Full throttle not always best when LOP?

[aaronk25]

Did some experimenting with MP today and as a always LOPer I was always a bit curious that at 4k and WOT 2500rpm the lowest I could get the fuel flow down to was about 8.9 or it started to miss. At that setting it was about 90 LOP so as to why it was starting to miss is expected.

My point is that I think after 20lop-40lop further lop actually reduces mpg when compared to reducing the MP.

I wasn't in any hurry today so at 4k I set the prop at 2300rpm WOT and leaned for 30lop, which was about 9.4gph and 26mp. Decided I'd like to get the fuel flow back to 8.5gph but the lowest I could before detecting a miss was 8.9gph and that was about 90lop. Was indicating 140kts. Probably about 153kts true.

So I decided to bring MP back to 24 by reducing the throttle then set the fuel flow to 8.5gph with the mixture (about 25lop) and waaaalaaaaa 140kts indicated which proves WOT is not always more efficient.

I think its less efficient to compress all the extra air if your not going to put more fuel to the fire vs reduce the MP and keep the mix about 25lop.

BTW I have 25 degree of timing perfect condition mags and 200 hour tempest fine wire plugs.

Anyone else notice it's better to reduce throttle than leave if wide open and lean it as far as it will go and still run smooth?

[Cruiser]

Spot on! 

 

and well explained.

[jetdriven]

below 3K or so i often set 27" MP and then run LOP / 10.0 GPH  on that.  Full throttle activates an extra circuit which gives it extra fuel, pulling it back a small bit defeats that

[xftrplt]

aaronk25 posted:

I think its less efficient to compress all the extra air if your not going to put more fuel to the fire vs reduce the MP and keep the mix about 25lop.

----------------------

Your empirical data is interesting, but I disagree with the explanation.

Pumping losses of a NA 4-stroke increase as MP decreases and more vacuum is being pulled.

Even if just being windmilled, the air compressed on the compression stroke rebounds elastically on the power stroke--returning most of the energy of compression to the crankshaft.

Note, Jake brakes work by opening the exhaust valve right after the compression cycle, before the compressed air can rebound on the power stroke.

[Shadrach]

BFSC maxes @ ~25 LOP as Dick states above. The MP setting that yields the HP required for the speed desired at 25LOP will always yield the best fuel economy at said speed..

Byron, I do not believe the fuel system on my aircraft has an enrichment jet. The Bendix system has provisions for such a jet, but it is application specific. Are you sure your J has an enrichment jet at full throttle?

[jetdriven]

BFSC maxes @ ~25 LOP as Dick states above. The MP setting that yields the HP required for the speed desired at 25LOP will always yield the best fuel economy at said speed..

Byron, I do not believe the fuel system on my aircraft has an enrichment jet. The Bendix system has provisions for such a jet, but it is application specific. Are you sure your J has an enrichment jet at full throttle?

I cant verify its specific to the RSA-5AD1 but this diagram shows an enrichment circuit. It seems to act that way, bringing the throttle off the stop leans it out quite a bit.

[rbridges]

Same here, except I use 25" MP and generally 8 to 8.5 GPH.

 

 

you lean of peakers make me sick.  lol

[aaronk25]

Was gonna say to that by all means when at a altitude that is conducive to WOT and keeping MP below 25" I use WOT.

If I wanna to fast at 4k I'll leave it wide open and run 25 lop.

[DS1980]

Aaron,

Does your engine have the enrichment circuit?

[aaronk25]

Aaron,

Does your engine have the enrichment circuit?

Not sure but I don't think it matters as the problem appears to be running 80lop is inefficient.

[pinerunner]

Did some experimenting with MP today and as a always LOPer I was always a bit curious that at 4k and WOT 2500rpm the lowest I could get the fuel flow down to was about 8.9 or it started to miss. At that setting it was about 90 LOP so as to why it was starting to miss is expected.

My point is that I think after 20lop-40lop further lop actually reduces mpg when compared to reducing the MP.

I wasn't in any hurry today so at 4k I set the prop at 2300rpm WOT and leaned for 30lop, which was about 9.4gph and 26mp. Decided I'd like to get the fuel flow back to 8.5gph but the lowest I could before detecting a miss was 8.9gph and that was about 90lop. Was indicating 140kts. Probably about 153kts true.

So I decided to bring MP back to 24 by reducing the throttle then set the fuel flow to 8.5gph with the mixture (about 25lop) and waaaalaaaaa 140kts indicated which proves WOT is not always more efficient.

I think its less efficient to compress all the extra air if your not going to put more fuel to the fire vs reduce the MP and keep the mix about 25lop.

BTW I have 25 degree of timing perfect condition mags and 200 hour tempest fine wire plugs.

Anyone else notice it's better to reduce throttle than leave if wide open and lean it as far as it will go and still run smooth?

I agree. My GAMI spread, even after a bit of injector swapping, doesn't allow me to go very low in the fuel flow WOT so I drop the MP too to get where I want to be. At the lowest power settings, like for landing, I actually go for peak EGT at 2500 RPM after dropping MP enough to get the prop to minimum AOA. I just want it simple for landing so, full RPM after dropping power back a bit, adjust throttle until 2500 RPM (now I can treat it like a fixed pitch prop), then (following Busch's advice to those with simple planes) I adjust mixture to maximize the RPM. I'm at a low power setting so can't hurt it and figure I'm minimizing lead deposits and keeping the CHT's from dropping more than they need to. I leave it alone throughout the rest of landing, using throttle to adjust RPM's just like a cub. For go around mixture full then throttle (but if I forget and do the throttle first not a disaster). 

 

I think WOT is possible and good for some systems and situations but not a rule that we have to follow. I did get brave and do a big pull WOT after a climb the other day. The air was a bit rough and I was into the yellow arc on my old M20E so backed off to my current favorite setting 24 squared with big pull and set to 8 GPH. I watch the CHT's a lot when in doubt.

 

Opps, I just noticed I wandered into a "Modern Mooney Discussion" 

[DS1980]

Not sure but I don't think it matters as the problem appears to be running 80lop is inefficient.

The "standard" best economy setting at the moment is thought to be around 75 LOP. After reading some posts on here, it seems their results have shown between 20-25 LOP yields best economy. While LOP isn't new, there hasn't been an all-out comprehensive test of actual figures by engine manufacturers. Rather it's just real pilots operating in real environments that have done the testing for their own accord. I've been out of flying for a while, but am eager to start back up to do some testing of my own. I expect to have the same results.  It is entirely possible that the power curve on the lean side of peak is much steeper than what we are lead to believe, and 80 LOP may be getting to the point where it is too lean to burn.  If stoichiometric (peak EGT) is somewhere around 14.7/1, then I could see how 80-90 LOP would be getting too lean to burn, which is 18/1.

[Shadrach]

DS, if we had some way of advancing timing, then I'm sure that BSFC would continue to fall past 25 LOP. As it stands, for a fixed timing, NA engine, it's about the best we can get. At 25 BTDC and 80 LOP, peak pressure (PP) is occurring after the optimal crank position.  Advancing the timing would retard the ATDC angle of PP thereby regaining some power and efficiency.  With TSIO applications or NA engines run at max power right on the deck the BSFC curve does not fall off as precipitously.

 

http://www.avweb.com/media/newspics/196816_bsfc_mixture_curve.jpg

 

[DS1980]

Hi Shad,

But doesn't the power curve for LOP just look "too pretty"? It seems that the math has been done for ROP operations because that's where the engine manufactures tested their engines. It isn't a nice neat line rather it has dips and bumps as if those were actual tested numbers. Then after peak EGT, it falls off at a nice neat proportional line, and other graphs are like this too. What if the power curve drops off at a much steeper rate and "it's about the best we can get" because it IS the best? Or closer to best economy than originally thought.

 

Just a theory. I would like to see someone with electronic ignition give this some real testing. I think there lies the truth. You're exactly right with the fixed timing, and I really haven't thought about it from a PPP standpoint, just a fuel flow and CHT standpoint. That earlier spark from the electronic ignition just might be what is missing to get the whole picture.

 

And to think there's still pilots arguing over high wing vs low wing.

[Shadrach]

I'm pretty sure just about every BSFC chart I've ever seen in related to avaition is "conceptual"...which is to say an illustrated approximation of the curve. I'd never trust the numbers unless they were engine model specific... 

[carusoam]

Doesn't George Brailey have the instrumented engine developing real charts?

Or they are all IO550s, and that won't count for some reason.

The chart defining what and where the red box is pretty, but based on real data, isn't it.

I can picture and memorize the red box chart based on a few points. I would be unable to memorize the same chart made from actual data points...

Fortunately we don't have to be perfect. Yes it helps, but it's not that critical....?

Best regards,

-a-

[DS1980]

Most (all?) of the charts I've seen are for Continentals. Not sure if they're specific to the big bores. But I believe the physics is the same for all Jurassic engines.

It would be nice to know where the red line is.

[pinerunner]

I'm just happy to be minimizing lead deposits and lowering CHTs a bit, even if I need a lower MP setting to get there.

[Awful_Charlie]

The Bravo Lycoming manual has enough figures and graphs in in to calculate BSFC, which gets down to 0.38 or so.  Interestingly, most of the car figures I've seen are similar, or much worse than this, only getting significantly better (0.33 or so) for diesels.  Aircraft BSFC tends to be poor at maximum power ratings, improves for a time as the power comes back, and then goes to a worse figure than full power when you get down to the low end, have a look at this car one for instance! Wikipedia has a few ball park figures here too.

 

My understanding for the improvement at 'just off WOT' was that the throttle plate introduces turbulence into the induction stream and helps fuel atomisation and distribution, and is allegedly more pronounced for carburettor equipped engines

[kellym]

below 3K or so i often set 27" MP and then run LOP / 10.0 GPH  on that.  Full throttle activates an extra circuit which gives it extra fuel, pulling it back a small bit defeats that

Do you actually see a rise in EGT when you pull the throttle back? I don't recall seeing that at all on my E model.

As some are missing, LOP for our 8.7:1 compression engines FF X 14.9 or 15= Horsepower. So 10 gph LOP, regardless of MP and rpm equals 75% if you are LOP. One can just do big mixture pull to around 9-9.5gph and then find peak from lean side and back off desired amount from first to peak.

[jetdriven]

Kelly, you and Ross got me wondering myself. I often cannot get down to 10.0 GPH when WOT at my typical local cruising altitude of 1500' and I always assumed it was because of an enrichment circuit. Perhaps it was just because of too much air. Next time I go fly, I'll take note of EGT and report back.

[aaronk25]

I don't have any hard evidence but my oil has been getting darker faster since lately most of my flights have been at lower altitude around 3000 and since I've been running full throttle LOP around 27" mp about 30-40lop 2400-2500rpm I've got a sneaky suspicion that the increase cylinder pressure due to low altitude wot high MP is blowing more combustion by products past the rings, regardless of the cleaner lop operations.

I just changed oil and I'm going to keep MP at or below 25" during cruise to see if it makes a difference.

[jetdriven]

Cylinder pressure directly correlates to fuel flow when lop. What's your FF at these altitudes?

[Shadrach]

Kelly, you and Ross got me wondering myself. I often cannot get down to 10.0 GPH when WOT at my typical local cruising altitude of 1500' and I always assumed it was because of an enrichment circuit. Perhaps it was just because of too much air. Next time I go fly, I'll take note of EGT and report back.

I have a transducer, but have not bought an instrument to hook connect to it, so I don't know my FF. I assure that the plane will quite nicely LOP with the balls to the wall (except the red one). When it's cold and clear it will run ~160KIAS at 1000 feet and 40 LOP RAO with cool CHTs (<330 on #3) below 2000ft. The ASI may be incorrect, but I know relative to other flights that its giving about all it's got. From the numbers, I assume I am making just under 90% power.

[jetdriven]

Actually 11.0 gph and even 11.3 are pretty tame and that's 83% power.