Rotating prop disc tutorial

Discussion in 'Building Questions, Tutorials and Guidebooks' started by Hamiltonian, Mar 23, 2014.

  1. Hamiltonian

    Hamiltonian Member

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    #1 Hamiltonian, Mar 23, 2014
    Last edited: Mar 24, 2014
    I'm not sure if anyone will be interested in this, but I thought it would do no harm to post it.

    I've been building some kits as in-flight aircraft, and I hate to see static propellers on flying aircraft. I'm also not very keen on "prop-blur" discs, which aim to produce a blurred sector for each prop blade, reproducing what we see in photos and movies, but not what I see with the naked eye.
    So I set about trying to produce discs with uniform rings of colour, the density at each radius matching the relative amount of prop blade and empty space at that radius.

    Here's how I've been doing it:


    1) Mark up and measure the kit propeller at regular intervals starting from the centre of the prop boss.
    Prop 1.jpg


    2) Calculate what proportion of the prop disc is composed of prop blade at each measured radius. 2 pi times the radius gives you the circumference at that radius, the measured blade width times the number of blades gives you the total amount of prop blade at that radius. Divide the latter by the former, and you have the proportion of the circumference at that radius which is occupied by prop blade. I also calculate a relative density - whichever radius has the maximum proportion, I set that proportion equal to one, and work out the value for all the other radii as a proportion of that. Here's my little spreadsheet, filled out with data.
    Prop 2.jpg


    3) I open GIMP, and create a colour gradient matching the radial densities I calculated above.
    GIMP is an open source image manipulation program, available here: GIMP - Downloads
    There's a tutorial on building GIMP gradients here: How to Make a Custom Gradient in GIMP - Using the Gradient Editor in GIMP
    In this case for a Luftwaffe prop my base colour is RLM71, and I don't need to worry about adding tip colours. I can find the RGB values for RLM71 here: Simmers Paint Shop - Luftwaffe RLM - RAL/FS 595b colorlist / aircraft colors (RGB values). The alpha channel ("A" in the GIMP tool) carries the densities I calculated above. In this case my prop disc is becoming steadily more transparent towards the rim. I add a small dense black region at the extreme left end, which will mark the centre of the prop disc - that'll make it easy to cut out with a scribing tool, and it will be obscured by the spinner in the final assembly.
    Prop 3.jpg


    4) Having built the gradient, I open a new document in GIMP, making sure to set the resolution in pixels per inch to match my printer. Using my newly created gradient, and the "radial" setting, I draw a circular gradient of the correct radius for my propeller - in this case 24mm.
    prop 4.jpg


    5) I duplicate this disc a few times, and print out. On this occasion, I've used overhead-transparency film - it's a little thin, but makes the job quicker and easier. I've sealed the printed side by airbrushing gloss varnish.
    I've also had success using printable decal film which I then transferred to thicker transparent plastic sheet. Another option (which I'll experiment with in future) would be to print to self-adhesive transparent labels, and then stick them to plastic sheet. In all cases, use a cutting compass tool set to the prop radius to cut out the disc (this is where that black centre mark comes in useful).
    prop 5.jpg


    6) Now remove the blades from the kit propeller, sand the spinner smooth, and divide with a razor saw. Some spinners with rear cut-outs will need filled before sanding. Sometimes it's easier to just remove the rear of the spinner and replace with a new part fashioned from styrene rod or tube of an appropriate diameter. Glue the rear and front parts of the spinner to the centre of the prop disc.
    prop 6.jpg


    7) And complete.
    bv p170 4.jpg
    bv p170 3.jpg


    Hope that's of interest or use to someone ...
     
  2. fubar57

    fubar57 Well-Known Member

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    Very interesting, thank you.

    Geo
     
  3. Crimea_River

    Crimea_River Well-Known Member

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    Thanks for taking the time to post that. They look pretty good.
     
  4. stona

    stona Well-Known Member

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    I'm a 'leave the blades/disc off altogether and fill the spinner' kind of a guy, but I reckon your discs are about the most convincing I've seen. Thanks for sharing.
    Cheers
    Steve
     
  5. Lucky13

    Lucky13 Forum Mascot

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    Cool stuff! :thumbright:
     
  6. Shinpachi

    Shinpachi Well-Known Member

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    Great job :shock: They look rotating!
     
  7. Hamiltonian

    Hamiltonian Member

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    Thanks for the kind comment. Yeah, it's that choice between "What's that thing where the propeller should be?" and "Where's the propeller?"
    So far, I'm reassured by the fact that casual passers-by don't comment on the props at all. So they seem to be doing the job well enough to avoid jarring people's attention away from the rest of the model.

    But I realized one problem last night, when I was watching the Australian aircraft setting off to look for the missing Malaysian airliner. Their prop tips seemed to marked with a white/red/white banding. You can't print white on a standard printer, so there's no way to reproduce white prop tips using this method.
     
  8. herman1rg

    herman1rg Well-Known Member

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    Nice idea, look great.

    BUT

    WTF is that model they are on?
     
  9. Hamiltonian

    Hamiltonian Member

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    :D
    It's a Blohm und Voss P170 "Schnellbomber". A high-powered and low-drag design intended to outdistance Allied fighters. The constant-chord wing sections were intended to be interchangeable, port and starboard.
    "The design studies submitted to the RLM in October 1942 met with no positive response," so sadly (or perhaps fortunately for anyone trying to land the thing) it never flew in real life.
     
  10. Wayne Little

    Wayne Little Well-Known Member

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    Interesting stuff!
     
  11. Wurger

    Wurger Siggy Master
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    I agree ... interesting. THX for sharing. :)
     
  12. Hamiltonian

    Hamiltonian Member

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    Glad that it's been of interest for some folk. :)

    While the print side of the overhead transparency film I used benefits from a little gloss varnish to seal the ink, the non-print side is *very* reflective, and looks better after a little matt varnish.
     
  13. Blue Yonder

    Blue Yonder Member

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    Very helpful!
    The plane looks great too...what kind of aircraft is that though?
     
  14. Hamiltonian

    Hamiltonian Member

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  15. Wolfman_63

    Wolfman_63 Member

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    I really like how you did that. Great job. As for the white/red/white, You could try painting the edge with a very thinned white wash then print the gray gradient and the red on top.
     
  16. swampyankee

    swampyankee Active Member

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    #16 swampyankee, Mar 29, 2014
    Last edited: Mar 29, 2014
    That has got to be one of the weirdest aircraft layouts I've ever seen. But wouldn't it have been a bit easier on the pilot if they put him a bit closer to the nose?

    The tutorial is great! It deserves multiple thumbs up.
     
  17. Hamiltonian

    Hamiltonian Member

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    Thank you.

    I guess then you'd end up with a disc that looked different front and back - more of the wash visible on the non-printed side, more of the printing visible on the printed side. But I suppose you could apply the white wash to clear plastic sheet, and then apply lightly printed decals to *both* sides of the sheet.

    There are printers that add white toner to the other colours, specifically for producing decals and printing on non-white stock. Oki make a couple. But unless you spend a lot of time printing decals, they're a prohibitively expensive solution.
     
  18. Hamiltonian

    Hamiltonian Member

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    I haven't read a detailed description of how they came up with that layout.
    They were aiming for something so fast it could outdistance Allied fighters, so it was all about high power, low drag and light structure. One priority seems to have been to ensure that the front end consisted of nothing but engines and lifting surfaces, to minimize drag. To keep the structural weight down, there was retractable undercarriage supporting all three engines, stowed in the fuselage space behind the engines. And with those big engines sitting forward of the centre of lift, they counterbalanced by putting the crew compartment on a long moment arm out the back. The pilot sits over a long window in the underside of the fuselage, which lets him look under the mainplane when on final approach to landing.
    There are some decent diagrams here: Blohm Voss BV P.170 Luft '46 entry. The Planet Models resin kit I built features a big central glasshouse for the navigator/bomb-aimer, as you can see. A later version move the pilot further forward and put the navigator/bomb-aimer behind him, with the pair of them sharing a longer canopy.

    Thanks. :)
     
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