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Old May 6th, 2012, 12:23 AM
Posted in reply to Ted DuPuis's post "Re: LOP operations"
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Re: LOP operations

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Your CFI's approach isn't what I'd do at all. For more info, fly on out to IPT.
Are you available tomorrow... err, today? (it's after midnight)

I'm not 100% sure, but if the weather holds and the forecast doesn't move the rain into this area too early, I might be able to fly over there early-mid afternoon. My mech's jacks are still sitting under the wings. He literally hasn't touched my plane yet.
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Old May 6th, 2012, 10:16 AM
Posted in reply to Dave Siciliano's post "LOP operations"
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Old May 6th, 2012, 10:22 AM
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Re: LOP operations

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My mech checked my ignition system thoroughly last fall, after I complained about large RPM drops on single mag operation when full rich. He'll be checking it again at annual (which should be NOW... he's gone incommunicado again, still trying to raise him), but I really doubt it's an ignition issue, more likely the injectors themselves. As I said, GAMIs are in my future, but so is a fuel totalizer and I'm not springing for either until I know what this year's annual is going to cost me.

Not sure what you mean by "go 75 before rough"?? 75F LOP or something else? My leanest is at 100 LOP at onset of roughness...
Yes, 75LOP, if you can get to 100LOP on your leanest before onset of roughness then you can easily get your hottest EGT to 15*LOP with a smooth engine; you don't have an issue so far as running a cylinder at risk. The issue is not making enough power on that little fuel. You don't necessarily need GAMIs, you can experiment with swapping nozzles on hot and cold cylinders first. You only have 16 combinations to try and often you can greatly improve the situation.
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Old May 6th, 2012, 10:55 AM
Posted in reply to Henning's post "Re: LOP operations"
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Re: LOP operations

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Yes, 75LOP, if you can get to 100LOP on your leanest before onset of roughness then you can easily get your hottest EGT to 15*LOP with a smooth engine; you don't have an issue so far as running a cylinder at risk. The issue is not making enough power on that little fuel. You don't necessarily need GAMIs, you can experiment with swapping nozzles on hot and cold cylinders first. You only have 16 combinations to try and often you can greatly improve the situation.
Well, that's just it, by the time #2 is 15 LOP #s 3 and 4 are so lean that it's getting rough. There might be power settings where it's a little better, but that's what I found doing the lean test. Bottom line is it ISN'T easy to get them all "safely" LOP without roughness. From what Ted says it might all be moot, as long as my #2 CHT is cool enough at peak EGT. With #3 @ 40 LOP I'm still making decent power with no roughness. But I've never tried it at higher than 65% by the book.

Trying different combinations at random, sure I can do that but it could end up costing as much as just buying GAMIs and still not narrow the spread much. I was going to ask my mech to try swapping the front and back cylinder injectors at annual and see if that makes a difference. If not, I think that's as far as I want to follow that idea.
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Old May 6th, 2012, 11:07 AM
Posted in reply to azure's post "Re: LOP operations"
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Re: LOP operations

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Well, that's just it, by the time #2 is 15 LOP #s 3 and 4 are so lean that it's getting rough. There might be power settings where it's a little better, but that's what I found doing the lean test. Bottom line is it ISN'T easy to get them all "safely" LOP without roughness. From what Ted says it might all be moot, as long as my #2 CHT is cool enough at peak EGT. With #3 @ 40 LOP I'm still making decent power with no roughness. But I've never tried it at higher than 65% by the book.

Trying different combinations at random, sure I can do that but it could end up costing as much as just buying GAMIs and still not narrow the spread much. I was going to ask my mech to try swapping the front and back cylinder injectors at annual and see if that makes a difference. If not, I think that's as far as I want to follow that idea.
It takes around an hour to swap though all the possible combinations of nozzle arrangement, it's about 10 minutes to swap hot for cold. If you can rearrange them and maintain the same results, you have a cam issue.
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Old May 6th, 2012, 11:48 AM
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Re: LOP operations

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It takes around an hour to swap though all the possible combinations of nozzle arrangement, it's about 10 minutes to swap hot for cold. If you can rearrange them and maintain the same results, you have a cam issue.
While this varies among engine (or more specifically intake system) designs, unbalanced mixtures in IO aircraft engines exists with perfectly good cams. It is true that given the somewhat loose tolerance on OEM injector flow rates, swapping can improve the overall balance but unless you happen to have a pair that inversely match the effects of the intake system on mixture you may see little improvement with swapping. That said, as you pointed out, swapping is relatively easy and costs nothing by time if you do it yourself (assuming you don't damage anything in the process).
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Old May 6th, 2012, 12:00 PM
Posted in reply to gismo's post "Re: LOP operations"
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Re: LOP operations

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While this varies among engine (or more specifically intake system) designs, unbalanced mixtures in IO aircraft engines exists with perfectly good cams. It is true that given the somewhat loose tolerance on OEM injector flow rates, swapping can improve the overall balance but unless you happen to have a pair that inversely match the effects of the intake system on mixture you may see little improvement with swapping. That said, as you pointed out, swapping is relatively easy and costs nothing by time if you do it yourself (assuming you don't damage anything in the process).
Not that big of an imbalance, not without other problems. If swapping around the nozzles has no effect then there is a cam or other induction failure issue.
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Old May 6th, 2012, 12:33 PM
Posted in reply to azure's post "Re: LOP operations"
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Re: LOP operations

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So basically then, on my engine, Mike Busch's approach is pretty much valid?
Mike's approach is "pretty much" valid but there are some caveats that would give me some concern if you focus exclusively on CHT. For one thing, CHTs vary considerably with the amount, density, and temperature of air flowing over the cylinders so I suspect (but honestly don't know) that you could experience damaging ICP with a CHT at or below 380F with some combinations of high power, adverse mixture, cold dense air, and relatively high airspeed.

Secondly, one goal of LOP operation is to learn "typical" combinations of power settings and fuel flow that yield safe and efficient engine operation so you don't have to go through a complicatetd leaning process every time you fly or change altitude. And I fear that relying entirely on CHT for that is likely to result in setting that work well under certain conditions but fail to give the same results in terms of efficiency and safe ICP under others (e.g. summer vs winter).

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Yep I know... the problem with my engine/injectors is the spread is such that I get roughness before I can bring #2 very far past peak EGT. Hopefully the CHT will be cool enough even so.
One issue that makes this unlikely without reasonably balanced mixtures is the fact that power falls off dramatically with fuel flow when LOP. As a result, even if the leanest cylinder is still firing consistently, there will be a large imbalance in the power produced between the richest and leanest cylinder and this will increase vibration as well as reduced efficiency.

One final note: It's pretty much accepted by all the "experts" that when the engine is producing less than 60 (some say 65%) power you cannot develop excessive ICP regardless of the mixture setting as long as there are no glowing deposits or small metal objects capable of igniting the fuel air mixture (pre-ignition) in the cylinders. And since most (non rental) pilots flying with normally aspirated engines typically operate at or below 65% in cruise flight this makes mixture management more of an efficiency issue than one of engine longevity. Also keep in mind that the %power is lower than the ROP based tables in your POH would suggest when you are on the lean side of peak EGT.
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Old May 6th, 2012, 01:04 PM
Posted in reply to gismo's post "Re: LOP operations"
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Re: LOP operations

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One final note: It's pretty much accepted by all the "experts" that when the engine is producing less than 60 (some say 65%) power you cannot develop excessive ICP regardless of the mixture setting as long as there are no glowing deposits or small metal objects capable of igniting the fuel air mixture (pre-ignition) in the cylinders. And since most (non rental) pilots flying with normally aspirated engines typically operate at or below 65% in cruise flight this makes mixture management more of an efficiency issue than one of engine longevity. Also keep in mind that the %power is lower than the ROP based tables in your POH would suggest when you are on the lean side of peak EGT.

I take some exception to that. While it is true that the detonation, heat, and immediate damage issues of longevity with LOP fall by the way side, the sticking/burned valves and engine deposit reduction advantages to engine longevity of running LOP are still very much in play even at reduced power settings.
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Old May 6th, 2012, 02:48 PM
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Re: LOP operations

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I take some exception to that. While it is true that the detonation, heat, and immediate damage issues of longevity with LOP fall by the way side, the sticking/burned valves and engine deposit reduction advantages to engine longevity of running LOP are still very much in play even at reduced power settings.
Yes, I agree. I was referring to the more immediate longevity issues involved with detonation, pre-ignition, cylinder head separation, and other failures instigated by excessive ICP. Also FWIW, very little mention has ever been made WRT the potential for harm caused by LOP induced/enhanced vibration when there is a mixture and power imbalance between cylinders and/or as a result of inter cycle variation.
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Old May 6th, 2012, 03:44 PM
Posted in reply to Dave Siciliano's post "LOP operations"
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Re: LOP operations

There really is no question as to the viability of running LOP, the question is one of running LOP under manual control at high power settings. With EFI & digital ignition it is the norm in any racing scenario involving multiple fuelings. You only give the fuel you need for the requirements and you don't go ROP until the very top.

To do that right now you have to cough up the money for the FADEC 94UL engines available. Teledyne Continental is ready to meet your FADEC needs. So we have to do it by hand, it still works. Thing is, consider how the digital system approaches the problem, from the lean side, not the rich. Once it's stable running at temp the only time the engine will go ROP is when required to make terminal power. It avoids detonation by limiting heat saturation as well, another ingredient required in the detonation cycle. This is the real reason you want to keep your CHTs low as the value is much lower than the thermal destructive value.
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Old May 6th, 2012, 04:53 PM
Posted in reply to Dave Siciliano's post "LOP operations"
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Re: LOP operations

So correct me is I am wrong but my take-away from this thread and Mike Busch's webinar is that I can do little if any harm by simply leaning to roughness and then enrichen until the roughness goes away. And I can do this at any time other than intial takeoff.

I am talking about my Arrow II with 200 HP IO-360-C1C.

I understand from this thread that I will not damage my engine by leaning to any degree at 65% power and below, roughness or not.

And from the webinar that I should lean all the time except TO.

So this morning I practiced that a bit. Used to be I would only lean at or above 3000' DA and use my single-cylinder EGT/CHT to lean ROP at cruise. Today I leaned starting at 500' as part of pulling the power for cruise climb and at every altitude just by feeling for the roughness. The EGT said I was just a little LOP but I was prolly a bit conservative when I enriched to remove roughness. I did my run-up leaned also.

How did I do?
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Old May 6th, 2012, 05:13 PM
Posted in reply to alfadog's post "Re: LOP operations"
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Re: LOP operations

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So correct me is I am wrong but my take-away from this thread and Mike Busch's webinar is that I can do little if any harm by simply leaning to roughness and then enrichen until the roughness goes away. And I can do this at any time other than intial takeoff.

I am talking about my Arrow II with 200 HP IO-360-C1C.

I understand from this thread that I will not damage my engine by leaning to any degree at 65% power and below, roughness or not.

And from the webinar that I should lean all the time except TO.

So this morning I practiced that a bit. Used to be I would only lean at or above 3000' DA and use my single-cylinder EGT/CHT to lean ROP at cruise. Today I leaned starting at 500' as part of pulling the power for cruise climb and at every altitude just by feeling for the roughness. The EGT said I was just a little LOP but I was prolly a bit conservative when I enriched to remove roughness. I did my run-up leaned also.

How did I do?
Very well, that is the WWII technique I was taught early in my training which consequently was also the system taught per 152 instructions for leaning however you are cutting a finer deal with advanced information and starting leaning earlier.
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Old May 6th, 2012, 06:00 PM
Posted in reply to gismo's post "Re: LOP operations"
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Re: LOP operations

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Mike's approach is "pretty much" valid but there are some caveats that would give me some concern if you focus exclusively on CHT. For one thing, CHTs vary considerably with the amount, density, and temperature of air flowing over the cylinders so I suspect (but honestly don't know) that you could experience damaging ICP with a CHT at or below 380F with some combinations of high power, adverse mixture, cold dense air, and relatively high airspeed.
I'd agree that this does present some possibilities for problems, and why strictly running on CHT isn't in all cases the best idea.

That said, Liz's engine is also quite stout, and the ICPs aren't that high on it, even at 75% power when leaned out. I wouldn't want to run it at 25-50 ROP, but I wouldn't want to run any engine there.

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Secondly, one goal of LOP operation is to learn "typical" combinations of power settings and fuel flow that yield safe and efficient engine operation so you don't have to go through a complicatetd leaning process every time you fly or change altitude. And I fear that relying entirely on CHT for that is likely to result in setting that work well under certain conditions but fail to give the same results in terms of efficiency and safe ICP under others (e.g. summer vs winter).
To be clear, that isn't what I was suggesting. What I was suggesting was that, so long as her CHTs are reasonable, the way she was talking about running it either LOP or ROP was fine.

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One final note: It's pretty much accepted by all the "experts" that when the engine is producing less than 60 (some say 65%) power you cannot develop excessive ICP regardless of the mixture setting as long as there are no glowing deposits or small metal objects capable of igniting the fuel air mixture (pre-ignition) in the cylinders. And since most (non rental) pilots flying with normally aspirated engines typically operate at or below 65% in cruise flight this makes mixture management more of an efficiency issue than one of engine longevity. Also keep in mind that the %power is lower than the ROP based tables in your POH would suggest when you are on the lean side of peak EGT.
I'd agree with this as well.

These sorts of questions are why I offer the service of providing individual engine instruction, as well as flying in the plane with pilots to teach them settings that work for them. The APS course I think is very good, but the lack of time in the plane I think is an important part for many pilots, especially ones who don't have a tremendous amount of engineering background. I suspect that you (Lance) knew a good bit about how engines worked prior to starting to fly, as did I.

It's pretty difficult to try to teach this stuff online.
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Old May 7th, 2012, 01:33 PM
Posted in reply to Ted DuPuis's post "Re: LOP operations"
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Re: LOP operations

For the record, I was never seriously considering leaning by CHT alone, that simply doesn't make sense. Mike Busch doesn't say that anyway -- what he does say is that you don't need to worry about the effects of excessive ICP (including, he seems to imply, detonation) as long as your CHTs are reasonable. I gather that although there are engines for which this might not be true, the 200 HP IO-360 isn't one of them, though it's possible to make it detonate if you try hard enough. My question was whether it's safe for me to operate with my leanest cylinder solidly LOP (40* or so) and my richest at peak or even slightly ROP, and I understand Ted to be saying, yes, as long as your CHTs are reasonable. That's the best I can do without incurring roughness. Of course it's safe to operate that way below 65% power, the red box disappears below 65%, but we weren't talking about that, we were talking at power settings that would normally yield 70-75% according to the POH. I haven't tried doing that at higher power settings yet so I'm not certain what my CHTs would be like, but I'd be very surprised if they were much above 350, except maybe during summer heat, OAT >30C or so.

Yes Lance, I know that the values in the book percent power table are for specific mixture settings -- I think they're at economy cruise in the Cardinal POH but I'll have to check to be sure (my spare copy is in my car). Since I don't know exactly how much power I lose LOP vs economy cruise, I use the book values to give me a safety buffer, knowing that my actual power output will be somewhat (though maybe not a lot?) less. As long as I'm below 65% book power, I consider running at any mixture setting to be safe, and my own experience shows that at 65% and around or even a little above standard temperature, there is no mixture setting where I'll see CHTs above 310F in cruise.
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Old May 7th, 2012, 04:36 PM
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Re: LOP operations

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And from the webinar that I should lean all the time except TO.

So this morning I practiced that a bit. Used to be I would only lean at or above 3000' DA and use my single-cylinder EGT/CHT to lean ROP at cruise. Today I leaned starting at 500' as part of pulling the power for cruise climb and at every altitude just by feeling for the roughness. The EGT said I was just a little LOP but I was prolly a bit conservative when I enriched to remove roughness. I did my run-up leaned also.

How did I do?
I don't like the idea of leaning just short of roughness at high power (e.g. climbout below about 8000 DA) as this has a pretty good chance of putting you squarely in the "red box" if the cylinder to cylinder mixture isn't well balanced. I'm also not a fan of reducing power for cruise climb beyond a slight RPM reduction for noise abatement. That said, I do believe that you should lean during climb, but unless you're very comfortable with LOP ops and have a decent engine monitor I wouldn't try climbing LOP with a NA engine. The proper technique for initial climb (works for high DA takeoff as well) is to lean to the same EGT indication that you normally see shortly after takeoff with a full rich mixture at or near sea level DA.
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Old May 7th, 2012, 04:49 PM
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Re: LOP operations

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For the record, I was never seriously considering leaning by CHT alone, that simply doesn't make sense. Mike Busch doesn't say that anyway -- what he does say is that you don't need to worry about the effects of excessive ICP (including, he seems to imply, detonation) as long as your CHTs are reasonable. I gather that although there are engines for which this might not be true, the 200 HP IO-360 isn't one of them, though it's possible to make it detonate if you try hard enough. My question was whether it's safe for me to operate with my leanest cylinder solidly LOP (40* or so) and my richest at peak or even slightly ROP, and I understand Ted to be saying, yes, as long as your CHTs are reasonable. That's the best I can do without incurring roughness. Of course it's safe to operate that way below 65% power, the red box disappears below 65%, but we weren't talking about that, we were talking at power settings that would normally yield 70-75% according to the POH. I haven't tried doing that at higher power settings yet so I'm not certain what my CHTs would be like, but I'd be very surprised if they were much above 350, except maybe during summer heat, OAT >30C or so.
FWIW, there are two somewhat related issues with high CHT. One is that abnormally high CHT is often an indication of detonation and if the CHT is rising rapidly it's quite likely that you are experiencing thermal runaway and pre-ignition, something that can happen with any high performance engine under the right conditions. The other is that the potential for many scenarios of combustion event induced damage (e.g. detonation, pre-ignition, etc) are "enhanced" by high CHT so keeping the CHTs in check will reduce the chances for such at the very least. Of course there are other issues associated with high CHT such as inadequate valve cooling, cylinder head structural weakening, but those are relatively independent of the bad combustion events.


[/quote]Yes Lance, I know that the values in the book percent power table are for specific mixture settings -- I think they're at economy cruise in the Cardinal POH but I'll have to check to be sure (my spare copy is in my car). Since I don't know exactly how much power I lose LOP vs economy cruise, I use the book values to give me a safety buffer, knowing that my actual power output will be somewhat (though maybe not a lot?) less. As long as I'm below 65% book power, I consider running at any mixture setting to be safe, and my own experience shows that at 65% and around or even a little above standard temperature, there is no mixture setting where I'll see CHTs above 310F in cruise.[/QUOTE]
Sounds good. Typically going from 50-100 ROP to the mixture for true best economy (which is usually leaner than the POH might suggest) will result in about a 10% reduction in power output so if you start with 70% and go safely LOP you'll be at or below 65%.

And I tend to believe Ted when he says your 200 HP IO-360 is fairly robust even though from a purely logical perspective it would seem to need more ICP than an IO-550 producing 300 HP since a six cylinder equivalent of your engine would be a 300HP IO540.
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Old May 7th, 2012, 05:55 PM
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Re: LOP operations

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FWIW, there are two somewhat related issues with high CHT. One is that abnormally high CHT is often an indication of detonation and if the CHT is rising rapidly it's quite likely that you are experiencing thermal runaway and pre-ignition, something that can happen with any high performance engine under the right conditions. The other is that the potential for many scenarios of combustion event induced damage (e.g. detonation, pre-ignition, etc) are "enhanced" by high CHT so keeping the CHTs in check will reduce the chances for such at the very least. Of course there are other issues associated with high CHT such as inadequate valve cooling, cylinder head structural weakening, but those are relatively independent of the bad combustion events.
All correct.

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Sounds good. Typically going from 50-100 ROP to the mixture for true best economy (which is usually leaner than the POH might suggest) will result in about a 10% reduction in power output so if you start with 70% and go safely LOP you'll be at or below 65%.
Correct, which is part of what helps you with LOP operations.

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And I tend to believe Ted when he says your 200 HP IO-360 is fairly robust even though from a purely logical perspective it would seem to need more ICP than an IO-550 producing 300 HP since a six cylinder equivalent of your engine would be a 300HP IO540.
I do agree that the ICPs should be higher. Of course, damaging ICPs for one engine may be fine for another engine. Diesels are a great example of this, and why the gasoline engines converted to diesel haven't worked well. I don't know what the numbers are in 520s/550s and what they are in 540s, nor do I know the comparative strengths.

That said, given the fact that Lycoming cylinders are typically known for better reliability than Continentals, it would stand to reason for me that they are built stronger, and thus would be more tolerant of higher ICPs. This is based strictly on what I've observed as an aircraft owner/operator.
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Old May 7th, 2012, 05:57 PM
Posted in reply to Dave Siciliano's post "LOP operations"
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Re: LOP operations

I need to finish this book of mine...
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Old May 9th, 2012, 05:55 PM
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Re: LOP operations

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Originally Posted by gismo View Post
Sounds good. Typically going from 50-100 ROP to the mixture for true best economy (which is usually leaner than the POH might suggest) will result in about a 10% reduction in power output so if you start with 70% and go safely LOP you'll be at or below 65%.
I didn't want to say anything here until I'd had a chance to check my POH to see what mixture setting Cessna bases their power table values on. It turns out it's not "economy cruise" at all but something called "recommended lean". I guess from the description that it's how they recommend you run your engine in cruise.

Guess what: it's not 50-100 ROP, it's 25 ROP. Right where Ted said he'd never want to run an engine.

But I'm really not sure whether I could still expect 10% less power as compared with "recommended lean", with the mixture set where I'm talking about, which isn't quite "safely LOP" but with the two back cylinders LOP and the front two at peak or slightly, maybe 10-15 ROP. That's the best I can do without roughness. It seems there would be a lot of variables going into a calculation like that and it would depend to some degree on the design of the cylinders.
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Old May 9th, 2012, 06:03 PM
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Re: LOP operations

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Guess what: it's not 50-100 ROP, it's 25 ROP. Right where Ted said he'd never want to run an engine.
This has been something that I've always wondered about since I got into aviation, and haven't found much in the way of good answers. We all know that's a terrible place to run an engine.

One old timer I talked to said the reason those recommendations were made was that they found most pilots who thought they were running at peak or 25 ROP with a traditional EGT gauge were actually running at peak or LOP. I'm not sure I buy that, but he was around then, I wasn't.

Anyway, so long as you're not at 25 ROP, you're doing better than what Cessna recommends.
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Old May 11th, 2012, 05:52 AM
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Re: LOP operations

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Originally Posted by azure View Post
I didn't want to say anything here until I'd had a chance to check my POH to see what mixture setting Cessna bases their power table values on. It turns out it's not "economy cruise" at all but something called "recommended lean". I guess from the description that it's how they recommend you run your engine in cruise.

Guess what: it's not 50-100 ROP, it's 25 ROP. Right where Ted said he'd never want to run an engine.

But I'm really not sure whether I could still expect 10% less power as compared with "recommended lean", with the mixture set where I'm talking about, which isn't quite "safely LOP" but with the two back cylinders LOP and the front two at peak or slightly, maybe 10-15 ROP. That's the best I can do without roughness. It seems there would be a lot of variables going into a calculation like that and it would depend to some degree on the design of the cylinders.
The number LOP or ROP is meaningless by itself. It represents a combustion condition in that specific engine is all. What that condition will qctually represnt in the way of harm available will be dependent on several factors, most important in this case is the compression ratio of the pistons and ignition / ICP timing (which you can vary the ICP timing in flight with the use of the prop handle.) If your engine is of such a design that it cannot get into detonation as equipped, then there is no risk operating 25ROP.
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Old May 11th, 2012, 10:00 AM
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Re: LOP operations

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If your engine is of such a design that it cannot get into detonation as equipped, then there is no risk operating 25ROP.
While there may not be a "risk" as far as an immediate catastrophic failure, it is worth noting that doesn't mean that one should operate there if avoidable. The higher ICPs and CHTs aren't going to be good for your engine longevity.
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Old May 11th, 2012, 11:09 AM
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Re: LOP operations

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While there may not be a "risk" as far as an immediate catastrophic failure, it is worth noting that doesn't mean that one should operate there if avoidable. The higher ICPs and CHTs aren't going to be good for your engine longevity.
Not necessarily true. As long as maximum achievable ICPs are within the design limits of the engine you have what we refer toa Continuous Duty Rating. If that happens with the throttle WFO and mixture set where it can possibly have the most damaging effect, it doesn't matter, you're not damaging anything.

Just because an engine is putting out everything it can doesn't mean harm is being done.
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Old May 11th, 2012, 12:18 PM
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Re: LOP operations

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Originally Posted by Ted DuPuis View Post
This has been something that I've always wondered about since I got into aviation, and haven't found much in the way of good answers. We all know that's a terrible place to run an engine.

One old timer I talked to said the reason those recommendations were made was that they found most pilots who thought they were running at peak or 25 ROP with a traditional EGT gauge were actually running at peak or LOP. I'm not sure I buy that, but he was around then, I wasn't.

Anyway, so long as you're not at 25 ROP, you're doing better than what Cessna recommends.
The story I heard (from APS) is that 25 ROP was dictated by the marketing department of the aircraft manufacturers against the engine mfg's engineers recommendations. 25 ROP was chosen because it represents a significant (and marketable) increase in range without a significant reduction in speed at typical cruising altitudes. I personally suspect that the widespread but incorrect belief that EGTs correlate with valve head temps and that peak EGT represented "worst case" conditions for the valves also led to the 25 ROP compromise.
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