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Thread: Somebody Please Help Me on Leaning and Prop Pitches

  1. #1
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    Question Somebody Please Help Me on Leaning and Prop Pitches

    I should have asked this years ago, but I have never understood how much to lean my mixture, and when; ditto altering the prop pitch.

    Earlier today, I was in the Beechcraft Baron, which has controls for both. At c. 15,000, I just could not get it over about 130 knots. Increasing the mixture % decreased my speed. Does it matter if you're ascending, and the rate of ascent? (I'm guessing yes, but how much difference does it make?) Does altitude matter; that is, will the same mixture percentage at 15,000 result in a different speed at 20,000?

    Are there instructional materials on all this?

    Thanks all,

    Mac6737

  2. #2
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    Leaning is done in order to maintain a near ideal mixture ratio of fuel to air. As the air gets thinner with altitude the fuel flow needs to decrease.

    Increasing the mixture % decreased my speed.
    Yes, because too rich a mixture reduces the amount of power the engine can put out, and at 15,000 ft. you are already above something over 40% of the atmosphere, meaning that the air is just short of half as thick (and half as much pressure) as at sea level, so the MAXIMUM power the engine can produce is waaay down. Even at 7,500 feet you're down to about 75 to 80 % of sea level power on engines that are not turbocharged.

    According to my Bonanza manual (same engine as the Baron), at 6º F (standard temperature) at 15,000 feet, you can set the power at 16.1" of MP and 2500 RPM and get 183 mph (158 kts) max TRUE airspeed and burn 11.3 GPH, which will mean that your INDICATED airspeed will be MUCH less. On the fuel flow vs brake horsepower chart the engine is putting out a max of about 150 HP, or about half of the rated power at sea level. So you don't have a lot of power available, and yes, climb cuts you airspeed since part of your power is diverted to fight gravity more so you can climb.

    that is, will the same mixture percentage at 15,000 result in a different speed at 20,000?
    Mixture isn't set in percentage, rather in fuel flow rate, gallons or pounds per hour (depending on the gauge). You may not GET TO 20,000, especially since you're getting into thinner air and need a leaner mixture, and you'll have still less power available at that altitude.

    So at those altitudes you'll need a turbocharged airplane.

    Larry N.

    As Skylab would say:
    Remember: Aviation is NOT an exact Science!

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    Sorry I got cut short in my previous post -- life happens.

    The actual leaning procedure for the Baron would be to lean the mixture until the fuel flow reads according to the charts in the manual, but many in real life would lean until the EGT peaks, then enrichen the mixture until about 50º rich of peak. With a fixed pitch prop, you can lean (in level flight) for max RPM, then enrichen just a tad.

    The constant speed propeller is a little like a continuously variable gear shift on a car (but only a little). With the prop full forward (max RPM) you are set for takeoff, and will develop the maximum power and get the best bite on the air with the prop for takeoff. This is a bit like low gear in your car. Once airborne with the gear and flaps up, (assuming near sea level), bring the throttle back to 25" MP and then the prop back to 2500 RPM for climb (cowl flaps open, please, for cooling). If you are climbing past 4,000 to 5,000 ft then you'll need to lean the engine somewhat in the climb, as indicated above for level flight, and keep making adjustments every couple of thousand feet, or so until you reach you cruising altitude. You also will probably have to add throttle periodically to keep that 25" of MP. As you get to 5,000 ft and above you'll be at full throttle to keep that 25", but as you get higher you'll no longer be able to get 25". By 15,000 ft you might not get more than 15-16", depending on specific conditions (temp, barometer, humidity)

    Once you reach your cruise altitude, nose over into level flight then as you reach cruise speed reduce power (throttle first, then RPM) to the desired power setting -- you may use pretty much whatever you want so long as you don't exceed 75% power (max continuous rating), and if you want to bring the prop back to 2300 RPM, or even 2100, do that, but don't get the throttle too high (at low altitudes-- not a problem at higher altitudes).

    I hope all this helps -- if not, please ask for more info, but please be specific on what you need help with, or what wasn't clear from the explanations above.

    Larry N.

    As Skylab would say:
    Remember: Aviation is NOT an exact Science!

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    Thanks, Larry.

    This gives me something to chew on -- tomorrow. Just a few follow-ups for now:

    What is MP?

    What is EGT?

    What does 25" mean?

    If the on-screen prompt reports mixture as a percentage, of what is it a percentage (if anything), and what has that to do with "gallons per hour"?

  5. #5
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    MP is Manifold Pressure
    EGT is Exhaust Gas Temperature
    25" is 25 inches which is a measure of pressure derived from the height of mercury in inches associated with that pressure.
    Last edited by plainsman; 05-30-2021 at 11:50 PM.
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    Plainsman has it right. There is a Manifold Pressure gauge where you can read the changes as you move the throttle, and it reads in inches of mercury (Hg" for abbreviation). The quote mark has long been an abbreviation for inches, just as a single quote has been an abbreviation for feet. There is also an EGT gauge for each engine in the Baron and many other aircraft with relatively high power engines -- possibly not in a Cessna 172 and similar, though.

    If the on-screen prompt reports mixture as a percentage, of what is it a percentage (if anything), and what has that to do with "gallons per hour"?
    I don't have the 2020 sim, so I don't know anything about that "on-screen prompt" or what they mean by percent for mixture -- on the instrument panel you'll read that as GPH (Gallons Per Hour) or PPH (Pounds Per Hour), depending on the aircraft.

    Larry N.

    As Skylab would say:
    Remember: Aviation is NOT an exact Science!

  7. #7
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    Quote Originally Posted by lnuss View Post
    The quote mark has long been an abbreviation for inches, just as a single quote has been an abbreviation for feet.
    Everybody knows that. It just made no sense at all to me in this context.

  8. #8
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    Quote Originally Posted by Mac6737 View Post
    Everybody knows that. It just made no sense at all to me in this context.
    Granted that most folks know that, but I didn't know your background or much of anything about you except that you are a simmer, not even whether English is your first language, and I'd have hated to leave a loose end, so I added that. Didn't mean to offend.

    Larry N.

    As Skylab would say:
    Remember: Aviation is NOT an exact Science!

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    this lady explains mixture really well....



    this guy goes into depth

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    Cool

    None taken.

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