Another constant speed prop question

Discussion in 'Flight Following' started by Rebel Lord, Aug 8, 2019.

  1. Rebel Lord

    Rebel Lord Line Up and Wait

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    Ive been flying the Cherokee 235 for 5 months now. Still learning the airplane but I generally feel very confident in it and moving the new to me blue knob. One question though is for producing a given HP percentage what are the pros and cons of using the different rpms and manifold pressures that I’m given in my tables. I’ve heard that people choose lower rpms and higher MP for a quieter prop, or is it better for the engine to run the prop at a higher rpm and lower MP?
     
  2. EdFred

    EdFred Touchdown! Greaser! PoA Supporter

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    What color is better? Green or Red?

    What are you trying accomplish?
     
  3. Grum.Man

    Grum.Man En-Route

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    1-300 rpm isn't going to make a huge difference in your engine life so I wouldn't really use that for a decision point. As long as you can make the power setting you are after it's advantageous to run a lower rpm. Propellers are more efficient at lower rpm as well as more quite. Pay attention to the engine though, there may be some MP/RPM settings that create a vibration or an undesirable temp spread you may want to avoid.
     
  4. Rgbeard

    Rgbeard Line Up and Wait

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    I’ve been taught that, in general, a fine pitch (higher RPM) is good for climb, and coarse is best for cruise.

    I also like the quieter operation at 2,300 when at cruise.
     
  5. bradg33

    bradg33 Cleared for Takeoff

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    With a NA engine, above about 6-7000ft you lose the ability to adjust manifold pressure to compensate for running a lower RPM (assuming you want 65%+ power for cruise). I think, in general, 2500rpm and full throttle will produce the best results at those higher altitudes. Down low, where you can pull the prop back some and push the throttle up to compensate, that's another matter.
     
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  6. luvflyin

    luvflyin Final Approach

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    That’s what I do. I have felt what feels like the engine being uncomfortable doing this before. Kinda like ‘lugging’ it, what happens when you are in to high a gear in a car. I just back off the MP and/or increase the Revs a little to make it feel happier
     
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  7. Rgbeard

    Rgbeard Line Up and Wait

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    The people that invented the “square” theories don’t have turbos.
     
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  8. bobmrg

    bobmrg En-Route

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    Old wive's tale. No basis in fact.

    Bob
     
  9. Rgbeard

    Rgbeard Line Up and Wait

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    Yes many still believe it and promote it. Sigh...
     
  10. iamtheari

    iamtheari Pattern Altitude

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    For cruising, I prefer to run maximum available MP and adjust the RPM for my desired power setting. This is usually the quieter way to get to my desired power setting. I usually cruise at altitudes where a wide open throttle gives me an MP well within my plane's power setting tables. For example, the ambient pressure at 7,000 MSL in standard atmospheric conditions is 23 inches and it's almost down to 22 inches by 8,000 MSL. Running MP below the ambient air pressure means that the engine has to suck in air against a partially closed throttle valve. Here's how I look at it: The airplane's engine is simply an air pump with a fan attached to it. The speed of the pump can be adjusted by restricting air flow into it or by adjusting the drag of the fan. To me, it makes more sense to accomplish that speed adjustment with the thing that makes the airplane go forward (induced drag of the propeller blades) instead of the thing that doesn't (throttle valve). I suggest that you experiment and find what you like. Just obey the limitations in the airplane and engine manuals and you should be fine.
     
  11. Rebel Lord

    Rebel Lord Line Up and Wait

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    Yeah I’ve also heard a lot of about this square stuff. The only square conditions I fly at is explicitly called for at 24 2400 for cruise climb
     
  12. Salty

    Salty En-Route PoA Supporter

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    I do 2500rpm WOT for cruise climb
     
  13. luvflyin

    luvflyin Final Approach

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  14. bflynn

    bflynn Final Approach

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    I've been told "over square" was a rule for DC-3s. The fallacy of teaching it persists through today. Any power combination listed in the POH is fine and many of them have lower MP than RPMs.
     
  15. CharlieD3

    CharlieD3 Pre-takeoff checklist

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    I always thought it was okay to run oversquare, as long as you were flying "on step"...

    Yet another myth busted...

     
  16. Juliet Hotel

    Juliet Hotel Cleared for Takeoff PoA Supporter

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    For a plane that isn't much of a speed demon like a 235, I don't know that are too many pros or cons of using one setting over another. Lower RPM is going to be quieter, but only a little.
     
  17. Clip4

    Clip4 En-Route

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    It is only 300 rpm, but over 2000 hours the crank will spin 18 million less revelations, the cam will not activate a lifter 9 million times and with .1 difference in fuel burn you will save $1000 in fuel.
     
  18. Ghery

    Ghery Final Approach

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    "Over square" is X amount of BS. I had a CFI get all over me during a flight review in the club's Arrow because I was using over-square settings. I reminded him that the settings I was using came straight out of the POH. I never hired him again, either.
     
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  19. denverpilot

    denverpilot Taxi to Parking

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    Just for a side note, don’t get hung up on that. There’s altitudes in a non-turbo aircraft where a cruise climb will get you nowhere fast.

    Just change it to desiring a maximum performance climb and magically the POH says you’re fine to go WOT and RPM to the redline in most aircraft if you need it.

    Which means, it’s not really a limitation. Any climb can be maximum performance if you want it to be. It’s just a noise and climb rate decision. Your real limit is usually temperature in a high deck angle climb more than anything.

    As for me, we live at 6000 so the blue knob has always been a noise abatement device more than anything. After that, it’s a “how much performance do you want” device.

    Down at sea level I like lower RPM within book limitations for cabin noise level, as long as performance isn’t drastically reduced. And I’ll burn whatever fuel needed to go fast, I didn’t buy it to fly it slow on an XC. Farting around doing maneuvers, sure.

    But do look at fuel burn. High power settings if the temps are reasonable, suck down gas at a high rate. More air and fuel pumped through the engine, is all that is.

    Up here we run out of air and have to reduce the fuel accordingly. We don’t even take off full rich in anything without a turbo.

    People say the 182 is a 13 GPH airplane, but ours has always lived up here, and we have the data to show it’s an 11.5 GPH airplane when living at this altitude. Heh. ;)
     
  20. ateamer

    ateamer Line Up and Wait

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    Over square - BS that needs to just die and be forgotten. If it was a rule of thumb for radial engines, it must have been in their very early days when they were making hardly any power. I think takeoff power in the planes with Turbo Compounds was something like 56”/2700+ RPM, and they went well over 2,000 hours, maybe up to 3,000 between overhauls.

    If the engineers who designed and tested the plane say you can do it, do it.
     
  21. flyingron

    flyingron Touchdown! Greaser!

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    It's a fallacy to think that engine lifetime is related to sheer number of rotations (or oscillations). There's a whole lot more to it than that (primarily temperatures). Running things at lower RPMs aren't going to change engine life.
     
  22. Chip Sylverne

    Chip Sylverne En-Route PoA Supporter

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    Fear is the poison of our lives.
    Any combination on the MP/RPM chart provided by the engine manufacturer for a given pressure/density altitude is just fine for the engine. Having said that, experiment with them until you find the combination that is smoothest. That's the one your engine likes best. Mine is 24/2275 or thereabouts.
     
    Last edited: Aug 8, 2019
  23. Clip4

    Clip4 En-Route

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    Good point, engine will run cooler too.
     
  24. midwestpa24

    midwestpa24 Cleared for Takeoff

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    Obviously over square never applied to turbo charged engines. For normally aspirated engines it is a good starting point. When learning the basics, then expand from there. Most light normally aspirated aircraft will have some type of limitation on how far over square you can safely run, and it's still a good principle to keep from running high manifold pressures with low rpm.
     
  25. flyingcheesehead

    flyingcheesehead Touchdown! Greaser!

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    There's several reasons to choose lower RPM and higher MP:

    1) It's more efficient. Lower pumping losses (not trying to suck air past a more-closed throttle plate), lower friction losses (lower RPM), generally more of your power becomes thrust than noise compared with higher RPM, etc.
    2) It's quieter.
    3) Less wear on the engine.

    There really isn't a good reason to go higher RPM and lower MP for the same power setting unless you're high enough that you can't make enough MP to get the power you need at a lower RPM.
     
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  26. deyoung

    deyoung Pre-takeoff checklist

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    It's also the money lever (or knob). Where I put it can be 15gph, or 11.5...

    Honestly, I never take off full rich here either. The field elevation is 2600 or so, but DA is regularly over 5000 or more. On run-up, I set the mixture full rich and RPM to 2000, and as I bring the mixture down, I get the RPM bump more than halfway through the lever's travel. Probably close to 2/3rds. Full rich is definitely not best power, here (and really I find that to be true even when it's not ridiculously hot).
     
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  27. denverpilot

    denverpilot Taxi to Parking

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    Yeah by the time I strap the airplane on, the money difference between 11.5 and 15 is kinda not worth worrying about much. Ha. Too many years spent flying slow airplanes, I’m over it. LOL.

    I’ll never buy a jet, but $20 more bucks an hour to fight the evil damn drag curve and physics, is fine by me. Ha.

    Old Cessna POHs say 3000’ for when to start to lean. They don’t specify MSL or DA. That was left up to the pilot and their brain cells in the day of tiny pamphlet sized POH books. Haha. :)

    As long as you’ve made about three half twists in from max power, ok most vernier style mixture controls, just to have some unknown... but clearly workable... margin against detonation, it’ll work out better than dumping massive amounts of fuel through the engine that it can’t even burn.

    One caveat to this, if the engine refuses to cool down in climb, I’ll let it have as much fuel as it wants with the red knob all the way forward and the climb shallowed and sped up as much as practicable.

    Overheated aluminum just isn’t very strong, and it’s no longer a fun day when the engine quits because of repetitive heat stress or worse, things melting. :)

    At sea level the mighty old O-470 will suck gas through a firehose compared to up here and it’ll get a lot hotter if you shove the nose in the air to impress someone with the climb rate of a fat nosed spamcan that has lots of yummy air to work with, both for the engine and for the lifty things we are hanging under.

    I’ll still climb it like a bat out of hell for just a minute or so, down there, just to prove the book isn’t lying about the climb numbers that I never see at home. And oh yeah; to clear obstacles. Right... obstacles. That’s it!

    Hahaha. But then ya gotta ease off a bit in hot weather. That fat six cylinder chunk of metal likes airflow.

    I feel cheated. I almost never get to use all 230 HP I paid for! Only in winter when I have to freeze my butt off at the hangar. :)

    The answer is always more money though...

    Heated, lighted, hangar... mo’ money. Mo’ money. :)

    My short time in twins made me quite thankful for 11-15 GPH in my Skylane. Hahahaha.
     
  28. danhagan

    danhagan Pattern Altitude

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    Might make a difference in endurance on a long XC;);)
     
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  29. Grum.Man

    Grum.Man En-Route

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    By .1-.2 gph for a given power setting so not real significant
     
  30. bnt83

    bnt83 Final Approach PoA Supporter

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    CS props are pretty sweet, was cruising at 11.5k feet away from the Denver area a few weeks ago, cruise climbing wide open throttle at 2550 RPM, got to altitude leveled off with pitch trim still wide open throttle and thought the CHTs were a tad high, pulled another 50 RPM off with the blue knob and it seemed to do the trick.
     
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  31. Skip Miller

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    None other than Charles Lindbergh demonstrated with a squadron of P-38 fighters, that running a low rpm/high manifold pressure saved fuel noticeably on extended missions (up to nine hours) in the Pacific.

    Why? The engines' cylinders are a vacuum pump of sorts. And using conventional guideance, it is more work for the engine to suck a high vacuum (higher rpm, lower manifold pressure) than the opposite - suck a low vacuum (lower rpm, higher manifold pressure). So according to one article I found, his squadron cruised at 1600 rpm instead of 2200 rpm.

    The naysayers worried about more load on the engine but the engines ran fine at 1600 and high MP.

    I wouldn't recommend running outside of the POH recommended RPM/MP settings, but within those POH recommended settings, fly like Lindbergh did!

    See post #27 in this message string. Flyingcheesehead has got it!

    -Skip
     
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  32. danhagan

    danhagan Pattern Altitude

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    My fuel burn at 2650 takeoff/climb RPM fuel burn versus 2300 in route doing 165 knots is more than .1-.2 gallons per hour, but I get to play with MP also and climb to altitudes the previous Tiger couldn't reach.
     
  33. Grum.Man

    Grum.Man En-Route

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    If you are running the same power setting just varying the rpm you shouldn't see much more than that. Takes pretty much the same amount of gas to make 65% power whether it's at 2700 or 2300. The slight decrease is from increased efficiency.
     
  34. Deano

    Deano Filing Flight Plan

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    Unless doing maneuvers, flying slow, or so low that the MP is out of the parameters on the POH for 75%, I am always full throttle and pulling back on the prop
    to obtain the desired HP. Naturally aspirated. I fly over or under squared if the engine manufacturer tells me I can.
     
  35. steingar

    steingar Taxi to Parking

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    I always run my prop at the lowest RPM I can for the percent power I want while following the tables supplied by the manufacturer. Less wear and tear on prop and engine, and it's a bit quieter.
     
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