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Thread: Airfoils, bulbs, graphs and a very long pre-amble - help!

  1. #1
    Join Date
    Nov 2010

    Default Airfoils, bulbs, graphs and a very long pre-amble - help!


    I have been building Claudio's Pico on my 3d printer and things are going very well in terms of weight.
    However, I have had some issues with foil shapes. Please bare with the long post.

    My first problem came with the fin. At this scale, It's incredibly difficult to get a carbon rod of any decent strength to fit the commonly quoted thin foil sections to stiffen the fin.
    For example, The PICO fin chord is 38mm according to the plan. That would mean a max fin thickness of 2.28mm at it's maximum if using a 6%.
    Via my very bad and un-scientific method...... anything under a 2mm carbon rod is extremely bendy - exponentially so.
    I am aware that once the rod gets the skins on it will have far less deflection, but for this exercise I'm trying to 3d print everything so I have some very real-world issues.
    My nozzle is 0.4mm so by the time I print the foil shape and give the rods their guidance holes, I will be way thicker than I should be for the 6% foil shape.
    So I hopped onto airfoiltools.com and looked for much thicker foils that would allow me a decent skin, with supports for the 2mm rods and the rods themselves.
    My thinking being that I would rather have a foil that is as designed rather than a weird, plasticky squished thing.
    This time I did however do things in a "slightly" more scientific method. I searched for Low Reynolds number foils and picked one.......It's a 12% foil with a very gentle front and tailing slope.

    It has been printed and I have a decent, thick enough fin that I'm happy will not snap - win for me at this point as this project is for me to learn on, not really race - with the bonus that I can now print new fins as needed as I made the keel trunk square so playing with different foils will be easy

    So I now moved onto the bulb.
    I have previously printed a bulb as a plug, made a mold with Plaster and poured a bulb that came out 4g off the designed weight, so I can do it that way but again, I'm trying to print as much as possible.
    So the plan is to print an empty bulb shell that I can fill with 2mm leadshot.... in a slightly more scientific method.
    Bulb weight is 380g so I measured that weight out in a cylinder using the leadshot I will be using.
    The radius Of the cylinder was 33mm and I filled it up by 19mm to get the weight.
    Yay for the internet and the online volume calculaters......... The volume used for 380g was 65cm/3
    So I divided 380 by 65 to get the density of 1cm/3 of my leadshot which is 5.84g.
    Put that value into CAD and start to get a bulb shape that will work.........

    Right, here we go with the actual questions......

    1. Was my maths correct for working out the density for g/cm3 of the leadshot?
    3. Do I have to use actual Reynolds numbers or can I use a rough guess?
    4. If I can use rough values..... what the heck are they for RG65, IOM, and Marblehead?
    2. If you need to get the actual number, do you work it out for the fin, rudder and bulb seperately or do you use a common number for all of them?
    5. This one is pretty embarrasing...... on the airfoil tools graphs, you get the CL vs CD, Cl vs Alpha etc. There are no labels so I have no clue which value is which. Is there a naming convention for graphs? is it taken that the first value is X and the second is Y or vice versa? I'm not supposing that I can interpret them correctly at this point , but that snippet of info will certainly help me on my way lol.

    Here's a pic of a random foil's graph

    Click image for larger version

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    To me the higher reynolds plot shows a much more predictable response to make a choice on vs. the lower number one. So it's obviosly important to know what one to be using
    Do I need to worry about the ncrit/mach numbers?

    Thanks, I've been reading up on this a lot for quite a while but just need help trying to get it to all click together.....hopefully.
    Attached Thumbnails Attached Thumbnails Click image for larger version

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  2. #2
    Join Date
    Apr 2005
    Toulouse (Fra)

    Default Re: Airfoils, bulbs, graphs and a very long pre-amble - help!

    don't worry about Reynolds at that size level.
    Chord percentage can be higher than 6% as used in competition boats, was told to me that some IOM went down to 4% and I wonder about Stall effects when turning has mentioned by Earl on my tread. See : 8% for the Fin and 12% for the Rudder can be acceptable.
    No need to have a removable Fin as well the Rudder.
    After all, PICO is not supposed to be the super Race Winner of the Year.

    Lead shot are an alloy mix of various elements and much lighter the real lead (~50%)
    Shots being almost spherical, there is a lot of air in between when put together (~30%)
    Probably a wooded Fin and Rudder covered with glass epoxy for protection and strength could be used.
    Aluminum sheet will be too heavy Fin.

    Some readings : http://www.onemetre.net/Design/FinForce/FinForce.htm

    NACA 0009 can go up to 8/9 before entering into Stall and NACA 0012 can go up to 12/14 before entering into Stall.

    Now it is very easy to swing the rudder more than 12 during sailing at a pond, (see image) while the Fin, being fixed to the Hull, is navigating at about 4 shift.

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  3. #3
    Join Date
    Nov 2010

    Default Re: Airfoils, bulbs, graphs and a very long pre-amble - help!

    Thanks Claudio,

    So my maths makes sense then about the leadshot being about 50% of the lead weight (my teachers would be so proud that I have finally mastered elementary math)
    Any idea on which axis those values are on?

    As for the PICO, I've started a thread and I believe that the very intention that the class was designed for is happening with me. I never would have attempted to understand these graphs If I wasn't building her.

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