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Thread: Advice and techniques for running at reduced power settings

  1. #1
    Green Goggles's Avatar
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    Advice and techniques for running at reduced power settings

    I started flying about 4 years ago, and have logged all of my 200 hours in an M20C Mooney.
    I have the carburated, 180-horse Lycoming engine.


    For all of those 200 hours, I operated with engine RPM between 2300 and 2500- the recommended operating range according to my owner's manual. (Generally I was going somewhere and wanted to get there.) 2000-2250 is the Red Arc, denoting no continuous operations.


    Recently, it seems I am never in a hurry, just sight seeing. And I want to burn less fuel. So what do I need to know to safely operate below that range?
    Is there anything to be aware of regarding mixture settings, carb heat, or the reduced cylinder temperatures?


    My owners manual gives powers settings for 1800 RPM, noting 16.9, 16.8, and 16.6 inches at 5,000 ft, 7500 ft, and 10,000 ft respectively, but that's it.
    Do I need to reduce power to 16 inches to safely buzz around at 1800 RPM? Am I over thinking this?


    Any thoughts or advice are appreciated!

  2. #2

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    For what you are doing, there is no advantage to running at low RPM. I believe that you are looking for low fuel consumption and you can get that at more typical RPMs.

    Lycoming will be happy to sell you an operating manual for the O-360 engine that contains a TON more operating info than you will find in the Mooney airframe manual. I highly recommend that you get it.

    I will suggest that you can happily run around at 2300 RPM with the manifold pressure set for 55% power with the mixture pulled way out to get low fuel burn at moderate speed. You could run 18" - 20" MP with lots of leaning if you want.

    So the next question is whether your airplane has an Insight or JPI or EI engine analyzer. With the temps for every cylinder you can really tell what the mixture control is doing for you and you can get your lowest fuel consumption.

    At low RPM, the airplane won't accelerate if you discover that you need to maneuver and you can hurt the engine by applying too much manifold pressure. Low RPM, low MP, cruise is usually done by the ferry guys trying to get the absolute max miles per gallon so that they can cross an ocean.

    Hope this info helps,

    Wes
    N78PS

  3. #3
    Green Goggles's Avatar
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    That all makes sense and is very helpful. Thanks! cheers2.gif

    FWIW, I don't have any type of engine analyzer in my airplane. I have one overall cylinder temperature guage, not individual readings.

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    I will suggest that a GEM or a JPI is a better investment than the latest sexy mode S transponder. With a light bar for every cylinder you can see exactly what is going on in your engine and lean very reliably. I own two Insight GEM 602's (two airplanes). If you foul a plug on startup and your engine runs rough when you do your mag check, you can see exactly which cylinder and which plug. Takes the sweat out of trouble shooting. Lets you get the best fuel consumption at any power setting.

    Best of luck,

    Wes
    N78PS

  5. #5

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    I owned a 1970 M20C.

    The prohibition of between 2000 and 2250 is probably due to some harmful vibration either prop or engine or the combination of both.

    As for as hurting the engine with too low of rpm if you need to add power, that is a partial truth. If you have to add any significant amount of power, like a go around, THE PROPER PROCEDURE IS FIRST PUSH UP THE PROP RPM TO FULL AND PROBABLY THE MIXTURE TO FULL IF AT LOW ALTITUDE AND ONLY THEN ADD THE THROTTLE.
    So going to more power lead with the rpm and mixture, then add throttle.
    The reverse is true when needing less power, bring throttle back as needed, then prop rpm, then set mixture leaner as needed for less power.
    It only takes a few seconds to lead with prop and mixture when adding power and even on a go around you can come in with a smooth addition of power and not need to slam the throttle open.
    It is one reason I like the Mooney style pedestal with the three levers better than the Cessna style push pull knobs; it is easier to make changes with one hand for all three levers.

    It might be nice to have an engine analyzer, but I think the 4 cylinder carbureted engine is a simple and reliable one as it is.
    My injected Lyc IO 360 in my next Mooney 201 went to 2000 hr tbo, used a lot of oil but kept running.

    By the way if anyone tells you those Mooneys are hard to land is almost surely not much of a Mooney pilot and probably trying to land too fast and/or with power on.
    Last edited by Bill Greenwood; 10-10-2012 at 08:35 PM.

  6. #6
    Green Goggles's Avatar
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    Many good thoughts there, Bill.
    Thank you.

    Quote Originally Posted by Bill Greenwood View Post
    By the way if anyone tells you those Mooneys are hard to land is almost surely not much of a Mooney pilot and probably trying to land too fast and/or with power on.
    I did all my flight training in my Mooney, and it is the only plane I've ever flown.
    So... even if they are difficult to land, I wouldn't know any better!

  7. #7
    kscessnadriver's Avatar
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    Quote Originally Posted by WLIU View Post
    I will suggest that a GEM or a JPI is a better investment than the latest sexy mode S transponder. With a light bar for every cylinder you can see exactly what is going on in your engine and lean very reliably. I own two Insight GEM 602's (two airplanes). If you foul a plug on startup and your engine runs rough when you do your mag check, you can see exactly which cylinder and which plug. Takes the sweat out of trouble shooting. Lets you get the best fuel consumption at any power setting.
    I could see this talk of an engine monitor if he had a fuel injected engine, which could likely be ran lean of peak fairly easily. However, I'm not sure that an engine monitor is really a worth while investment on a carburated engine, where running LOP is going to be a challenge no matter what you do.
    KSCessnaDriver
    ATP MEL, Commercial Lighter Than Air-Airship, SEL, CFI/CFII
    Private SES

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    The reason for the red arc on the tach is indeed due to destructive resonance between the 4 cylinder engine and the prop. Hartzell does a bunch of testing and in that RPM range the blades apparently are fatiguing at a rate that greatly shortens their fatigue life. It apparently has to do with resonance between the prop and crank, etc.

    An engine monitor is just as valuable with a carbureted engine, but if you do not need to worry about that last gallon-per-hour, or when the engine burps you just drop your ship off at the shop and wait for the bill, you don't need one. I can report that as soon as I installed my first one, fired up, and started seeing what it told me, I had an "A-HAH!" moment and never looked back. But I fly airplanes with limited fuel supply and when going cross country always have to optimize my fuel burn. If you fly an airplane where low fuel means that you still have 2 hours on board, then you don't need precise info on your engine performance.

    Fly safe,

    Wes
    N78PS

  9. #9

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    An engine monitor would be nice to have, just like leather seats and a new Porsche to drive to the airport. But if you run that O-360 engine carefully , it should go to TBO, either 1800 or 2000 hours with or without the monitor.

    If you have nothing to see egt, then once at reduced cruise power, probably at 2000 rpm, you slowlly lean until the mp just starts to drop or the engine seems a little rougher, then enrich just a bit. If you have an egt gauge you can see the temp rise as you lean, then peak, then fall and go a bit, say 50* cooler than peak.
    I think you will find that this setting will be close no matter if you are leaning with no gauge or just egt or a GEM which gives egt and cyl head temp for each cylinder.

    I am not sure what a "burp" is in this engine, might be that the pilot had the Taco Bell giant breakfast burrito, but it is unlikely that you are going to have a sudden engine disaster in a new or good OH Lyc O-360 that is flown at less than 75% power for cruise, and properly cared for. The is not an engine overheating problem in these Mooneys like in some tightly cowled planes and he doesn;t have a turbo or even fuel injector to deal with. Just a relatively simple and understressed engine.

    One big advantage of a Mooney is range. The M 20 c Ranger might cruise around 150 mph at 7500 feet, and will likley go for 5 hours or so, I can't recall exactly.
    My 201 had 64 gallons of fuel, and burned less than 10 gal per hour in cruise, thus fuel was not something you had to worry about, that is, of course if you used good common sense and filled up the tanks after each flight. I know I went from Colo to Austin once non stop, safely, but these days the pilot needs more stops. I recall heading directly to the mens room upon landing at Mueller.
    Last edited by Bill Greenwood; 10-11-2012 at 08:41 AM.

  10. #10
    Hank's Avatar
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    Goggles,

    My Owners Manual recommends 20"/1950 RPM for low-speed loitering and flightseeing. But I don't like the way the plane handles there. I often run 2300 down low, and put the throttle wherever I need it airspeed-wise. Food runs to the next county are 23/2300; flightseeing can be anywhere from 16-20" & 2300.

    Bill, my C holds 52 gallons, burns a pretty steady 9 gph [good for 5 hours], and won't run LOP without lots of vibration. 201's do it nicely if your injectors are balanced, but carbs are difficult to get there even with slightly reduced throttle and partial carb heat. I probably average ~140 knots groundspeed, indicating around 140 mph at 6000-8000 msl.
    Hank
    1970 M20-C

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