Impact of Altitude on LOP

[201er]

On a recent flight at higher than typical altitude for me (8000msl) I was trying to lean LOP but the engine was running awfully rough and had to revert to peak (55% powe). The instructor I was with told me it was because of higher altitude but I'm not really sure I understand how higher altitude impacts the maximum leanness you can achieve. So is anyone familiar with this and be able to explain it?

[jetdriven]

At high altitudes or lower power settings, LOP can be difficult in practice as the engine is running a rather low airflow and the difference between peak and 50 LOP might only be .3 GPH.  The flow divider also steps in somewhere at low fuel flow and attempts to even out fuel flow to injecctors, messsing with your fuel balance.  1450 EGT is 1450 EGT.  At 2500', that may be 100 LOP.  At 10K, it may be peak.  At 8000 or above, you migiht consider running it at peak and perhaps at 2500-2600 RPM. at 12K, definately ROP and 2700 RPM. Low alrirude LOP pays off well. High altitude it doesnt save as much. More testing is needed but below 55% power, peak may be more efficient. 

[carusoam]

Mike,

It is real easy to get 75 d LOP at 5,000'

It is a real challenge to get 50 d LOP at 12,000’

Yes, it is harder to lean deep into LOP at higher altitudes, but it is less important at higher altitudes.

For students of the red box theory...

The red box ends below 8,500’ it is OK to run at peak, thus negating the difficulty to leaning to a particular target at higher altitudes.

Explanation: Less air MP seams to make it more difficult.  Overall less HP available, less of a margin to work with.

Best regards,

-a-

[garytex]

Unless my understanding of several interacting processes is wrong (which makes error almost a certainty) it works something like this. We know that lower cylinder pressures slow the progression of the flame front.  Leaner mixtures also slow the flame front.  As soon as the piston starts down, pressures fall even faster, and the burn shifts from progressive (the faster it burns the more pressure it makes, the more pressure its under the faster it burns), to regressive.  The other variable is ignition timing, which may be set for either 20 or 25 degrees before top dead center.  See the timing thread in Vintage. So lower manifold pressures, leaner mixtures, and later timing all slow the burn, meanwhile the piston is busily "running away" from the combustion, also lowering pressures.  Thats why peak is sometimes the way to go at altitude for any except the most evenly mixture distributed engines.  Thats also why it is easier to run LOP down low.

I think this is what is going on, the mystical piece of it can be explained by understanding that too lean doesn't just mean that the mixture can't burn.  It also means that the mixture slows down in a fast moving environment and that can be another source of inefficiencies.

 

Lots of speculation here, anybody else see it that way?

Gary

[carusoam]

Gary,

Good enough for me.  Low oxygen, low energy environment, makes it more difficult to inititate an oxidation reaction.

It has taken me about 18 months of LOP operations to see this as clearly as you have written it....

Best regards,

-a-

[garytex]

I visualize it very much like reloading low expansion ratio (bottleneck case) versus high expansion ratio (straight wall case) ammunition. The burn speed of the powder must match the expansion ratio, the bullet is running away from the combustion chamber, you really do not want detonation, and the longer you can keep the burn progressive, the faster the bullet goes.  In my engine I try to increase the burn rate by increasing pressure (MP or altitude), running closer to stoichometry, slowing the prop, or with this weird timing issue, advancing the spark.  The reverse reduces burn rate and pressure.  LOP is another way to decrease pressure, with the added benefit of reducing the molecules of hydrocarbon out on the dance floor.  Every one of them gets to danceWe can substitute our higher compression ratios and earlier spark for gas and still keep the flame front moving along quickly enough to push the piston.  And slowing down airspeeds just a little bit generates geometrically larger reductions in the thrust demanded by our airframes.  Even less gas  It's all good good good.  I still have a lot to learn about LOP, but finally have a group of like minded folks to bounce ideas off, and a much better theoretical framework assembled in this forum within which to experiment and ponder, for which I am grateful.   

I previously only flew fixed props, and carburetor equipped (poor mixture distribution) engines, and am excited about the new number of variables to play with in my new F.  The down side is that my wife hates it when I adjust the mixture and the engine stumbles, and thinks that all that fooling around can't be good.  "Please leave all the controls alone.  Quit touching everything.  You're like a damn raccoon.  Quit fiddling with all the knobs and staring off into the distance.  Please, you're making me nervous."   Good arguments for the one big pull technique, I guess.

[jetdriven]

My wife used to say the same thing.  Now she fiddles and fidgets with the levers too.   We really can get a Mooney 201 to burn 5 GPH at 100 KIAS.  

[garytex]

Cool.  

I'm really ready to start flying.  I have to go AUS - Fort Stocton - Del Rio for work, then we are working cattle on Fri so on to Falfurrias - AUS , and am going to have to do it in my Cheetah, as 9504M isn't ready yet.  I could almost cry.

[201er]

I still don't understand why pulling MP back to 20" at sea level vs WOT at an altitude that yields 20" MP with the prop at the same RPM affect ability to lean LOP differently? Leaning LOP at low altitude and low MP isn't a problem yet that same MP at higher altitude and it's really rough.

[jetdriven]

I have never tried 20" at low altitude and 50 LOP.  I suspect that would be the same, it would get rough around 50 LOP.  How many degrees LOP did you get at the onset of roughness?

[201er]

I've done around 20"/2000RPM at 20LOP at low altitudes no problem. Then I tried 22"/2200RPM and 20LOP at 8000' and it just wasn't happening and had to advance to peak. I've been doing a lot of flying at 20-22"/2000-2200RPM and LOP lately while going through my instrument training. It's all about logging hours and not distance so 100-120kts on 5.5-6.5gph has been golden. Faster than a skyhawk while burning less gas! But just as well I can decide to go on a VFR xcountry the next day and do 150kts on 10gph.

I just couldn't understand why altitude made it so difficult to achieve a comparable power setting to down low.

[garytex]

Higher altitudes make it harder for your harness to transmit spark (less insulating air).  Lean mixtures are harder to ignite.  Maybe the combination is stacking up on you and showing you a weak ignition chain.  Any one cyl. go dead first?  I would clean the cigarettes with acetone, (because thats all I can think of that I could do, and it's a step most mechanics don't deign to do, and they may be dirty.  Fingerprints are enough to generate jump overs.) and suspect harness, then plugs, then mag.

Gary

[201er]

Good point, but that still doesn't explain why 20"MP down low doesn't have the same affect as 20"MP up high? The amount of air in the engine is the same regardless what altitude you're at if it's 20", right?

Is it temperature? Is it air resistance on the prop? What makes leaning at the same MP but higher altitude more difficult?