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7 Answers

Leaning RPM vs Tabulated RPM

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Aircraft Systems

Hi askacfi, great site. I've got a really simple question regarding leaning. I've dug through various responses on sites like these, and even went as far as the engine manufacturers' manuals, since they are often quoted as being the ultimate source of info on leaning. However, I keep ending up with the same old answer: "rich of peak/lean of peak/some temp here, some there/roughness/smoothness/etc". That's all fine well and true, and there is no confusion on the side of the story of literally physically leaning - Lean out till RPM increases or temp changes, then proceed with whatever increment or decrement to achieve whatever kind of performance. What I'm trying to understand however, is how the "process of" and the "end result of leaning" relates to the tabulated power settings. If in a really simple(and made up) example, at a pressure altitude of 3000ft, an RPM of 2400 will give let's say 55hp, how does the leaning relate to that RPM value? Is the RPM value of 2400 the one you start off with, and don't try and adjust for after leaning, OR is final RPM value after leaning the one that you are meant to keep, and not "work back"(somehow) to get the 2400? The bit that's causing me this dilemma, is the fact that on various performance pages, in the noted conditions, it'll note whether to lean or not, what weight it's at, whether fairings are on etc. Well, if one reads(for example) "lean above x ft", should I start off with some RPM value, then lean for whatever performance, and leave it at that, OR should the value in the table, be the one that I should achieve WITH leaning? It seems that if you lean, you'll no longer have the value in the table...even if it says to lean :S   Thanks in advance

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7 Answers

  1. Wes Beard on Aug 20, 2013

    It seems to me that if the performance data says to lean above 3,000 FT MSL then you will lean above that altitude to achieve the desired performance numbers.

    Meaning this: Lean the aircraft and then set the RPM to the stated value in the performance section for the fuel flow and TAS information.

    If you were to lean after setting power, you will find that your TAS is lower as well as the fuel flow rate. As a test, plan a cross country with one leg leaning before setting power and then filling up and the second leg leaning after setting power and see the difference.

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  2. Jim F. on Aug 21, 2013

    You need to read the notes in aircraft POH. I’ll use the Cessna 172S NavIII for example. In the cruise performance charts under the conditions, it states, “Recommended Lean Mixture.” Additionally, these charts have another note relating to leaning: “Maximum cruise power using recommended lean mixture is 75% MCP. Power settings above 75% MCP are listed to aid interpolation. Operations above 75% MCP must use full rich mixture.”

    So unless you tell us the exact aircraft and flight condition you want to know, we can’t answer 100%. You’ll have to read and learn your aircraft’s POH. Perhaps a little ground session with a CFI is in order to ensure you understand how to properly interpret and follow the POH information.

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  3. Aleksandar Stjepanovic on Aug 22, 2013

    Thanks for the replies. Unfortunately, testing is easier said than done. I’m coming from a place where GA aviation is not as accessible or as welcome as it is in the US, hence why the question. Going up without the owner, head of the unit I’d rent from, flight chief, ATC and possibly others, not probing into what was I up to and why I even had the need to do it, would be a miracle in itself.

    Wes you note exactly what I was guessing might be happening. However now that you’ve said it, it does bring about another two questions – How come the leaning process is not described as such elsewhere(especially in the engine manuals)? If I change the RPM after leaning, would I not be changing the mixture yet again, by adding/removing extra air(assuming a non-constant speed propeller case here)?

    Jim the quotes you noted are exactly what I noted in my question – I keep coming across them all over, including official documents, tutorials, other responses on sites like these, and so on. It is also the way I was taught to lean, which also goes in hand with everything else I’ve read and seen. There isn’t any specific case in mind, but rather I’m trying to better understand the fundamentals.

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  4. Best Answer

    Brian on Aug 22, 2013

    “If I change the RPM after leaning, would I not be changing the mixture yet again, by adding/removing extra air(assuming a non-constant speed propeller case here)?”

    Yes, but not for the reasons you noted. As you adjust the throttle the fuel is also adjusted with it. That adjust, however, is rarely perfect.

    Wes highlighted what appears to me as the exact answer to your question, to which I am in agreement. HP is directly proportional to RPM, so if your RPM, after leaning, is not adjusted to match the specific book value than your HP will not be higher or lower respective of your RPM.

    I would not worry about the small change in mixture on that final RPM adjustment. However, if you care to be more accurate then after you lean and make your RPM adjustment simply repeat your leaning process. Repeat this lean, adjust, lean, adjust as much as you like till there is no more adjustment. Like I said though, since you’re likely only adjusting 50 RPM after leaning. Is the plus or minus 0.1 gph fuel burn associated with the slight change in mixture going to matter? I tend to doubt it.

    What you need to know is RPM equates to HP. Set the RPM – get the HP. Fuel flow equates to leaning. So the notes on the chart talking about leaning only deal with the column related to fuel flow.

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  5. Aleksandar Stjepanovic on Aug 22, 2013

    Thanks Brian, that’s exactly the type of a tech-heavy answer I was looking for. I remember reading in FAA’s PHAK(or one of those), about relations of different kinds of thrust, power, torque, HP, fuel flow and so on, and how one would associate one with the other, but it was pretty dry so it didn’t register the way it obviously should have.

    The whole releaning and “What you need to know is RPM equates to HP. Set the RPM – get the HP. Fuel flow equates to leaning. So the notes on the chart talking about leaning only deal with the column related to fuel flow.” explains exactly what I wanted to know.

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  6. Brian on Aug 22, 2013

    You’re welcome. Keep in mind in a constant speed system, RPM and MP will equate to HP; not just RPM. If you care to read up again try this: http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/00-80T-80.pdf

    The section starts on page 135 and the information relevant to this discussion is primarily on page 137.

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  7. Aleksandar Stjepanovic on Aug 22, 2013

    Thanks I’ll have a look. Now that I’ve had a short bit of time for the info you provided to actually settle in, it all looks so ridiculously simple. As my technical graphics teacher used to say, “you’ll be kicking yourself when I tell you the answer” – And I am, believe me!

    For anyone else pondering the same thing as I was, I’ll note it as such:

    The engine will use up whatever *quantity* of the mixture to create a certain amount of power(ie RPM relation). In the meanwhile, the literal fuel-air mixture is adjusted such that it works “best”(with a strong caveat on that term) for certain conditions(ie roughness, EGT, RPM drop and other relations). Not too different from the way it works in a car, with the exception of having the ability to control the proportions 🙂

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