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Asked by: 874 views Aircraft Systems

POHs list takeoff distance for various pressure altitudes. But the higher the altitude the less performance/power a normally aspirated engine produces. POH says to lean for best power. Best power will be lower RPM at higher pressure altitudes so are the figures in the POH are assuming what RPM? Example: sea level normal max RPM is 3000 RPM, what should RPM be at 8000 pressure altitude for best power?

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4 Answers

  1. John D Collins on Mar 28, 2016

    For a fixed pitch propeller, the RPM for best power is what ever you get at full throttle and leaned for maximum RPM. If you are flying a constant speed prop, you will usually set the maximum RPM (usually full RPM) and use full throttle and leaned for best power appropriate for the altitude. On most NA aircraft, one leans using fuel flow or the EGT. EGT is normally set to a value equivalent to that achieved at sea level, for example 1200 EGT. I will either do a full power runup or make a leaning adjustment on the initial takeoff roll.

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  2. Dan Chitty on Mar 28, 2016

    Thanks John.

    Additional thoughts…….

    For fixed pitch props, do the calculations in the POH factor in a “rule of thumb” RPM decrease as pressure altitude increases? If “yes” a pilot should need to know what general RPM for a specific airplane is considered best power for a given pressure altitude. But this info. is not published. The only language I have ever seen in any POH is “above 3000 feet lean for best power” for takeoff. I know the engineers factor the standard decrease in air density to calculate performance but did the Cessna engineers figure 3000 feet of runway with also a factor of a decrease in horsepower at 8000 MSL that yields in 3000 feet of takeoff distance?

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  3. Best Answer

    John D Collins on Mar 29, 2016

    For the purposes of normal flying, I don’t know of any practical value needed to knowing the static RPM an engine will generate. Leaning the engine for best power is straight forward and does not specify an RPM. The pilot sets full throttle, lets the RPM stabilize, then leans until the RPM peaks and then sets the mixture back to the point it peaked. Then you release the brakes. As soon as you start moving, the air resistance to the propellor will decrease and the RPM will continue to increase on the takeoff roll.

    What I do want to know is if I am achieving the expected performance from the engine. I have already determined from the charts what distance to expect for the takeoff roll. Say it is 3000 feet. I need to be at 70% of my takeoff speed when half the takeoff roll is accomplished. So if my liftoff speed is 70 Kts, I need to be just under 50 Kts at the 1500 foot point. I use runway markings to locate a point that is 1500 feet. If I am not at 50 Kts, by this point, I know I am not getting the desired performance and can abort the takeoff.

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  4. Dan Chitty on Mar 29, 2016

    Thanks for the feedback. Much appreciated.

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