Blue Foam Building for Fun - RC Groups
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Blue Foam Building for Fun

Here's Clayton Greaves story on imitating a tried and true performer with his own foam working methods.

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Introductions

Electric flight has certainly brought new levels of challenge and diversification in my enjoyment of this hobby. My gassers continue to accumulate a thickening layer of dust.

Every once in awhile something new will come around and monopolize my interest for months, even years. Let me explain. Usually it’s something whimsical, a bit odd, but always something new and different. Foam and tape building- Zagis, and the like, would be one primary example. I continue to marvel at their durability. Another is the IFO and its amazing carbon fiber structure; so strong, so light. Another is the amazing duration available with lithium metal and lithium ion battery packs. As the saying goes, So little time, so many (electric) planes to try; it’s a great time to be alive.

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Back in December, a fellow flyer and professional model builder Tom Lazar of Classic Glass, showed me a 2.5 foot x 3.5 foot sheet of blue polystyrene foam, 1/8" thick. This material is used as soundproofing under wood floors manufactured by Award Hardwood Floors: http://www.awardfloors.com/ . I was intrigued and rushed out and bought several sheets from a local carpet dealer for $7 each. The bipe in this article was cut from two of these sheets. Now I have built a Folding Flier and a Pibros from foam core board with moderate success. These latest projects however, turned out lighter and thus they flew much better than prior attempts. This material has entertained me well, and most importantly, helped cultivate my interest in scratch building.

What followed initial shaping tests in December was a furious series of copies from successful designs shown in the pictures above: A Zagi "Lite" electric, a Twin screw Lite Stick, And the subject of this article, a traced copy of the just introduced Graupner WD Sunwheel Bipe

 

Kiting the Parts

At Christmas my flying buddy Peter (Double "A") Aarsvold asked to have his new kit shipped to me while he vacationed in sunny Arizona. With his permission I popped the carton’s seal to study the plan over a cup of coffee. Impressed with his kit and this new foam stock on hand, I went to work tracing his parts.

I started by tracing all the foam parts. The fuse is blow molded so I built mine up instead. Then I tried my hand at bending and matching the wire cabane struts and tail wheel. Light gauge wire is a far cry from the 3/16" found on my big birds! As I recall, I had the parts cut and the fuse framed in a single evening. I suppose the speed and easy shaping are a big part of the appeal for me.

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This Graupner kit is gorgeous. Having flown Peter’s, I highly recommend it. Speeds are quite slow, given the extra drag of a second wing but it loops nicely too. Perhaps the recommended power combination isn’t ideal; 3:1 gear reduction requires a 12-volt pack for solid performance. 5:1 wasn’t right.  Next try will be a 280BB and 4:1 reduction. Though a couple ounces heavier, the copy flies every bit as well as the original and demonstrates the creative opportunities available with this material. Here’s a Graupner link for details on the source kit: http://www.graupner.com/electricflight.html.

Fuselage

Fuselage assembly is straightforward and doesn’t warrant a lot of explanation other than to emphasize how fast it came together.

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Here’s a view of the inverted fuse and gussets. Very rigid once sheeted.

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Here’s a view of the fuse upright-nearly complete

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Here’s the fuse with control rods installed and drying. I chose to anchor with small EPP blocks. Note the elevator is routed to the top!

Tail Feathers

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Here’s a view of the drying empennage. I double laminated the foam; next time I’d save weight and make these from a single layer. These pieces were fun to shape though! For attachments like the tail feathers and formers, I chose to spray 3M 77 into an epoxy cup and apply it with a Q-tip. Foam-safe CA works, but be cautious with using accelerator; too much and it attacks the foam. I experimented with R/C 56 (good for hinges in EPP) I also tried some carpet and home craft foam adhesives, but I can’t recommend any of those. The shiny outer skin on the foam made the bonds pop loose when these adhesives were bend and twist tested.

Motor Mount

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The Graupner kit calls for the #1700 3:1 drive and speed 280. I shaped a block of scrap EPP foam as my motor mount and attached it to the fuse sides with 3M77. Motor mount is 1/16" ply. I like to carve this foam with a small diameter Dremel routing tip. I later added another vertical ply motor mount to the nose, not shown here but visible in shots of the finished plane. I needed it added for experimenting with 4:1 and 5:1 ratios and the popular MP Jet gear drives. Today the motor drive and prop choice is still a work in progress.

Wing Cutting

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Here’s a wing panel being cut out on my cutting mat. An Oleo rolling cutter is a real plus for this type of work. If you haven’t yet purchased a self-healing mat for cutting covering, get one- you won’t be sorry.

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Each wing is made up of two laminates of foam. If you look at Czech designs like the Chubby Lady you will see the identical treatment. Only their production grade bends are MUCH more crisp and uniform. Must be some sort of heated clam- shell; how do they do that anyway?  I added ribs both to lock in the shape and to tie together the interplane struts when mounted later.

Wing Forming Notes

I found a double laminate foam wing resistant to shaping after gluing, so I chose to shape each wing layer prior to bonding the two laminates together with 3M 77-spray adhesive. Once the two layers were bonded, I then tweaked the assembled airfoil shapes with more heat, as needed, to get that final 10% of the shape. Perhaps for a future project, jig or extra hands would help me heat form a double thickness. A further note: Over time the glue bonds in sharp bends have delaminated and required a smear of foam safe CA to lock it back together.

The Wing Forming Process

To form the airfoil I lift the leading edge off the table with a 3-foot level I have on hand. Any straight, fair stick, a 2x2 perhaps, would work. It’s extremely important to bend the panel as a unit or you will get wavy results. Then choose a mailing tube or the tube inside a roll of Ultracote to press in the airfoil shape- obviously, lots of things would work for this job. Extra hands are a plus here.

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To heat-form the foam I use a paint-stripping gun on the lowest setting. Too hot and the foam will melt. I bend the shape about 150% of what I want on the first attempt as it will spring back. Heat until the foam distorts slightly. Also, look for the surface to smooth under the heat. Patience is everything here. Slow heating and waiting between treatments are the keys to good looking, distortion free results. Go over the panels two or three times until they are within 10% of the final form.

Now lightly spray the bonding surface of the 4 panels with 3M77 or similar, wait 15 –30 minutes, and join. Get it right; there are no second chances!  This picture shows the assembled lower wing laminates before mounting and dihedral bending.

Dihedrally Challenged

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The Graupner plan calls for 3" wing dihedral. This was challenging, given the already heat formed airfoils. The dihedral bend creates a compound stretch on the foam that was constantly trying to distort the established airfoil shape. A flat wing or building the wing in halves separately is much easier. The lower wing dihedral was heat formed while mounted in its saddle as shown. Sandbags were placed atop the fuse to keep things in place. Then each wing tip was blocked up 3 inches.

After two tries at heating and cooling, the foam relaxes nicely to the 1.5 inches needed on each side (3" overall). I managed to overheat and curl the fuse foam in the saddle area. As when Monokoting, be careful with the heat in tight corners - the heat builds and burns the material. In truth, the markings left from heating are the only negative commentary I have on this method. With some finessing the shapes came out ok.

The cabane is a simple wire affair. The fact that the plane is so lightweight allows for simply cutting thin 1/32 blocks and gluing it to the side of the fuse. All in all, easier than it looked, and a lot of fun to construct. The top wing just sits rubber banded atop the cabane shown installed.

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Tailwheel installation was simple enough. I glued the wire to a triangle of ply and then bonded the ply to a block of EPP wedged in the back corner. Once sheeted, this proved a very solid set up. I cut up one of those Fingerboards – a miniature skateboard that comes with a Mc Donald’s Happy Meals- as a tail wheel. Don’t tell my son.

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Interplane struts

I made the struts double thickness; a single would have sufficed. The bottom wing is a +2 degree incidence, so I made the top wing +2 degrees. After initial flight-testing and too much down trim, incidence was reduced back to 0 degrees. The kit has the struts attached with a glued hinge. I chose to fasten mine with a length of filament tape. The bottom is fastened with a toothpick skewered through the strut. Works well, with easy disassembly.

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On Its Feet

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Finally, here I show the landing gear set up. Pretty straight forward.

 

Side by Side Bipes- Flight impressions and reflections

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First flight was February 4, 2001; that’s Pete holding the red Graupner version of the Sunwheel. Though heavier, (17 vs. 14 ounces), we agreed that the planes flew identically. Aside from some necessary tweaking, I consider this a successful effort. Extra down elevator was needed, so I will make an adjustment to the upper wing incidence to compensate.

My original choice of power system combination was all wrong. Power was barely sufficient to ROG on the first flight. The stock 280 has been upgraded to a fire-breathing Simprop 280 BB. Gear drive configuration was switched from 5:1 to 4:1. Esc went from a 5 amp Dymond Modelsports to a 14 amp Castle Creations Pixie 14. I continue to use a Graupner 9x6 Slim prop.

The negative spin to these changes is that nothing lighter than a 500 mAh pack can handle the peak 8.5-amp output of my latest motor. The combined addition of the ball bearing motor and heavier pack brought the overall weight to 21 ounces. A credit to the folks at Graupner, the kit version weighs in at about 15 ounces. If I were to start all over I would have been much choosier about adding unnecessary weight. The newly added thrust is very apparent on the bench, but I suspect that the slow side of the flight envelope will correspondingly deteriorated.

 

Conclusion

I have had great fun creating in blue foam over the past couple of months. I hope this example demonstrates the fun you too can have scratch building and copying proven designs with this versatile material.

Final power system configuration is a 280BB and 9x5 Graupner slim prop.  Ground handling and take-offs are no sweat, but the addition of the hot motor and bigger, heavier 500 mAh battery causes it to nose over on landing. I plan to open a port in the landing gear former and shift the pack back under the wing more.

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