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Brent Regan asks:
1) How does 150 degrees ROP compare in your ROP/LOP performance comparison.
We had so many people ask the question: "Where should I set the
mixture...?" that I thought about writing a book on that subject. It is a
very complex answer. But, it then dawned on me that if I could change the
question around, and ask a slightly different question, the answer would
convey some very useful information... useful every day in the cockpit.
So here is the "better" question:
Question: Where should I *NOT* set the mixture?
Answer: At very high power settings, do not set the mixture anywhere in
the range of mixture settings between about 10-15F LOP and about 100 to 150F
rich of peak. If the cylinder head temps are over 380F, then move further
away from this band of mixture settings in the lean direction or the rich
direction. At modest or lower power settings, set the mixture any place
you want, rich, lean, peak, but remember that if you use rich mixtures at
low power settings, you will likely, in time, cause the spark plugs to
collect gunk and they will have to be cleaned rather often.
And yes, that answer then begs the question as to what is meant by "high"
power settings. My response to that is that in almost all cases, anything
less than 60% power will qualify as a moderate to low power setting.
Anything above 65% will qualify as a high power setting. And we can quibble
about anything in between 60 and 65%.
2) In your opinion, what are the metallurgical consequences of transitioning
from ROP (Climb) to LOP THROUGH peak at high power settings (>75%). In
particular I am interested in the exhaust valve and turbine impeller as they
have low thermal time constants.
As you can tell from the question and answer set forth in 1) above, based
on the hardcore raw engineering data that applies to ALL SPARK IGNITION
INTERNAL COMBUSTION ENGINES, one should absolutely avoid prolonged operation
at high power in that band of mixtures centered around 50F ROP + - 65degrees
or so. For simplicity's sake, I sometimes call that the "forbidden zone".
The process often suggested by OEMs (most recently Lycoming) of *** S L O W
L Y *** leaning down to peak TIT and mucking around in the forbidden zone
is a recommendation that I find to be, shall we say, "exceptional". I
will only do that when necessary to perform the standard lean test required
to obtain the necessary data required to adjust the nozzle flows to obtain
improved fuel/air balance for an engine. Even then, I am careful to do it
at modest altitudes where the air is dense and provides improved cooling and
I do it with the cowl flaps wide open and still watch the CHTs like a hawk.
I do not agree that either the exhaust valves or the turbine impeller have
particularly short thermal time constants, but that may not be a real
disagreement, as we may just have a different point of reference as to what
is "short". I do agree that it takes less time to change the actual
temperature of a turbine wheel than it does, for example, the cylinder head.
On the other hand, the temperature of the exhaust valve is pretty closely
related to that of the cylinder head since the exhaust valve spends about
75% of its time closed and in close and intimate contact with the valve seat
and the valve guide, both of which are integral to the cylinder head.
There is actually pretty good old Lycoming data that tends to show that the
valve temperature is more closely related to cylinder head temperature
rather than exhaust gas temperature.
All of that aside, I favor a leaning technique for transitioning from a
very rich mixture during the climb to a very lean mixture that is, for lack
of a better descriptive term, what we call "The BIG mixture pull."
It goes like this:
A) Coming out of the climb, level off, punch on the autopilot
and.... do nothing for a while.
B) Let it accelerate... make a radio call... and if the CHTs are
coming down or stable, close the cowl flaps.
C) Reset the MP to either the final desired MP or to the final
desired MP minus 3 inches.
D) Reset the RPM to the final desired RPM.
Here is the important part:
E) Take a firm grasp on the RED mixture control, and smoothly (use
the button if it is a vernier) MOVE the mixture from the previously very
rich setting down through peak EGT/TIT and continue on to a fuel flow known
to provide a mixture *well lean of peak EGT/TIT*. NOTE: One does this by
reference to the fuel flow gage, NOT the TIT/EGT gage. The T IT/EGT is
much too slow to respond with relevant information in the time frame we use
for making this "big mixture pull".
This process should take no longer than approximately 5 to 7 seconds
and to do it smoothly, it should take at least four seconds. If you pull
the mixture too far, the engine will tell you about it by operating
somewhat rough. If you really pull it back to far, your wife will complain
as the engine will quit!<g>
If you do it "right" you will, towards the end, actually feel the
aircraft decelerate slightly as the horsepower is reduced during the
transition from below 50F ROP and on down to the end of the fuel flow
reduction.
Now... after doing this:
F) Do nothing for a while. Note the CHTs should drop steadily.
AFTER the CHTs have dropped and stabilized, take a hard look at your fuel
flow.
If your fuel flow is accurate and you are, in fact, lean of peak,
then you can quickly calculate your horsepower as follows:
For an engine with 8.5: 1 compression, multiply the fuel
flow in gph by 14.9.
For an engine with 7.5:1 compression (most turbocharged
engines) multiply the fuel flow by about 13.7 gph.
This calculation will likely be far more accurate than
anything you can get out of the POH.
Note: When operating on the lean side (and only on the lean
side) horsepower is proportional ONLY to fuel flow. NOT manifold pressure.
NOT RPM. Not OAT, and not the stock market. These simple and accurate
formula do not work when rich of peak.
G) If you are anal about the whole thing, (I plead guilty) then
when you get a chance, you can go find PEAK EGT/TIT, but you do that FROM
THE LEAN SIDE. Just slowly enrichen the mixture until the TIT/EGT peaks.
Note the value and then lean again until you are 20 or 40 or 60 or 80 or 100
or YES... even 120 or so LOP (Note: only engines with exquisitely well
balanced fuel/air ratios will routinely operate smoothly below about 90F
lean of peak. My personal engine will usually go to 150 to 170F lean of peak
before roughness sets in, but there is no operational reason to ever NEED to
operate these engines more than about 90 to 100F lean of peak. Anything
more is just a matter of demonstration.
H) Now.... If you now want:
i) More power: Then add some fuel and or add some MP and
some fuel. If you want some more power and you used the initial MP value
that was 3" lower than what you wanted, then, fine, now is a good time to
add 3" of MP. If you do, the fuel flow should rise, but the TIT should not
change much, and it certainly should not cause the CHTs to rise more than a
very few degrees. If the TIT rises significantly, then lean the mixture
some until it returns to its previous value plus maybe 10 to 15 degrees.
Since you already know where peak is, you can muck around while lean of
peak, pretty much at will without putting anything at risk. Remember, if
you are 50F or more lean of peak at high MP it is pretty much impossible to
hurt one of these engines. Some fuel servos may schedule fuel flows with
changes in manifold pressure that do not perfectly maintain the desired
fuel/air ratio, but most do. Just keep an eye on the TIT and stay away
from peak TIT and stay on the lean side and life will be nice to your
engine.
ii) Less power: then consider a) reducing the fuel flow;
or b) reducing the RPM; or c) reducing the fuel flow and the rpm. Reduce
MP only as a last resort. ALL internal combustion engines operate more
efficiently with the throttle wide open than at any other throttle setting.
Keep the throttle wide open or as close to wide open as is reasonable.
Modulate and adjust horsepower with the mixture knob and the RPM.
At any rate, we use and teach the "Big Mixture Pull" technique to all of
the pilots who fly the turbo-normalized 520 & 550 engines in the Bonanzas.
Works like a charm. I have done it in everything from Navajos, to Mirages,
to Malibus, to Mooneys to Cessna 414's. It simply works and works better
than any other technique that we know.
It also has the advantage of minimizing the time the turbo is exposed to
very high exhaust temperatures.
The result is that the consequences of leaning from rich to lean, in this
manner are that there is absolutely no adverse affect with respect to the
metallurgy of the valves and the turbine rotor. Nil.
Sorry to be so lengthy.
Regards, George Braly
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