flyingcheesehead
Taxi to Parking
Welcome aboard, Jenna!
You may have noticed, many threads here tend to "creep" as we discuss every last little thing that could possibly result in a different answer, and after a while we can get WAY off the original topic.
These questions aren't stupid, and this stuff isn't necessarily easy. It just seems easy to some people because we've spent years reading and increasing our understanding of it. So, I'll do my best to go back and answer all of your questions in as easy and reasonable of a manner as I can.
When we talk about ROP or LOP, the "Peak" involved is peak EGT.
As with so many thing in aviation, the answer is "it depends."
If you're trying to find best power mixture (for best climb performance), you'll lean to peak RPM. This is about 100 degrees rich of peak EGT. It's also probably not where you want to run your engine in cruise - You'll burn a lot of fuel.
Peak EGT is where you have the perfect stoichiometric mixture of fuel and air - All the oxygen will be used up, and all the fuel will be burned. Rich of peak you'll have some unburned fuel going out the exhaust, while lean of peak you'll have some oxygen going out the exhaust. Peak EGT is often where you'll find that the POH says to run for best range as shown in the performance charts, although you'll get even better range if you can run lean of peak EGT.
Try leaning it further - I've never seen an aircraft engine that runs smoothly and then just quits. Carbureted engines are notorious for having unbalanced air flow, so I'd be especially surprised if the engine in your 172 wouldn't get rough. So, I'm guessing that maybe you're seeing a "false peak" and you're not leaning nearly as much as you could. It's been a while since I did this in a 172 specifically, and I've never done it in a 180hp 172, but based on other experiences with the O-360 engine I'd say that you should be able to get the mixture knob at least 3/4 of the way out before it gets rough (depending on atmospheric conditions, of course). It'll definitely get much farther out than peak RPM - Remember that peak RPM is still about 100 degrees rich of peak EGT, so you've got quite a ways to go after you hit peak RPM.
What causes the engine to run rough is that one cylinder has gotten so lean that it's really not developing any power any more, so instead of the smooth-ish operation you normally get with all four cylinders going bang-bang-bang-bang in perfect sequence, you'll get bang-bang-bang-miss. It's quite noticeable, and you'll both feel and hear the difference.
Not sure where you're reading this and what the context is there, but the basic idea is that when you're properly leaned, if you add a significant amount of power, you'll be VERY lean and the engine may quit. This is also why we go full rich prior to landing - If you're still leaned out, and you go to full power for a go-around, your engine may quit right when you need it most.
It's very easy to see a demonstration of this while taxiing, and if you're leaning for taxi (you should be) and doing it right, your engine will quit if you go to runup or takeoff power. I lean the engine after start and as soon as I've checked to ensure I have oil pressure.
At idle power on the ground, lean the engine WAY out - Again, it'll be over 3/4 of the way back in most airplanes. Once you get past that, lean slowly so that you don't cause the engine to quit, but you should be right on the edge. Now, push the throttle in slowly - You'll notice the engine start to cough and run rough well before you reach runup RPM, if you did it right. If your engine doesn't quit by runup RPM, you're not leaning it enough for ground operation. (The bonus of doing this exercise is that you'll see what a rough engine feels and sounds like without any risk of it quitting in the air.)
You should go ahead and always lean it out during taxi operations. Up here in the cold parts, we get more lead fouling in the winter than the summer...
If you do the ground leaning as I described above, you'll need to richen the mixture just a little bit, or you'll actually end up with *less* power when you add throttle because the mixture is so lean! For that reason, I usually lean it as described above, except for at 1000 RPM. You shouldn't need more than 1000 RPM to taxi in a 172, and that mixture will allow you to make the power changes you need during taxi without needing to change the mixture.
What you read is right - If you keep it full rich during climb, as you climb the mixture will be getting richer and richer and richer as the air gets thinner and thinner, and your climb performance will begin to suffer. Full rich down low gives you a mixture with lots of power and good cooling. As you climb and the air thins, leaning the mixture a bit at a time to maintain that shortly-after-takeoff EGT will continue to give you the power and cooling you want as well as maintaining good climb performance.
Ironically, because we try to make it easy. None of this stuff is taught to student pilots for the most part, except the "lean til rough, enrich til smooth" technique that works in most trainers, and "always full rich in the climb" that works for most training and $100 burger flights that newer pilots are doing. The "why" isn't taught, all the other mechanics of it aren't taught, because they're not on the PTS and CFI's are trying to get you up to the PTS standards as quickly as possible. In addition, many CFI's don't know this stuff themselves, so they can't possibly teach it. And frankly, many people just don't care. So, those of us who do care are left to learn it on our own, and there's a lot of details to pick up along the way.
Since you only have one EGT gauge, you need to realize that all four cylinders will peak at different times - The airflow to the cylinders varies somewhat - and so it's difficult to run lean of peak EGT because you don't know when all your cylinders have gotten there. You essentially have four separate engines connected to the same crankshaft (and intake manifold). In theory, the EGT sensor is placed on the cylinder that runs the richest, but you really don't know for sure. That uncertainty is yet another reason why the lean-to-rough, rich-to-smooth technique is taught - It tends to work pretty well and it's hard to do better without better instrumentation.
The documentation for the JPI engine analyzer says that there may be "false peaks" as you are leaning, so you do an initial mixture pull that takes you up to the 1450+ range and then start looking for the peak. Not sure what causes that, but it may be at least part of what you're seeing.
Does the RPM vary quickly, surging for a few seconds and the like, or does it vary over time? With a fixed-pitch prop, many things can affect RPM, even if the engine is developing the same amount of power - Slight climb or descent, etc. so RPM can be quite variable in turbulence for example. Longer term, atmospheric conditions can change and other factors can affect the amount of power the engine is producing as well, which will also change the RPM.
If it's fluctuating fairly quickly, can you hear the change in pitch? If not, you might have a tach problem. If the tach needle bobbles across a range really fast, you definitely have a tach problem.
If you're at a high power setting with some cylinders ROP and some LOP, all of the cylinders are probably close to peak - That can be a bad place to run at high power settings. If you're below about 65% power, you can run the mixture just about anywhere and not cause problems.
Hope this helps!
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