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Posted for George Braly <gwbraly@gami.com>:
Marcelo,
Please... you have misinterpreted what Mr. Deakin has said. I suspect that
he would be horrified.
Neither Mr. Deakin nor I have ever suggested that ALL turbocharged engines
could be operated WOT and LOP using the BMP technique.
They cannot and should not. Emphatically!
Mr. Deakin and I and Walter Atkinson just spent a lovely three days together
making some video clips in a turbocharged aircraft to be used in the
upcoming Advanced Pilot Engine Management Seminar classes we are conducting,
starting next month. (Because of your obvious interest - - you might
consider attending!)
We did a LOT of the Big Mixture Pulls in those video clips - - but they were
not done at 38" MP or anything like that.
The "Big Mixture Pull" technique is *** WONDERFUL*** and it works very
very well for a very wide variety of situations. But *NOT* all.
For example, I would not think of attempting to operate Brent Reagan's
rather excellent Barrett Performance Aircraft built TIO-540AE2A engine at
WOT and LOP by doing the big mixture pull.
That engine SHOULD (I have extensively run another identical engine on the
test stand with full up internal cylinder pressure instrumentation) operate
very nicely while lean of peak at about 33 or maybe even 34 inches of MP.
Under the right circumstances, maybe at 35. BUT... I would not leave it
parked at 37 or 38" of MP and attempt the BMP. I will not even do that on my highly instrumented engine test stand.
This has nothing to do with detonation or pre-ignition (although it could -
- but there is no need to go there, as there are other compelling reasons.)
Then - - Why? Because when the mixture goes through the high power F/A ratio danger zone -
- from slightly lean of peak out to 150F or more - - even briefly (remember,
there are about 20-22 combustion events in each cylinder each second) the
PEAK cylinder combustion pressures at 38" MP and peak 25LOP or PEAK TIT or
25 or 50 or 100 or 150 or 200F ROP - - will be unacceptably high even for a
single combustion event... much less 20 or 22 of them during the one or two
seconds that you are passing through that mixture setting while you move the
mixture control.
This is especially compelling in the situation of Brent's engine, or similar
engines, with 8.7:1 compression, but it is also true of a stock TCM TSIO-550
at 38" with 7.5:1 and stock Lycoming engines with 42" MP and 7.3:1
compression ratios.
And, just for the record, heaven forbid you try this with a TIO-540
Lycoming with the MP up at 46" (Chieftain engine).
******************************
Now, that said, a turbo-normalized IO-550 with 30 or 31" MP and the CHTs
down in the sub-380F area can do the big mixture pull with extreme grace and
no chance of harm to the engine. Even 33" would probably be fine for that
engine. Anything more on the MP, and I would not do that. I would consider
setting the MP at 31" and doing the BMP - - and after being stable at 90F
LOP for a while, THEN increasing the MP up to 32 or 33 or 34". But THAT is
a very different technique and because the mixture is ALREADY very lean of
peak, the peak pressures are already well controlled by the slow burn time.
With a 285Hp TSIO-520 engine (a 520D, for example) with max MP of 32.5" Hg,
doing the big mixture pull with sub 380F CHTs is also not a problem.
But more powerful engines, or engines not as well behaved, or engines with
any CHT elevated (> 380F or so) should not be doing the BMP at those very
high manifold pressure settings. Regards, George Braly
I would climb to a safe altitude, then slow my rate of climb to 800fpm,
picking
up extra speed.
Then I would lean until 50 degrees lean of peak if your engine allows for
that
and you have an all cylinder probe system with EGT and CHTs for all of them.
If
you can't lean enough or can't see each cylinder's temp, I would stick to
full
rich until cruise.
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