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  1. #1
    bwilson4web's Avatar
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    Surprised by prop efficiency analysis

    Hi,

    Re-engine of N19WT replaces a 60 hp VW, 3,200 rpm, with a 60 hp, Hirth 3502, geared to 2,400 rpm. I've found a flight controllable IVOprop whose moment of inertia fits within the Hirth 3502 specs. But I wondered, 'What about prop diameter?'

    My first though was to keep the same pressure differential through the prop disk. The slower turning prop needs a larger diameter to move the same mass at the same pressure differential. It turns out this is the square-root of the ratio of the two rpm and moves the propeller diameter from 52" to 60". The landing gear would have to be 8"/2 or 4" taller. But is that enough?

    So I looked at propeller efficiency and this led to "advance ratio." There are multiple Google references but it all comes down to a non-dimensional formula:

    J = V / (n*d)

    J - advance ratio
    V - velocity feet/sec
    n - revs/sec
    d - propeller diameter in feet

    So I put together a quick spreadsheet and calculated propeller efficiency at three speeds and three diameters:

    • 60 mph - minimum flight speed
    • 80 mph - maximum rate of climb
    • 140 mph - maximum level speed
    • 48" - smallest diameter available
    • 52" - current prop diameter
    • 60" - calculated, constant pressure differential


    I also calculated the prop tip speeds to avoid high mach numbers.

    The prop efficiency surprised me at 60, 80, 140 mph:
    • Original 52" prop @3200 rpm, efficiency range: 27%, 38%, 68%
    • Alt 48" prop @2400 rpm, efficiency range: 46%, 55%, 79%
    • Alt 52" prop @2400 rpm, efficiency range: 38%, 55%, 79%
    • Alt 60" prop @2400 rpm, efficiency range: 38%, 46%, 72%


    I had expected the slower turning, 52" prop to be less efficient because of the higher pressure differential. Instead, it turned out to be more efficient across all speed ranges compared to the original 52" prop turning much faster. But even more surprising, the longer 60" prop was less efficient than the 52" and 48" props.

    The original, fixed pitch, wooden prop would limit the VW power at low speeds and let it generate more power at higher speeds. Due to the characteristics of the Hirth 3502, the replacement prop has to be flight adjustable to stay within a narrow rpm range. But finding that the prop efficiency decreased with larger diameter has me scratching my head. I would have thought that increasing the pressure differential across the smaller propeller disk would have reduced efficiency.

    The only thing that might make sense is the increased tip speed of the larger diameter prop may have a greater impact on efficiency than I originally thought:
    • 48" @2400: 504, 506, 512 ft/s tip speeds at 60, 80, 140 mph
    • 52" @2400: 546, 547, 553
    • 60" @2400: 630, 631, 636


    But these tip velocities are relatively narrow and I don't see enough, even raised to a power, to explain the lost efficiency of the larger diameter prop.

    Have I missed something?

    Bob Wilson
    Last edited by bwilson4web; 02-24-2013 at 12:10 PM. Reason: formatting

  2. #2

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    I'm not an engineer, but I have a few guesses!

    First, the props aren't pitched the same. You may be comparing apples to oranges (speed vs. climb props), as the variable pitch prop changes the dynamic across RPM. I wonder if the fixed prop were pitched to the same as the variable at 2400 RPM that they wouldn't match up more closely. Of course one would have to have three fixed props to match the variable one for more rigorous testing.

    Second, we're missing the thrust. Effectiveness beats efficiency! While the efficiency of the larger prop might be seven percent less than the smaller one, it may have ten or twelve percent more thrust....and that's all that really matters.

    Third, I'm betting that the long prop is vibrating more than the shorter one, which would make it less efficient...or not.

    I'm betting the change in pitch due to RPMs is the efficiency solution.
    The opinions and statements of this poster are largely based on facts and portray a possible version of the actual events.

  3. #3
    bwilson4web's Avatar
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    Thanks Frank,

    I'm still researching propeller sizing and dynamics. The pitch changes are absolutely necessary, especially at these lower rpms. Also, no problem finding efficiency and other analysis. But I'm running short of practical design rules on the size and number of blades as a function of HP.

    Bob Wilson

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    I'm guessing it's all about torque with more blades.

    Let's say we have high RPM's and plenty of torque. The length of the blade is limited by the RPM's (darned mach numbers!), but we have loads of power. Solution - add another blade! Three blades won't interfere with each other aerodynamically, and I get a third more thrust in the same disk.

    With lower torque it doesn't matter what the RPM potentials are; we just won't have the power to drive it through the air to keep the RPMs up. That's the "unloading" of the prop and rise in RPM's a lot of planes have after they leave ground effect.

    Looking back at WWI Dh4's and SE5a's we start seeing four blades. RPM's are lower, but they had monstrous torque available to them to overcome the resistance of the blades through the air.

    Working backwards, we see four bladed props on wind-driven generators for the same reason, but now resistance is what we want to drive the shaft as quickly as we can.

    The Kansas City Dawn Patrol guys did some back of the envelope stuff with props and then put pull testers behind their planes to validate differences. They were more interested in finding out minimum thrust numbers for safe flight than the science behind it, but they worked out min/max for prop pitch and length for their Nieuport 11's. Of course anything but a wooden two blade prop on a WWI replica would be heresy, so they had that constraint to work with.

    Again, I'm woefully ignorant on the science of this other than the basics - I'm hoping to spark an idea to help you out.
    Last edited by Frank Giger; 02-26-2013 at 07:31 AM. Reason: formatting
    The opinions and statements of this poster are largely based on facts and portray a possible version of the actual events.

  5. #5

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    Dang ol' double post!
    Last edited by Frank Giger; 02-26-2013 at 07:29 AM.
    The opinions and statements of this poster are largely based on facts and portray a possible version of the actual events.

  6. #6
    bwilson4web's Avatar
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    Thanks Frank,

    Somehow sleeping on the problem made things a lot clearer:

    1) blade area and profile defines the lift/drag per blade - this is the fundamental force the engine has to overcome.
    2) integrating the drag per slide of the prop times the radius - this gives the 'anti-torque' or the moment the engine torque has to overcome
    3) maximum static pitch is the angle that leads to blade stall - in effect the boundary between a prop and a club

    Now I was concerned that the 52", two-bladed prop might not have enough blade area to avoid prop-stall at 60 hp, take-off power, at 2,300 rpm. But with a better understanding of the physics, I should be able to work up a credible model.

    BTW, I did speak with Ivoprop yesterday and they think this is going to work based upon another customer's experience. But I also have the option of adding another blade later if testing reveals a problem.

    I'm fairly confident this this going to work but I will be thinking about how to test for torsional vibration . . . just to make sure.

    Bob Wilson

  7. #7

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    I don't know much about prop formulas, but I have read that the Wright Bros prop with no help from computer design was about 70 percent efficient and the best our most modern props can do is about 90% so not that much above them.

    Howard Pardue raced his Fury at Reno many years, top speed around 420 or so. One year some computer wizard boffin came up with a fabulous new prop design, and convinced Howard to buy it .
    He then went about 15 mph Slower with the new prop!

  8. #8

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    May I suggest a couple of resources. I used an Excel spreadsheet that I downloaded from the EAA web site. At the time, I was looking at props for a Jab. Neal Willford wrote it for S.A. in 2004. Search for "propeller spreadsheet" on the home page.

    Ed Sterba makes props down in FL. and his web page has some good info and rules of thumb. He has a link off the Great plains web site.

  9. #9
    bwilson4web's Avatar
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    Thank you Bob Dingley,

    The spreadsheet has helped. I am a great believer in research, study, and test.

    Bob Wilson

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