Matty's Marvelous Modular Monstrosity

Discussion in 'Composites' started by Foundationer, Jun 28, 2019.

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  1. Jun 28, 2019 #1

    Foundationer

    Foundationer

    Foundationer

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    I've not thought of a name for this thing yet so it's either MMMM or Iteration 9. I'm starting a sort of build log here because I've actually started building the thing and the bit I built first has just passed its first test so Iive run out of excuses for asking dumb questions and not showing any progress.

    300kg
    +6-3g
    18knot stall
    EDIT: 18m/s or 35 knot Stall - thanks to BoKu and Pictsidhe for pointing that one out...
    80knot cruise
    Electric(!)
    100kg batteries
    8.2m^2 wing area
    230km range (no reserves or anything like that!)

    This is an old render, gives you a rough idea.
    NEW FUS iteration 7.4 render-17.JPG

    So here's the horizontal tail. Load test to limit with no cracks or creaks or delams, my very inaccurate deflection measurements say 21mm versus 18mm (or under) expected. Each brick weighs on average 2.4Kg.

    HT construction is 600mm wide blocks of foam CNC hotwired, butt joined with epoxy/micro, carbon shear web running chordwise between the blocks at the centre to transfer loads to mounting points fore and aft then the whole thing wrapped in 265gsm carbon.

    Bit of background to the Modular part: All major assemblies need to be built mostly within an 8x4 footprint. I built an extension to the shed but it's not very large!

    IMG_20190628_160225.jpg
    IMG_20190628_160239.jpg
     
    Last edited: Jun 28, 2019
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  2. Jun 28, 2019 #2

    BoKu

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    When I run the pV^2 on that, It looks like you need a Cl max of 7 to reconcile your mass and wing area with your stall speed. My math is likely suspect, but unless it's very far off you'll need more Cl than you'll get from any known profile. Either that or a lot more wing area. Or accept a higher stall speed.

    For reference, I'm using:

    L = 1/2*p*v^2*A*Cl
    Where:
    L is lift, in this case 660 lbs
    p is the air density in slugs (0.0023 at sea level)
    v is velocity in ft/sec
    A is the wing area in ft^2
    Cl is the maximum coefficient of lift

    When I put in a reasonable Cl max of 1.6 with your wing area and mass, I get a stall speed of about 38 knots.

    When I put in a stall speed of 18 knots and a Cl max of 1.6, I get a required wing area of 388 ft^2 or about 36m^2.

    --Bob K.
     
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  3. Jun 28, 2019 #3

    pictsidhe

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    My project numbers are 11.4m2, 210kg, 24kts. With flapped CL a bit over 2. 18kts isn't happening for you with less wing and higher weight.
     
  4. Jun 28, 2019 #4

    Foundationer

    Foundationer

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    Hahaha! You're both right! That should have been 18m/s stall or 35 knots. As usual I'm 'effortlessly' switching between units.
     
  5. Jun 28, 2019 #5

    BoKu

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    Oh, man, I have similar mistakes so many times. My usual thing is to do the rho-vee-squared in imperial units, then fail to convert velocity from ft/sec into knots or mph.
     
  6. Jun 29, 2019 #6

    Hephaestus

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    Are you stealing my 4am math that makes the impossible possible? ;)
     
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  7. Jun 29, 2019 #7

    Victor Bravo

    Victor Bravo

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    Don't feel bad, that's the same math that some of the candidates used last night on TV to explain how everyone can get everything for free and nobody has to pay for it.

    Uh Oh... what's that red dot doing on my chest?
     
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  8. Jun 29, 2019 #8

    Foundationer

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    You're talking to a guy that lives in a democracy where our right is left of your left and I get medical treatment that's free at the point of use. It's best we don't discuss politics!
     
  9. Jun 29, 2019 #9

    mcrae0104

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  10. Jun 29, 2019 #10

    Foundationer

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    It's time to make some moulds! Here's the upper wing skin mould parts ready to go. Process is:

    Cut the foam, glue the chunks to the 12mm mdf (vacuum the whole sheet & clamp it to the table to keep it flat) then fill gaps, lightly sand and face with 1mm polypropylene. The cutter kerf is a very reliable 2mm so it's a perfectly shaped mould.

    Once that's done I lay up into it, bag the whole thing and clamp it back to the table. Parts come out SHINY with no release agent required!

    I modded the design to get rid of the tapered tips so I can use two moulds for all wing skins. As it's a four piece wing (! - build space limited) I can change to nicely tapered tips down the line.
     

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    Last edited: Jun 29, 2019
  11. Jun 29, 2019 #11

    Foundationer

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    My inspirational poster fell off the wall and the ceiling is radiating heat (unusual here in england to get above 90f out) so I'm giving up for the day.
    IMG_20190629_131130.jpg
     
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  12. Jun 29, 2019 #12

    tspear

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    Good luck. I like where you are headed with a reduction in molds.
    My favorite piston airplane is still the Aerostar by Ted Smith. It is very fast (still holds the record as fastest twin), and has a much lower parts count compared to other planes of its generation.
    For example, the tail feathers are symmetrical airfoils; and use the same parts for all the non-moving sections of the elevators and rudder. This drastically lowers build logistics complexity. There are other examples in the fuselage, the wings....

    Tim
     
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  13. Jun 29, 2019 #13

    pictsidhe

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    Being a British person living in the USA, I absolutely, errr, must not comment!
     
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  14. Jun 30, 2019 #14

    Jay Kempf

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    Really nice stuff.
    What adhesive and method did you use to bond the polypropylene into the foam cavity. Contact cement and a vacuum bag?
     
  15. Jul 1, 2019 #15

    Foundationer

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    Polypropylene to polystyrene is tricky - Epoxy doesn't stick it however much you rough up the PP so... Masking tape round the edges and a few strips elsewhere (not overlapping), paint with epoxy, place carefully into mould, vac down. It only has to stay put for four releases. My first go at it was to rough it up and use micro slurry but that stops the mould surface being perfectly flat (obviously, duh). My first first go was to lay up on flat PP then put the whole lot in the mould and vac down. Which sort of worked... Those bits are getting some aggressive trimming and becoming tips.
     
  16. Jul 1, 2019 #16

    Foundationer

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    This morning was spent cutting out carbon and foam for the four upper wing parts. 8 rectangles of bi-ax, 8 rectangles of foam. I think this device is the closest I've come to an innovation. Perfect foam edges in one go!

    The tips are very small, aileron authority should be ok but it's not going to have the fastest roll rate!
     

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  17. Jul 1, 2019 #17

    Jay Kempf

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    Why did you go with polypro? I have had the same experience with adhesive problems and arrived at thick mylar as a solution. There are versions mirror finish one side and matte finish on the other. Old school drafting stock. Those adhere well to normal foams. You need to get it in about ten mil thick to make a good mold. I see you already figured out laying up on the plastic directly. That I have done a lot. Harder to control the accuracy but makes a nice finish.
     
  18. Jul 1, 2019 #18

    Foundationer

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    PP was available and cheap! And I didn't think of Mylar!
     
  19. Jul 2, 2019 #19

    pictsidhe

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    Try a chlorinated polyolefin adhesion promoter. It comes in expensive rattle cans as 'plastic primer' for paint but you don't need much. I have used it with foaming gorilla glue on XLPE and it works well there. Not tried epoxy yet. Can also be had in small bottles with a brush for cyanoacrylate use. Using it as a loose stiff film would make demoulding a breeze. There are special polyolefin glues, but they are horribly expensive and total overkill for holding a mould skin on.
     
  20. Jul 2, 2019 #20

    Victor Bravo

    Victor Bravo

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    Take epoxy and microballoons and squeegee it into the surface of the foam core "bed", like they used to build Long-Eze's.

    Lay the plastic film into the wet micro, and stack up the bed, core, etc. and weight it down. When the epoxy cures, disassemble the stack. Carefully peel off the plastic film,leaving a very smooth surface of epoxy as a permanent surface on the foam.

    Now stick a new (un-crinkled) layer of plastic film down to the epoxy using transfer adhesive (aka super thin double stick tape with pressure sensitive adhesive). I use "Tesafilm" but I'm sure there are plenty of others.

    The double stick tape will hold the plastic release film in place better than contact cement or any glue. The epoxy/micro will get into the foam grain and become a good anchor.

    Actually, if you use .016" aluminum instead of plastic, it will be stronger, hold the airfoil better, and probably cheaper. Wax or spray the aluminum, but normal laminating epoxy doesn't like to stick to it at all. My guess is that the aluminum will be stiffer and give you a better airfoil than plastic.

    You should be able to pull several molded parts out of this without too much difficulty, especially with some sort of mold release spray or wax.
     

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