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4 Answers

What is the recommended propeller RPM for high density altitude takeoff?

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Aircraft Systems, General Aviation

When performing a takeoff from a high density airport, is it advisable to use a lower RPM setting when flying an aircraft with constant speed propeller?? I am well aware that I have to lean the mixture properly for taxi and departure at high density altitude, but what about the prop? Do I leave it full forward as I do at sea level airports or will I get more power by taking off at a reduced RPM setting? I assume that because I am already flying at a greatly reduced manifold pressure it won't matter if I takeoff with a lower RPM. Please advise and thanks for your very useful site!

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4 Answers



  1. Matthew Hammer on Aug 17, 2010

    You want a low pitch high RPM setting no matter what the density altitude is. At a high pitch low RPM setting the propeller produces a lot more drag and will put an unnecessary load on an engine which is already operating at high power. During cruise you are typically operating with lower engine power, and thus by increasing the blade angle the propeller takes a larger “bite” out of the air for each revolution. This increases efficiency for this stage of flight, but not for takeoff.

    An easier way to picture it is like the gears in a car. If you want to accelerate quickly you aren’t going to start with the car in 5th gear… you start in 1st and move up to a higher gear as your speed increases.

    Also, the purpose of leaning out your mixture during takeoff at a high density altitude airport is simply to maintain a proper fuel/air ratio. At a higher altitude the air is less dense, and so the result is that if you do not lean the mixture (reduce the amount of fuel relative to the amount of air) the engine will run rich (too much fuel for too little air) — which causes the engine to run rough, lose power, consume an excessive amount of fuel, etc.

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  2. Matthew Hammer on Aug 17, 2010

    I failed to mention one very important (but hopefully obvious) thing in my previous comment. *Always* check your POH for the appropriate power settings and procedures for your aircraft. If it specifies that you should use a lower RPM setting during takeoff, then that’s what you should do. The procedures listed in your aircraft’s POH *always* preempt general rules-of-thumb.

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  3. John D. Collins on Feb 05, 2011

    Generally speaking, you want to use maximum power.  This is achieved by setting the prop control to the maximum RPM allowed. Some engines have limitations on how long the maximum RPM may be used, but most GA piston aircraft allow continuous use of the maximum RPM.
     
    Maxim RPM generates the most power when the engine is operated rich of peak EGT. The best power mixture setting is normally 125 to 150 degrees rich of peak EGT.  At lower density altitudes, a richer mixture of 200 degrees rich of peak EGT is often recommended to reduce the possibility of detonation and most throttle linkages are designed to run extra rich at full throttle.
     
    As an aside, since maximum power is achieved with a rich mixture, if we had a single gage that registered how much air was being delivered to the engine, we could calibrate it to indicate power. That is because there is more than ample fuel being supplied when operating rich mixtures, what is limiting is the amount of air being supplied for combustion. So more air, more power.  Unfortunately we don’t have a gage that measures the air flow to the engine, so we have to know about all of the factors that affect the airflow quantity.  Those factors include, RPM, manifold pressure, altitude, and temperature.  That is why we have those complex performance tables that include all of those factors to determine how much power we generate when operating rich of peak.
     
    Although it is a different discussion on how, when, and where to operate lean of peak (or a lean mixture), one thing that makes determining power easy is that it is solely dependent on the fuel flow to the engine.  In other words, with a lean mixture, you are supplying more air than needed for combustion, therefore the fuel flow determines the power.  Now a days, many aircraft have a digital fuel flow meter that can be used to determine power when operating lean mixtures.  On my Bonanza with an IO520BA engine (285 HP), horsepower equals 14.9 time fuel flow in gallons per hour, so if I want to set 65% power for cruise, all I do is set my fuel flow to 12.5 GPH.  This works over a wide range of RPM’s and MP, as long as the combination provides sufficient air to allow the engine to run lean.  So at 3000 ft, 2500 RPM, 22 inches, 12.5 gallons per hour is 65% power. So is 2300 RPM, and 23 inches, so is wide open throttle and 2700 RPM, as long as the fuel flow is set to 12.5 gallons per hour.
     

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  4. Kent Shook on Feb 05, 2011

    I always like to show the “why” behind the answers, and I think it’s been somewhat glossed over here.
     
    The reason you always want maximum allowable RPM for takeoff: All else being equal, each combustion event is going to deliver a certain amount of power, which is generally limited by the amount of air in the cylinder. The more combustion events you can have, the more power you’ll be developing. (More revolutions per minute = more combustion events per minute). In a 4-cylinder engine, you have 2 combustion events per revolution, and in a 6-cylinder you have 3 combustion events per revolution. So, in terms of RPM, the higher RPM will always give you more power (again, all else being equal).
     
    The amount of power we can develop is limited mainly by airflow, which relates to all three engine controls. We want the most air molecules we can possibly get, and the right fuel mixture to develop the most power. Throttle controls airflow by changing the pressure of the air in the intake manifold. Higher manifold pressure = more air = more power. RPM controls airflow by allowing each cylinder’s volume to be filled with air a certain number of times per minute. Higher RPM = more volume = more air = more power. Finally, the mixture control allows us to balance that air with the right amount of fuel to develop the most power. Too lean = Not enough fuel to develop power (air cannot make power on its own, of course). Too rich = the volume of fuel in the cylinder is too high to combust properly and is simply displacing some of that air we need.
     
    So, for takeoff – Always the maximum allowable manifold pressure (full throttle in a normally aspirated airplane), maximum RPM, and best power mixture.
     
    Turbocharged airplanes with either fixed wastegates (a la the Seneca II and later) or wastegates linked to the throttle (a la the Turbo 182) will overboost if you push the throttle all the way forward below the critical altitude, so be careful to know your equipment, as always!

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