Mixture--science vs witchcraft?

"Jay Honeck" wrote in message
oups.com...
Any suggestions or comments?

Do you have an engine analyzer on board, Doug?

If so, this tool (we have the JPI EDM-700) lets you lean with
confidence that you're not harming anything...


I do--the Insight GEM 602. I'm doing my best to integrate that information
(I'm new to this instrument), too. My question is less about "harming" the
engine from over-leaning than it is about achieving max power for takeoff &
landing (i.e., potential go-around). Of course, failing to achieve that
power could result in harming the whole airframe, eh...?

--
Doug
"Where am I to go/Now that I've gone too far?" -- Golden Earring, "Twilight
Zone"
(my email is spam-proofed; read the address and make the appropriate change
to contact me)

"BillJ" wrote in message
...

Maybe flying with an instructor who knows about this critical question
would help?

Indeed!! Even--or perhaps "especially"--some ground school on the subject,
and I have been actively pursuing (not finding) just that. Know where I can
find such a CFI?!? Every instructor I've approached on the subject has one
of two answers: "if the POH says full rich, put it there," or "lean it, oh,
about *that* much...." ARGHH!!

Speaking as a big-iron guy w/ military procedures drilled deeply into his
skull from day one, I find the lack of information on this disturbing....

--
Doug
"Where am I to go/Now that I've gone too far?" -- Golden Earring, "Twilight
Zone"
(my email is spam-proofed; read the address and make the appropriate change
to contact me)

On Aug 19, 10:51 am, "Douglas Paterson"
wrote:
"Jim Carter" wrote in message

news:000d01c7e206$d059b690$4b01a8c0@omnibook6100.. .



The engine won't generate 250 HP at altitude will it? Isn't that a sea
level rating based on the pressure of the air available at sea level? So
the 20-22 gph would be for sea level wouldn't it?

That is precisely what's at the root of this issue.



At FL180 the pressure is 1/2 of sea level so we can interpolate that at
9000' the pressure would be only 3/4 of sea level, then factor in the
temperature for the density altitude the engine will really breathe.

I don't think the relationship is that linear, is it? (going from SL to
1,000' is a bigger change than going from 9,000' to 10,000', AIUI) But I
agree with you in general.

Doesn't this mean that the normally aspirated engine in the OPs question
will produce significantly less than 250 HP? I'm not going to do the
math because I'm sure to get it wrong and there are many others on here
more qualified than I, so I'm only guessing that we might see 200 HP. If
that's the case then instead of 20-22 gph wouldn't we be looking for
around 16 gph?

And, as it happens, that 16 gph is pretty much right in the ballpark of what
I've been using. The climb chart tells me I should be seeing around 14 gph
in a climb through 9,000' DA, so including the 2 gph "enrichening factor,"
16 is what I'm seeing (numbers from memory, I do not have the chart in front
of me).

This sounds a LOT closer to the "science" I'm looking for here!! What's
this math that you don't want to do in public? If there's some equation I
can plug the variables into & come out with the right answer, I'll be a
happy camper! Is there "someone more qualified" than Jim (your words! to
show me the math? Thanks!
--
Doug
"Where am I to go/Now that I've gone too far?" -- Golden Earring, "Twilight
Zone"
(my email is spam-proofed; read the address and make the appropriate change
to contact me)

According to my engr reference book the pressure at 9,000 would be
about 71% of the sea level pressure and the standard temperature is
only about 27 degF

Douglas Paterson wrote:

Speaking as a big-iron guy w/ military procedures drilled deeply into his
skull from day one, I find the lack of information on this disturbing....


I sympathize. Perhaps flying from mostly sea level runways has allowed
me some insight as to how much leaning I need to maintain best power
for my airplane. I start leaning on the way up and by experimenting
over time I've pretty much figured out where to set the mixture control
when climbing in high DA situations.

Its an "educated guess" but seems to work although the highet DA
I've actually had to depart from thus far was about 6K.

Douglas,

Every instructor I've approached on the subject has one
of two answers: "if the POH says full rich, put it there," or "lean it, oh,
about *that* much...." ARGHH!!


In Colorado? Yikes!

--
Thomas Borchert (EDDH)

"Douglas Paterson" wrote in message
...
Deakin's advice still falls on the
"witchcraft" side of the equation, in my book, boiling down to "put the
mixture where it feels right.

I believe that the best "science based" information about this topic is
available from the GAMI folks in ADA, Oklahoma
http://www.gami.com/frames.htm

They have perhaps the best instrumented piston airplane engine anywhere and
have collected lots of repeatable data that are the basis for their
recommendations. They have seminars that answer the kinds of questions
you're asking and now offer them online, too. Lots of graphs and tables. I
haven't attended yet, but I plan to.

In another message, you asked about the mixture setting for maximum power.
I think max power is in the vicinity of 25 - 50 degrees rich of peak EGT.
In cruise at altitude, pull the mixture out until the EGT starts decreasing,
push it back in until it peaks, then push it in some more to get 25 - 50
degrees less than peak. That procedure doesn't work on takeoff; the method
suggested by David Paterson in another post is a good one for takeoff.

Running full rich for takeoff at very high density altitude will put you way
too rich and you'll give up lots of horsepower when you need it the most.
Running full rich for low density altitude takeoffs is important because the
engine is producing at or close to maximum power, where cylinder cooling is
needed. The richer mixture effectively retards the timing, which causes the
peak cylinder pressure to occur later in the combustion cycle, which results
in cooler CHT.

Dennis

"Douglas Paterson" wrote in message
...
"Matt Barrow" wrote in message
...

http://www.avweb.com/news/pelican/list.html (Start with #18 and go
through the entire "Engine-Related Columns" series)



Matt, that was a great series, thanks for the pointer. I found #63,
"Where Should I Run My Engine? (Part 1)," to most closely address my
question. http://www.avweb.com/news/pelican/182179-1.html

However, I'm still disappointed. Deakin's advice still falls on the
"witchcraft" side of the equation, in my book, boiling down to "put the
mixture where it feels right. From that article:

The books are full of various techniques for this, but I find the
simplest and most effective is just add full throttle,
full RPM, then grab the mixture knob and move it aggressively from
full rich to whatever feels like "more power"
on the takeoff roll. You can't hurt the engine with momentary
mixture settings like this on normally aspirated
engines! Saw that mixture knob back and forth, and feel the power
change in the seat of your pants! At some
point as you pull the mixture out from full rich, you'll feel the
power first increase, then for a large part of the
movement you'll feel no power change at all, because the "best
power" mixture setting is very flat in that area. (In
other words, "best power" occurs over a fairly wide range of rich
settings, but not at full rich.) Go ahead, pull it a
bit too far, and you'll feel the power drop off from being not rich
enough. Push it back in to the point where you
first felt the best power, and forget it. It's quick, simple, and
very effective, and pinpoint accuracy is not necessary.

However well this may work (??), it hardly qualifies as "science." Anyone
have any suggestions on what "books" he may be referring to when he says
"the books are full of various techniques"?

The old P&W series from the 40's and 50s.

I think, though, you're confusing his method of getting from rich to lean.
Given his copious charts and lengthy explanations of each, and discussion of
the "Danger Zone", lingering in the peak areas is, his points are in fact,
well documented. The part you mention above is, IIRC, how to do it without
proper instrumentation.


A big take-away from these articles is that I'm probably wasting my time
tweaking for max power (rpm) during the runup. Also, reading between the
lines seems to indicate that using the climb fuel flow chart from the POH
is probably a decent starting point--though I'm not yet really convinced
of that. Thanks again for the discussion--I appreciate any & all
insights!

Read his articles about the lunacy in most POHs.

You might find it helpful to print out his entire engine series and read
them slowly, trying to integrate all that he has to say. There is certainly
a load of information there and it's more difficult to grasp in that so much
of it goes contrary to what most all of us have been taught over the past
couple generations. (Think of the Enlightenment and Renaissance after the
Inquisition).

Good luck with your new bird!!


--
Matt Barrow
Performance Homes, LLC.
Cheyenne, WY

"Dennis Johnson" wrote in message
. ..

"Douglas Paterson" wrote in message
...
Deakin's advice still falls on the
"witchcraft" side of the equation, in my book, boiling down to "put the
mixture where it feels right.

I believe that the best "science based" information about this topic is
available from the GAMI folks in ADA, Oklahoma
http://www.gami.com/frames.htm

Agreed!!!!

Also, their sister company, http://www.advancedpilot.com seminar is about
the best thousand bucks you'll ever spend. It's paid itself back many times
over in terms of 100LL $$$ and maintenance (when I traded my 1800 hr TN
Bonanza 36, there was virtually zero lead buildup in the engine and
compressions were all still well into the 70's).

"Newps" wrote in message
. ..


Jim Carter wrote:





The engine won't generate 250 HP at altitude will it? Isn't that a sea
level rating based on the pressure of the air available at sea level? So
the 20-22 gph would be for sea level wouldn't it?

You need that for proper cooling to make up for the little airflow you're
getting.

Or you can use a higher climb-out speed.

"Douglas Paterson" wrote in message
news
"Jay Honeck" wrote in message
oups.com...
Any suggestions or comments?

Do you have an engine analyzer on board, Doug?

If so, this tool (we have the JPI EDM-700) lets you lean with
confidence that you're not harming anything...


I do--the Insight GEM 602. I'm doing my best to integrate that
information (I'm new to this instrument), too. My question is less about
"harming" the engine from over-leaning than it is about achieving max
power for takeoff & landing (i.e., potential go-around).

The harm would come from too high a CHT, particularly over an extended
period.

Re-read: http://www.avweb.com/news/pelican/182176-1.html

I assume the GEM provides CHT temps?

Of course, failing to achieve that power could result in harming the whole
airframe, eh...?

Well, yes...if the engine caught fire. :~(


--
Matt Barrow
Performance Homes, LLC.
Cheyenne, WY