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Thread: Homebrew Airscrew

  1. #1
    Forum Reader
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    Sep 2016
    NE Iowa

    Homebrew Airscrew

    Part I: Design & Parameters: Part 1
    This guide will be for engines with 50hp or less, as you will see later.

    One thing I learned (as did the Wright brothers), it is more efficient to turn a big prop slow than a small prop fast. There is a neat article in Flying magazine from Feb. 1941 that shows the relationship of diameter vs thrust & rotating speed vs thrust. Thrust increases to the 4th power of the diameter (two times the diameter gives 2x2x2x2 thrust). Likewise rotating speed is to the second power of thrust (four times the speed gives 4x4 thrust). Of course I am not a aerospace engineer so some of this might be wrong...

    My main resource for propeller parameters is a couple of nomographs that came out of a Soviet modeler magazine from the '70s. The fella who wrote this article was the chief designer of the Tupolev A-3 Aerosledge, so it is pretty reputable and has been fairly accurate from my experience.
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    The above graph shows the thrust to horsepower curves with various sized diameter props. The x-axis is horsepower and the y-axis is thrust in kilograms. The prop diameters are in meters. As the example shows, you find your horsepower, draw a vertical line up until it intercepts you propeller diameter curve and go horizontal to find your theoretical thrust. The author claims a 5-7% accuracy, I found it to be 10-13%, never the less it may be due to my workmanship.

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    The above graph will finally allow us to find pitch & blade width and speed to rotate the prop. You can easily go either way this graph (unknown prop dimensions vs unknown horsepower). To clear some things up the right x-axis is the horsepower, left y-axis is rpm. The example shows the difference of prop diameters per a given horsepower. The author chose 25 hp and drew a line to intercept with the 2 meter prop curve, then you draw a line leftward until it intercepts with a curve on the left graph either A.) the rpm you want to spin the prop at or B.) the pitch and width of prop you want to make. It's not definite but it allows you to have a little more freedom in choosing what YOU want for performance.
    So now you found your intercept on one of those left curves, what are those values? Those pair of numbers define your pitch and width of the prop. Sigma (the one in percentage), is the width of the prop in terms of the radius of the prop. The h is the pitch of the prop in terms of the diameter. For example, for a 2 meter prop powered by a 25hp motor swinging at 1500rpm you would find sigma=16.5% and h=.52. This means our blade width equals .165*1meter and our pitch equals 2 meters*.52.

    Now the pitch and width is found, we can start the details (called a planform I believe?).
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    The above image is what I love about this guys method. The planform has five stations that are located by perentage of radius again (areas which have airfoil dimensions, everything else between is just blended). All the dimensions are spelled out for you! Basically all the numbers you see are percentages of radius. So like we found before our max blade width was .165*1 meter so we can leave the numbers in the above image unmodified. However if you have a big paddle prop width with like a 20% radius, then what you have to do is find the ratio between the above "master" plan and your number (station locations remain the same). If you had 20% radius you divide it by 16.5%, which equals 1.21. Thus we would multiply all those airfoil dimensions and blade widths by 1.21 to give you your correct percentage ie, 13.5% becomes 16.33%. I usually don't have the taper at the end of the prop for manufacturing reasons and to me the efficient gains aren't worth it. It's a lot of info to take in, but it definateley beats guessing! Alright thats it for now, next week I can show how to construct a planform (a prop 'blueprint'). I just have to find my old one I made a year ago lol

  2. #2

    Re: Homebrew Airscrew

    That's way over my head.
    But thanks.

  3. #3
    Super Moderator Corky's Avatar
    Join Date
    Jul 2014
    Susquehanna river, Dauphin, Pa

    Re: Homebrew Airscrew

    Thanks for sharing the graphs that gets someone in the ballpark for sizing ,pitch and the blade width...

    Aluminum .120 14' X 76" hull
    Teledyne 4a084-4 engine
    Circle S 1.69:1 reduction
    67" Whirlwind "Razor X" prop


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