Lean Of Peak

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awash2002
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Lean Of Peak

Post by awash2002 »

Is it possible to run this L049 lean of peak?
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Paughco
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Re: Lean Of Peak

Post by Paughco »

I'll have to check with the flight engineer on our next flight. He manages the engines; I fly the airplane. Also, I believe that the mixture controls on the Connie have three positions: auto-rich, auto-lean, and idle-cutoff. Dunno that you can peak and tweak 'em the way you might with a Bonanza.

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awash2002
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Re: Lean Of Peak

Post by awash2002 »

OK
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alan CXA651
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Re: Lean Of Peak

Post by alan CXA651 »

Hi Awash.
The connie has off/auto lean /auto rich settings , as does the B17/B377.
Regards Alan. 8)
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CAPFlyer
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Re: Lean Of Peak

Post by CAPFlyer »

LOP isn't recommended for most large displacement engines anyway. For small engines, it works fine, but big engines you're putting a lot of stress on the system which is part of why as the engines got more advanced, the use of automatic mixture controls became more and more prevalent.
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AKar
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Re: Lean Of Peak

Post by AKar »

CAPFlyer wrote: 13 Jul 2020, 18:35 LOP isn't recommended for most large displacement engines anyway. For small engines, it works fine, but big engines you're putting a lot of stress on the system [...]
In fact, generally speaking this is false. I've got no data on the specific engine models used in L-049, but the operating instructions for some other versions, turbo-compounds included, of R-3350 specifically recommend leaning manually for 10 % drop from the peak BMEP for all cruise operation at maximum cruise power and below. This puts the engine right into the best economy range, which in terms of EGT would be approximately 50 °F lean-of-peak.

The operating instructions carry on explaining that "[t]his procedure will place the mixture in the Best Economy range and is desirable since it results in a direct saving in fuel. It has the additional advantage of cooling the engine by means of lower combustion and exhaust temperature due to the excess air which absorbs some of the heat of the combustion resulting in optimum cylinder, turbine, nozzle box and exhaust system durability." Underlining is mine.

C-69 manual I have available hand-leans to specific fuel flow figures for different engine settings. I haven't done the math on that, but I'd guess it resulting in a mixture setting that would be lean-of-peak in "today's" terms.

As far as I have entertained myself reviewing these old monsters as a purely technical subject, it appears that what is today referred to as "lean-of-peak running" was precisely how the engines were intended to be ran during cruise. Even if leaning in reference to the EGT was not the method of choice back then, probably because better engine instrumentation was available.

Some P&W documentation recommends against using the manual leaning range below the Auto-Lean setting. This, apparently, has to do with imprecise mixture control over that limited range, and resulting need for attention when operating the engine. In overall, it appears that Wright encourages use of lean fuel mixtures whereas P&W tends to put more emphasis on the use of rich mixtures. I don't know if Auto-Lean settings were different as a rule between Wright and P&W engines.



Edit: Several engine installations had a linear mixture ramp leaning the mixture further from the Auto-Lean when the mixture was pulled further back towards the cut-off. This has been referred into as "Hand-Lean" and "Manual-Lean", and perhaps something else as well. Whether this ramp was an intentional feature, as in to provide a mean to fine-tune the mixture, or an engineering leftover within the mechanical function of the mixture lever from the "Auto-Lean" to the cut-off, seemed to have some relevance to whether this was to be utilized or not.

-Esa

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CAPFlyer
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Re: Lean Of Peak

Post by CAPFlyer »

Huh, interesting. I'd always been told by the "old hands" that lean-of-peak had started resulting in significantly lower TBOs and this is why auto mixture was introduced. Lean of Peak operation was common on "light" and training aircraft during the 1920s and 1930s, so it would make sense that as pilots advanced, they would also go LOP with the bigger engines as well. They said that scoring was found in the cylinders and the belief was that the LOP operation was causing more oil to be sucked through the rings resulting in less lubrication, allowing metal-on-metal contact. Thus, with later radials the mechanical auto mixture was introduced. Maybe with the Turbo-Compound there were changes made that ensured this wasn't a problem? I can't find any references that show a non-turbocompound R3350 FE or Throttle Quadrant to verify that there was/wasn't a manual leaning capability like there is in the Super Constellations.
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flapman
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Re: Lean Of Peak

Post by flapman »

Here's a description of operation of the R-4360 Turbosupercharged and single stage Supercharged engine from the Aircraft Engine Historical Society: https://enginehistory.org/Operations/R- ... ops1.shtml

Under the section "Manual Mixture Adjustment with Spark Advance" it describes a procedure which involves finding Peak BMEP, and continuing to lean to a specified setting. That might not be Peak EGT, but it is Lean of Max power.

I was under the impression that LOP was okay with supercharged radial engines, as the supercharger mixed and evenly distributed the fuel/air charge to all cylinders. But this is a problem when the practice is carried over to NA horizontally opposed GA engines because the fuel (and air?) distribution is not so even.

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AKar
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Re: Lean Of Peak

Post by AKar »

The main problems with very lean mixtures were back then very much the same ones as today, that is, the issues with mixture distribution. Not so much with the single cylinders, but it appears that with some individual engines, there were difficulties in achieving even mixture distribution for both cylinder rows ("both" as I'm referring to R-3350). This either left the other cylinder row, apparently almost always the rear one, to run too rich, hence possibly overheating and forcing the front row to run leaner than desired. This was something to look for, and R-3350's documentation mentions some stuff that warrants maintenance action on the fuel injection system, such as high CHT split between the cylinder rows.
flapman wrote: 17 Jul 2020, 17:25 Here's a description of operation of the R-4360 Turbosupercharged and single stage Supercharged engine from the Aircraft Engine Historical Society: https://enginehistory.org/Operations/R- ... ops1.shtml

Under the section "Manual Mixture Adjustment with Spark Advance" it describes a procedure which involves finding Peak BMEP, and continuing to lean to a specified setting. That might not be Peak EGT, but it is Lean of Max power.
Interesting to read about P&W's rival to the big Wright - I've got pretty good collection of Wright's documentation, but not too good about P&W, R-1830 aside. Note how they came up with this alternate ignition timing setting of as much advance as 30°, much like Wright did in the R-3350. This is a specific adaptation to take full advantage of the very lean, slow burning mixtures. Of note is also that the selection of such timing with any cruise mixture not leaned sufficiently far away from the peak power/pressure would be outright destructive to the engine. I wonder if many engines met their end that way.

-Esa

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