Lean of Peak revisited.

[jkkniese]

The April 2007 PLANE & PILOT has an article starting on page 56: Lean of Peak, the Red Knob of Doom. It could: a) reinforce your beliefs,
b) Change your thinking,
c) Make you more confused.

Actually, I thought it did a good job of explaining ROP and LOP and where not to operate {50f ROP because of high ICP (Internal Cylinder Pressure)}.

At least I don't have to deal with Mixture and Carb heat on those SWA
B-737's that I'm fortunate to fly! :)

[kwmoore]

There has been a lot of discussion of this subject on the Cirrus Owners' and Pilots Association forums, including extensive participation by Walter Atkinson, author of the Plane and Pilot article and a principal of Advanced Pilot Seminars. (http://www.advancedpilot.com) A comprehensive "SOP" with accompanying diagram has been worked out for the SR20 and SR22 for pilots who want to operate LOP and avoid the so-called "red box" or "red fin," where CHT and ICP are higher than desirable. Note that on the lean side of peak, horsepower is directly proportional to fuel flow: gph x 14.9 = net hp.

However it's possible to simplify this a great deal if one is not determined to extract every last erg of performance while remaining just on the lean side of the "red fin." In the SR22 (310 hp IO-550), this amounts to the following:

Takeoff and climb: full throttle; keep all EGTs in the 1275-1300 range. This means leaning in the climb to keep EGTs at 1275-1300 (lower is OK, higher not so OK). Sea level fuel flow should be 29 gph or so to keep EGTs in this range.

Cruise: full throttle (2700 rpm) or slightly backed off to 2500-2600 rpm, adjust mixture to the following fuel flows:

4000 feet or below: 15-15.5 gph (15.5 gph is about 75% power)
4000-8000 feet: 14-14.5 gph (14.5 gph is about 70% power)
Above 8000 feet: 13.5 gph or less (13.5 gph is about 65% power)

For our 260 hp IO-470, this would presumably extrapolate to:

Takeoff and climb: full throttle & rpm (or back off rpm to ~2500 for noise abatement); keep all EGTs in the 1275-1300 range. This means leaning in the climb to keep EGTs at 1275-1300 (lower is OK, higher not so OK). I'm guessing sea level fuel flow should be 24.5 gph or so to keep EGTs in this range.

Cruise: full throttle and rpm (2625) or backed off to 2400-2500 rpm, adjust mixture to the following fuel flows:

4000 feet or below: 13 gph (13 gph is about 75% power)
4000-8000 feet: 12 gph (12 gph is about 69% power)
Above 8000 feet: 11.3 gph or less (11.3 gph is about 65% power)

It's important to keep the throttle wide open and control power with the mixture knob! If you back off on the throttle you will reduce air flow and could in some situations put yourself back in the red fin area by adjusting to the above fuel flows! :(

In the descent you can leave the mixture knob alone as you reduce throttle except for enriching if necessary for smooth operation, all the way to short final.

I emphasize that I have yet to verify this, but it's how I plan to operate the new plane--except when I'm racing Glen--after confirming that these settings are the appropriate amount LOP. I have indicated to Todd that I want sea level max power fuel flow to be no less than 24 gph, preferably 24.5. :)

[nworth]

Cruise: full throttle and rpm (2625) or backed off to 2400-2500 rpm, adjust mixture to the following fuel flows:

4000 feet or below: 13 gph (13 gph is about 75% power)
4000-8000 feet: 12 gph (12 gph is about 69% power)
Above 8000 feet: 11.3 gph or less (11.3 gph is about 65% power)

Kevin,

How many degrees LOP would you expect those values to equate to? I regularly run LOP but only at <65% power. My understanding of "LOP theory" would tell me that to run 75% you would need to run fairly LOP (like 80-100 degrees). I question whether that's going to be 13 GPH even at low altitude.

I am not totally clear on this topic and I have been looking for some more info to help me understand how this all works at higher % of power.

[kwmoore]

Kevin,

How many degrees LOP would you expect those values to equate to? I regularly run LOP but only at <65% power. My understanding of "LOP theory" would tell me that to run 75% you would need to run fairly LOP (like 80-100 degrees). I question whether that's going to be 13 GPH even at low altitude.
At 75% power you only have to be 40-45 deg LOP, 50 is good too. At 85% power you'd want to be more like 80 deg LOP (only achievable near sea level in a non-turbo'd plane). More this evening when I'm home and can access my relevant info.

[nworth]

At 75% power you only have to be 40-45 deg LOP, 50 is good too. At 85% power you'd want to be more like 80 deg LOP (only achievable near sea level in a non-turbo'd plane). More this evening when I'm home and can access my relevant info.
My EDM-930, now that it's working well, does a great job of finding lean from either side and then displaying LOP degrees and FF. However, I really wish it would also display where on the "red fin" the engine was running. I would really like to run LOP in all regimes and at high powers but I haven't had the time to really figure out what settings were safe.

I don't know if I am super lucky or not but I seem to be able to run very LOP (100 degrees) without engine stumbling even at "normal altitudes" (5-6k). But, this has always been at 65% or less power settings. Are you saying it will be more difficult at higher power settings (more throttle) but same LOP degrees?

[witrakw]

I run LOP all the time. I get more bang for the airspeed buck by more prop efficiency at ~2200-2300 rpms. I set rpms not only by vibration feel but more objectively at the lowest amount of vibration noticed on the canard.

LOP 11.3gal/hr 65% power is the magic cruise " keep my engine happy"number i use for fuel flow per the formula that Kevin noted and I don't worry too much about MP/rpm or looking up the performance chart cookbook numbers which seem to be rough estimates, anyway, especially depending on OAT.

Kevin, I don't understand your statement about "keeping the throttle wide open and control power by fuel flow" lest some untoward "red box" damage happens. I think I understand basic LOP theory and I reread the P&P article by Atkinson...he is the expert on this stuff but his comment that he runs his turbo'ed engine at WOT and uses 85% power doesn't make theoretic sense even using his LOP combustion theory.......taken to an extreme, you could theoretically overboost an engine to 50" MP, run it at2700 rpms and run it at effectively 170 % of rated power with sufficient fuel flow...but to keep it out of the red box you'd have to run it so far LOP that you'd need "super Gami's" to keep it running smoothly.. if you could,,as a consequence your power would be sorely curtailed because you would be so far LOP that your fuel flow would be severely diminished and therefore you couldn't really be getting that high % of engine power. How can you run an engine at 85% power, give it enuf fuel to develop that 85% and stay out of the red box?? Seems like a catch 22. The only thing I can figure out is that the LOP linear relationship between hsp and fuel flow is nonlinear with respect to the number of degrees LOP you have to be to stay out of the red box. Atkinson's landmark chart in P&P doesn't substantiate this latter explanation.

Running LOP and given equal fuel flows and therefore equal power %'s,,,,what does WOT do for you that other nonWOT settings don't?

Looking for enlightened answers.
thanks Bill

[kwmoore]

I don't know if I am super lucky or not but I seem to be able to run very LOP (100 degrees) without engine stumbling even at "normal altitudes" (5-6k). But, this has always been at 65% or less power settings. Are you saying it will be more difficult at higher power settings (more throttle) but same LOP degrees?
If you're lean of peak EGT, the only way to have 65% power is with an 11.3 gph fuel flow. Note that at 11.3 gph, as Bill points out, it's pretty much impossible to harm your engine no matter what the throttle setting. To be 100 deg LOP at 11.3 gph means you'd be at pretty low altitude.

At lower altitudes, it is possible to safely run LOP at higher fuel flows as I pointed out. Near sea level you could run at 80-85% power (up to 14.8 gph) as long as you're sufficiently lean of peak EGT (80 deg or so IIRC but I'll check when I get home tomorrow evening).

I will provide a table that Cirrus folks use, calibrated for our 260 hp engines.

I don't understand your statement about "keeping the throttle wide open and control power by fuel flow" lest some untoward "red box" damage happens.
The throttle actually primarily controls air flow, not fuel flow. If you retard throttle, you're effectively constricting airflow to the combustion chamber. Thus at a partial throttle setting, 13 gph would no longer be as lean of peak as at full throttle, and you could be in a region of undesirably high CHT/ICP.

I know it's confusing; it took me a while to grasp as well. More later after I'm back home.

[witrakw]

how do you guys insert those handy quotes from other members in your replies??

anyway, as per nworth, I too can run very smoothly LOP at 65% power and 11.3 gal/hr. I can continue leaning to about 7 gals before the engine finally kills but with no roughness. Stock injectors,,,, they actually seem to have improved over the years with diminishing roughness the leaner i go...go figure!!

Kevin, I think i understand the basics of throttle/throttle plate/air restriction/MP relationship. Theoretically as per "the formula" 14.9 xFF= hsp, you'd have to be running LOP approx 14 gal/hr FF in our io470s for 80% power. In the io550 you'd have to be running ~18gal/hr for 85%. are they running that in the sr22's? This sounds like what Deakin was talking about when he talks about the BMP(big mixture pull) on almost full power LOP climb in his bonanza

And again, once the engine is lean of peak,,,any benefits of running WOT vs. nonWOT at the same fuel flow(i.e.same power)..or said another way, WOT but way lean of peak vs nonWOT but lesser lean of peak? Theoretically, I wouldn't think so.

Anyone out there with IO470's who goes 14gal/hr and 80deg lop?

thanks bill

[nworth]

I will provide a table that Cirrus folks use, calibrated for our 260 hp engines.
I really look forward to seeing that table. I've seen something like this at the COPA site but it was in the form of a red fin graph, showing FF, LOP degrees and % power. Outside the red fin, good. Inside the red fin, blow up your engine! :rolleyes:

[nworth]

how do you guys insert those handy quotes from other members in your replies??
Click on the Quote button inside the post you want to quote. You will see the text inside some quasi-HTML codes. Delete all the stuff you don't want and put your own stuff outside the bracketed original text....

[kwmoore]

I really look forward to seeing that table. I've seen something like this at the COPA site but it was in the form of a red fin graph, showing FF, LOP degrees and % power.
I have attached tables for our 260 hp IO-470 and the 310 hp IO-550 as installed in the SR22. There is also a red fin diagram for the IO-550. I explicitly acknowledge Cirrus Owners and Pilots Association (Gordon Feingold I believe) as the source of the IO-550 data.

A couple of points are worth noting. Note that at 75% power LOP, one has 13 gph fuel flow and should be at least 45 deg F LOP. 70% power is 12.1 gph and you should be at least 26 deg F LOP. 11.3 gph is 65% power and at least ~9 deg F LOP is recommended. At 11 gph (~63%) or less it really doesn't matter whether you're ROP or LOP, you can't produce enough heat or internal pressure to do harm.

I have described the general procedure for getting to the desired LOP setting in an earlier thread, post #13. (http://www.260sepilots.org/forums/showthread.php?t=207) For the 260se, I'd leave throttle wide open and just pull the mixture back to a fuel-flow of ~11 gph (63% power) or until I felt a perceptible loss of speed. After letting the engine equilibrate for a minute or two, I'd then activate the lean assist function on the engine monitor. As I'd already be on the lean side of peak EGT, I'd enrich the mixture until the first cylinder peaks, then lean to achieve the desired number of degrees LOP depending on altitude and fuel flow (% power and thus target degrees LOP is determined by fuel flow). At low altitudes (below about 4000 feet) ~12.5-13 gph should work well; at 9000-11,000 feet in cruise likely 11.3 gph or so would work.

Importantly, for example, if you lean to 13 gph (75% power) but are still less than 45 degrees LOP, you need to lean even further to stay out of the red fin. This will likely be required above about 4000 feet and of course will generate less than 75% power. :)

Disclaimer: I haven't yet verified all of this in my own 260SE, although much of it worked well in 812KT. I do know that the IO-550 settings work well for the SR22.

Outside the red fin, good. Inside the red fin, blow up your engine! :rolleyes:
For our normally aspirated engines, I think we can take these as guidance rather than as warnings about the engine blowing up. ;) The best way to look at the red fin, especially near the edges, is as "a region of undesirably high CHT/ICP." When consistently operated in this region over many hours, the engine will experience more wear and heat-related stress/fatigue, and the chances of something wearing out, cracking, etc. will be higher. Of course, even without turbocharging/turbonormalization, one should avoid the heart of the red fin. :)

For turbonormalized/turbocharged engines it's a little more critical, since they always operate as though they were at sea level (turbonormalized) or better (turbocharged).

[kwmoore]

I think I understand basic LOP theory and I reread the P&P article by Atkinson...he is the expert on this stuff but his comment that he runs his turbo'ed engine at WOT and uses 85% power doesn't make theoretic sense even using his LOP combustion theory.......taken to an extreme, you could theoretically overboost an engine to 50" MP, run it at2700 rpms and run it at effectively 170 % of rated power with sufficient fuel flow...but to keep it out of the red box you'd have to run it so far LOP that you'd need "super Gami's" to keep it running smoothly.. if you could,,as a consequence your power would be sorely curtailed because you would be so far LOP that your fuel flow would be severely diminished and therefore you couldn't really be getting that high % of engine power. How can you run an engine at 85% power, give it enuf fuel to develop that 85% and stay out of the red box??
This is fine in turbonormalized engines that always develop sea level manifold pressure. In fact, for the turbonormalized IO-550 in the recently marketed TN-SR22 (and I think also for Tornado Alley Turbo's turbonormalized Bonanza conversion), recommended cruise setting is WOT and 17.5 gph (85% power) at all altitudes. Of course, you can lean further for less speed/greater range, but the engine is nonetheless outside the red fin at 17.5 gph (80 deg F or greater LOP).

Turbonormalized engines are in fact simpler to operate than our normally aspirated powerplants! :cool: In the TN-SR22, the recommended procedure is full throttle/full rich for takeoff and all the way to altitude. After leveling off for cruise, one simply backs off the throttle a bit to 2500-2600 rpm, leans to 17.5 gph and that's it! (There's an alternate procedure for LOP in the climb but it doesn't save all that much fuel due to slower climb rate so many owners don't bother.) In our NA engines manifold pressure decreases with altitude and it's a little more complex to find the correct combination of fuel flow and degrees LOP at any given cruise altitude. Nonetheless following the procedure that I described in the above post should get you there with a minimum of fuss.

Note that turbocharged (not turbonormalized) engines have a lower compression ratio, are less efficient, and the "magic number" of 14.9 no longer applies. For engines like the turbocharged IO-550 in the Col400, the magic number is instead about 13.8 IIRC.

The most recent issue of Aviation Safety (March 2007) has another Walter Atkinson article that's quite good on p. 8: "Top Five Engine Myths."

[witrakw]

thanks for the info.
nice tables.
bill

[nworth]

I have attached tables for our 260 hp IO-470 and the 310 hp IO-550 as installed in the SR22.
I have a cross country flight for business planned Wednesday. I will give this a try by matching up what the EDM930 says about FF, LOP degress and engine power (supposedly it's smart enough to properly computer % HP on the lean side) say by comparison to the chart (or at least portions of the chart).

Thanks, Kevin, for taking the time to put this together.

[kwmoore]

I will give this a try by matching up what the EDM930 says about FF, LOP degress and engine power (supposedly it's smart enough to properly computer % HP on the lean side) say by comparison to the chart (or at least portions of the chart).
If so, the EDM930 is quite a bit smarter than the Emax in the Cirrus, which does OK ROP but is completely clueless about %power calculations on the lean side of peak!

[nworth]

Did a little "clear my head" after work flying today and brought along Kevin's chart.

Flew between 3000 and 8500, full throttle, using the chart and it all seemed to work out just right. The EDM930 seemed to agree, within 1% or so, at each FF and LOP degrees setting.

Very great.

I even tried climbing LOP. It worked, but I don't think I will make a regular practice of it -- the LOP degrees changes rapidly even at relatively sedate rates of climb and, full throttle, it's easy to find yourself in red fin territory.

Thanks Kevin.

- nw

[kwmoore]

Whew! :p Glad to hear it--thanks for doing the experiment Norm.

How did the simplified version seem to hold up (13 gph below 4000, 12 gph 4000-8000, 11.3 gph above 8000)?

At what altitude was it no longer possible to be 45 deg F or more LOP at 13 gph (75%)?