M20M LOP Discussion

[LANCECASPER]

When the TLS first came out in 1989, to squeeze out as much performance and range, Mooney/Lycoming put the redline on the TIT at 1750 and the CHT at 500. That was crazy, cylinders were lasting about 250 hours, turbos about 500 and TIT probes in some cases less than 100 hours. The Bravo conversion helped with cylinder life.

In the 90's at the MAPA homecomings there used to be model specific seminars. All of the shared knowledge plus factory input helped everyone realize that 1650 was a more realistic TIT and 400 on the CHT was going to help a lot with cylinders. Lycoming came out with a little booklet that advocated 1650/400 for M20M owners and for Piper Mirage owners. By the time I went through the Flight Safety Training in 1996 after buying a new Mooney Bravo, they stressed 1650 and 400 even though the POH said otherwise.

1750 on the TIT will drastically reduce the exhaust system life and maybe your own life if you don't carefully inspect the exhaust system regularly.

[KSMooniac]

What is "blade stretch?"

FWIW, the engineering phenomenon that is the issue here is called "creep." Metal parts that are operated at elevated temperatures and sustained tensile loads actually creep...that means their length will permanently stretch, even if the tensile loads are below the elastic limit where they normally return to their un-loaded length. Turbos are designed with very small tolerances between the blades and housing for maximum efficiency, but since the blades are under tensile load from the centrifugal forces due to rotation, and of course high temps, they creep in normal operation. That limits the life of the turbo. Higher temps, shorter life of the blades, but I don't know the details specifically in our typical Mooney applications.

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[aviatoreb]

50 minutes ago, KSMooniac said:

 

FWIW, the engineering phenomenon that is the issue here is called "creep." Metal parts that are operated at elevated temperatures and sustained tensile loads actually creep...that means their length will permanently stretch, even if the tensile loads are below the elastic limit where they normally return to their un-loaded length. Turbos are designed with very small tolerances between the blades and housing for maximum efficiency, but since the blades are under tensile load from the centrifugal forces due to rotation, and of course high temps, they creep in normal operation. That limits the life of the turbo. Higher temps, shorter life of the blades, but I don't know the details specifically in our typical Mooney applications.

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True - but aren't those modern turbos coated with ceramics with amazing thermal properties.  Maybe 1750 is too much but anyway the treatments on a M20M turbo is more modern and different than that on a turbo such as my TSIO520NB.

[KSMooniac]

I don't think so, but I'm not sure either. Nothing about turbos in our world are modern, I'm afraid. I don't think turbo fan blades are ceramic coated either, but it sounds like a great idea.

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[aviatoreb]

21 minutes ago, KSMooniac said:

I don't think so, but I'm not sure either. Nothing about turbos in our world are modern, I'm afraid. I don't think turbo fan blades are ceramic coated either, but it sounds like a great idea.

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I don't know what they actually do - I am just speaking about what they can do.  My old roommate from college is the chair of material science at a fancy famous school (not saying which for his privacy) (and I am not in material science), and he told me what they can do these days over a bowl of cheese fondu about two years ago during casual conversation.

[KSMooniac]

I don't know what they actually do - I am just speaking about what they can do.  My old roommate from college is the chair of material science at a fancy famous school (not saying which for his privacy) (and I am not in material science), and he told me what they can do these days over a bowl of cheese fondu about two years ago during casual conversation.

I don't doubt that there are some amazing material possibilities today, but getting amazing thru the FAA is tougher than solving difficult technical challenges like this, unfortunately.

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[donkaye, MCFI]

I would think the weakest link controls, and for the Bravo it's the exhaust system.

[thinwing]

On 12/25/2016 at 10:52 AM, donkaye said:

The spring (?) freezes up and the watergate won't function.  This has happened on both engines just after mid time.

Hi..this something I check every oil change...I take a wrench that engages the flat part of the waste gate arm next to the spring and manually engage it against spring pressure...I also mouse milk the waste gate pivots as well as every slip joint on the exhaust tubes..

[jlunseth]

Well written Dave.

Educate me on a couple of things.  I have a 231 and run it LOP most of the time.  I am tring to correlate our experience to what I do.  When you lean out the Bravo engine ( I assume you are leaning from the rich side over to the lean side), does the MP change as you change fuel flow, or does the MP stay where you set it.  In other words, if you start with, say, 29" of MP and 18 GPH, and then just lean the fuel flow down to your 15 GPH setting, does MP fall as you lean or does it remain at 29".  

You have an absolute pressure wastegate controller and the controller in the 231 is a differential pressure controller (Merlyn), so maybe your MP stays constant as you lean.  Mine does not.  There is a crosslink between the MP and fuel flow which tries to maintain the same fuel/air ratio if you start at a particular setting and then change one or the other.  In other words, if you start at an MP of 30" and a fuel flow of 13.5 GPH, and then lean to 11 GPH, you will have an MP of somewhere in the vicinity of 27-28.  I just adjust the two until I get my target settings for fuel flow and MP, but it does take some adjusting.  

Also, in my experimentation to get to LOP, I remembered the lesson from APS that power changes with fuel flow on the lean side, not with MP.  And their are two ways to get further LOP, decrease fuel flow at a constant MP, or increase MP at a constant fuel flow.  So I do a combination of the two, and wind up using a fuel flow of 11-11.1 GPH (a 125 ROP flow would be 13.3), and I increase MP to 34 (give or take a few tenths).  Max MP in my engine setup (Turboplus intercooler) is 36-37", and a normal MP for 125 ROP would be 29-30" depending on alt. and RPM, so my MP is high.  If I were running ROP it would correlate to about a 90% HP setting.  LOP, what I am doing is driving the engine further from peak.  I have to watch my TIT, the turbo is working harder and this does not work very well up in the flight levels because of lack of cooling.  But in the mid-teens and below it seems to work really well.  You might experiment with it in your Bravo.

I was out flying yesterday just to play around.  Low altitude.  Using my 11-11.1 GPH and 34" MP setting, 2450 RPM, my hottest CHT's were in the 380's and my TIT was 1600 or less.

Increasing MP is an option the NA guys don't have for the most part and I think that is why APS does not talk about it much.  They talke WOTLOPSOP, but even WOT in an NA you can't get above ambient so the only option is less fuel.

Edited May 29, 2017 by jlunseth

[donkaye, MCFI]

40 minutes ago, jlunseth said:

Well written Dave.

Educate me on a couple of things.  I have a 231 and run it LOP most of the time.  I am tring to correlate our experience to what I do.  When you lean out the Bravo engine ( I assume you are leaning from the rich side over to the lean side), does the MP change as you change fuel flow, or does the MP stay where you set it.  In other words, if you start with, say, 29" of MP and 18 GPH, and then just lean the fuel flow down to your 15 GPH setting, does MP fall as you lean or does it remain at 29".  

You have an absolute pressure wastegate controller and the controller in the 231 is a differential pressure controller (Merlyn), so maybe your MP stays constant as you lean.  Mine does not.  There is a crosslink between the MP and fuel flow which tries to maintain the same fuel/air ratio if you start at a particular setting and then change one or the other.  In other words, if you start at an MP of 30" and a fuel flow of 13.5 GPH, and then lean to 11 GPH, you will have an MP of somewhere in the vicinity of 27-28.  I just adjust the two until I get my target settings for fuel flow and MP, but it does take some adjusting.  

Also, in my experimentation to get to LOP, I remembered the lesson from APS that power changes with fuel flow on the lean side, not with MP.  And their are two ways to get further LOP, decrease fuel flow at a constant MP, or increase MP at a constant fuel flow.  So I do a combination of the two, and wind up using a fuel flow of 11-11.1 GPH (a 125 ROP flow would be 13.3), and I increase MP to 34 (give or take a few tenths).  Max MP in my engine setup (Turboplus intercooler) is 36-37", and a normal MP for 125 ROP would be 29-30" depending on alt. and RPM, so my MP is high.  If I were running ROP it would correlate to about a 90% HP setting.  LOP, what I am doing is driving the engine further from peak.  I have to watch my TIT, the turbo is working harder and this does not work very well up in the flight levels because of lack of cooling.  But in the mid-teens and below it seems to work really well.  You might experiment with it in your Bravo.

I was out flying yesterday just to play around.  Low altitude.  Using my 11-11.1 GPH and 34" MP setting, 2450 RPM, my hottest CHT's were in the 380's and my TIT was 1600 or less.

Increasing MP is an option the NA guys don't have for the most part and I think that is why APS does not talk about it much.  They talke WOTLOPSOP, but even WOT in an NA you can't get above ambient so the only option is less fuel.

For the record the Bravo has two controllers; a density controller for controlling maximum power and a pressure differential controller for maintaining constant power at different altitudes.  The density controller works well from my experience, the pressure differential controller not so well, in fact down right lousy.  Even after overhaul it won't maintain constant MP, as it is supposed to as altitude decreases.  I've given up trying to get it to work properly and just monitor the MP in descents.

[DVA]

 

6 hours ago, jlunseth said:

Well written Dave.

Educate me on a couple of things.  I have a 231 and run it LOP most of the time.  I am tring to correlate our experience to what I do.  When you lean out the Bravo engine ( I assume you are leaning from the rich side over to the lean side), does the MP change as you change fuel flow, or does the MP stay where you set it.  In other words, if you start with, say, 29" of MP and 18 GPH, and then just lean the fuel flow down to your 15 GPH setting, does MP fall as you lean or does it remain at 29".  

Thanks for the compliment!

First, before I go down any paths (or rabbit holes lol) we have to remember that we do not have a direct “Manifold Pressure” control on our airplanes, we have an indirect throttle control that we choose to monitor via a MP gauge. So when we try to “lock” MP in one place we are using devices (density controllers, diff controllers, etc) that don’t actually adjust all the right stuff - just some stuff.  That said, anyone who has flown a manual waste-gate engine (Twin Comanche) really appreciates the convenience and safety of the somewhat sloppy but mostly okay automatic boost controllers you and I fly behind.

So why the drop in MP you see as fuel flow changes? ... mainly for the reason above.  As you lean the mixture you change the dynamics of actually what propels the hot side of the turbo - pressure, air density and heat of the exhaust gases. All three combine to provide the energy, mostly thermodynamic, needed to spin the turbine. Add to that the tight relationship of the cold side changes that are made as a result of changing the hot side and you have an inherently unstable positive feedback loop. Tracking these different functions and how they affect the turbo system requires a lot more brain power than the simple density / differential and/or absolute controllers we fly with to provide precise control.

I think I have my Bravo tuned up pretty well, and I get similar variations as you do. When I adjust mixture LOP and the MP drops an inch or two, I just bump it up with throttle and turn up the XM classic hits and fly on. There is a logarithmic widening of the detontion margin on the lean side of peak, so even 10-15 dF past peak on the lean side puts you in a fine safe area.

Just a note, I don’t run LOP all of time, when I want to go TAS fast I run 100-125dF ROP in cruise, but I nearly always run LOP in the decent, approach and landing portions which is usually at least 1/3 of the trip time. I would say I have run 50/50 LOP vs ROP so far, and I’ve my Bravo now for over a year. A recent look with the bore scope showed a carbon free clean set of heads and pistons, no valve issues, and picture perfect spark plugs. I routinely run TiT at 1650dF up high and a little lower down below and the exhaust is in fine shape.

DVA

 

 

[carusoam]

Great details, Dave!

Best regards,

-a-

[Awful_Charlie]

12 hours ago, donkaye said:

For the record the Bravo has two controllers; a density controller for controlling maximum power and a pressure differential controller for maintaining constant power at different altitudes.

Not quite - the differential controller is there to limit the difference between upper deck and induction pressure - no point in compressing to say 34" and then the throttle only letting 27" into the engine - the differential controller should limit the turbo output to around 2" above the MP. Unless you install a second gauge between the turbo and throttle, you won't know how well this is working.

I fully agree that the exhaust is rather fragile though, and this alone makes it worth being conservative with EGT/TITs. FWIW I'm a staunch LOP runner, but then usually cruise at 30" and 2200 to boost efficiency (interestingly the engine POH has figures for 2000RPM, but the Mooney POH is moot about that, and I'm reluctant to try that for the cruise in case it is a prop/engine harmonics limitation). For the climb I shoot for 34", 2400RPM, 120KIAS, TIT max 1500, cowl flaps full open, CHT max 400, and for that 20-22GPH usually fits. My #1 cylinder is the hottest by far, substantially so in the climb, so if often the limiting factor. (my #2 used to be the coolest by far, so I added a bit of speed tape to blank off quarter of an inch of fins to bring it in line with the others)

At 30"/2200 I'm normally in the high 12 to low 14 GPH area for about 165-185 KTAS depending on altitude (I prefer FL100-190) and don't have a problem keeping TITs sub 1650 and CHTs in the 350-380 area with the cowl flaps closed in up to ISA+20: when it's much warmer than that I may need to open the cowl flaps half an inch or more to keep the CHTs down. Moving up to 32" MP is more difficult for me to keep under 1650TIT/400CHT without going ROP, and then jumping to 17+GPH for a few more knots, so only do that when there's a stonking headwind or I'm desperate to make up a few minutes. 2100 hours on this engine so far, hoping to make it 2200! (and around 900hrs on the fine wire plugs)

[carusoam]

Great details, AC! 

Best regards,

-a-

[jlunseth]

@DVA .   Well, ok then, in the Bravo, if you reduce fuel flow the MP also reduces.  It does what my aircraft does, tries to maintain the fuel /air ratio from your starting setting.  So then the question is, how do you know how far LOP you are?  It probably helps somewhat to use TIT (provided you know where your EGT's are so you don't have cylinders running where they shouldn't).  BUT (that's a big but), degrees from peak is an "all other things being equal" number.  In other words, leaning "x" degrees from peak assumes that other things such as MP don't change.  If they do change during the leaning process, your original peak is no longer valid and you don't know how many degrees you are from peak.  Yes, your EM gives you a number.  Or you calculate that number yourself.  But it is not a valid number for purposes of knowing where you are LOP.  Another way to look at it, is that if you make a setting and find peak at, say, 1650, and then you pull the MP back until the number is 1600, you are still at peak (more or less, your engine has tried to maintain the same fuel air ratio).  What you have accomplished is simply to cool your peak off, not to reduce your fuel/air mixture so you are degrees LOP.  

The LOP leaning function in the JPI (probably in the EI, but I don't know that monitor), isn't particularly helpful because of that, as far as I am concerned.  You have to lean too far, from ROP across peak to lean, with too many things changing.  So the "downward icicle" display (what you get when you use the LOP lean function) looks nifty, but is not very meaningful.

I use a different method.  I use the ROP lean function from the lean side.  In other words, I am already running LOP, I have done the big pull to get over there and I have made a lean setting that works decently.  Then I use the ROP lean function because all it does is find the first cylinder to peak vs. the last (which is the LOP function), it has no idea whether the engine is LOP or ROP and it doesn't care.  I then enrichen back to peak, and lean again, watching the degree differential until I get to about 20, that's as far as it is worth going in my engine, too much power loss after that.  The difference between this and the "lean across" method, is simply the "distance," that is, you aren't having to go very far to find peak, and therefore your peak number is going to be reasonably valid as you lean.

[carusoam]

JL,

once you have leaned this way...

You are LOP, rpm and FF (hp) are set... do you then add additional MP and then adjust out the additional FF that accidently comes with the additional MP.

This would be the equivalent of only adding excess air into the system.  The benefit is additional cooling that should be evident on the TIT indicator...

There are two issues that could get in the way...

1) extreme LOP would go even more LOP making the ignition of fuel more challenging.

2) compressing extra air can't be free.

I have heard of this being done.  But so few people have the equipment and experience to actually pull it off.

Your thoughts?

Best regards,

-a-

[thinwing]

On 5/29/2017 at 8:41 AM, donkaye said:

For the record the Bravo has two controllers; a density controller for controlling maximum power and a pressure differential controller for maintaining constant power at different altitudes.  The density controller works well from my experience, the pressure differential controller not so well, in fact down right lousy.  Even after overhaul it won't maintain constant MP, as it is supposed to as altitude decreases.  I've given up trying to get it to work properly and just monitor the MP in descents.

Mine seems to do a better job Don...a descent say from 15 k 32 in to 5 k only increases MP .5 in...I like you manually dial it back but it's not a huge increase

[Danb]

Mine is also similiar to thinking, I generally do not have to make any adjustments for ascent or descent.

[jlunseth]

@carusoam

Yes, that's what I do.  If my goal setting is 34", 2450 RPM (my max is 2700), and 11.1 gph, I set the MP at about 35" and dial the RPM's back first to 2450, and then readjust the MP to make sure it is at 35".  I do the big pull to get the fuel flow down around 11 gph, and then adjust the MP back up to 34".  In pulling the fuel flow down the MP has gone down with it and is probably in the 32" range.  I have to adjust MP and fuel flow each, a little bit, to get where I want to be.

That's the trick though.  I have read lots of posts here where the pilot finds peak and then leans the fuel and thinks at a temp of, say, 50 degrees less (either the chosen EGT or TIT), and then they believe they are 50 LOP.  They aren't, they are more or less at peak still, but at a lower power setting.  Just because the temp is reduced from the peak you found, does not automatically mean you are LOP at all.  You are LOP only if you have maintained the MP where it was, and reduced fuel flow, in other words if you have changed the fuel/air ratio.  

My turbo seems to be fine at 34", but it may be part of the reason why my setting does not seem to work well in higher altitudes.  The turbo has to work pretty hard to produce 34" in the high teens, flight levels, and probably is producing some heat.  The cylinder that becomes a problem first is #2, which I believe is the one parked above the turbo.

[gsxrpilot]

On 5/31/2017 at 3:33 AM, jlunseth said:

@carusoam

Yes, that's what I do.  If my goal setting is 34", 2450 RPM (my max is 2700), and 11.1 gph, I set the MP at about 35" and dial the RPM's back first to 2450, and then readjust the MP to make sure it is at 35".  I do the big pull to get the fuel flow down around 11 gph, and then adjust the MP back up to 34".  In pulling the fuel flow down the MP has gone down with it and is probably in the 32" range.  I have to adjust MP and fuel flow each, a little bit, to get where I want to be.

That's the trick though.  I have read lots of posts here where the pilot finds peak and then leans the fuel and thinks at a temp of, say, 50 degrees less (either the chosen EGT or TIT), and then they believe they are 50 LOP.  They aren't, they are more or less at peak still, but at a lower power setting.  Just because the temp is reduced from the peak you found, does not automatically mean you are LOP at all.  You are LOP only if you have maintained the MP where it was, and reduced fuel flow, in other words if you have changed the fuel/air ratio.  

My turbo seems to be fine at 34", but it may be part of the reason why my setting does not seem to work well in higher altitudes.  The turbo has to work pretty hard to produce 34" in the high teens, flight levels, and probably is producing some heat.  The cylinder that becomes a problem first is #2, which I believe is the one parked above the turbo.

This behavior is more applicable to the 231 with the Conti GB and LB engines. In the 252/Encore with the MB and SB engines, leaning is just a matter of pulling the mixture. The MP will remain the same throughout the big mixture pull to lean. This works well up through at least FL240 in my 252.

[jlunseth]

That was my understanding also, but the Bravo guys seem to be saying that MP will change when you dial back the fuel flow.  I don't fly a Bravo so I don't know.

Edited June 2, 2017 by jlunseth

[DVA]

22 minutes ago, jlunseth said:

That was my understanding also, but the Bravo guys seem to be saying that MP will change when you dial back the fuel flow.  I don't fly a Bravo so I don't know.

It’s not really a Bravo thing, but most of the Bravo’s are not set up using the procedure outlined in the Lycoming MM for adjusting the density controller - rather it is just eyeballed and set by a few ground runs. The density controller (DC) must be adjusted properly for the rest of the system to work well and track the changes in, well density, which can be influenced by a ton of things: temperature, humidity, mixture, Trump, (sorry that just blurted right out for some reason), etc.

I have seen first hand people setting the DC so that (on that day of tweaking) they produce 38” at full throttle because they believe that’s what it should be because that is the “max”. They are dead wrong, they are setting the DC incorrectly and they are causing the system to be inherently unstable as a result.

The DC is set by properly measuring inlet air temperature at the correct location and adjusting the DC to produce a MP in concert with that temp on the chart for the AF1B. This is most often no where near 38” on the ground roll and more often closer to 35” at WOT; I wrote extensively about this elsewhere. If your MP on a Bravo is 37-38” on take off on a standard day I can virtually guarantee that you will not make critical altitude before you run out of boost and start to bootstrap due to the incorrect DC setting.

That said, the combination of turbo control systems on the Bravo make it more difficult to balance than single systems, hence the variations you see Bravo to Bravo.

I lose/gain about .5” or so +/- as I ascend or descend from the low FL20’s to earth, and often will not adjust anything at all. Some birds are better some worse.

 

[Awful_Charlie]

2 hours ago, DVA said:

It’s not really a Bravo thing, but most of the Bravo’s are not set up using the procedure outlined in the Lycoming MM for adjusting the density controller - rather it is just eyeballed and set by a few ground runs. The density controller (DC) must be adjusted properly for the rest of the system to work well and track the changes in, well density, which can be influenced by a ton of things: temperature, humidity, mixture, Trump, (sorry that just blurted right out for some reason), etc.

I have seen first hand people setting the DC so that (on that day of tweaking) they produce 38” at full throttle because they believe that’s what it should be because that is the “max”. They are dead wrong, they are setting the DC incorrectly and they are causing the system to be inherently unstable as a result.

The DC is set by properly measuring inlet air temperature at the correct location and adjusting the DC to produce a MP in concert with that temp on the chart for the AF1B. This is most often no where near 38” on the ground roll and more often closer to 35” at WOT; I wrote extensively about this elsewhere. If your MP on a Bravo is 37-38” on take off on a standard day I can virtually guarantee that you will not make critical altitude before you run out of boost and start to bootstrap due to the incorrect DC setting.

That said, the combination of turbo control systems on the Bravo make it more difficult to balance than single systems, hence the variations you see Bravo to Bravo.

I lose/gain about .5” or so +/- as I ascend or descend from the low FL20’s to earth, and often will not adjust anything at all. Some birds are better some worse.

 

Well said DVA - I shudder when I hear of people asking for 38" of MP and wonder if they comply with the SB for overboost!

I put in the IAT probe for the JPI to the measurement port, and can then use ExTrends to check MP vs IAT over a range of lights to see if it drifts off or not (been stable for a while now)

Like DVA I get a small variation in MP in the climb/descent, maybe up to 1" for an altitude change of 15,000', for the descent my SOP is to trim for 600FPM at TOD, let temperature stabilise, and try to get down to 24" and 150 ROP for the IAF or a few miles before the pattern on a visual, so I'm normally fiddling with MP and mixture on the way down anyway

[jlunseth]

@DVA

I think we are apples and oranges.  Here is what I was trying to find out.  Pick an MP, let's just say 36" hypothetically.  If you set the Bravo at 36" and you have the fuel set a full rich and you decide you want to lean to, say, 20 degrees LOP.  So you leave your MP knob alone and you dial the fuel flow back.  Does the MP fall in the Bravo?  It does in my 231, it does not in the 252.  The question I am asking is whether the Bravo maintains MP at the number you initially set, as fuel is dialed back. 

I am not asking you the controller is set on the ground, by the mechanic.

[DVA]

I lose/gain about .5” or so +/- as I ascend or descend from the low FL20’s to earth, and often will not adjust anything at all. Some birds are better some worse.
 

My other point was that the DC, when adjusted correctly, all else considered equal, will allow the Bravo to maintain MP pretty tightly up to the the FL20's. Sadly most are not adjusted correctly from the start.