Scratch built radial engine, using straws

For an upcoming project, and just for the absolute fun of scratchbuilding, I made this 9 cylinder radial engine after being inspired to use pieces of flexible drinking straws for the cylinders.

Starting with a stiff paper, from a microwaveable dinner box, I carefully layed out a pattern, with 9 rectangles spaced every 40° apart (the center lines) around a circle sized to a proportion from a typical radial engine. The rectangular “arms” or “spokes” are sized so that the plastic drinking straw sections slide tightly on to them. Once I had that all figured and tried out, on the brown paper pattern, I traced the pattern onto a piece of .030″ thick styrene sheet. The #1 is for knowing where you started working, as you work your way around the pattern.

Ok…. I Photoshopped two pictures together here, to show the drinking straws with the flexible section compressed [top, green], and the straw with the flexible section opened up, or extended [below, blue].

I will probably be making a 6 cylinder inline airplane engine, like the WW1 era Mercedes D engine using the same method, as can be seen at the top of the photo, and using straight, non-flexible sections of the straw for the cylinders.

A bit of information. After sliding the straw sections onto the styrene form- with the holes drilled at the center top of each “arm”- I glued a piece of styrene rod in the hole, cut long enough to extend beyond the diameter of the straw, to hold it in place. Not being sure if the straw plastic being compatible with my glue, I find ways to make it work, without CA or Super glues, as I am extremely sensitive to those fumes.

I cut a balsa wood disc out, thick enough to extend beyond the cylinder depth, front to back, and glued it to the styrene using Tacky Glue. I followed that with a basswood disc, for a sturdier mounting plate, also Tacky glued.

I have Photoshopped some color into this picture to help explain some things….

The yellow domed section of the engine was cut from the Ping Pong ball in the background.

The red “hub” is 3-4 sections of styrene tubing glued inside the next bigger size, as a way of getting an absolutely centered hub.

The light blue seen at the ends of the cylinders is the .030″ styrene. I decided to not fill in the tops of the cylinders, as not needed for my purposes.

I used different diameter styrene rod for the valve rods, the exhaust (? I think?) pipes, and for the “bolts” in the top ring, which get cut and sanded down later. The exhaust pipes, I made by heating and bending the larger rod, then sanded the curved end to taper them.

The valve rocker arms were cut from the same .030″ styrene sheet. I use double sided tape to stick the 9 pieces-blanks together, and cut & shape them all together before separating them and doing final touch up.

The wooden base/block here is the solid wood base from a sports trophy leftover from my time coaching my son’s soccer team. It is a fixture on my workbench, and used quite often.

Finally, in primer. I thought maybe flat black instead of this gray, as a better under cover for dry brushing silver and other metals on, but figured I would give the gray a try.

And here’s the final, ready for installation on something. Not sure what yet, but a propeller driven sled in this big 1/12 scale is currently top of the list.

I pretty much use only craft acrylic paint anymore, for all my modeling. Even the cheapest acrylic craft paint from Walmart is good. Nothing like the early days, when craft paint began. Painting the metallics does take some learning. Basically you paint them on top of a black, and you paint layers, building up the eventual opaqueness and color of the metal you’re after.

I really haven’t yet found a good acrylic brass or bronze paint, so I used the gold acrylic paint I got at Walmart- (one of 3 shades) and I mixed some gloss orange and flat red & yellow to the gold, until I was satisfied with the look. I then added a little more yellow, and/or white to lighten it, or black to darken it.

Thanks for looking. Hopefully we’ll see a propeller on this soon, and it on some sort of funky sleigh 😊


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