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Good run down, Mark.
Gary does mention the numerous parts on
the rotor itself – and while each rotor does have a high part count,
you have to consider that each rotor is the
equivalent of 3 pistons – so in that context the parts count
is actually lower, not higher – its very seldom you ever hear of any
failure of rotor parts other than the occasional apex seal – wear yes,
failure – seldom. plus I have never heard of a rotor coming
through the block {:>) So, good questions and good answers from
you.
One saying does come to mind – from
our good friend, Tracy Crook. “…If you’re asking if you should do it,
you probably shouldn’t. If you should be doing it, nobody can talk you
out of it..”. For 90% of homebuilders, its probably
not appropriate.
Ed
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Mark Steitle
Sent: Friday, April 10, 2009 9:26
AM
To: Rotary
motors in aircraft
Subject: [FlyRotary] Re:
[Lancair_ES] Re: Rotary Engines
Thanks for adding a more technical tone to this discussion. Yes,
I was not accounting for all the misc pieces needed to make the rotary run, but
then I wasn't considering all the little pieces needed to make a conventional
piston engine run either. Of the pieces that normally fail and end up
poking out through the engine case, I think you'll agree that the rotary has
significantly fewer of those. In fact, I have never seen a rotary with a
thrown connecting rod. ;-)
Having a liquid cooling system is a two-edged sword, but its
not anything that can't be overcome with good engineering. For
coolant lines on my installation I used aluminum tubing connected to the engine
and radiator via "Wiggins" couplings. I
monitor coolant pressure, coolant level, and coolant temperature. Of
course, if I catch a Canadian goose in the radiator, it will likely loose its
ability to cool the engine, but then you have the same risk with an air-cooled
engine.
As for the bsfc, do your numbers reflect the modern EFI
systems, or carbureted engines. Tracy Crook realized a significant
improvement in bsfc when he switched from carburetors to EFI. The
new "Renesis" rotary engine has a better bsfc due to the side
exhaust ports. Anyway, I prefer to consider it
in "dollars per air-mile". By the time you factor in the
cost savings for purchasing and maintaining a rotary engine over a certified
engine, and that the rotary runs happily (prefers) on 89 UL fuel (half the
cost of avgas), the cost per mile tips significantly in favor of the
rotary. (Reading
the recent post about the $2300.00 oil pan practically brought tears to my
eyes.) I guess its the German in me that caused me to seek out something
better, or different.
Ahhhh... you mentioned the magic word, "turbo-charger".
I built my engine with the intention of turbo-charging as it was
initially turbo-charged in its former life. After much thought,
I decided to follow the KISS principle and go N/A. But
there's a little voice in my head that keeps whispering
"turbo-charge". With the rotary's high energy exhaust gasses,
turbo's are a natural solution. Yes they add weight, but not much
more than my current exhaust system. Stay tuned...
One thing that I hadn't mentioned that could be considered a negative
for the rotary engine is that very few A&P's know anything about rotary
engines. Heck, very few auto mechanics know how to work on a rotary
engine. But, if I'm there with my repairman's certificate in hand, who
needs an A&P? Also, rotary parts are less plentiful if you get stuck
in some hole-in-the-wall town. But there is always UPS overnight.
Gary,
thanks again for your thoughtful post. I'm not trying to convert
anyone to a rotary engine, I only want to see it get a fair
shake.
P.S. I've CC'd the Fly Rotary group as they need something to
talk about (the list has been rather quiet lately).
On Fri, Apr 10, 2009 at 7:13 AM, Gary Casey <casey.gary@yahoo.com> wrote:
I'll certainly have to commend Mark on the great work
with the rotary engine. I agree with his comments on almost every count.
But...
You probably should count ALL the parts in and around
the engine to have a fair comparison. For example, the air-cooled
aircraft engine cooling system has essentially no moving parts, unless you
count the vernitherm. Yes, the 3-rotor engine has only 4 MAJOR moving
parts, but each rotor has about 50 components. While that's not
necessarily good or bad, it's not an inherently simple solution. It
rejects more heat to the coolant and more of that to the oil (rotors are
oil-cooled), making the cooling system larger and potentially more complex.
And the exhaust is hotter and contains more aggressive pressure pulses,
which have to be taken care of by some sort of muffling. The ideal
muffler is probably a turbocharger, which can work very well on account of the
pressure pulses, but it probably takes a special high-temperature turbo that
can tolerate the up-to 2000 degree exhaust. The turbo adds weight and
complexity, but perhaps not more weight than an effective muffler. The
fact that the engine is inherently round and concentric with the output shaft is
a good thing, but probably more attractive for a wing-mounted engine than one
in front of the fuselage. The rotary engine almost requires a speed
reduction unit to make the power/weight come out favorable, and I was not
impressed with the design of the then-currently available units, although they
seem to work okay in practice. One big thing that bothered me is that the
efficiency is inherently lower than that of a good piston engine, partly
because the compression ratio is limited to less than about 9 and the surface-volume
ratio the combustion chamber is higher. This penalty is probably 5 to
10%. All that being said, the big attraction to me was, as Mark said, the
rotary will rarely completely fail, even if the coolant is lost. The apex
seals might disintegrate and parts warp, but it will most likely continue to
produce power for some time, unlike a piston engine. A long time ago we
were testing many rotaries and occasionally we would see a loss in power.
When the engine was shut down it welded itself together even though it
was still producing power. And the very things that make it less
efficient contribute to the fact that it can tolerate a variety of fuels.
And with boosting it can be made to produce a lot of reliable power.
I seriously looked at 3 different approaches - a
standard aircraft engine, a direct-drive automotive piston engine, and a
rotary. The eventual deciding factors were that the automotive engine
came out heavy and the rotary engine burned more fuel. I really do like
the rotary, though.
From: Mark Steitle <msteitle@gmail.com>
To: Lancair_ES@yahoogroups.com
Sent: Thursday, April 9, 2009
2:27:21 PM
Subject: Re: [Lancair_ES] Rotary
Engines
Since there were no other replies, I figured I would
give my 2-cents worth.
I have been flying a 3-rotor Lancair ES for almost 2
years now with a total of 110 hrs on the Hobbs.
While it hasn't been without some teething pains, all-in-all, it has been a
very positive experience and I would choose a rotary again if/when the
opportunity presents itself.
While I did the FWF myself, my installation and
the Mistral are both closely related. As an example, I could bolt a
Mistral intake and/or exhaust directly to my engine, and probably
interchange many parts with the Mistral 3-rotor. The Mistral folks have
taken much of the rotary engine technology, and refined
and pakaged it into a (soon to be) certified product.
My reasoning is based on my belief that the rotary is
inheretly a stronger engine (pistons are cast iron vs.
aluminum), with only 4 moving parts. If you read the
recent AOPA story about the Cessna 400 blowing an engine over Pennsylvania in the night, well, I had a
similar experience in a Cessna 152, only not at night. Like the chap in
the AOPA story, we too just barely made it to the nearest airport, with
oil pouring out from the cowl onto the runway. Since that incident, I have
been very leery of all conventional piston engines. Hence my
decision to go with a rotary.
Gross weight on my ES was 2060 lbs. I typically
climb out at 7000 - 7200 rpm (2400 - 2500 prop rpm), climbing at between
1000 fpm and 1300 fpm, burning 16 - 18 gph, 15 gph in regular cruise (6000
rpm) and around 10 -12 gph in economy cruise (5100 rpm). (Keep in
mind that the pistons (rotors) turn at 1/3 the speed of the crankshaft, so they
are only turning 1733 rpm in economy cruise.) I can run either 100LL
or mogas (w/o alcohol) without worry and can lean the mixture aggressively
without worry of hurting the engine (no exhaust valves to burn). I can
pull the throttle to idle whenever and not risk shock cooling
the engine. Being fuel-injected, it will start cold, hot, or
anywhere in between. What's not to love?
I mentioned some teething pains... those consisted of
an early cooling problem which was solved with an auxilary water-to-oil
exchanger and a cowl flap. I have also had a series of oil leaks, all
from the oil pan not being properly sealed. I finally pulled the pan,
cleaned and resealed it. Problem solved. The
toughest issue to resolve has been finding a muffler that could withstand the
pounding of the rotary's exhaust. I'm pretty sure that issue
has been resolved by switching to a DNA racing muffler, but I don't
have enough hours on it yet to state for certain.
Hopes this helps answer your question(s).
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