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PostPosted: Sun Sep 17, 2017 11:04 am 
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I have the Skylane in mind, but it seems like this applies generally to prop aircraft.

The manual for the 182 says:

Quote:
Recommended Lean 50° Rich of Peak EGT
Best Economy Peak EGT
Best Power 125° Rich of Peak EGT


Isn't the peak EGT getting as high as you can on the indicator? I can do that. How can I go 50° degrees above the peak if the peak is the highest possible on the indicator? I must be missing something obvious.

I know there are performance charts, but I'd like to be able to do this anyway. Thanks in advance.


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PostPosted: Sun Sep 17, 2017 11:36 am 
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CarbonBasedLifeForm wrote:
Isn't the peak EGT getting as high as you can on the indicator? I can do that. How can I go 50° degrees above the peak if the peak is the highest possible on the indicator? I must be missing something obvious.
The simple bit you're missing is that for both rich of peak (ROP) recommendation in the manual, we adjust the fuel mixture so that EGT peaks, and after it's had time to stabilise, we enrichen it again slightly so that EGT drops a bit. (i.e. by either 50°C or 125°C.)

As well as ROP, it's actually possible to run lean of peak (LOP) too. Here we adjust the mixture for peak EGT again, but this time we go on to lean it slightly, and the EGT also drops.

The whole ROP vs. LOP debate is one that's been discussed here quite a bit and I understand it can be a somewhat controversial topic amongst real flyers. You could start with this topic: Understanding the Basics of Leaning. :wink:

Nick


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PostPosted: Wed Sep 20, 2017 7:53 am 
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CarbonBasedLifeForm wrote:
I have the Skylane in mind, but it seems like this applies generally to prop aircraft.

The manual for the 182 says:

Quote:
Recommended Lean 50° Rich of Peak EGT
Best Economy Peak EGT
Best Power 125° Rich of Peak EGT


Isn't the peak EGT getting as high as you can on the indicator? I can do that. How can I go 50° degrees above the peak if the peak is the highest possible on the indicator? I must be missing something obvious.

I know there are performance charts, but I'd like to be able to do this anyway. Thanks in advance.
Rich of peak means you reduce temp by richening the mixture instead of leaning which would also reduce temp but the unburned fuel caused by richening the mixture has benefits to engine life


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PostPosted: Wed Sep 20, 2017 8:44 am 
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Shalomar wrote:
Rich of peak means you reduce temp by richening the mixture instead of leaning which would also reduce temp but the unburned fuel caused by richening the mixture has benefits to engine life
Hmm? :) I don't quite follow what specific mechanism you are after.

-Esa


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PostPosted: Wed Sep 20, 2017 8:52 am 
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I believe that they are just describing some of the benefits of running rich vs lean of the peak EGT.


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PostPosted: Wed Sep 20, 2017 9:14 am 
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Oracle427 wrote:
I believe that they are just describing some of the benefits of running rich vs lean of the peak EGT.
Yes, but I'm a bit confused because there is no way of simply saying that running rich of peak will provide better engine longevity. Under heavy load, yes, rich mixtures are beneficial in couple of ways, but at lower engine powers, slightly lean mixtures (LOP) do the trick very well. Running rich for extended periods of time actually brings in its own issues. It is not a comparison problem, but an optimization one for any given set of flight and engine conditions.

-Esa


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PostPosted: Wed Sep 20, 2017 10:05 am 
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I see, yes the dynamics are certainly much more complex such that an answer such as richer is better can't be applied to every circumstance and outcome.


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PostPosted: Wed Sep 20, 2017 11:16 pm 
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ROP rich of peak vs LOP lean of peak is and has been a discussion in the pilot community for a long time.

The LOP/ROP sequence begins by leaning the mixture and watching the EGT rise in temperature.

Continuing the leaning process will eventually result in a highest or "peak EGT" temperature followed by a decrease in EGT temperature.

Lean Of Peak advocates leave the mixture at the setting or lean a little more.

Traditionally with Rich Of Peak leaning the mixture would be made richer
Enriching the mixture would bring the temperature through the peak EGT and then the EGT temperature will cool.
The mixture is made richer until the EGT temperature is X degrees cooler than the peak EGT temperature noted earlier.

+++++
Many pilots will lean the mixture until the engine runs rough and then make the mixture richer until the engine runs smooth. Although this is an empirical method for finding the optimum fuel-air mixture it seems to work very well.
Using this method accomplishes the same goal but removes the need to watch the EGT temperature or set the operating conditions according to the EGT guage.
+++++


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PostPosted: Thu Sep 21, 2017 6:45 am 
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Yeah, lots of this stuff actually run around a couple of physical issues.

The usual recommended cruise mixtures somewhat rich of peak are actually near the maximum power mixture, or just on the lean side of it. This apparently has to do with nice cruise figures more than of any engine longevity reasons. At very high cruise powers, it actually can be the opposite.

The best power mixture is somewhat richer than theoretical stoichiometric ratio, because of that the combustion is slightly incomplete in real engines: excess fuel makes sure that all the oxygen is used from the charge, while a bit of fuel remains unused. Where exactly the best power is reached, depends on the engine. Running richer still, like during the takeoff, uses the same principle: evaporation of the excess fuel cools down the combustion.

Maximally fuel-efficient mixtures introduce excess air instead, which in turn makes sure that all the fuel is burned. Efficiency starts to drop with still leaner mixtures, because of few reasons, one being that a slower combustion leads into larger heat transfer during the combustion event.

If the engine did have closer to perfect homogeneous charge that would burn completely, the two would approach each other, making the life much simpler.

-Esa


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PostPosted: Sun Sep 24, 2017 9:06 pm 
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I recommend that we all watch two YouTube videos by Mike Busch of Savvy Aviation:

Leaning Basics: https://www.youtube.com/watch?v=_VfiPuheeGw&t=3489s; and
Leaning - The Advanced Class: https://www.youtube.com/watch?v=X-tKyiUZ3ts.

One of the points that impressed me the most was an overlay of curves for exhaust gas temp (EGT), cylinder head temp (CHT), brake horsepower (BHP), and brake specific fuel consumption (BSCF) at a given power setting, with fuel mixture on the X-axis, and temperature scales on the Y-axis.

The main thing I got out of that discussion was that a parameter that you want to absolutely minimize for engine longevity is cylinder head pressure (CHP). Unfortunately, we don't measure this parameter in our aircraft, but a very good surrogate for that parameter is CHT. You want to keep CHT below 400 degF for sure, and below 380 degF as much as possible. According to the curves shown in the Leaning Basic video, you get peak CHT right around 50 deg ROP (rich of peak EGT). Mike B says that you want to climb at full power (with climb rpm in the Comanche or 182) and lean during climb to maintain a constant EGT, then at cruise altitude you want to reduce power and rpm then pull the red knob to the onset of roughness, then slide it in just enough to smoothness. AVOID that area of peak EGT, and the slightly ROP zone, as that's where you get high CHP, as indicated by high CHT.

I'd like to hear comments from "somebody who knows something," such as Scott or Dudley on the aforementioned videos. Now I plan to sign off and fly my Comanche from KAWO to somewhere fun to try out this technique.

Seeya
ATB

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PostPosted: Mon Sep 25, 2017 12:06 am 
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Paughco wrote:
I recommend that we all watch two YouTube videos by Mike Busch of Savvy Aviation:

Leaning Basics: https://www.youtube.com/watch?v=_VfiPuheeGw&t=3489s; and
Leaning - The Advanced Class: https://www.youtube.com/watch?v=X-tKyiUZ3ts.
I didn't know that full-length videos by these guys were on Youtube, thanks for linking. Of course, they have published quite a bit of written work, and all of it is highly recommended, even if not an absolute point-of-view that everyone must take.
Paughco wrote:
The main thing I got out of that discussion was that a parameter that you want to absolutely minimize for engine longevity is cylinder head pressure (CHP). [...] AVOID that area of peak EGT, and the slightly ROP zone, as that's where you get high CHP, as indicated by high CHT.
There is one big "but if" in this one, which at least in their written work they sometimes mention (no time for the videos right now). We need pressure to produce torque, and thereby any usable power. In particular in the simulator world, we see a tendency of folks flying pretty high for naturally aspirated piston airplanes. At high cruise altitudes, where we are running the engine at its maximum output for the conditions, we actually want to maximize the cylinder pressure to get maximum performance. The peak pressures will remain tolerable, as you really can't push the engine further with thin air. At sensible airspeeds, it is impossible to bring the CHTs harmfully high either.

Another point that is not too often brought up is the importance of ignition timing, which is often simply noted to be fixed in these engines. This is true obviously, but it still varies in between engine models, and not always hand-in-hand with rated RPM. Engines with relatively late ignition timing would generally speaking tolerate best power mixtures better, if everything else is assumed unchanged. Further, the issue of having fixed ignition timing is one big reason for that all this trickery with mixture is needed at all. For instance, automobile engines that vary their ignition timing dynamically as required, don't vary their mixture much at all for most part, albeit reasons for their design choices are entirely different. Anyways, where the timing is set precisely in any given airplane engine will surely have certain consequences on its nature.

-Esa


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PostPosted: Mon Sep 25, 2017 4:25 pm 
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Those videos touch upon ignition timing, and how mixture and rpm affect spark timing. I actually didn't watch all of the first video. I listened to it while walking yesterday afternoon. Both videos are copies of webinars, so consist of a long series of Powerpoint slides (well done, IMHO), and plenty of explanation for each one. I was able to pretty much form mental images and concentrate on what was being said while getting in some good exercise. :D Mike Busch seems to be interested in saving his engine, and in saving gasoline.

In the first video, he discusses how aluminum weakens as temperature increases, which goes to keeping low CHT.

There's a whole series of other videos by Savvy, and they look pretty good. Now I have another aviation YouTube subscription, to go with SimCFI, FlightChops, Steveo1Kinevo, and Zenos.

Seeya
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