Mixture--science vs witchcraft?

[Douglas Paterson]

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)

[Matt Barrow[_4_]]

"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

[Newps]

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!!
> :)

[Jim Carter[_1_]]

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

[Jay Honeck]

> 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"

[Thomas Borchert]

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)

[BillJ]

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?

[Newps]

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.

[Douglas Paterson]

"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)

[Douglas Paterson]

"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)

[Douglas Paterson]

"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]

"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)

[nrp]

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

[kontiki]

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.

[Thomas Borchert]

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)

[Dennis Johnson]

"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

[Matt Barrow[_4_]]

"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

[Matt Barrow[_4_]]

"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).

[Matt Barrow[_4_]]

"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.

[Matt Barrow[_4_]]

"Douglas Paterson" > wrote in message
...
> "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

[[email protected]]

Douglas Paterson > wrote:
: "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....

As a avid "Deakin-reader" and a cheap-******* pilot/owner, I can try to paraphrase as follows:

- It's impossible to hurt the engine at or below approx 70% power no matter what you do with the mixture, PROVIDED you keep
the CHT below 350-380 as measured at the bayonette lugs.
- For normally-aspirated engines, it's impossible to exceed 75% power below about 6000-8000' DA.
- EGT will remain roughly the same for a given fuel/air ratio.... SO
.... find out what your full-rich EGT is at a sea-level takeoff and lean to that as DA increases.... whether it's at a high DA
takeoff or as you do a long climbout. It's generally about 200-250 rich of peak.

e.g.... On my plane, I see about 1400 EGT on a sea-level, standard-day full-rich takeoff. During a long climb I'll lean to
the same EGT as I climb. On a hot summer day (2100' MSL field, 4500' DA takeoff) I'll see 1300 full rich and doesn't make
full power. Leaning to the 1400 before takeoff I get full power, and still have good cooling.

In cruise below 70%, I'll lean until it wheezes so long as CHT stays below 375.

-Cory


--

************************************************** ***********************
* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

[Montblack]

wrote)
> For normally-aspirated engines, it's impossible to exceed 75% power below
> about 6000-8000' DA.


"...impossible to exceed 75% power below about 6000-8000' DA."

Below or Above?


Paul-Mont

[Doug[_1_]]

Here is what I would suggest. Fly at the field elevation in level
flight (obviously somewhere where the ground is lower). Lean for
maximum RPM. At that altitude (7000+'), you don't have to worry about
overleaning. Just lean to peak rpm. Now go and land at THAT setting.
Now leave it at THAT setting and go do a runup AT THAT SETTING. Turn
the mixture knob some both ways to see what the effect on peak rpm is
at your runup rpm. NOW USE THAT mixture/rpm behavior to lean. It will
probably be right at peak, but if it slightly below then lean to that.
Hope this makes sense. Basically what I am advocating is leaning to
peak in the air at full throttle, then go on the ground and learn how
to recognize THAT mixture setting based on runup rpm behavior. Good
luck!

If all else fails, just runup at full rpm and lean to peak rpm. That
will work also. (A bit risky with the prop blast at full rpm, but if
you watch where you do it and are careful can be done).

[[email protected]]

Montblack > wrote:
: wrote)
: > For normally-aspirated engines, it's impossible to exceed 75% power below
: > about 6000-8000' DA.


: "...impossible to exceed 75% power below about 6000-8000' DA."

: Below or Above?

Above, of course. The "rule of thumb" that I've heard is 8000', but I think that's at best-power
mixture. The best-economy mixture setting is more like 6500' I think.

-Cory

--

************************************************** ***********************
* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

[[email protected]]

On Aug 18, 3:43 pm, "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.

The people who manufacture that engine, and who have lots to
lose if it quits on you, have a procedure here:

http://www.lycoming.textron.com/support/publications/service-instructions/pdfs/SI1094D.pdf

Dan

[Denny]

That ol black magic has me in it's spell
That ol black magic that we know so well
Those quivering needles up and down my spine
That same ol back fire when it hits red line

<lots more snipped>

So darlin down and down we go
Round and round we go
in a spin
wonderin the spin were in
Starin at that ol EGT against the pin

the devil made me do it - honest!

[gpaleo]

Excellent!!!!!!!!!!!!



Ï "Denny" > Ýãñáøå óôï ìÞíõìá
oups.com...
>
> That ol black magic has me in it's spell
> That ol black magic that we know so well
> Those quivering needles up and down my spine
> That same ol back fire when it hits red line
>
> <lots more snipped>
>
> So darlin down and down we go
> Round and round we go
> in a spin
> wonderin the spin were in
> Starin at that ol EGT against the pin
>
> the devil made me do it - honest!
>
>

[Michael[_1_]]

On Aug 19, 10:58 am, "Douglas Paterson"
> wrote:
> > 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?!?

Yes. Walk around the hangars, and find someone who owns an airplane,
flies it a lot, works on it himself (preferably a lot), has a day job
(preferably outside aviation), and teaches on the side. There are
some out there. For that matter, a CFI ticket isn't really
important. They're not very hard to get and don't assure much in the
way of knowledge or instructional ability. Just find an owner who
flies a plane in the same performance class as yours (a lot) and works
on it himself. Buy him lunch (or better yet, help with the
maintenance) and you will get all the ground school you need.

As for your specific situation, since your engine is instrumented with
CHT/EGT/fuel flow, I can give you a prescriptive system for
determining and applying the proper procedure, as well as the reasons
that underlie it. Here goes.

Aviation engines at takeoff are normally operated 200+ deg F ROP (rich
of peak). Contrary to popular belief, this is neither maximum power
(which occurs about 25-50 ROP) nor primarily for cooling (although
cooling can be a factor). The primary reason we do not lean for peak
power for takeoff is engine timing. The spark advance on aviation
engines these days is fixed some number of degrees (of crankshaft
rotation, not temperature) before (typically 15-25 degrees before) the
piston hits top dead center (TDC). Since it takes some time for the
flame front to propagate from the ignition point (spark plug) to the
piston surface, by the time the flame front hits the piston surface
and transfers the energy to the piston (yes, I am simplifying
tremendously here, but go with it) the piston is already well on the
way down, meaning past TDC. This is critically important -
transferring energy to the piston before that is terrible - it can put
excessive stresses on the crankshaft, rods, etc. It can damage the
engine, much like pre-ignition and detonation.

The time the flame front will take to travel through the combustible
gas is primarily a function of two things that are somewhat under your
control - charge density (meaning basically how much air is there) and
mixture. The denser the charge (meaning the denser the outside air
and the more open the throttle) the faster the flame front moves. The
closer the mixture is to stoichiometry (meaning just enough air to
burn all the fuel and no more) the faster the flame front moves. At
sea level on a standard day at full throttle, you can actually damage
the engine by leaning - because the flame front will arrive too
early. At a density altitude of 10,000 ft this is impossible unless
something is really wrong (for example, the bolts securing your
magnetos are loose and the magnetos have moved increasing the spark
advance - it is actually true that you set the spark advance on these
engines by loosening those bolts, moving the magneto by hand until the
points open the number of degrees before TDC you want, then securing
the bolts again).

Now I have neglected the cooling aspects somewhat here, but they are
important. You really don't want to weaken your cylinders by letting
them get above 400F. 380F is better. 350F is VERY conservative.
This is because the aluminum your jugs are made of does start to lose
tensile strength appreciably above these temperatures.

So having laid out the background, here's what you do. Pull into
position, hold the brakes, full power, lean until EGT's peak, then
enrich (for how much - see below), and take off. No need to do this
every time - pretty soon, you will know where peak EGT is for your
engine at a given density altitude, so you will just pull the mixture
back to that number. You will also soon learn the position of the
mixture control that is appropriate, so you will be able to do it
quickly. Do your initial practice at strips with plenty of room for
error. Once you get to where you can set what you need quickly, you
are ready for the challenging ones.

So how far rick of peak should you be?
Al less than 5000 DA, leave it alone. At 8000 ft DA, probably about
100-200F. That's because you can still hurt your engine by
overleaning. By the time you hit 10,000 DA maybe 50-100. Above that
25-50. Even peak EGT can't hurt above 10,000 DA or so, so as long as
you haven't leaned enough to lose power, anything you do at 10,000+ is
OK. Interpolate between the given points however you want to - the
error in the method of interpolation will be less than the error of
setting the mixture, which is why there is some room for error in the
numbers I give you.

Now this isn't going to be enough for long term cooling - but you're
not going to overheat your cylinders in two minutes by doing this -
and after that point, you should be solidly climbing, absolute best
performance won't be critical anymore, and you can monitor CHT's and
increase fuel flow and/or airspeed as needed.

Is that sufficiently scientific for your tastes?

Michael
CFI, ATP, A&P, PhD, and other good alphabet soup

[Frank Ch. Eigler]

Michael wrote:

> [...] The time the flame front will take to travel through the
> combustible gas is primarily a function of two things that are
> somewhat under your control - charge density (meaning basically how
> much air is there) and mixture. [...]

It may be interesting that the vulnerable time between the spark and
TDC is also to some extent under one's control, with the blue knob.
Thus here's another reason for using maximum RPM for takeoff.

- FChE

[Michael[_1_]]

On Aug 23, 9:24 pm, (Frank Ch. Eigler) wrote:
> It may be interesting that the vulnerable time between the spark and
> TDC is also to some extent under one's control, with the blue knob.

Good point, and you are of course correct. This takes us into a whole
'nother discussion - not can you run oversquare safely (because of
course you can, and virtually every flat-four, six, and eight out
there does) but how much oversquare you can run and under what
circumstances. This is precisely the factor that limits how much
oversquare you can run in most circumstances.

I left that part of it out mainly because I've never heard anyone
advocating making high density altitude takeoffs at reduced RPM, and
the post was already longer than most people will read.

Michael

[[email protected]]

Michael > wrote:
: On Aug 23, 9:24 pm, (Frank Ch. Eigler) wrote:
: > It may be interesting that the vulnerable time between the spark and
: > TDC is also to some extent under one's control, with the blue knob.

: Good point, and you are of course correct. This takes us into a whole
: 'nother discussion - not can you run oversquare safely (because of
: course you can, and virtually every flat-four, six, and eight out
: there does) but how much oversquare you can run and under what
: circumstances. This is precisely the factor that limits how much
: oversquare you can run in most circumstances.

: I left that part of it out mainly because I've never heard anyone
: advocating making high density altitude takeoffs at reduced RPM, and
: the post was already longer than most people will read.

: Michael

...but it does bring up an interesting point. One that I have yet to see adequately explained anywhere. I've seen a
few engine operating curves that limit max manifold pressure as a function of RPM, there are many that don't. My fixed-pitch
Cherokee doesn't have the option, but I'd be interested to know any source that explains it adequately.

-Cory

--

************************************************** ***********************
* Cory Papenfuss, Ph.D., PPSEL-IA *
* Electrical Engineering *
* Virginia Polytechnic Institute and State University *
************************************************** ***********************

[randall g]

On Thu, 23 Aug 2007 08:44:27 -0700, wrote:

>On Aug 18, 3:43 pm, "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.
>
> The people who manufacture that engine, and who have lots to
>lose if it quits on you, have a procedure here:
>
>http://www.lycoming.textron.com/support/publications/service-instructions/pdfs/SI1094D.pdf
>
> Dan


That Lycoming document describes leaning for takeoff at high DA for
these cases:

- fixed pitch - lean for max rpm
- constant speed but no engine instrumentation - lean with smooth
operation of the engine as a deciding factor

What if I have a CS prop plus EGT, MP and fuel flow instrumentation?






randall g =%^)> PPASEL+Night 1974 Cardinal RG
http://www.telemark.net/randallg
Lots of aerial photographs of British Columbia at:
http://www.telemark.net/randallg/photos.htm
Vancouver's famous Kat Kam: http://www.katkam.ca

[Newps]

wrote:


>
> ...but it does bring up an interesting point. One that I have yet to see adequately explained anywhere. I've seen a
> few engine operating curves that limit max manifold pressure as a function of RPM, there are many that don't. My fixed-pitch
> Cherokee doesn't have the option, but I'd be interested to know any source that explains it adequately.


One of the absolute worst sources of information on how to properly run
an engine is the engine manufacturer. They are utterly uninterested in
proper engine management, only reducing liability and costs. For proper
engine operation you need to talk to the GAMI guys in Ada, OK. For
example Continental or Lyc will never tell you that the absolute worst
place to run your engine is 40 degrees rich of peak at any power
setting. Or that Continental flat out refuses to even discuss LOP
operations on any engine other than the 550. But install their
electronic engine controls and where do you think your engine is
running? Yep, LOP. We've all been taught that detonation is bad.
Turns out detonation is a trivial concern for our engines. GAMI has set
up an engine to detonate for days at a time of constant running. End
result? No damage whatsoever. Now preignition is a completely
different animal.


http://www.gami.com/frames.htm

[Matt Barrow[_4_]]

"Newps" > wrote in message
. ..
>
>
> wrote:
>
>
>>
>> ...but it does bring up an interesting point. One that I have yet to see
>> adequately explained anywhere. I've seen a few engine operating curves
>> that limit max manifold pressure as a function of RPM, there are many
>> that don't. My fixed-pitch Cherokee doesn't have the option, but I'd be
>> interested to know any source that explains it adequately.
>
>
> One of the absolute worst sources of information on how to properly run an
> engine is the engine manufacturer. They are utterly uninterested in
> proper engine management, only reducing liability and costs. For proper
> engine operation you need to talk to the GAMI guys in Ada, OK.

Completely agree!

Quite frankly, from what Deakin points, manufacturers recommend operating in
the worst possible ranges as far as mixture goes, both for ICP and for
fouling. Then they try to claim you didn't follow their directions.

At least TCM is (slowly) getting its act together in the Columbia's 550
series.
--
Matt Barrow
Performance Homes, LLC.
Cheyenne, WY

[Michael[_1_]]

On Aug 26, 6:53 pm, wrote:
> ...but it does bring up an interesting point. One that I have yet to see adequately explained anywhere. I've seen a
> few engine operating curves that limit max manifold pressure as a function of RPM, there are many that don't. My fixed-pitch
> Cherokee doesn't have the option, but I'd be interested to know any source that explains it adequately.

The main reason such a chart is not universal is because there is no
requirement to publish such a chart, not because it's not relevant. I
suppose there might be a (horizontally opposed) engine installation
out there that will comfortably run at sea level with the throttle
control all the way forward and the prop control all the way back, but
I doubt this is the norm.

While few manufacturers provide a limit chart, most provide
recommended power settings (generally with quite a few 'oversquare'
options) and other guidelines (for example: Do not exceed 25" MP at
2000 RPM or less).

Michael