Mixture--science vs witchcraft?

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)

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 *
************************************************** ***********************

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

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 *
************************************************** ***********************

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

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

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