X-Mozilla-Keys:
Fred wrote:
I know the Lancair guys well (I am building one) and spent a lot of time with
Ross at Reno. (Ross did the modifications for racing.) Here is the scoop as
I heard it.
True, fuel burn was 70 GPH, but keep in mind it was running VERY rich for
cooling and antidetonation. (Pratt and Whitney 2800's operate at a
stoichiometric ratio of 1.65 at take off, and use 48% of the energy available
in the fuel as available combustion energy. SFC is awful, but only for take
off and initial climb.) Manifold pressure was 54 inches as opposed to 38
inches for the stock blown Continental. This alone would yield a bit less
than 500 HP at the normal 2700 RPM, neglecting the effect of higher inlet
temperatures due to additional turbo compression and a warm day. Include the
effect of spinning the engine 3100 RPM and 500 HP would be my guess based on
these numbers alone.
HP = #fuel/BSFC. Working backwards 500 HP gives a BSFC of 0.84. I have
never heard
of an engine sucking down so much fuel. Even if you give it a BSFC of .7
that
would be 600 HP. As I mentioned before the intercoolers were probably
cooled
by spraying or dribbiling alcohol over them. PL
There were four nozzles spraying water in the plenum over the cylinders, plus
nozzles over intercoolers and oil cooler, also spraying water. Cylinders
operated at about 375-400F (normal range). A 50-50 water-methanol mix was
sprayed into the intake manifold just downstream of the throttle plate,
injection rate was one quart per minute, or about 25% of the fuel flow. The
big Merlins use about one part water/alcohol to one part avgas at about
120-130 inches of manifold pressure (no intercooling).
I don't know about RPM, but Ross said speculation was 500 HP which agrees
with the numbers above. Compared to stock of 350 HP, flat out speed
improvement would be cube root of HP ratio, or about 13% faster. My guess is
that this is about the margin Dave Morss had over the other Lancair IVs based
on my recollection of the space between Dave and the "stockers" (less than
half a lap at the end of a 6 lap race). Dave did start to lap the slow
planes (not Lancairs) in the last lap.
On the topic of opposed air cooled engines being superior to other
configurations for aircraft, I think you have missed a few points in your
praise of opposed air cooled engines. (Full disclosure: I have concluded the
opposite: they are in fact inferior to some other designs.)
1) Being air cooled, the cylinder to cylinder spacing must be larger. Thus
the crank shaft must be longer and it is thus prone to wind up like the long
crank of a V-12, although probably not as bad.
OK lets get down to specifics. The torsional twist is proportional to
the length but inversely porportional to the fourth power of the radius.
So a small increase in journal diameter can make up for the length
increase. What you don't get in a stupid siamesed cylinder, water
cooled automotive engine is enough crank length to put a decent sized
fillet radii to mitigate the stress concentrations where
the journal meets the cheeks. Furthermore if you increase the
journal diameter to compensate for the increased stress concentration
the journal width to diameter ratio becomes unfavourable to maintain
the oil film in the bearing. We discussed all this a year or so ago
and it is on the ACRE NL CD. The formla is deflection in degrees
equals TL/KG G is the torsional modulas of elasticity and
K = 1/2 * pi * r^4. T is torque and L is the length.
2) Opposing cylinders do not eliminate loads on the crank case. On a 4
cylinder engine, they fire every 90 degrees, not at the same time. When
cylinders fire, the thrust of the piston is delivered to the crank throw, and
reacted by the crank case half opposite the cylinder. The load path is
through the piston, through the wrist pin, through the rod, to the crank
through, to the main bearings, to the opposing crank case half, back to the
crankcase tie bolts to the half attached to the cylinder, to the cylinder tie
down bolts, then to the cylinder and head. Every compression and combustion
event creates load that is transmitted through the crank case tie bolts. It
is VERY COMMON to see opposed engines torn down and have significant fretting
damage between crank case halves because of relative motion between them in
spite of all the cross tie bolts. The crank case halves are trying to part
company while the engine is running. Viewed in this way, I concluded that
opposed engines bolted together are the LEAST durable way to build an engine
to react loads. However, they evolved from a long tradition of radial
engines (which at least have single piece crankcases) but present less
frontal area which is why they dominate today. Thank history and not the
basics of the design.
Yes but the opposite crankcase half has a nice large bending section
moment
of inertia that is idealy designed to resist these loads and at the same
time
provide crank crank clearance for the other cylinders.
The indisputable facts of the matter are opposed engines of a given duty
cycle have better power to weight ratios than inline engines. PL
I have been told that if you see a big opposed aircraft engine operating on a
dyno with a strobe light to permit one to see vibratory motion, it is
possible to see cylinders flailing up and down visible amounts at high power
as the entire structure shakes and flexes.
Never the less they are designed for 2000 hour TBO and they achieve that
in many cases. My crankcase or cylinder heads never cracked in 3000
hours
of operation. There is no other piston engine type in existance that
has any where near the continuous power to weight ratio as the opposed
air cooled aircraft engine. The closet thing in the automotive world is
the
over the road truck engine with 350 HP or so but weighs 1500 to 2000
pounds.
PL
3) With sufficient beef in the crankshaft, one can handle lots of horsepower
as the drag race guys have demonstrated by passing 4000 horsepower (plus the
power needed to drive the supercharger, probably another 200-400 HP) down the
crankshaft. I'll grant you that lifetime is short, but 500 to 1000
horsepower in the same sized crank cuts the stress by a huge factor resulting
in effectively infinite lifetime.
HP for ten seconds does not count. It is the fatique life of the crank
that
we are dealing with here. As you well know a peice of steel can take a
few load
reversals but it is another matter entirely when the load is reversed
millions
of times. PL
4) The German Jumo diesel engines were 6 cylinder, 12 piston engines with the
pistons tops opposing one another connected to crankshafts at opposite sides
of the engine connected by a gear train. The BMEP was about 10 bars,
comparable to modern piston engines so piston loads and pressures were
comparable to current. Here we have crankshafts at opposite ends of the
engine trying to part company. These engines flew with specific fuel
consumption below 0.35 pounds/hp hour regularly across the Atlantic from
Europe to Argentina on flying boats with an excellent reliability history and
limited weight penalty (liquid cooled). It shows that with good design you
can make nearly any configuration work well. It is a tradeoff based on
weight, frontal area, experience of the designer and preferences of the
company, etc. And, of course, money.
The Junkers Jumo 205 develped 600 HP and weighed 1147 pounds. Not an
exceptional power to weight ratio. The Curtiss R-3350 TC had a BSFC of
0.28 gave 3700 HP and weighed 2800 pounds. A power to weight ratio
well over one. See the ACRE NL web site for fuel consumption charts and
pictures of this engine. A Lycoming O-720 horizontal opposed air cooled
engine gave 520 HP with a weight of 597 pounds. PL
5) In my view, based on the mechanical stresses, temperatures, and
thermodynamics, the "perfect' aircraft engine using today's available
technology (and ignoring turbines doe to cost and fuel consumption reasons)
would be a two stroke piston diesel because of: high efficiency due to diesel
cycle, low exhaust temperatures due to high expansion ratio (diesel cycle
again), ability to use fixed intake and exhaust ports (no valve train,
although there are questions about hot spots above the exhaust port, like the
Wankel), and opportunity for light weight if properly designed. With not hot
exhaust ports, air cooling should be adequate. (It is not adequate around
the exhaust port on current turbo engines.)
Funny you should mention that. Our man on here Jeff Spitzer just got
done
designing and building such an engine although it was water cooled. PL
Michael Zoche is pursuing this path in Germany, but being a perfectionist, he
may never get to the end. Rather than try to get 300 HP from 271 pounds (his
current design), if he went to beefier structure and accepted the consequent
weight increase (while still staying below conventional gas engine weights)
he would take the world by storm because of ability to use jet fuel. But his
design stretches everything to the limit, and as a result he is fighting
constant quality problems from his suppliers. Crankcases must be X-rayed,
heat treated, and samples from the casting subjected to pull tests to verify
metal strength. I say make it steel, pay the 50 pound penalty, and get on
with it. But that is not for Michael.
I have been going to Oskosh and seeing the Zoche engine for it seems
like
fifteen years. I have yet to see it fly. PL
So instead we have the French four cylinder four stroke diesel. If you knew
what a conventional four stroke four cylinder does to a prop, you would gaze
at this engine in amazement, particularly if you start to think about torque
reversals in the gearboxes planned for the higher power versions. But that
is another story.
My conclusion: there is nothing inherently inferior in a V-8 (for example)
over an opposed engine for aircraft use if appropriately designed. I
conclude they can be superior. Liquid cooling will probably require a weight
penalty compared to air cooled particularly if a gearbox is used due to
smaller displacement, but I believe that the longer hot section life time
(cooler hot end) and lower fuel consumption (no need for rich operation, use
of higher compression due to lower head temperatures) offset this
disadvantage. Having a uniblock construction with closer bore spacing due to
liquid cooling results in a more compact package with superior aerodynamics
possible, and one can design the radiator to have cooling drag much lower
than that achievable with an air cooled engine which has very high pressure
drop on the cooling air circuit.
Opposed engines have too many bolts holding things together (crankcase half
to crankcase half, cylinder to crankcase) which lead to stress concentrations
and cracks. We have opposed air cooled engines for historical reasons, not
good engineering reasons applicable today.
I totally disagree with this too. Fifty million has been spent so far
on the Orenda V8 engine and it is still a failure. When I was racing the
life expectancy of our Chevy aluminum block V8's was two hours. At the
end
of that time there were almost no parts that were not cracked. A
much heavier NASCAR engine has a life expectancy of about three hours.
PL
And if rotary engines had better
fuel consumption and lower emissions, and if they could reliably put out
300-400 HP, they would dominate instead. But they do not and so they have
not. However, for sport aircraft where cost counts and emissions don't, they
are a nice option, have attractively low frontal area, and are fun to work
with.
Getting four hundread HP out of a normally aspirated wankel type
rotary is no problem just add another rotor. In 1991 Mazda won Lemans
outright with a 650 HP four rotor engine and they did it with a fuel
quantity limitation.
I firmly believe the rotary has a much better chance than any piston
engine.
Give the three rotor a chance. No attempt was made to lighten the
engine so far for aircraft use. Emissions will never be a real issue
for aircraft engines. They are a drop in the ocean compared to cars and
trucks on the road. Wait another month a new Mazda rotary engine will
be announced at the Tokoyo Motor Show. PL
Be thankful for the diversity available to us as airplane enthusiasts.
Fred Moreno
Yes I agree. If you want some facts on aircraft engines I highly
recommend
Herschel Smith's classic work "A history of Aircraft Piston Engines"
Sunflower University Press. Manhattan Kansas.
Fred, I highly recommend that you buy the ACRE NL CD and read the past
issues
We are going over old ground here. Most of these issues have been
discussed at
length and I don't like wasteing time going over them again. See the
ACRE NL
web site for information on how to order the CD.
Paul Lamar
The Aircraft Rotary Engine Newsletter. Powered by Linux.
http://home.earthlink.net/~rotaryeng/ http://www.linux.org