Leaning - Cessna manual vs. Pilot's notes

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cristi.neagu
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Leaning - Cessna manual vs. Pilot's notes

Post by cristi.neagu »

Hello,

Reading through the actual POH for the 172R (as well as the manual that comes with the A2A version) i noticed that it says that "continuous operation at mixture settings lean of peak EGT is prohibited". But the pilot's notes gauge inside the plane says to set the mixture 50deg lean of peak for cruise. Is there a reason for this? Is it an error?

Thanks.

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mallcott
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Re: Leaning - Cessna manual vs. Pilot's notes

Post by mallcott »

cristi.neagu wrote:Hello,

Reading through the actual POH for the 172R (as well as the manual that comes with the A2A version) i noticed that it says that "continuous operation at mixture settings lean of peak EGT is prohibited". But the pilot's notes gauge inside the plane says to set the mixture 50deg lean of peak for cruise. Is there a reason for this? Is it an error?

Thanks.
Welcome to the `lean` debate, it's raged for dozens of years and involves the manufacturer recommending only a safe option, while the aftermarket and users recommend a more aggressive method. BTW, the debate was instigated by techniques developed for WWII fighters by Charles Lindbergh to maximise range by operating very LOP. It was what enabled the Lightnings to catch Yamamoto, for example. Lindbergh wasn't interested in enhancing lifespan, one might say...

Why should it be any different in the sim? :wink:

The essence of the argument rests on the ability to monitor EGT and CHT more accurately and with multiple measuring points so as to gauge the exact temperature and the finesse for the adjustment.

In the sim I always use LOP. In real life I don't often as the cost of fuel saved is offset by the potential for overhaul costs and parts replacement. The exception is when the aircraft is equipped for proper LOP operation, when you invest in GamIjectors and multi-point digital measurement for example.
http://www.gami.com/gamijectors/beforeandafter.php

It's pretty pointless to invest thousands in those things and NOT run LOP. :lol:

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AKar
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Re: Leaning - Cessna manual vs. Pilot's notes

Post by AKar »

This mixture stuff (and engine management in overall) is something I've been studying extensively from automotive point-of-view due to general interest related to real-life commitments. It has been a source of huge debates in aviation world, somewhat stupidly, IMO. It is common that an attempt to push a point overcomes any attempt to understand, or to provide understanding.
cristi.neagu wrote:i noticed that it says that "continuous operation at mixture settings lean of peak EGT is prohibited"
I don't know what the current state of things is, but at some point, it seemed that this kind of notes were raining down into the POHs, while little explanation of the background was provided. The recommendation in POHs appear to be to set the mixture to peak or 50 °F rich. Such leaning is effective for cruise performance, but is not the best way to "un-stress" the engine or maximize economy.

-Esa

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mallcott
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Re: Leaning - Cessna manual vs. Pilot's notes

Post by mallcott »

Good point Esa.

I don't see how this is awfully relevant to water-cooled automotive applications where high power states are hardly ever maintained for any length of time, but its certainly the case that your average Lycosaur or Contiplodocus dino engine uses unburnt fuel as part of the spot cooling process and ROP can prevent `hot spots` inside the engine, sometimes in areas not monitored by temperature-measuring devices.

Differential expansion and contraction can be an issue in an aircraft engine when operated in extremes of power, (weather) temperature, or with rapid transitions from low- or no-power state to high power, low-airflow scenarios such as experience flights lasting only a few minutes. Its the cycling that actually damages the engine, as much as the peak temperature.

As we've already noted, the manufacturers have to consider liability and `inability` (cretinous pilots & idiot mechanics) issues so will always take a conservative stance in the POH. This will often mean the limits set are actually a long way from the damage zone, and there is room for refinement.

Of course, with 20th Century engines (LycoNentals are basically 19th Century technology) and modern measurement and calibration tools, devices and monitoring allied to advanced design techniques to optimise performance and cooling, it is normally perfectly acceptable to operate a modern GA engine to considerably LOP - in fact that's exactly what the latest Rotax 912iS engines do - the ECU leans the engine significantly to deliver power from around 75% and down. (of course it's a moot point in the Rotax because the mixture is controlled by the ECU, not the pilot).

This is because Power can be reduced from 100% by any combination of lower RPM, reduced throttle or weaker mixture. Older engines tend to concentrate on the throttle and lower rpm, simply because the finesse to monitor and control the mixture isn't there.

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AKar
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Re: Leaning - Cessna manual vs. Pilot's notes

Post by AKar »

mallcott wrote:I don't see how this is awfully relevant to water-cooled automotive applications where high power states are hardly ever maintained for any length of time, but its certainly the case that your average Lycosaur or Contiplodocus dino engine uses unburnt fuel as part of the spot cooling process and ROP can prevent `hot spots` inside the engine, sometimes in areas not monitored by temperature-measuring devices.
While engine management principles are entirely different in automotive (and work machine) use, the properties of different mixtures are the same. They are just employed differently, because in automotive use, the emissions are of much importance, and the combustion event is controlled in real time by varying ignition timing. In airplane use, the prioritization, so to say, is left to pilot.

The cooling effect of rich mixtures are to a certain extent often misunderstood, in that it is thought to "directly" cool the engine. The more correct idea is effectively the same that is behind water injection: the evaporation of liquid fuel cools down the mixture, in effect working as an anti-knock feature. The extra fuel does not take part into combustion, which is why the evaporation of it "steals" energy from the fire, cooling down temperatures and slowing down the combustion. For maximum power, the mixtures are somewhat "rich-of-peak": this is because some extra fuel is necessary to ascertain that all the oxygen is utilized.

At lean mixtures, the cooling is achieved by intuitively reducing the gas into the flame.

There is more into it, the speed of combustion of different mixtures, and how the ignition timing changes the equation. What is noteworthy is that in airplanes, the timing is relatively aggressive; in an automotive solution, should the maximum power mixture start to show signs of knocking, the timing is immediately relaxed. In airplanes, it is pilot's responsibility to alter the other parameters to keep the engine running happy. Generally this is achieved by going very rich, and it can be achieved also by leaning over the peak, but the required power may not be achieved that way anymore.

-Esa

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mallcott
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Re: Leaning - Cessna manual vs. Pilot's notes

Post by mallcott »

AKar wrote:
mallcott wrote:I don't see how this is awfully relevant to water-cooled automotive applications where high power states are hardly ever maintained for any length of time, but its certainly the case that your average Lycosaur or Contiplodocus dino engine uses unburnt fuel as part of the spot cooling process and ROP can prevent `hot spots` inside the engine, sometimes in areas not monitored by temperature-measuring devices.
While engine management principles are entirely different in automotive (and work machine) use, the properties of different mixtures are the same. They are just employed differently, because in automotive use, the emissions are of much importance, and the combustion event is controlled in real time by varying ignition timing. In airplane use, the prioritization, so to say, is left to pilot.

The cooling effect of rich mixtures are to a certain extent often misunderstood, in that it is thought to "directly" cool the engine. The more correct idea is effectively the same that is behind water injection: the evaporation of liquid fuel cools down the mixture, in effect working as an anti-knock feature. The extra fuel does not take part into combustion, which is why the evaporation of it "steals" energy from the fire, cooling down temperatures and slowing down the combustion. For maximum power, the mixtures are somewhat "rich-of-peak": this is because some extra fuel is necessary to ascertain that all the oxygen is utilized.

At lean mixtures, the cooling is achieved by intuitively reducing the gas into the flame.

There is more into it, the speed of combustion of different mixtures, and how the ignition timing changes the equation. What is noteworthy is that in airplanes, the timing is relatively aggressive; in an automotive solution, should the maximum power mixture start to show signs of knocking, the timing is immediately relaxed. In airplanes, it is pilot's responsibility to alter the other parameters to keep the engine running happy. Generally this is achieved by going very rich, and it can be achieved also by leaning over the peak, but the required power may not be achieved that way anymore.

-Esa
True, true, but increasingly as we come out of the Stone Age we will see (automotive style) ECU control taking the choice away from the pilot and giving it to the planes' own `brain`. Manufacturers of aircraft and engines can then be more confident in extending service intervals, costs should fall and we can finally get away from anachronisms such as magenetos and manual enrichment. The Austro engines of the DA40-42-62 family being the case in point.

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cristi.neagu
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Re: Leaning - Cessna manual vs. Pilot's notes

Post by cristi.neagu »

Thank you for the replies, guys. Very interesting info.

The one thing that got me asking this question is that when i started flying the 172 i tried to go LOP 50deg. At around 30-40 or so, i started getting rough running, to the point of misfires. Since then i've been running ROP, with obviously ok results. I'll try running LOP again. Maybe i wasn't paying enough attention to my temps and i could have leaned a bit better.

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mallcott
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Re: Leaning - Cessna manual vs. Pilot's notes

Post by mallcott »

cristi.neagu wrote:Thank you for the replies, guys. Very interesting info.

The one thing that got me asking this question is that when i started flying the 172 i tried to go LOP 50deg. At around 30-40 or so, i started getting rough running, to the point of misfires. Since then i've been running ROP, with obviously ok results. I'll try running LOP again. Maybe i wasn't paying enough attention to my temps and i could have leaned a bit better.
50 deg LOP is a target, not a fixed point. If your plane, on that day, in those conditions will only go to 25° LOP before running rough, that's the limit.

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AKar
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Re: Leaning - Cessna manual vs. Pilot's notes

Post by AKar »

cristi.neagu wrote:The one thing that got me asking this question is that when i started flying the 172 i tried to go LOP 50deg. At around 30-40 or so, i started getting rough running, to the point of misfires. Since then i've been running ROP, with obviously ok results. I'll try running LOP again. Maybe i wasn't paying enough attention to my temps and i could have leaned a bit better.
The easiest way to lean for economy in A2A is to lean until you can just note the roughness, and enrich just enough to make the engine smooth again. Speaking of real engines, the "lean misfiring" is not the cause of engine roughness at usual mixtures, but uneven mixture distribution and therefore uneven power production in between the cylinders is. The effect in the simulator is of course rather generic to serve an overall purpose of indicating "too lean" mixture.

Lean misfiring can occur: leaner the mixture gets, the harder it gets to ignite, and eventually it may fail to ignite altogether. This is sometimes mixed up with afterfiring or even backfiring that is associated with lean mixtures in some engines as well. As lean mixtures burn slower, it can be possible that the combustion process is still in place when the exhaust or even intake valve, depending on the engine's valve timing, opens, which leads into aforementioned effects. The tendency of somewhat lean mixtures to produce knocking relates to the slow burning characteristics of a lean mixture: slower the combustion, more time there is for the end gases, as the cylinder charge still outside the flame front is called, to heat up and reach the state of auto-ignition. With rich mixtures, this is avoided by cooling effect of the extra fuel evaporating has on the end gases. With very lean mixtures, assuming fixed timing, the same is achieved by reduction in internal pressures and prolonging the combustion event to more occur later on the piston's downstroke, and that way keeping the end gases out of the auto-ignition regime. How it all balances out depends on the engine design. In automotive world, quick combustion event carefully timed by the ignition timing is preferred; with Lycosaurs, one must use sensible MP-RPM combinations and judge the mixture setting accordingly.

-Esa

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Re: Leaning - Cessna manual vs. Pilot's notes

Post by flaminghotsauce »

You guys are SUCH geeks! I love these threads. Great read.

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mallcott
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Re: Leaning - Cessna manual vs. Pilot's notes

Post by mallcott »

Glad it's of use and interest.

Certainly makes a change from the repetitive banality of most other flight simulation fora... :( :D

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