? Do you really want to be overhauling that on a
rock at 8000 ft. with the sun going down?) Similarly, at high density
altitude, I would rather have the possibility of compensating with both an
adjustable prop _and_ manual mixture control, not just the former.
After all, the good thing about the constant depression Bing carb is you
don't have to adjust the mixture. _One_ of the bad things is -- you can't
adjust the mixture! When paying who knows how much for fuel, I would rather
be able to fine tune mixture instead of having to rely on what is likely to
be an unmanageable (automatic) compromise in pressure/prop load compensation
across a wide performance range with the Bing(s). Imagine a Bing and a
manual are both trying to take off at high density. They've both adjusted
their props. Now what else is the Bing driver going to be able to do? Or,
suppose the adjustable prop or alignment tools have broken and they're both
stuck with whatever prop pitch they rode in on.
The Jabiru may not have the published torque of the Rotax -- especially when
cherry picking points in the power curve. But last I heard, HP is still
relevant, particularly at the top end of the speed range. As a matter of
personal preference, in the boonies (or anywhere for that matter) I'd rather
sacrifice a little torque somewhere for what I see as some of these other
advantages.
IMHO, to claim that the Rotax is per se, absolutely, positively, in all
conditions and under all considerations, a better lightweight (LSA) engine
than the Jabiru is, well, narrow minded at best. What is best for some, may
be relatively undesirable for others.
The defense rests, but reserves rebuttal time. :-)
OK, Ben, the incoming has drawn me out of my foxhole. :-)
Sorry about that. Gotta admit, you raise a lot of good points. BTW tho,
the 912S clutch protects the engine from a prop strike, so IMO the shorter
prop on the Jab is no advantage (besides cost of prop).
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