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<< Lancair Builders' Mail List >>
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[If the people on this mail list believe I spend the kind of time these
email exchanges require to prepare, just in order to promote some product
that is still in certification, then I have badly underestimated the folks
who read this mail list, and I will be glad to drop offline and go back to
much more productive work. Just say the word, somebody, anybody, and I'm
gone.]<<
No, don't ever be gone - I for one appreciate all the input I can get. I
think, though, that everyone has a point of view and typically zealously
promotes that. Zealous is good, over-zealous is bad. Unfortunately, the
dividing line is in the perception of the beholder. I have yet to find
anyone that is out there "pushing the envelope" that isn't over-zealous by
some perceptions. Maybe that's why the arrows are in the back...
In response to another post from Fred:
The Continental 470 and 520 engines use a log manifold design under the
cylinders as opposed to the Malibu 520-BE and the 550 engines which have
a top intake manifold that has been more carefully designed to equalize
air flow and mixture. The later engines are reputedly capable of MUCH
smoother operation suggesting the truth of assertion that mixture
uniformity contributes to more equal combustion events cycle to cycle,
and thus more smoothness.
...
In my mind, this differs substantially from the nice, regular, standing
wave behavior one would see from a modern auto engine with electronic
fuel injectors and carefully designed intake manifolds with equal length
runners, carefully designed inlets, and other careful attention to
detail. I gaze upon these new plastic intake manifolds with their
aerodynamic sophistication with some admiration.
....
Related: would you care to comment on the famous "Lycoming knock"
reported by many? For those not "in the know," this refers to the
tendency of Lycoming engines to cruise nicely, and then make about 10-20
knocking noises in a row which then disappears immediately after your
ears prick up in concern. In my plane, the frequency seemed to
correspond to one cylinder making the offending noise a few times in a
row, and then returning to normalcy. It happens often enough that it is
not just "automatic rough" (happens also day VFR), and it remains a
mystery as far as I know.
Your comments and contributions are most appreciated. Keep up the good
work.
Fred Moreno<<
Yes, I believe the cycle-to-cycle variations are maybe as important as the
cylinder-to-cylinder variations in fuel delivery and maybe air delivery.
Example: In our SkylaneRG with the infamous side-draft carburetor we would
lean to "roughness." We later got a mod done on the carb that was intended
to improve the "smoothness" of fuel delivery - a nozzle with better
atomization. There was a noticeable fuel consumption reduction as leanest
smooth operation. I am convinced it was because there was less
cycle-to-cycle variation in fuel delivery. The roughness you feel on
carbureted engines is mostly this effect - it is a random variation in
engine torque, not a regular one that you would get with
cylinder-to-cylinder variations.
"Lycoming knock" exists, I believe - and Lycoming maintains - because of
carbon build-up in a cylinder. A hot spot is generated and creates either
detonation or auto-ignition. As soon as that happens the pressure waves
knock off the carbon and it returns to normal. So, yes what you are hearing
is likely a single cylinder going into detonation for a few cycles. This
repeats at some interval. On the O-540 I noticed it only when cruising or
climbing at 25 inches manifold pressure - anything less than that and it
wasn't there.
And finally, in response to another comment (from George as I recall):
>>... 45" MAP on a crate motor ("no high-tech parts") with 9.0 to 1
calculated CR isn't likely
to produce the kind of reliability that I'd fly behind. <<
Well, the proof will be in the test. And I agree, it will be a challenge.<<
I don't see where this operation is too far out of reason. A lot of
turbo/intercooled passenger car engines operate at about 60 inches. And the
cruse condition will be more like 31 inches. A normal passenger car engine
with a 10:1 compression will pretty much run forever at 2800 rpm wide open
throttle. With a 9:1 compression that isn't off scale or anything - just a
little outside of conventional automotive practice.
Gary Casey
ES project
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