Which covering is right for your model project?

Which covering is right for your model project? by Louis Joyner ([email protected]) As seen in "Free Flight Duration" in the May 2009 issue of Model Aviation When I started in Free Flight, there were only a few covering-material choices. Japanese tissue offered light weight and a wide range of colors, but it was easy to puncture. Silkspan, available in several weights, was stronger than tissue and had enough wet strength to allow wet covering. But the color choices were limited, and it required more dope to fill the pores. Silk was great for wet covering but was expensive and took a lot of dope to fill.

Bud Romak covered his original Moffett with yellow Japanese tissue accented with dark-blue tissue. Yellow artist’s ink added to clear dope provides a brighter, fade-resistant finish.

Wet covering used to be the best way to deal with curved surfaces, elliptical wingtips, or built-up fuselages. The process involved wetting the silkspan or silk with water, blotting off the surface water, and then draping and pulling it over the model structure. Dope was brushed through the covering material, to adhere it to the balsa. Pros could cover a polyhedral wing, top and bottom, with one piece of silk. Now, a half century later, we have almost too many choices. Tissue, silkspan, and silk can still be found. In addition, a wide variety of plastic films offer resistance to moisture and high-nitromethane fuels, as well as faster covering. Which covering material is best? The answer is, “it depends.” For lightweight Rubber designs, Japanese tissue is still a good choice. In addition to the wide range of colors, it provides a fair amount of torsional strength when tight. This can be important for models with simple wing structures that aren’t inherently strong. Most wings with straight ribs and a single spar fit this category. Tight tissue covering will dramatically increase the stiffness of such a wing. However, a damp morning or a light rain will cause the tissue to loosen and sag, with an almost total loss of strength. Extra coats of clear dope help seal out the moisture. For additional protection in the rain, spray on a coat of 3M Scotch Guard. This fabric protector will cause the raindrops to bead, making them easier to wipe off with a paper towel. Another problem with tissue is fading. Keeping the model out of the sun as much as possible will help, but the best solution for long-term protection is to add color to the clear dope used to coat the tissue. One of the most popular techniques is to use Higgins fade-proof ink, which is available at most art-supply stores. The suggested mix is one part ink, 9-10 parts thinner, and one part clear dope. Spraying will give a more even finish than brushing. The Higgins method works best if you use the same color ink as tissue (such as a yellow ink mixture over yellow tissue). On his Moffett design, Bud Romak added wide strips of dark-blue tissue to a mostly yellow model. To save weight, the blue strips were not applied over yellow tissue; instead, Bud covered the model with yellow, omitting tissue where he wanted blue strips. Then he covered those wing bays with blue tissue. He sprayed the entire wing with the yellow ink-dope mixture, which had only a slight effect on the color of the dark-blue tissue. But little can be done to increase the tissue’s resistance to punctures, except to fly over the proverbial tall grass. The plastic covering material that is closest to traditional tissue and silkspan is Polyspan. Available only in white, this material resembles silkspan.

Bob Mattes used Polyspan to cover his scaled-up Amazoom’s wing and stabilizer. Red paint on the rudder, forward fuselage, and underside of tips aid in visibility. Power is a Fox .35 he’s had since he was 16.

Paul Crowley’s Moffett Rubber model is covered with a few materials. Wing inner panels are covered with metalized Mylar. Tips and stabilizer are covered with red-dyed clear Mylar. The fuselage is covered with Polyspan and sprayed with clear dope that has red dye added.

Polyspan is applied with dope, as is tissue, and then tightened with an iron, as are plastic film coverings. Then several coats of clear dope or lacquer are applied to seal the pores. It’s important to make sure that the smooth side of the covering is on the outside and that the covering runs spanwise as it comes off the roll. The finished weight for Polyspan is nearly 3.6 grams per square foot—approximately double the weight of doped tissue. However, Polyspan is much tougher than tissue. Stubble that would puncture tissue usually leaves only a slight depression in Polyspan, and those are easy to remove with the tip of a hot iron. Many Rubber fliers use Polyspan to over built-up fuselages for extra strength and less tendency to absorb rubber lube. For coloring Polyspan, you can use the same ink-dope technique I mentioned or one of the special dyes available from FAI Model Supply. Another approach is to leave most of the wing white but add areas of bright color to the tips, to increase visibility. Design Master florist’s spray, available at most craft stores, is available in bright colors as well as a surprisingly opaque black. As in the old days of tissue or silkspan, you should dope Polyspan with butyrate dope when used on a fuel-burning model. There are also other synthetic tissues, such as Lite Span and Air Span. Where weight is critical, 1/4 mil Mylar is almost as light as you can go. Available in both clear and metalized versions, this plastic film is not precoated with adhesive. Instead, the framework of the wing or tail must first be coated with a heat-sensitive adhesive, such as Balsaloc. After that’s dry, the Mylar is attached using a trim iron and heat-shrunk. Also available are 1/2 mil and 1 mil thicknesses that give a bit more strength. Although these thin Mylar coverings work well in the rain, they are prone to tears or punctures. The thin Mylar provides little torsional strength. The clear Mylar can be colored with fabric dye before covering. An interesting approach is to combine Mylar and tissue. To do this, cover the model with clear Mylar and then give it a coat of clear dope. Then smooth tissue over the Mylar. Thinner applied with a brush activates the dried dope, attaching it to the Mylar. A final coat or two of clear further seals the tissue. For larger models, the heavier, adhesive-backed plastic coverings provide extra strength and resistance to fuels. That’s why many Power fliers have gone to coverings such as Hangar 9 UltraCote. “It’s like MonoKote, just better,” says Ronnie Thompson. “Film is the only way to go.” On his larger AMA Gas models, Ronnie uses transparent UltraCote for the flying surfaces. But he also uses a traditional covering material. The balsa D-box on his Sunbird series of models is covered with silkspan, and then the entire wing is covered with UltraCote. “The silkspan really adds to the strength,” he says.

For Power models, heavier coverings such as UltraCote offer strength and resistance to high-nitro fuels. An O.S. Max .25 ducted-fan engine powers Ronnie Thompson’s 528-square-inch Sunbird.

Dick Hall used UltraCote on his 655-square-inch Jaysbird powered with an O.S. Max .35. “It’s got a nice short nose,” says Dick. “I use 4 ounces of lead in the nose to get up to the 35-ounce weight.”

Tips from Dick Hall: Another Power flier who really likes plastic coverings is Dick Hall. I asked him a few questions about his use of those materials, and he generously replied with three pages of tips and suggestions. Following are some of them. “I’ve gone entirely to polyester film covering. There are several reasons: The plastic films are not susceptible to rain or humidity; in our area test flying in the evening in summer was useless with tissue and dope because the covering would get so loose. Flying in high humidity or any kind of rain was also a problem. With plastic covering I have flown in the rain many times. “The many coats of dope required for a large engine-powered model took a lot of time to apply and even more time to cure. The fumes permeated the whole house. “I tried the polyester tissue coverings [i.e., Polyspan] and found them great for rubber models but not for power models. I covered an Old Timer with polyester tissue and used what should have been plenty of coats of dope, but the oil from the fuel soaked through to the balsa beneath. “I have been using UltraCote exclusively for many years. Everyone I have talked to has found it superior to anything else. I use regular weight [roughly 5 grams per square foot] on large models and the lightweight version [approximately 3.4 grams per square foot] on 1/2A and 1/4A models. “I like to use red on the wing and stab for visibility against the sky and white on the fuselage for visibility down low with a tree line background. The fluorescent red is a more visible color than the darker normal red, but it’s not available in the lighter weight film. “Since I like to have the wing and stab the same color, I use the regular weight film for all the surfaces. Ronnie Thompson sometimes uses the lighter weight material on the stab and the heavier one on the wing. “The UltraCote lasts very long. I have not seen any indication of color fading as with tissue. Sometimes the covering will loosen slightly over time; it is best to check and retighten those areas. “The loosening may actually have been caused by the covering not being uniformly tight to begin with. I use the maximum temperature to try to get the tightness uniform; the tighter the covering the better. “I have a DC to AC converter that works off the battery in my vehicle. With the plastic materials I can adjust warps and do repairs in the field and do not have to wait for dope to dry. For small holes or tears I use clear plastic package sealing tape, which can be removed back at home with the covering iron. “The use of a plastic film without the adhesive backing would probably provide some weight savings, but I have not found it worth the trouble. The brush-on adhesives I tried were temperature-activated adhesives and I couldn’t use a heat gun or a shrinking iron close to the edges without the covering coming loose. “One tip is to tack the covering around the periphery at many places before trying to iron down all the way around the edges prior to shrinking. This is particularly important when covering sheeted areas. “I use the plastic films on the fuselage where it is convenient. I use Klass Kote epoxy paint to foolproof areas that are impractical to cover such as stab platforms, engine mounts, and wing pegs. “With UltraCote, I have not found it necessary to seal the edges and I do use a lot of nitromethane in my fuel. Using CA [cyanoacrylate] to seal the edges is useless since nitromethane is an excellent solvent for CA adhesives. I have a bottle of solvent for CA removal and it says on the back ‘100%Nitromethane.’ “Perhaps the biggest problem with plastic coverings is that they do not have a ‘grain’ like tissue and will sag between ribs. This is not a problem with sheeted leading edges but can be of major significance with some Nostalgia designs that don’t have spars on the upper surface. The Nostalgia rules allow adding extra spars for strength and one ‘turbulator’ spar in the first third of the upper surface. “I add one additional ‘non-sag’ spar instead of ‘turbulator’ spar between the leading edge and main spar. I also locate the main spar at the high point of the airfoil and use an I-beam spar configuration with vertical grain webbing and the top spar thicker than the bottom one. I will not build an airplane anymore without an I-beam spar; I have folded too many wings with just a bottom spar while returning on my chase bike.”

Sources:

Hangar 9 (available through Horizon Hobby) (800) 338-4639 www.hangar-9.com Polyspan Lite Span, Air Span, Mylar films, Japanese tissue, Polyspan (which can be found in the Starline section of the website) FAI Model Supply (570) 882-9873 www.faimodelsupply.com Klass Kote (612) 243-1234 www.klasskote.com