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Thread: Surprised by prop efficiency analysis

  1. #11

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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.

  2. #12
    bwilson4web's Avatar
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    Thanks David,

    A forum can often be a good place to find credible references to more detailed resources. To help explain the constraints, N19WT has 150 hours and I am the third owner. So a few constraints fall out:

    • 52" propeller diameter - a function of landing gear length, the original propeller diameter, 52x40, wooden prop
    • 60 hp engine - the rated power of the original VW, HAPI engine, corresponds to similar Great Plains engine specifications
    • 140 mph top speed - from the original flight test results of N19WT
    • 80 mph best angle of climb - from the original flight testing
    • 60 mph lowest flight speed - from the original flight testing


    The prop efficiency numbers comes from "Advance Ratio," a non-dimensional number that is used to lookup propeller efficiency. But I agree, there are other aspects that need to be resolved to complete the design.

    Interesting, I too came up with a 60" diameter prop based upon equivalent mass-flow through the propeller arc. But adding 4", (60-52)/2=4, to the gear requires a non-trivial change and N19WT ground handling becomes even more challenging. For other reasons, I am looking at landing gear changes but this is not a commitment.

    Propeller selection is not trivial. Friday I had an excellent visit with the engine distributor, Matt at Recreational Power, Tifftin OH. So I have new data points to consider. In 3-4 hours, we covered a lot of territory and I have enough to proceed with detailed design.

    Bob Wilson
    Last edited by bwilson4web; 03-02-2013 at 12:12 PM.

  3. #13
    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

  4. #14
    David J. Gall's Avatar
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    Bob,

    I think you misunderstood my post.

    Good luck with your Dragonfly.

  5. #15

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    Here's a very informative & quite usefull in explaining/quantifying this lost art:
    http://www.propellersexplained.com/
    http://www.youtube.com/watch?v=ldBBEwi401o
    Some modern prop mfgrs use these methods already like WhirlWind aviation, but not sure they make something for your application.
    Hope this helps!
    Last edited by D.A.R.E.; 03-04-2013 at 12:47 PM. Reason: typo

  6. #16
    bwilson4web's Avatar
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    Hi David,

    There is an 10,000 character limit so I've had to trim your quotes to preserve my reply.
    Quote Originally Posted by David J. Gall View Post
    . . .
    First of all, you don't say how you arrived at your effficiency numbers. J = V / (n*d) is only one of the formulae needed for propeller calculations. . . .
    Two sources including a 1947 NACC report on light aircraft propellers. I've also ordered Weick's book, "Aircraft propeller design." I simply transcribed the curve and formula into a spreadsheet so I could quickly compare alternate configurations.

    But I could tell it was not providing guidance on how how much torque/hp was being absorbed by the prop. BTW, I found a copy of Goldstein's "Vortex Theory of Screw Propellers" which looks like an interesting read.

    . . .

    Quote Originally Posted by David J. Gall View Post
    . . . But why are we even looking at efficiency?
    The 30+ year history of the Dragonfly has seen a lot of re-engine efforts to replace the VW with more powerful engines. But without exception, these higher power, engines have reduced the range and often added significant weight. In contrast, if I can get more thrust per unit HP, I have the option of achieving flight performance similar to what the larger engines accomplished but without the weight and range penalty.

    I am looking at a possible landing gear modification so I may have the option to increase the prop clearance. What I'm finding in the absence of clear and unambiguous specifications:
    • 60" - equivalent mass-flow through the propeller disk area
    • 52" x 4 blade - PowerFin table projected from their small prop


    But other 'back of the envelope' data suggests more is going on. Worse, I'm not finding credible sources for the pitch vs power absorbed by a prop and this is what has me still looking.


    Quote Originally Posted by David J. Gall View Post
    . . . You've already established your constraints: You're using 60 hp, and you're limited to 52" diameter . . .
    Now you've touched on an unknown area, the degree that pitch can change to absorb the power. But obviously, there is a limit when the propeller becomes a 'club', in effect stalled out at maximum power.

    Quote Originally Posted by David J. Gall View Post
    . . . Power required to turn a propeller varies in proportion to the fifth power of the diameter. But you're restricted to not changing the diameter.

    Just for fun, let's see what change in diameter would be needed . . . a 52 x 1.19 = 61.9 inch diameter.
    Within the margin of error of my equivalent 'mass flow' calculation.

    Quote Originally Posted by David J. Gall View Post
    . . . Another good rule of thumb is that the rpm will increase by about 100 rpm if the blade width is decreased by 1/6. Turning that to our advantage, we'll increase the blade width to absorb more power. 3200 - 2400 = 800 rpm, so it looks like you might try increasing the total blade width by about 8/6 = 4/3 = 1.33. You could do that by increasing the width of each blade by 1/9 and then adding a third blade, or by decreasing the width of each blade by 1/6 and then doubling the blade count to 4 blades.
    This is good. In effect, blade area can be increased by the number. However, to do a credible analysis I need the blade area for:
    • existing prop - I still have it and can measure it. We are talking about the area normal to cord-line.
    • PowerFin - no dimensions listed on their web site
    • Warp - no dimensions listed on their web site
    • IVOprop - same problem but they show three styles of blades: ultra, medium, and magnum


    Quote Originally Posted by David J. Gall View Post
    . . . You are at a definite advantage at this stage since you already know the expected Vh of your plane with the 60hp VW, and you're (supposed to be) getting the same hp from the Hirth, so you can figure the prop pitch from the simple rule of thumb that pitch in feet = MPH x 100 / RPM. So using 140 mph as Vh, you prop pitch should be 140 x 100 / 2400 = 5.83 feet = 70 inches.
    Good, this is within the specifications of all prop vendors.

    Quote Originally Posted by David J. Gall View Post
    . . . Let's back up a moment and do that same calculation for your old engine/prop: 140 x 100 / 3200 = 4.375 feet = 52.5 inches. . . . a 52D x 52P.
    It is listed in the engine log as 52x40. Given the measured rate of climb, this appears to be a 'climb' prop.

    Quote Originally Posted by David J. Gall View Post
    . . . So, we've narrowed it down to a 52D x 70P three-blader with slightly wider blades, or a four-blader with slightly narrower blades (same pitch and diameter).
    No problem so far.

    Quote Originally Posted by David J. Gall View Post
    . . . What about that efficiency?

    Some people have said that the pitch/diameter ratio is the primary influence on efficiency, and that the ratio should be kept near 85%. . . .
    I have noticed more than a little 'buzz' about various props. So I do see merit in adding blade(s) but I'm resigned to having to do my own experimentation and soon.

    Quote Originally Posted by David J. Gall View Post
    . . . There is some consideration needed for the Reynolds number of your new prop. The lower rpm definitely means that you'll want to lean toward the wider blades of the three bladed prop, although small four bladed props have been successful on some VW powered planes (higher rpm).
    Interesting as N19WT came with a spare prop that obviously has a smaller, blade profile but I've yet to find papers with its specification. This second prop never flew (or it does not show up in the documentation.)

    Quote Originally Posted by David J. Gall View Post
    . . . Due to its construction, I question the ability of the IVOprop electric prop to achieve the high pitch you need, . . .

    Finally, the power curve of a two-sroke engine can be difficult to match to an airframe and propeller, especially a fixed-pitch propeller. . . .
    You've hit the nail on the head . . . why a pitch adjustable prop is so important for this two-cycle engine. I'm lucky because the Hirth 3502 has an exceptionally flat power curve after 5,000 rpm. But I am fully aware that two-cycles really work best in a narrow power band. A fixed pitch prop limits the ability to match the prop-to-engine-to-airspeed. I actually see the flight adjust prop as critical to making this geared, two-stroke work.

    Quote Originally Posted by David J. Gall View Post
    . . . Those are my thoughts on your prop design/selection problem. Use them at your own risk. . . .

    Then again, wouldn't it be easier to just get a smaller output pulley for the Hirth so that the prop rpm is 3200? No need for a new prop at all....
    I've gotten the smallest reduction ratio for this engine. There is another gear but it only mounts on their three-cylinder model. But I throughly agree, I would have preferred spinning at the VW speed and solve the problem. But propeller options are limited.

    You've given me good advice on the blade area. Sad to say, I was in a hurry in my last reply and didn't address your post in detail. I have the option of going both three-blade IVOprop or four-blade, fixed pitch PowerFin. But you've also given me an idea of how I might estimate based upon 'mass flow' the effective power handling of the IVOprop as a function of blades and diameter. As you've pointed out, what we are doing is 'experimental' and there may be another way.

    I am not discounting the hub area as much as sometimes we have to live within the constraints of the available products. I've only seen a few, more radical, highly pitched, inner diameter props and all appeared to be fixed pitch and often turning at high rpm.

    Thanks,
    Bob Wilson

  7. #17
    bwilson4web's Avatar
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    Hi David,

    I just stumbled across your 2010, EAA propeller workshop charts:
    http://propellerdesignworkshop.com/a.../2010-archive/

    There is a lot of meat to digest including the references. This will help.

    Thanks,
    Bob Wilson

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