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<< Lancair Builders' Mail List >>
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Somebody pointed me to this excellent mail list and suggested that I come
over and read through the mail postings and comment.
I am the engineer at GAMI and Tornado Alley Turbo. We have been correcting
the fuel/air ratio imbalances on an increasing number of TSIO-550 and
TIO-540 engines lately, and have probably seen more hard data from more of
these two engines than even the OEMs.
In addition, we are currently operating a TIO-540 Lycoming on our engine
test stand doing certification work for our PRISM electronic ignition
project. The test stand is the most thoroughly instrumented test stand for
piston engine detonation work we know of anywhere in the world. This test
stand was recently used for a complete FAA monitored certification run of a
new engine.
I would be happy to answer any questions anybody has. If the question is
one about an area of engine operation about which we do not already have
hard data, I will be glad to go out and fire up an engine on the test stand
and collect the data and post it back here.
In a message posted this morning, Brian Barbata refers back to a previous
post of Brent Regan's and states:
>>
1) All the technical discussions of temps, range, etc. are minor
compared with the detonation
risk, which increases with LOP power settings.
2) The fact that someone has been running LOP for hundreds of hours
successfully is beside the point.
3) If you run LOP at 80% and forget about it because of some
distraction,
detonation may be your reward upon descent.
4) The other LOP detonation scenario is a small piece of dirt
getting into an injector nozzle.
Normally, you might get an almost unnoticeable sputter, soon clearing
itself. But if you are LOP, you might get detonation....a one-way ticket.
5) It's theoretically a manageable risk, but so are a lot of other
things that
bring airplanes down. How many "manageable risks" would you like in your
cockpit? The fewer the better for me! <<
Let me respond to these one at a time:
1) The idea that detonation risks increase during LOP operations is simply
not supported by any known data. I will pay $1,000 bucks to the first
person that can supply me some hard repeatable data that supports that
notion. Let me elaborate a bit. The FAA has recently adopted a
detonation testing standard that includes a hard mathematical determination
of a detonation index from calculations made from the internal combustion
pressure data. Nothing "subjective" about it.
The widespread confusion in this area grows out of the history of general
aviation mixture use instructions. Mechanics for years have said, "...
don't run it too lean you will burn it up!". But what a mechanic typically
means by "lean" is a reference to something between peak EGT/TIT and 75 or
100F rich of peak. With all due respect to my many mechanic friends, MOST
mechanics had, prior to 1997, never seen an engine that was ACTUALLY
operated lean of peak (except for the TSIO-520BE's on the Malibus, and many
of those pilots had a really, really bad habit of adding some fuel flow
"just to be safe" and thus, ended up operating at peak or somewhat rich of
peak.)
Many mechanics had seen engines that had been "operated too lean", but that
is a completely different issue than an engine operated lean of the
stoichiometric fuel/air ratio. This confusion in terminology is responsible
for a lot of the problems many pilots have understanding this issue. I
know this is confusing because I periodically teach a session for the FAA at
one of the A&P/IA continuing education programs. I do surveys of 100
mechanics at a time on this subject. Very very few of them have a good
grasp of the engineering issues involved in this discussion. I give a quiz
at the beginning of those seminars and ask the mechanics to do a "multiple
choice" test to select one of four sets of curves that depict the correct
relationship of EGT,CHT,Hp, and BSFC. I have never seen even as many as
25% get it right. Think about that. On a multiple choice test with four
choices, you ought to be able to get 25% correct blindfolded. That means
that there is some serious mis-information out there that is causing the
mechanics to carry around an erroneous understanding of this whole issue.
This is NOT to criticize the mechanics. It really isn't their fault. They
have just never been exposed to the hard data have no experience with it.
So with that preamble, let us take a closer look:
Assume you have a 350 Hp engine, operating with a constant airspeed, and at
280 Hp. I picked 280Hp because the person who wrote the message referred to
an 80% power setting and .8 X 350 = 280.
A) Mixture set to 50F ROP (pick your number, the results are the
same for 100F or 25F or even peak TIT) rich of peak. Let us assume that the
hottest CHT is 420F. If that engine is one of the existing class of engines
in this horsepower range, it will be very close to or actually be
experiencing light detonation. Peak combustion pressure values will be in
the 850 to 1000+ PSI range.
****** The TIO-540 now operating on our engine test stand, in stock
configuration, using 100LL, and under these conditions will run one or
two cylinders continuously in light detonation, as measured by the FAA's
current detonation criteria. ****
B) NOW, set the MP up and reduce the mixture to 50F LOP, so that
the engine is again operating at the same 280Hp. Change nothing else.
What happens? First, the CHTs drop by 30 to 40F. Second, the combustion
burn time slows down. Third, the peak combustion pressure values drop to
the 750 to 850 PSI range. (The MEAN combustion pressure remains the same,
because the Hp is the same.) Fourth, the engine no longer operates in light
detonation.
Whenever we have an engine up and running on the test stand, we routinely
demonstrate these results on a routine basis to people who happen to be
passing through our facility.
Those are the hard engineering facts we are able to see and repeat on a
daily basis. We did it yesterday in a demonstration for Terry Capehart who
was passing through town.
2) I essentially agree with the second statement made by Brian. That is
nothing but anecdotal data. However in science, if the anecdotal data is
not consistent with the hard science data, then one has a problem. To that
extent, it is scientifically significant. The fact that people have been
able to operate lean of peak for hundreds of millions of successful engine
hours is beside the point in many respects. However, it *IS* consistent
with the observed engineering data, as reflected in 1) A) and B), above.
3) In the third item, Brian suggests that if you start a descent while at
80% power LOP and forget to make a MP reduction, you might induce
detonation. For a variety of reasons, this is unlikely, but let us
assume it to be true.
Now consider, if you are operating 25F or 50F RICH of peak and you "forget
about it" due to some distraction, and descend, then you have an even higher
chance of experiencing an adverse event than if you started out 50 F LOP.
I don't understand the point to these kinds of comments. For every comment
about LOP operations that asserts "... but what if you do this or that
'wrong' " one can also point to at least one or two comparable and equally
erroneous actions that the pilot can take while operating rich of peak, that
have an equally or more adverse outcome. When one considers two
alternative courses of action, one has to assess the "fault tree" for each
alternative way of operating the engine. The side of the fault tree that
flows down from the ROP side has a lot more branches that lead to trouble
than does the side the fault tree that flows down from operating LOP.
4) This is a classic variation on the item discussed above in 3). "What
if..." a piece of dirt partially clogs a nozzle while lean of peak? Good
hypothetical. Has anybody ever actually experienced this? I have. The
engine instantly operates so rough when this happens that the pilot knows he
has a problem. His natural reaction is to power back. No problem. If he
has the presence of mind to look at the JPI before he powers back, he will
easily see the cylinder that has partially plugged as the EGT on that
cylinder will have dropped ... a LOT, maybe even off scale. So, following
the methodology outlined in 3) above, play the same "What if..." game, but
start from an engine operating 75 to 150F ROP. Safe, right? Well, no.
Suppose a nozzle partially plugs (the most common event). THAT cylinder may
then move back to 25 to 50F ROP, and thereby cause the CHT to rise by 30 to
40F, and THAT may cause that cylinder to detonate. Worse, suppose it
happens while you are at takeoff power? Then you are likely in legitimate
serious risk of detonating one of the high powered turbo'd engines.
5) I submit that the hard engineering data clearly shows that the number of
"risks" that have to be "managed" when operating lean of peak is reduced as
compared to the number of "risks" that have to be "managed" when operating
rich of peak.
These issues are really matters that are subject to routine verification by
reference to hard engineering data. Somebody once said that it is pretty
pointless to argue about matters that are subject to factual determination.
I sure agree with that observation.
If you are ever in Southeastern Oklahoma, drop in and see us. We are on
the airport at Ada, Ok. (ADH). We have some very good Bar-B-Q down the
street. I'll buy lunch.
Regards, George Braly
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