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Thread: 3-Blade vs. 2-Blade Prop

  1. #1
    Senior Member jiott's Avatar
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    Default 3-Blade vs. 2-Blade Prop

    Every now and then I hear from people that all "bush pilots" use 2-blade props because they pull better for STOL and floatplane operations. I'm not sure I really believe this. How could a modern 3-blade prop, correctly sized to the engine, and pitched properly not "pull" as well as a 2-blade? I think it is agreed by most people that the newer 3-blade props are more efficient, quieter and have less vibration than 2-blades. I would like to hear some engineering facts on this subject, rather than hearsay.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

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    Senior Member jrevens's Avatar
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    Default Re: 3-Blade vs. 2-Blade Prop

    I think that it is pretty hard to make a blanket statement about that also, Jim. There are just so many possible variables in engine/prop/airframe combinations, making it hard to compare apples to apples. Just strictly from a number of blades standpoint, fewer blades equal greater efficiency because of disturbed air from preceding blades. The most efficient in that regard would be a single blade like was (is?) used on competition rubber band powered models - a single blade with a bob weight at 180 deg. for balance. Being able to utilize horsepower at an efficient engine speed and keep the blade tips from going supersonic sometimes requires more blades and a smaller diameter. I wonder why the 3-blade props are perceived as being quieter. Maybe because they can have a lesser pitch & smaller diameter to provide the same thrust as a longer 2-blade with greater pitch? I've got a 70" 3-blade Whirlwind for my Fox. I like the way they look and purportedly perform.
    Last edited by jrevens; 03-20-2016 at 08:36 PM.
    John Evens
    Arvada, CO
    Kitfox SS7 N27JE
    EAA Lifetime
    Chap. 43 honorary Lifetime

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    Default Re: 3-Blade vs. 2-Blade Prop

    The problem with 3 blade props is that no matter how you position it, it will always be in the way, when walking around the plane.

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    Senior Member PapuaPilot's Avatar
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    Default Re: 3-Blade vs. 2-Blade Prop

    Three blade props are typically quieter because the blades are shorter and the tip speed is lower. Most of us have heard a 2 bladed prop C-185 taking off, they are really loud at max RPM because the tip speed is at or above supersonic. Going to a 3 blade on this makes it a more neighbor friendly plane. I know some 185 pilots just bring the prop RPM back a little bit with the 2 blade prop to cut down on the noise with very little difference in takeoff performance.

    Sometimes three or four blade props are used to get the required or desired ground clearance. This can be a regulatory thing or because the operator is trying to reduce rock damage.

    More blades do make it challenging for positioning. A four blade prop can never be positioned out of the way. On the Quest Kodiak I always position a blade halfway between 12:00 and 3:00; if you run into it you will hit it at your waist. If you put two blades horizontal you are going to run into them with you head/face/eyes. Safety first!
    Phil Nelson
    A&P-IA, Maintenance Instructor
    KF 5 Outback, Cont. IO-240
    Flying since 2016

  5. #5
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    Default Re: 3-Blade vs. 2-Blade Prop

    If a two blade prop is in the vertical position, you would have to try pretty hard to bump into it.

  6. #6
    Senior Member av8rps's Avatar
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    Default Re: 3-Blade vs. 2-Blade Prop

    The best way I ever heard it explained is this:

    What is more efficient, a triplane, a biplane, or a monoplane? (Hint: there are no biplane fighters or airliners ) Fewer wings going through the air is less drag. The same is true with a prop.

    But that worsens even further once the prop speed gets close to or beyond supersic at the tip. Once a blade tip gets close to or breaks the sound barrier, much of the engine power is being applied to that, so engine and prop efficiency drops dramatically. 3 blades vs 2 beating the air into submission would be even worse, so prop makers typically will make the 3 or 4 blade shorter.

    Oh, and don't forget that 80% of the thrust is created at the last 20% of the blades tip.

    So with that pretty well known, a two blade generally does make more thrust as long as tip speeds are kept under supersonic numbers. So it works well on slow airplanes that want to get in and out of short places, and if you want to haul a lot of weight, or both. Shorter props however work much better on fast airplanes that need lots of runway. They can get away with a shorter prop running higher rpms because the lesser diameter keeps tip speeds down. Good case in point are the engines that need over 3k to make max power but do not use a prop speed reduction unit. Those engines with a short prop will work great on a low drag design like a Sonex. But they generally don't work as well on a less streamlined design like a Kitfox that needs a longer prop turning less rpm to get best overall performance.

    BUT, now consider our little geared engines running prop speeds in the low 2k range, which makes almost any prop more sfficient. And now throw in a ground adjustable prop design. While it certainly is more convenient, not to mention cost effective, even though blade design has come a long way, a ground adjustable prop is hardly the most efficient propellor. Fortunately, the geared engines we typically use on our Kitfoxes compensate for all that.

    The Highlander guys have been playing around with really long 2 blade props( 84 INCHES) for a while now, and have discovered a Zipper Kitted 110 hp 912 can make nearly as much thrust as a 150 hp lycoming can with a standard prop (for real STOL power the Lycoming guys run a really long 2 blade Borer prop). I would attribute getting that much thrust out of a 110 hp 912 mostly to the fact that the blade is turning so slow compared to a regular aircraft engine. If you follow AV8R3400's progress on his flight testing of his new/rebuilt Model 4 we will eventually learn what a long 2 blade Prince prop can do on a Kitfox. He also has the Zipper kit so I think it will be interesting.

    For me, I know the long 2 blades work better for STOL as well as for seaplane ops. But the shorter 3 blade still works really well overall on the Kitfox, providing exceptional performance compared to most similar powered aircraft. So for now I'm gonna stick with the more practical and smoother running ground adjustable 2 or 3 blade with a diameter of 72 inches or less. I enjoy the cruise speeds from the shorter props, and the Kitfox is still a way better than average performer in the boonies or on floats, even with the less efficient props. If I ever desire ultimate short field performance I'll bolt on something long with 2 blades.

  7. #7
    Rooster's Avatar
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    Default Re: 3-Blade vs. 2-Blade Prop

    Wow!
    Thanks so much!

    The teaching power of Team Kitfox!!!

    Reid

  8. #8
    Senior Member av8rps's Avatar
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    Default Re: 3-Blade vs. 2-Blade Prop

    You might also check out the latest Kitplanes magazine. Baranaby Wainfan has a really great article about props in there, covering in greater detail much of what we talked about.

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