"David Megginson" wrote in message
...
"Tom S." writes:

There may be planes where it's not OK to lean at 75%, but all of the
POH's I've looked at so far *require* leaning at 75% power to get the
performance numbers (fuel burn and range) that they list.

http://www.avweb.com/news/columns/182152-1.html (Article 57, but read 58
&
59 as well).

[As far as I understand, you're saying that people should run engines
full rich at 75% power. If that's not your point, apologies in
advance for the misunderstand.]

No...you snipped the part I was responding to (note the mark
indentations).


I'm a big fan of Deakin, and it's worth reading this article more
closely -- he's *not* recommending flying full rich at 75% power, but
rather, not leaning to 50 degF ROP. Here's the part that I'm guessing
you were looking at:

Take an engine set up at 50 ROP (rich of peak). Is it "too rich" or
"too lean"? We cannot say, without knowing what else is going on. At
any power setting above about 60 or 65%, 50 ROP is BOTH "too rich,"
and "too lean," in that it is the worst possible mixture setting for
the engine, and moving the mixture EITHER way will improve
things. On the other hand, at about 9,000 feet and above, with no
turbo, 50 to 80 ROP is an EXCELLENT mixture setting, for it will
produce just about the maximum possible power, and it won't overheat
anything at and above that altitude. You NEED 50 to 80 ROP up there!
But 50 to 80 ROP and full power at sea level will destroy your
engine.

To understand Deakin's point, you have to read this part as well:

The more accurate statement is, "Mixtures that are not rich enough,
or mixtures that are not lean enough may be harmful to your engine."
This is NOT a contradiction! The "harmful point" is a CENTER
POINT. Again, look back at the graph.

If you lean to best power or peak EGT at 75% (especially at lower
altitudes), you're going to be very close to that zone (on one side or
the other) -- in fact, some of your cylinders may be in it. That's
his point.

If you lean to 75% and enrich until you see a slight RPM drop (and
adjust throttle, etc. until you're back at 75%), you should be running
rich enough to be clear of the danger zone. If your engine runs rough
LOP, this is probably your best choice.

If you leave your throttle wide-open and lean until power settles at
75%, you should be running lean enough to be clear of the danger zone.
If your engine runs smoothly LOP, this is the absolute best choice.

If you leave the mixture full rich at 75%, you'll also be well clear
of the danger zone, but you'll burn lots of extra gas and are more
likely to have to deal with annoyances like fouled plugs and stuck
valves at some point down the road. More importantly, you're ensuring
that your engine is producing as much CO as it possibly can, so if
you're flying in the winter or at cold altitudes, you'll want to be
particularly careful that that heater shroud doesn't have any leaks.

David, what he's doing is reiterating, then dispelling, the conventional
wisdom.

Read the article in there where he does a 1100nm CC with the mixture set 250
degrees LOP.