Oberursel UR III WW I Rotary Engine (Model Airways; 1:16) [COMPLETED BUILD]

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20220423

Decided to tackle another WWI rotary engine as my next project – the Oberursel UR III that was used to power the Fokker Eindeckar. This is a double row, 14-cylinder engine so plenty of opportunity for large fingers to get in the way of placing small parts.

Started a couple of evenings ago with the crankcase body and covers. Not much clean-up to do other than trying to straighten up some of the cylinder seats so the cylinders can be glued on. I decided to not paint the crankcase so it was just polished up with a metal brush. Once the engine is fully assembled it’ll be clear coated so there shouldn’t be any issue with later discoloration.

Today was given over to prepping the 14 cylinders. Typical casting marks on small pieces to be filed off and the occasional extra flashing to remove. The cast spark plugs at the tops of the cylinders were masked off (they will be painted later) and the cylinders have been sprayed a flat black. They’re being left to dry overnight; will assemble them to the crankcase body tomorrow.

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20220424

Finished painting the cylinders yesterday evening. Cylinder bodies are black & spark plug legs are white. Glued the cylinders to the crankcase with medium CA and accelerator. (This is the first time I’ve used the accelerator; definitely made the work easier and quicker.)

After getting all 14 cylinders glued to the crankcase it was time to start drilling holes in the Cam Cover for the pushrods and the Igniter for the spark plug wires. Drilled the first one with the twist drill and then decided to use the drill press. Drilled 28 x 0.5 mm holes about 6 mm into the push rod and spark plug wire extensions. 0.5 mm works for the spark plug wires but is too small for the push rods so I expanded the 14 holes on the Cam Cover to 0.8 mm. After test fitting the nominal 0.8 mm (1/32 inch) brass rod used for the push rods, I enlarged the holes to 0.9 mm so there wouldn’t be any issues installing the push rods. There is a hole in the end of the rocker arms for the top of the push rods so 14 more 0.9 mm holes were drilled.

The cylinder heads require some work since the return springs are molded onto the top – with lots of flashing that needs to be removed (x-acto blade and small triangular file took care of the flashing). There is a recess on the top of the cylinder head that accepts the stud in the center of the rocker arms; I drilled a 0.8 mm hole through these so I could mount them on round toothpicks for painting.

The push rod connections on the Cam Cover should be brass but I didn’t have that color in my paint stocks. Mixed up something that is, to me, acceptable. The spark plug wire connections on the Igniter should be copper; bronze is what I had in my stash so bronze is what was used. While I was painting, I painted the front & rear propeller hubs a steel gray. The push rods will all be painted black. I’ve decided not to paint the bolt heads and nuts on the various parts of the engine black so they are being left in the color of the part they are on.

I decided to complete one cylinder at a time – from Cam Cover through push rod to cylinder head to rocker arm. The last engine photo shows the first one finished…only 13 more to go.

The last 5 photos show the painted propeller hubs & Igniter and the propeller laminations glued and ready to be carved to shape.

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Interesting new project ....... I am looking forward to see the prop sanded....
 
20220508

Finished installing all pushrods and rocker arms to the remaining 13 cylinders. Used the CA accelerator when installing the rocker arms to the cylinder tops. With the cylinder fronts complete it was on to installing the spark plug wires from the 14 igniters to the spark plugs. As I’d painted the igniters and spark plugs, I started by cleaning the previously drilled hole with a 0.5 mm drill bit. The wire is thin & bends easily; difficult to keep it straight. By the time I got to the 12th cylinder I had the installation process down – insert the wire end into the igniter, add a spot of CA, spray with accelerator, wait a few seconds for glue to set, stretch wire to spark plug, clip it off just above the spark plug end, insert the wire into the spark plug end, add a spot of CA, spray with accelerator, repeat with the next spark plug wire. Engine proper finished.

Next was the engine stand. When I had it dry fit together, I realized that there was no way that I would be able to get the engine into is as I’d installed the machine gun interrupter cam on the back of the igniter cover; the engine assembly was too long. Time to make a modification to the engine stand so I increased the overall length and it could be disassembled into 2 units so the engine could be installed. Minor woodworking and installation of cut down straight pins to index the 2 halves together.

Instead of leaving the iron cross as just a punched-out hole in the rear of the stand, I created a couple of indentations in the inside of the cutout so that I could install a rod (cut down round toothpick) that I could glue the iron cross to thus creating a 3D cross on the rear of the engine stand. After gluing on the cross I filled in the hole on the inside with wood putty. I decided to paint the stand green (allowed the laser etched engine information on the front of the stand to be readable) and the surface of the cross black. Engine stand finished.

Carved and sanded the propeller as best my knife skills allowed. Overall OK although when I was done, I realized that I had carved the propeller backwards according to the photos in the manual. Since the error probably went all the way back to when I glued the blanks together, I decided to leave it as is. I glued on the front and rear propeller hubs and then the propeller nut. Propeller finished.

Placed the front of the engine in the stand, placed the brass support tube into the rear of the engine crankcase, added the rear of the engine stand and then press-fit the propeller assembly onto the propeller shaft. Complete engine finished and in place on the display shelves.

Next up will be a ship; time to get back to that genre.

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