Leaning by POH?

drgwentzel

drgwentzel

Pre-takeoff checklist
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Kobra
Flyers,

Sorry to beat an over posted topic, but I'd like to hear the consensus of the group.

I fly a 1971 Cardinal RG with an IO-360, 200 HP, CS prop, with an installed EGT, digital fuel flow computer, a digital tachometer and no engine monitor.

The POH has tables grouped by altitude, such as: 2500, 5000, 7500, 10,000, 12, 500, etc. Down the left side you pick your MP and RPM and off to the right it will give you the % power, GPH, TAS, duration and range.

Ok…fine, let’s take a very common situation: I am at 2500 feet MSL, 24 inches MP and 2400 RPM. If I look that up on the table it says I am at 71% power, I should expect a TAS of 154 mph, 4.9 hours endurance and a 755 mile range with no reserve.

More to the point it says I should lean to 10.2 GPH. When I set the GPH to what the POH recommends I notice that on the EGT that it is somewhere between 25 and 50 degrees ROP. I think even rarely it can be right at peak EGT.

I’ve heard that at or below a certain % power you can’t hurt the engine no matter where the red knob is and you can run LOP or even peak EGT and have no problems or concerns. Some have said that figure is anything below 70% power and others have said it’s closer to 65%.

I’ve heard it stated that 25 to 50 degrees rich of peak is the absolute worst place to be. It should either be 100 degrees ROP or 50 degrees LOP.

I’ve heard it said that never run LOP unless you have an engine monitor.

Can the group help me dig out the gold from the OWT’s? Should I follow the POH no matter what? Are the POH’s updated to modern data and wisdom or am I flying with 1971 information?

Can’t wait to hear the groups thoughts.

Gene
‘71RG out of N14
 
Remember what one of the criteria for writing a POH is..."marketing" ;)

Actually, your tables are quite likely based on either "best economy" or "best power". "Best power" is about 50 degrees rich of peak, IIRC...one of the worst places to operate your engine in reality. "Best economy" is right at peak.

Just like you are not REQUIRED to takeoff and land on runways exactly the length required by your performance charts, you're also not REQUIRED to operate your engine in the manner implied by those tables. But if you adjust your mixture either side of best power, you're going to slow down, and you won't "make your charts". If you adjust your mixture richer than best economy, your mileage will go down, and you won't "make your charts". (I'll stay out of the whole LOP discussion here. ;))

It's up to you to operate your airplane in a fashion that you feel exercises the proper care of your equipment.

Fly safe!

David
 
The reason you don't is because you do not know what the other cylinders are doing. One cylinder might be 50 ROP but others may be in the danger zone.

It is easier to set the mixture with a CHT and fuel flow. I would not fly HP with out a CHT on all cylinders even if you had to have a switch to go from cylinder to cylinder.

The biggest thing is to keep the CHT below 380. Full throttle and set your mixture for the fuel flow you want, the leaner you go the less power you will get but the more efficient you are. As long as the CHT is below 380 you are OK. If it is not running smooth you need to balance the injectors or don't get so LOP. The EGT just gives a more immediate response and shows you where peak occurred.

Once you have set it a couple of times you can just set the fuel flow from memory or a chart then fine tune it with the CHT. Hotter days and colder days. I know what my cruise flow is at 8000' full throttle so all I have to do is set it with the mixture then just watch the CHT. If I am not in a hurry I can pull the mixture back some and go slower or get the best speed that the CHT will allow by going more towards peak on the lean side.

Dan
 
Read the Lycoming Service Bulletin, it has a lot of good information and is a very comprehensive discussion on leaning, IMO.
 
If you really care about this read John Deakin's articles on AvWeb. Your Lyc IO360 will run great LOP, you will save fuel, keep your oil and plugs cleaner, etc, etc. A rule of thumb for a NA engine is FF*14.9=HP. Therefore at 10gph LOP you are at 75% HP. Nice and simple. Forget the POH tables LOP. You do not need an engine monitor to run LOP, but they are GREAT devices for detecting problems before they become serious and for trouble shooting.

10.2GPH slightly ROP (right in the red box which gets smaller as you get higher) is probably ~71% power as your POH states. From that point try leaning down to about 9.2gph (68% power). You may lose a few knots. I'll bet you drop 10+dF on the CHT. An excellent trade off in my opinion. And while you're at it, just to see how well your particular engine runs LOP keep leaning while monitoring fuel flow. It may actually die from fuel cutoff before it gets rough (at this point I recommed pushing the mixture back in a bit :D). It may not. Note where it does get rough and use the formula above to see what your power was at that point. Note your FF, your CHTs, your airspeed. BTW, I recommend doing all of this at WOT to take that variable out. And give us some feedback of what you've learned.
 
Last edited:
tonycondon said:
Read the Lycoming Service Bulletin, it has a lot of good information and is a very comprehensive discussion on leaning, IMO.
Click to expand...
IMO, it does not (other than the chart). My opinion, study and experience is limited to NA Lycomings.
 
drgwentzel said:
Flyers,

Sorry to beat an over posted topic, but I'd like to hear the consensus of the group.

I fly a 1971 Cardinal RG with an IO-360, 200 HP, CS prop, with an installed EGT, digital fuel flow computer, a digital tachometer and no engine monitor.

The POH has tables grouped by altitude, such as: 2500, 5000, 7500, 10,000, 12, 500, etc. Down the left side you pick your MP and RPM and off to the right it will give you the % power, GPH, TAS, duration and range.

Ok…fine, let’s take a very common situation: I am at 2500 feet MSL, 24 inches MP and 2400 RPM. If I look that up on the table it says I am at 71% power, I should expect a TAS of 154 mph, 4.9 hours endurance and a 755 mile range with no reserve.

More to the point it says I should lean to 10.2 GPH. When I set the GPH to what the POH recommends I notice that on the EGT that it is somewhere between 25 and 50 degrees ROP. I think even rarely it can be right at peak EGT.
Click to expand...

I don't believe that you should ever lean by setting fuel flows taken from a POH. For one thing as David pointed out those numbers were put together with a lot of influence from the marketing department which may be contrary to your best interests. In addition, your instrumentation just isn't likely to be accurate enough and it doesn't account for humidity or SLP. If your tach or MP reads a bit low or the fuel flow reads high you could be setting yourself up a lot closer to peak EGT on the rich side than the POH would have you believe. For ROP ops you should use the EGT and add a fudge factor for the expected difference in fuel flows. Set the mixture to obtain your desired ROP EGT and then add at least an additional .5 GPH to ensure that the leanest cylinder is at the mixture you intended.

I’ve heard that at or below a certain % power you can’t hurt the engine no matter where the red knob is and you can run LOP or even peak EGT and have no problems or concerns. Some have said that figure is anything below 70% power and others have said it’s closer to 65%.
Click to expand...

The power setting below which any mixture is benign varies with the engine type. For turbocharged engines I'd say that anything above 60% could get things hot enough to matter and for your engine I'd use 65%. The reality is that this threshold isn't a cliff, it's a gradual slope. As you raise power from 60% to 75% on any engine you increase the peak cylinder pressure at any mixture and the closer you are to about 40 or 50 ROP on any given cylinder the higher the peak cylinder pressure. The result of high ICP is all bad, high CHT, high mechanical stress, high piston temps, decreased detonation margins, and worst of all, greater chances of pre-ignition.

Some folks will tell you that as long as you keep the CHTs below 380F you'll be OK and while there is some validity to that it's not all that matters. If you exceed 380 F you are treading on shaky ground, but it's quite possible to have excessive ICP with CHTs that are below 380F, especially when the OAT is much colder than standard. I recommend using 380 F as a sort of cross check on your mixture, not the primary means of establishing it.

I’ve heard it stated that 25 to 50 degrees rich of peak is the absolute worst place to be. It should either be 100 degrees ROP or 50 degrees LOP.
Click to expand...

There's some decent data supporting that postition. IIRC, at somewhere between 25 and 50 ROP is where the peak ICP is the highest and that combined with high power settings is one of the main things you should try to avoid. Personally, on your IO-360 I think you'd be OK running peak EGT (on the richest cylinder) at 65% power and below all day long, certainly it should be fine below 60%. You can get a slight efficiency improvement by going a little leaner, say 20-30 LOP with a slight sacrifice in power (which can be mitigated by rasing RPM) but I doubt that you'd hurt anything at peak EGT. At lower power settings (<60%) you'd be fine going to 30 ROP and the worst case ICP.

I’ve heard it said that never run LOP unless you have an engine monitor.
Click to expand...

I've heard that too but it's wrong. It's fairly simple to determine if your engine is flow balanced enough to run sufficiently LOP without a monitor and if you find that to be the case, you should be able to run LOP all day without concern. The method I have in mind involves determining how much airspeed drop you can produce by leaning before the engine begins to run rough. Set up the plane in normal cruise configuration with the mixture approximately 100 ROP (the exact mixture won't matter just make sure you're at least 50 ROP and not so rich that you're not so rich that you're losing power). Let things settle out long enough for the airspeed to stabilize (at least a few minutes) while carefully holding altitude as precisely as possible. While still holding altitude, lean the mixture until the engine starts to miss and then enrichen it slowly until you just reach the point where it smoothes out. Let the airspeed stabilize again for a few minutes. If the result is at least 3% slower than it was with the rich mixture your engine is sufficiently balanced to run LOP. If it won't run smoothly while lean enough to slow down that much you can still probably get away with running barely LOP if you keep the power below 60-65%.

One other quick and dirty opposed digit rule is that for an engine like yours, if your fuel flow is less than 8.8 gph (assuming your fuel flow measurment is accurate) you are producing less than 65% power. If you set the mixture to get that or slightly less fuel flow with the throttle wide open and the engine is running smoothly, you're probably doing pretty good.

Can the group help me dig out the gold from the OWT’s? Should I follow the POH no matter what? Are the POH’s updated to modern data and wisdom or am I flying with 1971 information?
Click to expand...

You can assume that your POH power setting data is based on misconceptions.
 
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