4500rpm propeller to negate the PSRU

Discussion in 'General Auto Conversion Discussion' started by pfarber, Jan 24, 2020.

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  1. Jan 24, 2020 #1

    pfarber

    pfarber

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    I've been reading that quite a few airplanes are swinging much smaller props much faster... 4000rpm and above and still seeing good cruise speeds.

    Sensetech and Warp drive make 4000rpm props for the Rotax and the performance charts look decent:

    https://www.propellor.com/Content/I...cles/CFC/Airmaster AP332 WD to WW upgrade.pdf

    Air racers also seem to live quite well with high rpm/small diameter props.

    Rough numbers would be a 45-50 inch prop (I know, crazy tiny when compared to engines turning half the RPM)

    Does anyone have any info to point me to regarding successes and failures along this line? A PSRU is all of 30lbs so it not really sure the effort is worth then weight savings or trouble to find such a unique propeller.
     
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  2. Jan 24, 2020 #2

    Dana

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    It depends on the aircraft. If it's a tiny slippery fast ship you can get away with spinning a toothpick of a prop real fast. For a slow ultralight, it doesn't work nearly as well.
     
  3. Jan 24, 2020 #3

    bmcj

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    Static thrust may be low, so takeoff acceleration might suffer, especially if the plane is heavy. It may also be noisier than a standard slow turning prop, endearing you to the neighbors.
     
  4. Jan 24, 2020 #4

    Doggzilla

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    Ive always wondered about this as well. Hummel aircraft reach ridiculous speeds with these little tiny props and VW engines.

    https://en.wikipedia.org/wiki/Hummel_Bird

    HenryK always posts these little counter rotating props that perform very well. He posted a video of a smoke test and the stream was remarkably tight. Much closer to a shock diamond than the wide cone of thrust for many props.

    Im curious what a hummel would be capable of with one of those props, since they already use little tiny things.
     
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  5. Jan 25, 2020 #5

    TFF

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    The Rotax are spinning thin light composite blades at less pitch than a conventional airplane engine. Not a lot of mass to swing at regular diameters.
    Air racing is a whole different animal. The first thing of note is the engines only have to last the race weekend. Most trailer the planes today. Of course they want them to last longer, but racing 50 hours is probably five years of racing. The ones who do fly to races put on regular props run at regular RPMs. Anybody competitive takes it apart at least once a year.
    Most Rotax are pitched for normal flying. Race props are not climb friendly. Watching the racers takeoff is a little scary on how much they wallow around trying to get into the air.
     
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  6. Jan 25, 2020 #6

    lr27

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    I followed the link, but they're talking about the Rotax 912, which has a PSRU, so the prop rpm's would be in tbe 2000's. If you run a 50 inch prop at 4500 rpm, the tips will be at something like Mach 0.87. That's going to be very loud, and possibly inefficient. Maybe if you swept the prop tips or something.
     
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  7. Jan 25, 2020 #7

    cluttonfred

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    Using this calculator suggests that you’d have to get down to 40” diameter to keep the tip speed efficient at 4,000 rpm direct drive, 36” at 4,500 rpm. It might be worth the reduction in total thrust for something like a pseudo-jet.

    https://warpdriveprops.com/propspd2.html
     
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  8. Jan 25, 2020 #8

    BJC

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  9. Jan 25, 2020 #9

    Riggerrob

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    You might need 5 or 6 propeller blades to gain the same disc-loading and blade area of a larger-diameter, slower-turning propeller.
     
  10. Jan 25, 2020 #10

    Dan Thomas

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    It was learned long ago that accelerating a large volume of air to a slower speed was more efficient than accelerating a small volume of air to higher speeds. It's the reason why STOL airplanes tend to have larger, slower-turning props. It's also the reason why a helicopter has a large, slow-turning rotor instead of a fast, small-diameter rotor. It's the reason why Lycomings and Continentals and Franklins and most radials are designed and propped the way they are.

    Small props are fine for fast cruise, but for takeoff they're lousy. I've seen VW-powered airplanes take a lot of runway, which tends to limit the places they can go. Or, at least, where they can go and still come back:)
     
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  11. Jan 25, 2020 #11

    pfarber

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    But most GA aircraft are not as small as E/ABs. My aircraft is a Bede BD-4B so its pretty large, but fairly clean and fast. I think a small high rpm prop would be the best option for the very small frontal area planes... like a Long EZ/Sonari/RV-8 but thinking and looking at my BD the propeller would be smaller than the diameter of the fuselage of 42 inches.

    I understand the tip speed issue, and the reasons for it. What I can't find (at a level I understand) is how the propeller would work when its barely larger than the body behind it.

    Also PSRUs have the useful side effect of raising the propeller thrust line. If you go direct drive, and get rid of the PSRU, you now need a dry sump oil system when you invert the engine and a way to ensure there is no oil in the cylinders prior to start. A dry sump could add back most of the weight you save by dropping the PSRU.
     
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  12. Jan 26, 2020 #12

    TFF

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    It doesn’t work well overall. Whatever rpm you decide on does not matter. The engine loaded to rpm knows nothing about what it’s driving. A small diameter prop making the same load as it’s normal size cousin, will have tons of pitch. Any reasonable slow speed it’s cavitating because it has not moved into a range it can bite. You are trying to accelerate air faster to go faster but you have less area that is pushing. With very little sticking out , the thrust column is always fighting skin and interference drag components and never just pushing free air. Diminishing returns. At some point it’s just a spinning stick.
     
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  13. Jan 26, 2020 #13

    BBerson

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    Figuring out the best prop diameter and rpm is what real airplane designers do.
     
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  14. Jan 26, 2020 #14

    Marc Zeitlin

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    You mean to say that the prop blades are stalled because they're at a very high AOA. Cavitation can only occur in liquids, not in gas, since the definition of cavitation is the formation of gas bubbles in liquid due to large drops in pressure. Water propellers most definitely can have cavitation issues - air propellers just stall.
     
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  15. Jan 26, 2020 #15

    lr27

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    Oh geez. Now you've opened a can of worms. Next you'll be telling us to think of centripal acceleration, rather than centrifugal force, and after that you'll mention a certain aeronautical maneuver that must never be named. ;-)
     
  16. Jan 26, 2020 #16

    TiPi

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    look at a test club, they are the extreme of an inefficident prop as they are designed to absorb the engine power without making excessive thrust (or none at all for the square wood bar dyno).
     
  17. Jan 26, 2020 #17

    pfarber

    pfarber

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    Good thing I have a pretend airplane otherwise things could get wierd.

    Small diameter props are a thing. Maybe not common, but they are out there.

    If they are a good fit, why not try them. But it seems that the required changes (mainly inverting the engine) means that the gains would have to be significant enough to justify the extra work. Right now I think it would be, at best, an equal exchange, but adding in the engine inversion makes it a no-go. The thrust line would be to low. I guess the next question is, given that engines have been mounted on pylons several feet above the longitudunal axis, would a prop thrust line 12-18 inches 'low' be a huge issue?

    I think people would not accept this simply becuase it's 'different'. Could be 100% safe, just different and that would be enough to sink even a properly designed package.
     
  18. Jan 26, 2020 #18

    BBerson

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    Yeah, if you mess with weird props on someone else's design you are an experimenter. The results are likely less optimal than the designers original. If you want more optimal results with these weird engine/props you will need to match the whole design. So you need to become an airplane designer (like Wittman or Beachner).
     
  19. Jan 26, 2020 #19

    Dan Thomas

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    And that gets way beyond most of us. Way back when I joined EAA in 1972, they sent out a lot of interesting advice, and one of those bits of wisdom was that one shouldn't build an unknown design and put an unknown engine conversion in it. Even the major manufacturers don't do that. Besides adding together so many possibilities for difficulties in flight, structure and reliability, one has no scale to measure the effectiveness of the conversion. Beachner got away with it (for a while anyway), but so many such projects end up with massive cost overruns and reversion to more typical powerplants, or they get scrapped in frustration.

    If I was to convert an engine, I'd put it in a known design so I could compare performance and handling. If I wanted to design an airplane, I'd start with a Lyc or Continental to eliminate a whole bunch of hassle. Then once I had the airplane flying and its bugs worked out, I could do my conversion and fool with that.
     
  20. Jan 26, 2020 #20

    Dan Thomas

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    Low thrust lines were more common 80 years ago, before inverted engines became available.

    [​IMG]


    [​IMG]
     

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    Last edited: Jan 26, 2020

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