Rotary Engine Cooling Fins for World War I Paper Model Aircraft - Part I

[Gil]

Rotary Engine Cooling Fins for World War I Paper Model Aircraft - Part I

A Paper Model Technique Tutorial

By Gil Russell
Review Contributions by Leif Ohlsson

Background
Modeling World War I rotary engines is a challenge by itself for most paper modelers. The most common cylinder design solution consists of rolled cylinders with black and grey stripes easily identifying the model as paper. Stacking disks can be significantly better but only if the disks are laser cut and aren’t over handled during assembly.
With this background I embarked on a search for another “more realistic appearing” technique.
The first line of reasoning mimicked the flex joint of a flexible straw. It’s a pretty neat solution for flexible hose modeling situations but falls short of being a recommended solution for cylinder cooling fins (this is another tutorial that’s been completed but not published). Although an interesting and useful result it fell short of satisfying the “ease of construction” requirement.
I had been thinking about making cylinder cooling fins by using a “comb” to form a paste on a strip of paper much like spreading floor tile mastic with a serrated trowel but on a much smaller scale. This actually works quite well but has one drawback. The formed strip doesn’t easily conform to shaped cylinders well, a major sticking point of the method.
The idea of directly applying “combed paste” to the cylinder required a slower rotating source than a variable speed Dremel tool. The Dremel can be used at slow speed but is somewhat more difficult due to the high rotational speed even with the speed control set on low. An old plug in, reversible variable speed Black & Decker (B&D) drill motor is the perfect solution for this task. It has an “On Switch” lock that allows hands free operation. Many drill motors have trigger style speed controls. The full on speed of these motors is well within the range for use in this tutorial. You’ll need to jury rig a way to keep the switch in the “on” position. A multi speed drill press will also work on low or medium speeds.

Preparing the Cylinder

The cylinder used in this tutorial is based on a Clerget 7 & 9 cylinder engine at 1:16 scale (same cylinder design was used on both engine types). The cylinder consists of 24# paper tightly wound around a 7/32 inch diameter brass tube mandrel (0.21875 in ~ 5.56 mm). Use PVA cement spread over the entire length of paper. Allow the white glue cement to dry thoroughly. Note also that varying the mandrel size plus or minus 10% doesn’t seem to have much of an impact on the finished cylinders scale appearance. A copy of the cylinder pattern can be found in the download section.

Figure 1 Spiral Wound Cylinder

Mount the rolled cylinder on a suitable mandrel for the drill motor. A wooden dowel of the same diameter as the brass tubing was used in this tutorial but on reflection I realize that the smooth shaft end of a 7/32 inch (0.21875 in ~ 5.56 mm) drill would make an ideal mandrel as would a short section of the brass tubing used to roll the cylinder originally (I’ll be using the drill mandrel in the future). A strip of Teflon plumbers tape wound on the wooden dowel mandrel prevents the cylinder from sticking to the mandrel.
Start the drill motor on medium speed and paint a layer of card modeling paste (40-50% PVA 60-50% Spackle paste). Use a heat gun on low or a hair dryer to speed the drying of the paste (highly recommended).

Figure 2 Applying Paste to the Cylinder

Sand the cylinder smooth with a strip of 120 grit sandpaper (Note that ship model gun barrels are great candidates for this method).

Figure 3 Smooth Sanding the Cylinder

Figure 4 Smooth Sanded Cylinder Ready for Finning

Making the Combing Tool
Finding a way to make the comb forming tool consumed a disproportionate amount of the time devoted to this exercise. A number of different methods were tried and abandoned for one reason or another. The technique for building up the cooling fins occurred over a series of experiments that, in turn, affected the comb forming tool. The resulting tool described here proved to be the most effective one found for “turning” the cooling fins.
A #10-32 screw shaft was heated with a butane micro torch till hot enough to melt polyethylene sheet (0.625 in ~ 1.5 mm thick). An alcohol lamp, heat gun or candle can also be used as a heat source.

Figure 5 Heating #10-32 Screw Shaft

A strip of polyethylene is laid over the hot screw shaft and worked with a burnishing tool to force the plastic into the threads of the screw as shown in Figure 6.

Figure 6 Burnishing Threads into the Plastic

Use a wet rag to cool the screw and plastic down. Peel the plastic off the screw shaft. The resulting rough comb should appear similar to Figure 7. You may need several tries to obtain a workable result.

Figure 7 Rough Comb Forming Tool

Trim the front edge and both sides of the comb straight with a pair of sharp scissors. The finished comb should look similar to that shown in Figure 8.

Figure 8 Finished Comb Forming Tool

(Continued in Part II)

[Gil]

Rotary Engine Cooling Fins for World War I Paper Model Aircraft - Part II

A Paper Model Technique Tutorial

By Gil Russell
Review Contributions by Leif Ohlsson

Forming the Cooling Fins
The process of forming the cooling fins is similar to forming clay on a potter’s wheel. Start by filling the edge of the comb forming tool with paste. Apply the paste to the rotating cylinder spreading it out to cover the areas of the cylinder with cooling fins (see Figure 9).

Figure 9 Applying the Paste to the Cylinder

Form the cooling fins by judiciously applying the comb forming tool edge to the cylinder. This takes a little practice. The fins do not have to be perfect at this stage. The idea is to apply the paste to form ridges on the cylinder which will act as guides for building up smoother more cosmetically acceptable fins on top of the base structure. The cylinder should appear similar to that shown in Figure 10.

Figure 10 Forming the Cooling Fin Ridges

Continue to apply paste to build up the ridges. They should appear similar to those in Figure 11 after the cooling fin application is dry. Note that ridges have been applied over the full length of the cylinder.

Figure 11 Completed Paste Cylinder Ridges

Sand off the ridges on the bottom of the cylinder barrel with a strip of 120 grit sandpaper. Finish sand with a strip of 220 grit sandpaper.

Figure 12 Removing Bottom Ridges from the Cylinder Barrel

The next step builds up a smooth layer of PVA (Polyvinyl acetate or White Glue) on top of the paste cylinder ridges. Begin by applying a bead of PVA on the comb tool teeth.

Figure 13 PVA Bead on Forming Tool

The PVA will wet the surface of the ridged cylinder smoothing the surface. The PVA will at first tend to spread out if it’s too wet. Just apply the forming tool from time to time till the glue gels and holds the ridged structure. Apply heat to accelerate drying or allow time for the PVA to dry completely.

Figure 14 Forming PVA Ridges

(Continued in Part III)

[Gil]

Rotary Engine Cooling Fins for World War I Paper Model Aircraft - Part III

A Paper Model Technique Tutorial

By Gil Russell
Review Contributions by Leif Ohlsson

Forming the Final Cooling Fin Layer
The next step applies Aleene’s Tacky Glue as the last layer in the buildup process. It produces a very smooth final surface detail necessary for the cooling ring effect to work. Apply a bead of Aleene’s Tacky Glue to the forming tool and apply it to the cylinder. You’ll notice immediately how effective this layer is in bringing out the crisp detail of the ring structure (see Figure 15).
An alternate to Aleene’s Tacky Glue is Acrylic Tub & Tile Caulk (Also known as seam sealer - water cleanup type). Failing this you can use white glue that’s been thickened by allowing it to dry out some. Several coats will be required to build up the fin into a smooth crisp structure.

Figure 15 Applying Aleene's Tacky Glue

Heat dry or wait for the Tacky Glue to thoroughly dry before cleaning up both ends with 120 grit and 220 grit sandpaper.

Figure 16 Finish Sanding the Ends of the Cylinder

The completed cylinder should appear similar to that shown in Figure 17.

Figure 17 Completed Cylinder with Cooling Fins

The completed cylinder was painted with Testors Metalizer Lacquers.
Base coat: Aluminum Non-Buffing Metalizer (entire cylinder barrel)
Fin Paint Only: First Coat - Steel Non-Buffing Metalizer
Second Coat – Gunmetal Buffing Metalizer

The cooling fins were then buffed (while rotating) to remove the buffing gunmetal layer on the outer edge of the fins to reveal the steel metalizer highlight underneath. Figure 18 shows the completed cylinder. Acrylic paints will work just as well but will require experimentation to a attain the same effect.

Figure 18 Completed Cooling Finned Cylinder

[Texman]

Outstanding Gil! Now if I could figure out how to do that for 1/144 engines,
I wouldn't have anything else to master! Truly well done.

Ray

[Leif Ohlsson]

Gil, the result is stunning, and it has been a privilege to witness the development of the technique. Have you thought about publishing it on some general scale modelers site as well?

It just may be that the technique is somewhat beyond what most papermodelers would find appealing.

I know I would have to think twice or more before venturing an attempt.

Leif

[cjwalas]

Brilliant. Great to see such a fascinating technique! I do agree with Leif, though. For most of us modeling this sort of thing, we're just not advanced enough to tackle this one. But maybe someday! Thanks for sharing this.
Chris

[Gil]

Originally Posted by Leif Ohlsson

It just may be that the technique is somewhat beyond what most papermodelers would find appealing.

I know I would have to think twice or more before venturing an attempt.

Leif

Hi Leif,

I'll keep this in mind.
I agree that an unused technique is worthless to the average modeler and it appears this falls into that category. I apologize for having attempted it here.

+Gil

[Leif Ohlsson]

Gil, please remember the part about it having been a privilege to witness this, and I do believe many will remember it, and the many elements of it, for future creative use. I know I will.

Just the process of guessing at what kind of material might be utilized for a completely new purpose, then realizing that a special tool is needed, then figuring out how to make that tool, and then make it work - that's inspirational, as well as challenging. I can only guess at how many failed attempts are marking the laborious road to final success.

Then we have your unique ability & knowledge of materials like glue, paste, tapes, and all kinds of stuff. To me that's invaluable, and I can assure you I have learned tons. There is already a little can of white glue & carpenters spackle mixture now incorporated in my row of cans most readily at hand, and the purchased modeling paste is awaiting its turn. Never would have known about these things, were it not for your expertise, and your willingness to share it.

A final example, which I think many would gain from, is the very intriguing painting technique at the end. Using one non-buffing paint, and then a buffing paint above it - that's really something to remember.

I can only imagine what kind of concentrated effort has gone into this piece, including the care taken about writing it up, and the extremely instructive photography.

So, a big fat belated Thank You! - that's what's in place here.

Leif

[B-Manic]

Gil

I find your experiments at the deep end of the modeling pool fascinating. I hope you keep at it and continue to generously share them with us. Some I use, some not, and some I use parts of but, they are all great.

~ Cheers

[redhorse]

Your stuff is always on the cutting edge, Gil. Don't know if I'll use every tip, but then again I might... especially with some of my sculpting stuff. The polyethelyne tool is quite impressive and something I've never thought of. I've referred people from other forums to some of your threads here for techniques, can't find this stuff anywhere else.

Keep up the good work!