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<< Lancair Builders' Mail List >>
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agony of the thread...
[Is the subject "technical". Sure. Most people building airplanes have
some interest in technical matters and those that do not on this subject can
scan the message header and skip it. Easy.]
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Gary,
There is no agony from my point of view. One of the hard and fast rules of
"discussion" that has always worked well over on the Compuserve AVSIG forum
for the last 15 years (proven by the test of time) has been an absolute ban
on "personal" attacks, but everyone should feel free to be highly critical
of the ideas. Thus, when somebody responds and says, "you are crazy to think so and so..."
then one is transgressing on good mail list etiquette. OTOH, if one says,
that the concept you just stated does not make since and here is why...."
well, that is just fine. That is how we all learn. Snide remarks are
usually considered to fall into the personal attack category, but there is
some grey area there.
In any event, going back to your message about BMEP,... We probably agree about more than we disagree. There are some good "short
hand" uses for BMEP. However, in the context of the use of high powered
piston engines for aircraft like the Lancair, a Malibu, or a Mirage, it is
MY OPINION, for the reasons stated in my message to Jack, that the term is
often used in an inappropriate and broader than warranted manner.
Unfortunately, it is sometimes used, with other technical jargon, to try to
make an argument appear to some non-engine-head type person to be more
persuasive than the combustion science would support. Originally, I jumped into this because Jack Kane had said in response to one
of your messages (in which, from my interpretation, he was seriously
questioning your ability to operate an engine at rather high BMEPs and keep
it together on the durability issues), the following: "If you are turbo charging to get there , I'd have some questions about
longevity. Remember, a TSIO-550 making 325 Hp at 2700 RPM only needs 176
BMEP, and guess how long one of them will run at max power (Or at 65%, from
the reports we hear)."
It was because I perceived that Jack, and to some extent, you, were only
focused on the issue of the absolute value of the BMEP, that I responded
and tried to point out some examples of an engine operating at exactly the
same BMEP, but with entirely different combustion pressure characteristics
that will have VERY VERY different consequences with respect to engine
longevity. One of the reasons I did that is because it is easy for people that are not
deeply involved in these issues to see a number, like the 176BMEP number for
the TSIO-550 that Jack included in his message, and to "stick that number"
in their head as "bad" and then, forever, thereafter, when any discussion
about high powered engines comes to their attention, the first thought that
goes through their mind is, "Oh!... my goodness, engines that operate at
cruise at 176 BMEP are 'bad' and I don't want mine to do that..." So I posted up some data designed to illustrate the point I was trying to
make.
As it happened, the formatting of that short A) compared to B) table was
screwed up in the translation from y screen to the mail list. Let me try it again, and hopefully the formatting will be better:
Example (these numbers are approximations, from memory, from recent
experience on the test stand):
Peak Peak Torsional
Combustion Crankshaft Stress BMEP Pressure Theta(p-p) Reversals
A) 175 1100 psi 5-8deg ~5 x mean B) 175 850 psi 17-19deg ~3.5 x mean Where "Theta(p-p)" is the rotational angle between TDC and the peak of the
combustion pressure event.
In an effort to be still more precisely clear than my earlier message, I add
that the example above, does, in fact, come from the SAME engine. The same
engine at the same RPM and the same spark timing. MP and Fuel Flows are
different.
Now, let me try to address, specifically, one of your questions.
How can one engine produce the same power with a lower peak pressure?<<
Because you (and a lot of others at other times) have asked that precise
question explains why I went to all the effort to follow up on this
discussion about the use and mis-use of BMEP numbers! The same BMEP can
give very different results in terms of what is going on in the combustion
chamber, as my example above, indicates. I wish I could post up a couple of graphic examples to the board. I will
email directly a couple of *.jpg attachments that show examples of the same
engine, operating at exactly the same HP, RPM, BMEP, AND the same spark
timing, but having two very different profiles on the combustion pressure
curves. To try to further answer your question, if you do the math and integrate
the area under the combustion pressure curve, you get to indicated mean
effective pressure. If you know the friction horsepower curve, you can get
to Brake horsepower. However, two combustion pressure curves, with very different shapes, can
each have the same area, and thus, the same horsepower. The examples I am
sending you illustrate that.
Regards, George
LML website: http://members.olsusa.com/mkaye/maillist.html
LML Builders' Bookstore: http://www.buildersbooks.com/lancair
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Please send your photos and drawings to marvkaye@olsusa.com.
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