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If my employer asked me to analyze the reliability
of two different systems. I then come back 10 minutes later and describe system
A is safer because it has 10 less moving parts. He'd kick me in the ass and show
me the door. There is no such method used by anyone in the reliability field.
If you want to convince someone you have a more
safe design, just measure how often it fails. Just add up the total flight
hours, add up the number of failures. Failures / hours = MTBF. This is a very
high number on the rotary, but the cool thing is it looks like it's getting much
better. It's very important to consider the severity of the failure. The rotary
has some positive characteristics there...where you can still limp home. So the
FMEA is an excellent tool.
----- Original Message -----
Sent: Friday, April 10, 2009 7:29
AM
Subject: [FlyRotary] Re: [Lancair_ES] Re:
Rotary Engines
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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