Mixture--science vs witchcraft?

Still getting to know my new baby (1990 TB-20, normally aspirated 250hp
Lycoming IO-540). I imagine my question here must apply to most non-turbo,
non-FADEC pistons (though I gather there's some sort of altitude compensator
on some airplanes' engines?). I live in Colorado, which means routine
high-elevation airport ops.

I'm looking for guidance on proper mixture setting for takeoff & landing at
high-elevation (with correspondingly high DA) fields. What's the best way
to achieve maximum power in these conditions?

The "book" answer, per the POH, of full rich for takeoff and landing is
clearly wrong--indeed, I stalled the engine on my first landing roll-out
back here (I was lean of full rich, but, obviously, not enough!). I'm
looking for some "science" to put behind this, instead of "mmm, about
*there*".... I've been tweaking the mixture for highest rpm during the
run-up (2,000 rpm), then looking for a couple of gph above the book's climb
fuel flow for the existing DA on takeoff roll. That seems to work OK for
takeoff, but, of course, I'm somewhat back to guessing for landing
(especially at a different field or if the DA has significantly changed).
Any suggestions or comments?

FYI, the field I'm basing from is 7,030' elevation, with 9 - 10K' DAs
typical; and we've been to Leadville (LXV)--elevation 9,927', North
America's highest municipal airport & highest paved runway, DA of 11,700'
when we visited. This is far more than just an academic discussion for me!!

--
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)

"Douglas Paterson" wrote in message
. ..
Still getting to know my new baby (1990 TB-20, normally aspirated 250hp
Lycoming IO-540). I imagine my question here must apply to most
non-turbo, non-FADEC pistons (though I gather there's some sort of
altitude compensator on some airplanes' engines?). I live in Colorado,
which means routine high-elevation airport ops.

I'm looking for guidance on proper mixture setting for takeoff & landing
at high-elevation (with correspondingly high DA) fields. What's the best
way to achieve maximum power in these conditions?

The "book" answer, per the POH, of full rich for takeoff and landing is
clearly wrong--indeed, I stalled the engine on my first landing roll-out
back here (I was lean of full rich, but, obviously, not enough!). I'm
looking for some "science" to put behind this, instead of "mmm, about
*there*".... I've been tweaking the mixture for highest rpm during the
run-up (2,000 rpm), then looking for a couple of gph above the book's
climb fuel flow for the existing DA on takeoff roll. That seems to work
OK for takeoff, but, of course, I'm somewhat back to guessing for landing
(especially at a different field or if the DA has significantly changed).
Any suggestions or comments?

FYI, the field I'm basing from is 7,030' elevation, with 9 - 10K' DAs
typical; and we've been to Leadville (LXV)--elevation 9,927', North
America's highest municipal airport & highest paved runway, DA of 11,700'
when we visited. This is far more than just an academic discussion for
me!!

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


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

250 HP needs about 20-22 gph for takeoff. After takeoff and throughout
the climb set mixture to achieve 1250-1300 EGT, any altitude. For
landing leave the mixture where it was for cruise. If you need to do a
go around then make sure to advance the mixture a little as you ease the
power in.



Douglas Paterson wrote:
Still getting to know my new baby (1990 TB-20, normally aspirated 250hp
Lycoming IO-540). I imagine my question here must apply to most non-turbo,
non-FADEC pistons (though I gather there's some sort of altitude compensator
on some airplanes' engines?). I live in Colorado, which means routine
high-elevation airport ops.

I'm looking for guidance on proper mixture setting for takeoff & landing at
high-elevation (with correspondingly high DA) fields. What's the best way
to achieve maximum power in these conditions?

The "book" answer, per the POH, of full rich for takeoff and landing is
clearly wrong--indeed, I stalled the engine on my first landing roll-out
back here (I was lean of full rich, but, obviously, not enough!). I'm
looking for some "science" to put behind this, instead of "mmm, about
*there*".... I've been tweaking the mixture for highest rpm during the
run-up (2,000 rpm), then looking for a couple of gph above the book's climb
fuel flow for the existing DA on takeoff roll. That seems to work OK for
takeoff, but, of course, I'm somewhat back to guessing for landing
(especially at a different field or if the DA has significantly changed).
Any suggestions or comments?

FYI, the field I'm basing from is 7,030' elevation, with 9 - 10K' DAs
typical; and we've been to Leadville (LXV)--elevation 9,927', North
America's highest municipal airport & highest paved runway, DA of 11,700'
when we visited. This is far more than just an academic discussion for me!!

-----Original Message-----
From: Newps ]
Posted At: Saturday, August 18, 2007 8:21 PM
Posted To: rec.aviation.owning
Conversation: Mixture--science vs witchcraft?
Subject: Mixture--science vs witchcraft?

250 HP needs about 20-22 gph for takeoff. After takeoff and
throughout
the climb set mixture to achieve 1250-1300 EGT, any altitude. For
landing leave the mixture where it was for cruise. If you need to do
a
go around then make sure to advance the mixture a little as you ease
the
power in.



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?

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.

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?

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...
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

Jim,

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?


You're dead on.

--
Thomas Borchert (EDDH)

Douglas Paterson wrote:
Still getting to know my new baby (1990 TB-20, normally aspirated 250hp
Lycoming IO-540). I imagine my question here must apply to most non-turbo,
non-FADEC pistons (though I gather there's some sort of altitude compensator
on some airplanes' engines?). I live in Colorado, which means routine
high-elevation airport ops.

I'm looking for guidance on proper mixture setting for takeoff & landing at
high-elevation (with correspondingly high DA) fields. What's the best way
to achieve maximum power in these conditions?

The "book" answer, per the POH, of full rich for takeoff and landing is
clearly wrong--indeed, I stalled the engine on my first landing roll-out
back here (I was lean of full rich, but, obviously, not enough!). I'm
looking for some "science" to put behind this, instead of "mmm, about
*there*".... I've been tweaking the mixture for highest rpm during the
run-up (2,000 rpm), then looking for a couple of gph above the book's climb
fuel flow for the existing DA on takeoff roll. That seems to work OK for
takeoff, but, of course, I'm somewhat back to guessing for landing
(especially at a different field or if the DA has significantly changed).
Any suggestions or comments?

FYI, the field I'm basing from is 7,030' elevation, with 9 - 10K' DAs
typical; and we've been to Leadville (LXV)--elevation 9,927', North
America's highest municipal airport & highest paved runway, DA of 11,700'
when we visited. This is far more than just an academic discussion for me!!

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

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.

"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"?

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!
--
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)

"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)