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5 Answers

Why Does Max Engine RPM Increase with Altitude

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Aircraft Systems

Cessna 172 PIM states that normal engine speed operating limit changes with altitude: 2500 rpm for sea level to 5000 ft in standard conditions, 2600 rpm for 5000 ft to 10,000 ft, and 2700 rpm for above 10,000 ft. Why? Is it due to the increased density preventing the propeller from spinning up to 2700 rpm at low altitudes?

Engine is rated at 180 HP at 2700 rpm. Does that mean I'm not producing max power any time I'm below 10,000 ft?

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5 Answers

  1. Nibake on Sep 01, 2014

    What you are talking about is probably “max allowable continuous” engine output. You can do 2700+ rpm at almost any altitude by diving with full power, but the POH is giving you a guideline as to what your maximum cruise should be. And yes, you are correct, density altitude is the key factor here. It doesn’t require as much fuel burn to maintain a given RPM at higher DA, but if your RPM stays the same as you climb you will be producing less thrust since the propeller has thinner air to work with. Therefore, to maintain a set percentage of engine power/hp say 65% or 75%, you will increase your rpm with altitude. I would think that should be pretty well spelled out in the POH, what year is your 172?

    Don’t confuse these settings with max power, these are almost certainly cruise limitations. You probably won’t be produce max power unless you are at sea level DA, which is probably what the 180hp figure is based on.

    In a nutshell, there is a difference between maximum engine performance and max allowable settings with regard to engine operation. Hope this helps.

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  2. Kris Kortokrax on Sep 01, 2014

    You didn’t mention which model you are flying, but I will make the assumption that it is a 172S.
    The POH lists in Figure 2-3 normal operating ranges (green arc) of 2100 to 2500, 2600, 2700.

    Your use of the word “limit” indicates a bit of a misunderstanding. The limit is 2700 (red line).
    This is also the maximum continuous RPM at any altitude. It has nothing to do with diving at full power.

    If you look at the performance data in Figure 5-8, you will see that the power developed at the high end of the green arc will be right around 75% for standard conditions. The POH recommends cruising at a power setting of 75% or less.

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  3. Drew on Sep 01, 2014

    I was about to say that the phrase “normal engine speed operating limit” was taken straight from Section 7 of the C172S PIM, but upon reading it again, I realized that it says “top of green arc” in parenthesis right after that.

    Thank you for your answers.

    Talking about engine RPM, could you please enlighten me on why we don’t achieve max RPM/power (180HP with 2700 RPM) on takeoff roll?

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  4. Kris Kortokrax on Sep 02, 2014

    I’m not an aeronautical engineer, but I would believe that it has something to do with the change in relative wind that the propeller blade sees as the airplane speed increases. The angle of attack and hence induced drag on the blades would decrease with an increase in forward speed.

    Sometime, have your instructor do a takeoff and you can monitor RPM during the takeoff roll. As speed increases, you should see an increase in RPM.

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  5. Brian on Sep 17, 2014

    “but I would believe that it has something to do with the change in relative wind that the propeller blade sees as the airplane speed increases.”

    It’s just windmilling. Air forced perpendicular over a pitched airfoil will make the airfoil move. This is exactly the same concept you use when you auto rotate.

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