Mixture--science vs witchcraft?

"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.

"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)

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

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"

"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)

"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

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?

"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)

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)