[rebel-builders] Rebel - engine choices - fuel economy --- was 0-2
Posted: Sun Feb 19, 2012 5:02 pm
Ben
I do think that a good Lyc. can cruise almost as efficiently as a car
engine. Lycoming says you can't harm their engines by leaning if below
75% power so lean of peak is fine. But there is a large variety of those
engines and little consistency even between new factory identical model
engines. Most of the intake manifolds are not well suited for a carb
IMO and even the better systems may have "golden" combinations of rpm
and throttle. Several large independent rebuilders and probably the
clones make better engines IMO. "The Major Overhaul" by Kas Thomas is an
eye opener into the world of piston aircraft engines. All I'm saying is
don't count on being able to run them at very low power unless you have
matched injectors and electronic ignition. Injectors will keep all
cylinders closer to the right mixture than a carb. The ignition advance
of electronic ignition will significantly improve fuel efficiency at low
power. In fact ignition advance is most beneficial at low power. Some of
the proposed new electronic control systems control each cylinder's
mixture and timing independently.
The soob has many features that I absolutely love. The gearbox is my
only concern. The main reason for my low burn is simply my low cruise
power. The burn goes up fast if I match Bob's cruise and I suspect it
would be within 5 or 10% of his at his cruise speed. The soob is as
heavy as an 0-320 and seems to match a fixed pitch 0-320 for takeoff and
climb but it is happy to cruise at low power. Mind you low power is 4200
rpm and 21" of MAP which is still two to three times what the car uses.
That gives me the same cruise as the rotax. I'm still on wheels (like
Bob) so don't know about float performance yet.
Ken
bransom@dcsol.com wrote:
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I do think that a good Lyc. can cruise almost as efficiently as a car
engine. Lycoming says you can't harm their engines by leaning if below
75% power so lean of peak is fine. But there is a large variety of those
engines and little consistency even between new factory identical model
engines. Most of the intake manifolds are not well suited for a carb
IMO and even the better systems may have "golden" combinations of rpm
and throttle. Several large independent rebuilders and probably the
clones make better engines IMO. "The Major Overhaul" by Kas Thomas is an
eye opener into the world of piston aircraft engines. All I'm saying is
don't count on being able to run them at very low power unless you have
matched injectors and electronic ignition. Injectors will keep all
cylinders closer to the right mixture than a carb. The ignition advance
of electronic ignition will significantly improve fuel efficiency at low
power. In fact ignition advance is most beneficial at low power. Some of
the proposed new electronic control systems control each cylinder's
mixture and timing independently.
The soob has many features that I absolutely love. The gearbox is my
only concern. The main reason for my low burn is simply my low cruise
power. The burn goes up fast if I match Bob's cruise and I suspect it
would be within 5 or 10% of his at his cruise speed. The soob is as
heavy as an 0-320 and seems to match a fixed pitch 0-320 for takeoff and
climb but it is happy to cruise at low power. Mind you low power is 4200
rpm and 21" of MAP which is still two to three times what the car uses.
That gives me the same cruise as the rotax. I'm still on wheels (like
Bob) so don't know about float performance yet.
Ken
bransom@dcsol.com wrote:
Ken,
Impressive! You make it hard to stick to this idea (which I've come to
believe) that a Lyc/clone can be operated as fuel efficiently as a car
engine. Also, the posts on rough running Lyc's at lower rpms add to the
difficulty in equating Lyc fuel economy to Soob.
Do you think your plane goes a little faster than Bob's 0320 Rebel as he
posted (about last year's ramble) ...and that being even yours on floats,
his on wheels?! Do you think that's mostly prop pitch?
I really appreciate the recent posts on this subject, and also hearing what
others are really doing for insurance.
-Ben/ 496r
Actually the Soob definitely used less than $1,000. of mogas on that
trip. 1023 liters in 58.4 flight hours. Don't have the actual cost handy
but mogas was under a dollar per liter back then and well under a dollar
in places that had regular without alcohol. So far it still hasn't
tasted avgas.
Ken
Bob Patterson wrote:Hi Keith !
I'm with Ken ! I don't have the fancy electronic ignition,
just stock mags & carb, on my O-320E2D. It burns 8 gph
at 2,450 rpm, and is very happy there. On a previous Ramble,
I ran it at 2,350 - it burns about 1 gph less, and is reasonably
happy there, but 2,250 or 2,100 are not so good ... it seems
rough, and get harder to start. There's no question that the
old Continental O-300 6 cyl burns less fuel....
On the subject of fuel .... On Ramble 2007, we flew out to
the West Coast, and back to Oshkosh. This involved a lot
of flying in 95 - 100 + F. temperatures, through the mountains,
from airports near 4,000 ft. ASL. We all carried 2 passengers,
and camping gear. I put $2,630 worth of 100 LL in my O-320
(those were lower prices back then ...). The Rebel with the
80 hp. Rotax 912 used about $880 worth of mogas, the Rebel
with the 100 hp. Rotax 912 used about $1,100 worth, and
I believe the Rebel with the Subaru 2.2L used about $1,200 - $1,300
worth of mogas .... SO -- we all got there, and it cost me over
DOUBLE the gas.... Yes, I could carry a bit more weight, and
climb a bit faster sometimes (although the Subaru wanted to
pass me !), and cruise a bit faster ....
My NEXT Rebel will have a 100 hp. Rotax 912, and might
go on 1500 Murphy amphib floats....
I think Tom summed it up quite nicely - he said (approximately):
"I chose the Rotax 912 because I couldn't think of anywhere I
wanted to go that quickly that was worth the extra fuel ! "
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