Leaning Mixture Questions

[cocolos]

I ran across this short article and it got a bunch of questions I wanted to ask about leaning:

1. What is rich of peak and lean of peak?

2. On run-up when it's hot the article says to lean the mixture, do you so by setting RPM to 2500 or at the normal run-up RPM?

3. Following the previous questions, on the run-up on hot days it says 'After you're done with the runup, push the mixture lever forward "a bunch"'. Why? don't you get the best performance when it's leaned?


4. Richer mixture cools your engine better? why?

5. What are your thoughts on descending do you just run full rich?

Thanks!

 

[poadeleted20]

1. What is rich of peak and lean of peak?

As mixture is leaned from full rich, the exhaust gas temperature (EGT) will rise until the mixture reaches the "perfect" (stoichiometric) combustion mix, and then will fall again after reaching that peak EGT. Any setting richer than that to give peak EGT is "rich of peak" and anything on the lean side is "lean of peak." These are abbreviated ROP and LOP.

2. On run-up when it's hot the article says to lean the mixture, do you so by setting RPM to 2500 or at the normal run-up RPM?

Neither. If the density altitude is high enough to lean for best power for takeoff (3000-5000 depending on the aircraft/engine combination), the engine manuals tell you to run up to full throttle and then lean. With a fixed pitch prop, you lean to peak RPM. With a constant speed prop, lean to the recommended fuel flow for that density altitude. If you lean at normal run-up RPM, you'll be leaning too much and will have less than best power at full throttle.

3. Following the previous questions, on the run-up on hot days it says 'After you're done with the runup, push the mixture lever forward "a bunch"'. Why? don't you get the best performance when it's leaned?

Ignore that. If you lean at full throttle as the engine manufacturers recommend, you'll be right where you want to be. Of course, if you lean at normal run-up RPM, you'll be too lean and will have to enrich to get best power, but there is no good way to know just how much to move the mixture control. That's why the engine manufacturers recommend leaning at full throttle, just like you'll have when you take off.

4. Richer mixture cools your engine better? why?

That actually depends on where you start enriching. Peak cylinder head temperature (CHT) typically occurs when the mixture is set so the EGT is about 75-100 degrees rich of peak. If you're lean of that point, then enriching will increase CHT, but if you're rich of that point, enriching will reduce CHT.

5. What are your thoughts on descending do you just run full rich?

That's the worst choice. As you reduce power with a carbureted engine, the mixture will go richer all by itself. If you started with the mixture properly set for cruise, reduce power, and then enrich, you'll go way too rich. The first thing that does is create an environment highly conducive to plug fouling, which means when you add power later you might not have it to add. Second, if you start at a high/hot enough position, the engine might go so overrich that it goes rough, and you'll have to lean more to keep it running smoothly. My personal technique is to just leave it where it was in cruise until I have to advance the throttle again.

All these issues are addressed reliably in your engine Operator's Manual as well as the Lycoming Key Reprints and Lycoming Service Letters 185, 192, and 197, which you can find on the Internet or obtain from Lycoming. Similar pubs are available from TCM for Continental engines.

 

[Apache123]

http://www.avweb.com/news/pelican/182544-1.html
That's a good column (in my unqualified opinion) to give you the data on engine management.

1) When the red knob is all the way in, you're putting more gas in the engine than can be burnt by how much air comes in. This is considered rich. When the red knob is all the way out, you don't have enough gas to burn with the amount of air that's in the engine. This is lean. There's a way to reach the "perfect ratio" of gas to air with that red knob. If you have excess fuel OR excess air that aren't burnt, it actively absorbs some of that energy, cooling the overall combustion. At this perfect ratio, your exhaust gas temperature will reach the hottest it will get for that power setting. This is considered peak.

If you run your engines so that there's excess gas you are running Rich of Peak -- in this case you would lean and watch your exhaust gas temperature (EGT) gauge, when it reaches peak, you would then push the red knob back in to enrich it. 75º Rich of Peak (ROP) would mean you push the mixture knob back in until the temperature cools by 75º. The same is true for running Lean of Peak (LOP), except that when you hit peak EGT you keep leaning as the temperature cools.

2 & 3) Lycoming Service Instruction 1497 ( http://www.lycoming.com/support/publications/service-instructions/pdfs/SI1497A.pdf ) recommends doing your run-up lean at all altitudes, and only operating full-rich at take-off and climb power below 3000'MSL. Be sure to research the performance charts for your specific engine before doing any of this, though! I'm a stranger without any reputation for aircraft/engine-operation knowledge on the internet.

4) Richer cools only while you are ROP. Leaner cools when you're LOP. If you want hotter while rich, lean some. If you want hotter while lean, enrich. This cools the engine because if you're not right at peak, it means something isn't combusting. Regardless of whether it is gas or air, those molecules which don't get burnt up will absorb some heat produced by those that do get burnt.

5) I'm full rich during take-off/climb below 3000'MSL (which includes go-arounds). Other than that, I'm running pretty much as lean as I can. Without an EGT on carbureted engines: During climb I lean until rough, then enrichen slightly. I repeat this every 2000'; At cruise I lean until noticeable powerloss, and then adjust the mixture for airspeed while holding altitude. I must be one of the lucky ones with carbs because my engines will lose power then shutdown before they ever start to shake. My original training was done in Cessna 172s and 150s, they would start to shake like a wet dog at some point, and then I'd enrich until smooth. Doing this, I'm "most likely" running LOP, but without an EGT I cannot be 99% certain.


6) You're welcome! And you also just asked one of the most heated questions in aviation =D

 

[Dan Thomas]

That's the worst choice. As you reduce power with a carbureted engine, the mixture will go richer all by itself.

Not for all. I have flown carbureted engine that gave an EGT rise on power reduction. That was from a ROP setting, so it was leaning as the throttle closed.

It all depends on how the carb is designed internally.

Dan

 

[poadeleted20]

Not for all. I have flown carbureted engine that gave an EGT rise on power reduction. That was from a ROP setting, so it was leaning as the throttle closed.

Unless you started from from full throttle so the enrichment feature was engaged to start with (an exception to the general rule), if that happened, the carb was probably misadjusted.

 

[flyingcheesehead]

5. What are your thoughts on descending do you just run full rich?

No! I leave the engine controls alone at the top of descent, and as I descend I slowly pull the throttle to keep the same MP I had at cruise. Since MP and RPM are the same, the mixture remains the same as well, no need to change it.

As I get close to my destination, I'll reduce to 20", then 17", then 14" as I slow down. 14" is about as low as I can go with the gear up without setting the horn off, so once I'm below gear speed, the gear comes out. When I hit the pattern, I'm at 2000 RPM (below governing range, it's the throttle that gets me there, not the prop control) and at that point I can push the prop forward without it speeding up. I choose to enrich the mixture at the same time, both in preparation for a go-around. When descending from TPA, I further reduce to 1500 RPM, which generally gives me the glide path I want.

Of course, these settings are specific to each plane - You'll need to determine what works best in the plane you're flying, but there's no reason to descend full rich, or even to change the mixture at the top of descent unless you're reducing power as well.

 

[poadeleted20]

Here's a link to Lycoming Service Instruction 1094, which covers all this in detail.
http://www.lycoming.textron.com/support/publications/service-instructions/pdfs/SI1094D.pdf
The figure on page 6 explains a lot about the relationship of power, EGT, and CHT to mixture setting.

 

[Tom-D]

Unless you started from from full throttle so the enrichment feature was engaged to start with (an exception to the general rule), if that happened, the carb was probably misadjusted.

Not really, from idle, your engine should give you a 10-50 RPM rise in RPM when you pull the mixture to close the mixture valve.
When you don't get that rise in RPM, your idle mixture setting is too lean, if you get more than 50 RPM your idle mixture setting it too rich.

 

[poadeleted20]

Not really, from idle, your engine should give you a 10-50 RPM rise in RPM when you pull the mixture to close the mixture valve.

While that is true, we were talking about what happens when you reduce throttle for descent from cruise flight, not what happens when you lean the mixture at idle. Other than with the enrichment valve engaged, if you reduce throttle from cruise power without touching the mixture control, the mixture in the cylinders will get richer. If it gets leaner when you reduce throttle in that situation, then something isn't right.

 

[Jaybird180]

When reducing throttle from cruise on a FP Injected engine, should the mixture be enriched? My current technique is to turn the venier every so often until I'm in the pattern.

 

[poadeleted20]

When reducing throttle from cruise on a FP Injected engine, should the mixture be enriched?

Only if the engine instruments indicate it's running too lean after you reduce throttle.

My current technique is to turn the venier every so often until I'm in the pattern.

That is one technique, but I suspect you're encouraging formation of lead deposits by doing that. Probably better to leave it alone until you go around, and then advance both controls (throttle and mixture) together.

 

[MAKG1]

I doubt it's going to make much difference at 3000 AGL, but when I'm descending from 10,500 down to an 800 MSL pattern altitude, I'll reset the mixture for best RPM (fixed pitch) every few thousand feet. I have a hard time believing the mixture setting at that high an altitude will result in a running engine at sea level.

With a CS prop, keeping manifold pressure constant, the mixture needs should be identical. That's what manifold pressure measures. But that may not represent the best forward speed; you'll have pretty low throttle if you started at high altitude (at least naturally aspirated).

 

[poadeleted20]

With a CS prop, keeping manifold pressure constant, the mixture needs should be identical. That's what manifold pressure measures.

You're confusing pressure with density. Even at the same MP/RPM combination, the denser air at lower altitudes requires a richer mixture unless you have a turbocharger to pack that air more densely.

 

[Dan Thomas]

Unless you started from from full throttle so the enrichment feature was engaged to start with (an exception to the general rule), if that happened, the carb was probably misadjusted.

No, one of them was one of those lousy Precision Airmotive HA-6 carbs.

There's no way to get really consistent performance from float-type carbs. I've even seen two exact same carbs, same part number, brand new, give different results.

Dan

 

[RV10flyer]

I doubt it's going to make much difference at 3000 AGL, but when I'm descending from 10,500 down to an 800 MSL pattern altitude, I'll reset the mixture for best RPM (fixed pitch) every few thousand feet. I have a hard time believing the mixture setting at that high an altitude will result in a running engine at sea level.

With a CS prop, keeping manifold pressure constant, the mixture needs should be identical. That's what manifold pressure measures. But that may not represent the best forward speed; you'll have pretty low throttle if you started at high altitude (at least naturally aspirated).

I leave the mixture setting from cruise at 12.5-13.5' the same until entering the pattern. The engine runs fine. My cruise mixture setting is also used for taxiing and runup. I dial rpm back to 2100 from 2350 for the descent. I pull throttle out in very small increments.

 

[denverpilot]

Neither. If the density altitude is high enough to lean for best power for takeoff (3000-5000 depending on the aircraft/engine combination), the engine manuals tell you to run up to full throttle and then lean. With a fixed pitch prop, you lean to peak RPM. With a constant speed prop, lean to the recommended fuel flow for that density altitude. If you lean at normal run-up RPM, you'll be leaning too much and will have less than best power at full throttle.

Dang. Now I need to go install a fuel flow gauge since I don't have one, but do have a constant speed prop, and make sure I only fly on cold days, since my airport is at 5885'.

 

[L.Adamson]

Dang. Now I need to go install a fuel flow gauge since I don't have one, but do have a constant speed prop, and make sure I only fly on cold days, since my airport is at 5885'.

And all that time, I never used my fuel flow gauge for leaning purposes. But I was only at 4607'.