How To Model Rotating Propeller Discs

There are a couple of things I hate about “in-flight” models of piston-engine aircraft. One is when the aircraft appear to be flying without a pilot; the other is a stationary propeller.

Modellers have a couple of ways of dealing with this second problem. One is to simply remove the propeller blades, leaving only the filled and smoothed spinner visible—it’s a well-recognized technique which many feel produces the most realistic appearance. But it always makes me think, Where’s the propeller? I find the complete absence of anything in the space where the propeller should be is a little distracting. I’m also not very keen on the photo-etched “prop-blur” option, which aims to produce a blurred sector for each prop blade, reproducing what we see in photos and movies, but not what we see with the naked eye.

So what I want to see is a transparent disc of the correct propeller colour(s), with the colour density at each radius matching the relative amount of prop blade and empty space at that radius.  A while ago I posted a tutorial about this on WW2Aircraft.net; now that I have my own website I thought I’d reproduce it here in a slightly revised version.

Here’s what I do:

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

Prop disc tutorial 1
Click to enlarge

2) Calculate what proportion of the prop disc is composed of prop blade at each measured radius. Multiplying the radius by 2π gives you the circumference at that radius, and 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 disc tutorial 2
Click to enlarge

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.
There’s a tutorial on building GIMP gradients here.

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 consult William Marshall’s excellent Digital Colour Charts page to find the Red/Green/Blue values corresponding to various paints used by various air forces during the Second World War. So I find that RLM71 corresponds to RGB 82/88/86. The alpha channel (“A” in the GIMP tool) should be set to the densities I calculated above. In this case my prop disc becomes 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 disc tutorial 3
Click to enlarge

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 disc tutorial 4
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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. In either case, 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 disc tutorial 5
Click to enlarge

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 disc tutorial 6
Click to enlarge

7) And complete. Here are the prop discs fitted to the Planet Models resin kit of the Blohm & Voss P.170 Schnellbomber. (A bizarre design, with the pilot sitting at the back of the aircraft, which never got off the drawing board in real life.)

Blohm & Voss P.170 (1)
Click to enlarge
Blohm & Voss P.170 (2)
Click to enlarge

There are a couple of disadvantages. I’d say that 1/72 is about the largest scale on which this works—beyond that, the disc are too thin to be realistic. And if your propeller has a white stripe in the safety markings at the tip, you’re probably out of luck unless you have a very expensive printer with white toner.

4 thoughts on “How To Model Rotating Propeller Discs”

  1. Hey mate, first of all thank you for the tutorial!
    Unfortunately I can’t get this to work at all!
    My problem is Gimp. I don’t really know how to handle the color gradient stuff. I read tutorials, but to no avail 🙁
    I might kindly ask you for help.
    The color of my prop blades is RLM 01 (silver) it’s a 1/72 BF 109 from the Legion Condor.
    I did my measurements and calculations in Excel, that was the easy part. But I can’t get this stuff in GIMP at all.
    Please help 🙂

    Regards ANthony

  2. Oooh, it’s a while since I’ve used GIMP – I’m going through a phase of building static 1/48 aircraft. But I’ll see if I can steer you through.
    How far have you got with GIMP? Where are you stuck, in other words?

    1. I’m stuck at building the colour gradient.
      Here are my questions:
      1) Is the number of fractions from the Excel spreadsheet the number of segments I have to set up? I’ve noticed, you have 8 fractions in Excel and 8 segments in the gradient, plus that tiny black one for the centre point.

      2) Which values do you use for the alpha values in gimp? The proportion values or the fraction of max. values?

      3) Building the gradient, is the color always the same for each endpoint? E.g. 8 segment = 16 endpoints, so 16 times RLM 01 in my case?

      4) How do you match end point alpha.values? Does the left endpoint of segment 1 start with alpha 100 and the right endpoint og segment 1 ends with alpha 80 in your example, or also with alpha 100?
      Where does the left endpoint of segment 2 start? At the alpha of the right enpoint of segment 1?

      I hope this is somewhat comprehensible 🙂 I don’t know how to explain it in an easier way, without actual examples.

      Do you, by any chance still have the gradient from the example?

      Thanks mate!

      1. 1) Yes, each segment measured on the prop corresponds to a segment of the gradient. A measured point halfway along the prop is matched to a segment junction halfway along the gradient, and so on.
        2) I use the max value, meaning that part of the prop is printed with a maximum density of colour. It depends on your printer, but that generally comes out mildly transparent for dark colours, and markedly transparent for pale colours. Test printing and fine adjustment may be required.
        3) The colour at each endpoint should match the colour of the prop at that point. So if you have RLM01 from end to end of the prop, that’s the colour for all points on the gradient. (Obviously, if you were building a prop with yellow tips, you’d need to change the gradient colours accordingly, at the correct point.)
        4) The central dark dot, generated at left of the gradient, has 100% opacity at both endpoints. For the prop blades themselves you’re aiming for a smooth transition of transparency, so the right endpoint of segment 2 should match the left endpoint of segment 3, both values equal to the calculated opacity for that measured point on the prop.

        I suspect I’ve now got rid of the old gradients I made. I’ll take a look, though.

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