What CHT requires cowl flaps in cruise?

[Shadrach]

1) My Mooney POH recommends 100ROP or peak in cruise, it makes no recommendation to run 50 or 75 ROP.  

2) Both Lyc and Cont are both horizontally opposed, fixed timing, air-cooled engines...in terms of combustion theory, they are the same... The fact that the author operates a Beech and uses Conti as an example is anecdotal...

3) Every question you've asked in this thread is answered in the articles that you've "already read". Perhaps you should read them again...and then maybe again.

4) If you're looking for a power setting cookbook, than keep using the recipes in your POH...

5) If you're concerned about an GAMI/APS conspiracy, your hottest cylinder must be in on the conspiracy by "conspiring" to run 20df...

Lastly, I have never taken nor given APS or GAMI a penny, but that "group of people" have been rather generous with the material they've assembled. Moreover, they freely admit that almost everything that they are advocating was SOP for Pratt & Witney before the first Mooney or Bonanza rolled off the line. All they've done is put a finer point on it and disseminate the info.  

You've been led to water (or kool-aid as some believe), whether you drink or not is up to you... Either way, your little Lyc will likely be OK...for a while...Good luck!

 

[danb35]

To maybe add a little to what Ross said, LOP does require a new way of thinking for most pilots, so you probably would need to read that material a few (or several) times for it to sink in.  However, the question on power settings is easy--once you're LOP, any setting that burns 10 GPH is giving you 75%; 8.7 GPH is 65%.  For any other fuel flow, multiply by 14.9 to determine horsepower, and then if you're really worried about percentage, divide that number by 200.  I normally fly WOT, 2500-2600 RPM, leaned to ~ 9 GPH.

[Jeff_S]

"Already read all that. What I'm not liking is that pretty much the entire source of info about LOP operations comes down to the same group of people (GAMI and APS). If it's so great, then I don't understand why the manufacturers aren't recommending it or at least other pilot groups that reach the same conclusion independently. Also, most of the info about it seems to always point to continental."

Lycoming has a circular on it. I'm pretty sure it's already been linked, but if not you can find it on their site. They generally acknowledge that LOP is well-known and not a problem, but their official position has been that it's too complicated to fly that way without the appropriate engine instrumentation, which wasn't available to GA pilots until the last few years.

"Anyone know of any good info for LOP on Lycoming in Mooney? Some power setting tables for LOP operations, graphs, etc? Cause I've seen nothing but continental based stuff. Yet it seems that the Lycoming has a better shot at running LOP with stock injectors."

Read through the suggested material again and particularly find the excellent chart for the red-box zone. It will show you exactly where to set your fuel flow for either ROP or LOP to stay out of the red-zone. As long as you're doing this you will be safe from detonation and pre-ignition. You may still not like your temps, though, so you still have to experiment for yourself.

"If I'm having these problems with CHT from 25-75 ROP, how did folks fly Mooneys for the last thiry years without engine analyzers or LOP?"

A well-maintained Lyc engine in a well-ventilated Mooney is pretty impervious. People just flew, kept it generally ROP and didn't worry about it except at annual time! But that doesn't mean there isn't a better way...

[201er]

To the person who said to set by GPH. How does this correlate to altitude? If you set by fuel flow alone, wouldn't it mean you'll be more LOP at lower altitudes and less LOP at higher?

[danb35]

Yes, a given fuel flow (in a normally-aspirated engine) will be less LOP at higher altitudes, but fuel flow is nonetheless the only way to determine power when LOP--MP and RPM are (nearly) meaningless.

I also find that I cruise at 7-8k 90% of the time.  At those altitudes, burning 9 gph (67% power), I'm comfortably LOP, as determined by having previously leaned to peak and seeing how far the temperatures drop.  Once you've done that, and figured out the fuel flow that corresponds to (say) 30 LOP at 8k, you can just lean by fuel flow.  Of course, if you cruise at widely varying altitudes, you may need to do more dialing-in of the process.

[takair]

I am currently neutral on the LOP/ROP argument.  Does anybody have statistics on making TBO with one method vs the other?  (I appologize if it is in one of the links).  Most folks don't fly enough to have those statistics, flight schools and charter operators might.  Many aircraft don't make TBO due to inactivity, most flight schools go past TBO running ROP.  However, very few bother to teach leaning so there are would be few if any schools running LOP to compare to.  Would be curious if any of the Cirrus charter folks have statistics. 

[Shadrach]

Yes, in a word. But to understand requires a bit more study.

LOP uses air in addition to what's used for combustion to manage the combustion event.

ROP uses fuel in addition to what's used for combustion to manage the combustion event.

With either method, power decrease with altitude. This means that as you climb, the engine will require less surplus fuel (if ROP) or less surplus air (if LOP) for cooling. Said simply, as you climb you don't need to be as far LOP or ROP (depending on what your using) to maintain cool CHTs.

 

IIRC the lowest "Brake Specific Fuel Consumption" (BSFC) occurs at around 45 LOP. But the difference in BSFC between peak and 45 LOP is not huge. BSFC simply means the most hp for a given amount of fuel.

In a normally aspirated AC it is impractical to fly at 45 LOP at altitude because the power loss at altitude; it's also unnecessary from a cooling standpoint .

I run about 5-10LOP at altitude and 45-55 LOP if I'm running down the coast at 250ft msl.  

To put a finer point on what Dave said calculating power when LOP is a function of fuel flow. This is because when LOP, all of the fuel running through the engine is being burned to produce power as opposed to ROP, wherein some of the fuel is not burned. Nit picking just a bit, but the actual LOP multiple for 200hp Lyc is about 15.1 X FF = HP. The 14.9 number is for 8.5 to 1 compression ratio engines; 200HP Lycs are 8.7 to 1 C/R.

All of the above may feel like drinking from a fire hose. When I said you needed to read the articles and then read them again (3rd time would not hurt), I was not trying to be snarky. There are multiple concepts at play. It is easier to understand them individually and then bring them together. Deakin does a great job of breaking it down. Whether you run ROP or LOP, the articles will give you a great deal of info about what is happening inside your engine.  Good luck! 

 

 

 

 

but when higher, you will be developing less power and will not need to be as 

[Shadrach]

I doubt that LOP/ROP makes a huge difference in attaining TBO if both are done properly. Heat is the enemy of engine life and it can be controlled either way.  I will say that our oil stays much cleaner LOP.

[201er]

I'd like to thank everyone for their advice. I flew again today and tried some more LOP stuff (right now just playing with things and getting a feel for leaning and the plane since I just got it recently). I'll read the articles again cause after the tests I've done recently, they may apply to me afterall. When I read them, I didn't take them too seriously because I didn't think I could fly her Lean of Peak. Here's what I got today:

3000ft, 26", 2600RPM, 150ktas, 45LOP, 367CHT (hottest cylinder, others were near 340), 9.7GPH. What my %HP was I have no idea, but I'm guessing 75% based on what you guys tell me with FF. Let me add that density altitude was more like 4500ft.

Here's the procedure I used to lean. Leveled off, left throttle full but pulled prop back to 2400RPM. I leaned mixture a reasonable ballpark toward peak. I set EDM700 for LOP and continued leaning through peak and until richest cylinder peaked. Then I continued leaning to 50LOP. I went past to test for roughness which occured by around 60LOP so I went back in to about 45-50LOP on richest cylinder. Fuel flow was reading about 8.5gph so I added RPM until I was close to 10GPH in hopes of achieving 75%. Then I closed the cowl flaps and was pretty happy the hot cylinder stayed below 380F. Here's a glimpse at the EGTs for all cyldiners at one moment after leaning:

1390, 1328, 1395, 1377. Peak was 1440. Should I be concerned about #2 running a lot leaner if engine isn't running rough? Am I doing things right? What am I at risk of screwing up if this is done incorrectly? Can I get any more speed out of her?

[jetdriven]

Joined: Jun 15, 2011

Posts: 14
Location: New York NY

Re: What CHT requires cowl flaps in cruise?
Posted Jun 19, 2011 11:15 PM

I'd like to thank everyone for their advice. I flew again today and tried some more LOP stuff (right now just playing with things and getting a feel for leaning and the plane since I just got it recently). I'll read the articles again cause after the tests I've done recently, they may apply to me afterall. When I read them, I didn't take them too seriously because I didn't think I could fly her Lean of Peak. Here's what I got today:

3000ft, 26", 2600RPM, 150ktas, 45LOP, 367CHT (hottest cylinder, others were near 340), 9.7GPH. What my %HP was I have no idea, but I'm guessing 75% based on what you guys tell me with FF. Let me add that density altitude was more like 4500ft.

FF (GPH) X 14.9= crankshaft horsepower. 9.7 GPH  is 144 HP or 72%.  72% to the airframe is 72%. 

 

Here's the procedure I used to lean. Leveled off, left throttle full but pulled prop back to 2400RPM. I leaned mixture a reasonable ballpark toward peak. I set EDM700 for LOP and continued leaning through peak and until richest cylinder peaked. Then I continued leaning to 50LOP. I went past to test for roughness which occured by around 60LOP so I went back in to about 45-50LOP on richest cylinder. Fuel flow was reading about 8.5gph so I added RPM until I was close to 10GPH in hopes of achieving 75%. Then I closed the cowl flaps and was pretty happy the hot cylinder stayed below 380F. Here's a glimpse at the EGTs for all cyldiners at one moment after leaning:

1390, 1328, 1395, 1377. Peak was 1440. Should I be concerned about #2 running a lot leaner if engine isn't running rough? Am I doing things right? What am I at risk of screwing up if this is done incorrectly? Can I get any more speed out of her?

 Your procedure is basically corrrect.   Richest cylinder is the last one to peak.  Lean to LOP from this cylinder.  Changine RPM changes fuel flow, and your degrees from peak probably will change. Re-lean.  As I have learned, leaning past 50 LOP is less efficient and much slower. Most benefit is from peak to 40 LOP.  EGT numbers mean nothing.  The value they are in relation to peak is all that matters.  If #2 is leaner than the otehrs whle LOP is no problem.  Above 6000-8000' you rapidly paint yourself into a corner while LOP, meaning you end up only being able to produce 55-65% power.  Speed suffers and peak or ROP might be more beneficial.

A rough rule of thumb is LOP, you can add 2" of manifold pressure to the book charts for the same percentage of power.  IE 23" and 2500 RPM ROP is 65% power, it takes 25" to make that 65% power if LOP.

Your goal is peak to 40 LOP on your richest cylinder, 75% power or below, smooth engine, and no CHT over 380.  360 is preferrable.  Do NOT exceed 400 degrees CHT on any cylinder.   I am in the camp that you are not harming anything in that regime.   That is peak BSFC and near the best efficiency and cost per mile. Our plane this weekend ran 380 CHT (#4), 40 LOP, 6500' altitude, 65 degrees temp, 2500 RPM, full throttle, and 141 MIAS.   ff was 8.4 GPH, so ~62% power. New baffle seals go in this week.  I'd like to run a little richer, with cooler CHT and a little more speed. 

If you cannot satisfy all these parameters, go to 75-100 ROP, richen to cool and later investigate why not.

 

UPDATE:  40-50 LOP is the lean end of the range. Best BSFC is arund 15-40 LOP.  Our newest procedure is 15-20 LOP.

 

[201er]

Wait so is it 14.9 or 15.1 times FF? I have 200HP Lycoming on '78 M20J.

How come in your set up you didn't add RPM? Or were you trying to conserve on gas? For now I'm just trying to find out how to go fast without exceeding CHT (it doesn't seem to be so much of an issue to find a lower/leaner setting to save fuel)?

[jetdriven]

Its 14.9.   We have the same angle valve IO-360-A3B6D you do.   Adding RPM yes does increase % of power but 2500 RPM is a good cruise RPM.  2700 gives you a couple knots but more fuel flow.  At 10,000'  that 2700 RPM is useful.  We were logging different RPM's at cruise vs. IAS to see what the real world values for our plane were. (6500')

You will be limited on the lean end by slow airspeed, and on the rich end by CHT.  You could simply richen to 380 and take what you get.  IN our case, today it was 40 LOP.

[Shadrach]

The difference is minuscule in the real world, but the 15.1 is correct for your engine, as well as jetdriven's, as well as any other angle valve Lyc IO360 with an 8.7 to 1 compression ratio.

The multiplier used is based on compression ratio. A great many of currently normally aspirated AC engines have compression ratios of 8.5 to 1 for which the multiplier is 14.9; That number has been repeated so many times that pilots use it without really understanding why. Higher compression engines are more thermally efficient, that is why the multiplier goes up.

You will find that the Turbos have low compression engines (usually 7.3 or 7.5) to protect against detonation.  This is why they must be boosted well past 29.9inches of MP to match the power of their naturally aspirated counterparts SL max rated power. It is also why the LOP multipliers for those engines are lower (13.5 and 13.7 respectively).  Don't sweat the multiplier, just use 15 it will be within 2%.  Which is close enough given that it's a quick and dirty method that fails to account for friction losses (maybe another 2%).

Focus on the general stuff, questions regarding the minutia of combustion science will come up as you learn more... It will take a while to totally grasp all of the things happening in your engine, but only a short time to learn what you need! 

[danb35]

It'd sure be nice if the Quote function worked again...  Anyway, in response to "1390, 1328, 1395, 1377. Peak was 1440. Should I be concerned about #2 running a lot leaner if engine isn't running rough?"

Absolute values of EGT are virtually meaningless, as is the difference in EGT from one cylinder to another.  The fact that #2 is 60 deg cooler than #1 means nothing at all, certainly not that #2 is at all leaner.  At what fuel flow did #2 peak?  What about #1, #3, and #4?  If #2 peaked first, at 1.5 GPH higher than the others, that would indicate that it's running leaner (though if it's running smoothly LOP, there's no way that there's that much difference between the cylinders).