# How To Model Rotating Propeller Discs

A shorter version of this post was published on 11 October 2017.

There are some 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.  Like this:

Years ago, I posted a short tutorial about this on WW2Aircraft.net; and shortly after I started this blog, I put up a slightly revised version. But it was light on detail—so now this is the extensively revised, expanded and updated version.

I appreciate that some people can’t be bothered doing all the measuring and editing I’ll describe below, which is intended to produce a prop disc that matches the specific measurements of the propeller. If you’re such a person, I encourage you to skip past the measurement and mathematics and read the start of the section about creating a GIMP gradient. Once you’ve seen how I create a gradient of the correct colour, skip ahead to the section on using that gradient to draw the prop disc, and follow on from there. (To skip the first dose of maths, follow this link.)

The first thing I do is to measure the radius of my kit propeller, and then divide it into eight equal sections, from boss to tip, marking off the divisions with a felt-tip pen. Here, I’m marking up two different kinds of propeller for a Blohm & Voss 138 seaplane:

Then I measure the propeller blade width at each of my marked locations. To calculate what proportion of the prop disc is occupied by prop blades at each of my eight points along the radius of the propeller, I first work out the radius at each marked distance, which is just the radius of the propeller multiplied by the number of eighths—the first mark on my 24mm propeller is at 3mm, the next at 6mm, and so on. Then I multiply each of those distances by 2π to derive the local circumference. To find out how much of that circumference is occupied by propeller blades, I take the measured width of a propeller blade at that distance, and multiply by the number of blades. Dividing this length by the total circumference tells me what proportion of the prop disc is occupied by propeller blades.

I use a little Excel spreadsheet to do the calculations for me:

The first column is the number of eighths, measured from the hub of the propeller. The second column converts this to a radial distance. In the third column I’ve entered my measured blade widths, and the fourth column does the maths—multiplying the local blade width by the number of blades, and dividing by 2π times the local radius. You’ll see these numbers turn out to be relatively small, and I find that discs printed with opacities to match are fairly unimpressive, visually—primarily because you can see through even a “zero transparency” layer of ink. So in the fifth column I take the highest number in the fourth column, and express all my numbers as a proportion of that. These are the numbers I’m going to feed to my printer, with 100% representing maximum opacity printing, and 0% full transparency.

I also need to specify a colour for my prop disc. The BV 138 aircraft I’m modelling would have had propellers painted in Luftwaffe Schwarzgrün, which was coded RLM70. I can use the handy digital colour charts created by William Marshall, available here, to discover that RLM70 translates into RGB values of 56/62/50.

So now I’m ready to create my prop disc in a graphics program. I’m going to use GIMP, which has all the tools I need. You can download it here, for Linux, Apple or Windows. (My screenshots and menu sequences below are from version 2.10.)

After opening GIMP, I go through Windows/Dockable Dialogs/Gradients, which brings up a list of GIMP’s built-in colour gradients. I right-click on the list, and choose New Gradient. This gives me a simple colour gradient, from black to white, ready to be edited. Here, I’ve renamed it “BV 138 three-blade prop”, and I’m ready to edit:

First, I tell GIMP my chosen colour, by right-clicking on the gradient and selecting Left Endpoint’s Color…, which brings up a colour dialogue box. All I’m interested in setting are the R, G and B values, for my colour, and the A value (for opacity). But first I need to ensure I’m using the right scale—the figures given by Marshall are based on a range of zero to 255, so I need to click on the 0..255 button before I start entering numbers. Then I set R equal to 56, G to 62 and B to 50, leaving A with its default value of 255 (that is, completely opaque). Here’s what that looks like:

Then I go through the same process with Right Endpoint’s Color…, except setting A to zero, for full transparency. So now I have a colour gradient that is RLM70 throughout, fading from complete opacity to complete transparency. That would actually produce a pretty convincing prop disc, just as it stands, but I can make it more physical accurate by adding the opacity profile I derived from my little spreadsheet. (To skip that detail and go straight to instructions on how to draw the prop disc using a gradient, follow this link.)

To give the gradient a more realistic opacity profile, I need to split my colour gradient into eight equal parts, to match the eight measurements I made above. That’s easy to do. I right-click on the gradient again, and click Split Segment at Midpoint—now I have three little black triangles at the base of my gradient, splitting it into two segments, and two white triangles marking the midpoints of these segments. (These triangles can be dragged around to modify the gradient, but they can stay right where they are at present.) Right-clicking again produces Split Segments at Midpoints, and creates four segments; and again, and I have the eight segments I need. Clicking between two black triangles in the bar at the bottom of the gradient selects one segment to be edited—it appears bright while the others go dark. Like this:

Now I can edit the colour and opacity within that segment. The RLM70 colour I set at the start has been inherited by all the segments I’ve created, so all I need to do is tweak the opacity. For the first (leftmost) segment, I leave the left endpoint unedited, but bring up Right Endpoint’s Color…. Now, because my calculated opacities from the spreadsheet run from 0 to 100, I click on the 0..100 button, and then set A to 80, the figure for the first eighth of my 3-blade prop.

(Notice how the RGB numbers seem to have changed from the original 56/62/50—that’s just the effect of changing the scale from 0..255 to 0..100, and the underlying colour has stayed the same.)

Then it’s just a matter of selecting each segment in turn, and editing the opacity. For the other segments, I need to make sure the left end of the segment inherits its opacity from the right end of the previous segment—I do that by using the sequence Load Left Color From…/Left Neighbor’s Right Endpoint from the right-click menu. Then I repeat the process above to set the opacity for the right endpoint. And so on. Here’s the final result, with a distinctive dark band a quarter of the way out, created by the broad bases of the paddle blades on this propeller.

There’s one last thing to do, which will come in handy later. I select the leftmost segment, and then right-click to Split Segment at Midpoint. I select the leftmost of these new segments, and set its left and right endpoints to black (RGB 0/0/0) with maximum opacity. Then I select the new segment immediately to its right, and set the opacity of its left endpoint to maximum. That ends up looking like this:

The narrow black segment at left is going to end up as a black dot marking the centre of the propeller disc. But it’s a little too broad at present. I can fix that by dragging the leftmost white triangle all the way to the left, and then dragging the neighbouring black triangle so as to close up the black segment while expanding the segment to its right.

Now I’m ready to use my gradient to create a prop disc.

I start with a new blank image using File/New… from the menu bar. Since I measured my propeller radius at 24mm, I make my blank 60mm across, to give plenty of space. And I note that the default resolution is 300 pixels per inch, which matches my printer settings.

Then I go through Tools/Paint Tools/Gradient to bring up the gradient tool at the left of the work area. I set Shape to “Radial”, and check that the Gradient is “BV 138 3-blade prop”. (If it isn’t, clicking on the little square to the left of Gradient brings up a list of available gradients.) I’ve captured all that in the screen-shot below:

Once all that’s set up, I plonk my cursor in the middle of my new blank image, hold down the left mouse button, and drag it outwards towards the edge of the image. And my propeller disc appears.

To get the size right, I need to glance at the bottom left of the screen, where GIMP gives me a readout of the radius of the pattern I’m creating—I need to tune it to 24mm, give or take a hundredth of a millimetre.

And then I’m done. I go through File/Export As… from the menu bar, and click on Select File Type (By Extension) so that I can save a *.jpg image to some directory where I can find it again.

Having gone through the same process for my 4-bladed propeller, I can print a test sheet to check that my prop discs are the right size:

Then I can print the discs on to a transparent sheet. Here, I’ve used an ink-jet compatible overhead transparency. My compass cutter is set to 24mm, the measured prop radius, and I can use the black dot I created in my colour gradient in GIMP to ensure that my cut disc is properly centred.

I don’t use laser transparencies, because the laser printer tends to a bake a curve into the transparent sheet which is very difficult to remove. I’ve had success in the past with printing on to transparent decal paper, and then laying the decals on to discs cut from transparent plastic sheet—you can produce a thicker and more rigid final product in that way, at the expense of several more production steps. And be sure not to use plastic so thick the compass cutters won’t go through it!

Preparing the propeller bosses and spinners is very much kit-dependent. Obviously, the propeller blades need to be removed from the boss, and the boss split so as to accommodate the plastic prop disc. For the BV 138 I trimmed a millimetre or so off the back of the spinners, replacing it with appropriately sized styrene tube, and filled the holes in the side of the spinner, producing a two-part assembly in which I could sandwich the prop disc.

I then used a hole punch to make appropriately sized holes in the middle of my discs, so that I could slide them on to the propeller spindle.

On other occasions I’ve simply had to glue the discs in place within the split boss and spinner.

Here’s the final result, with the spinners painted up:

And here they are on my BV 138 mine-sweeper:

There are a couple of limitations to this approach. It’s visually unsatisfying once you get up to larger scales (but then again, the alternatives are, too). And if your propeller has white markings on it, you’re out of luck unless you have an expensive printer that uses white ink.

I’ve also had a request for a tutorial on how to add yellow safety tips to propellers, for instance of the kind used by the RAF and USAF during the Second World War. It’s easy enough to do, and all the necessary knowledge is actually present in this post, but I’ll add a supplement at some time in the future that shows how I produced the yellow tipped propellers on my Supermarine Walrus:

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