Kevin Wickart's Tech Tips #6

 

A TISSUE O FLIES

or: What Is The Sound Of One Wing Snapping?

 

by Kevin Wickart

 

            To start, I want to give credit where credit is due. Many of the techniques described in this article have been passed along from rocketeer to rocketeer over the last several decades. I learned them from Mark Bundick, who learned it from Guppy Youngren and Chris Flanigan.

 

            The tissuing of model rocket glider wings is a useful skill to learn. Properly applied tissue strengthens the wing, so you can use lighter or thinner balsa without worrying about your model shredding. It provides a much smoother finish than raw balsa and is lighter than paint, giving better performance in the air. It also adds color to your glider, improving that all-important visibility.

            Proper tissue is available from most any hobby shop that caters to airplane enthusiasts, and comes in several colors. There are two main types, domestic and Japanese. The domestic is typically more readily available and comes in more colors, but the Japanese tissue is lighter and molds around curved surfaces better. Both are sold by Peck Polymers as of this writing.

            There is a third covering, Silkspan, which is applied using the same techniques as tissue. Silkspan is available only in white and is quite heavy compared to tissue, but it adds tremendous strength to gliders using D motors and above.

 

            The first thing to do is to choose the tissue color(s) for your glider. Common wisdom says to use a dark color on the underside and a bright color on the top. Both types of tissue come in black for the bottom surface and red, orange, and several shades of yellow for the top surface.

            Okay, I take it back. The actual first thing to do is to build trapezoidal wings—not elliptical. If you want to know why, just ask any rocketeer who’s ever tried to tissue around those curves. These people are easy to identify by their twitching.

 

            Now it’s time to get the chemicals out. You will need at the very least a bottle of clear dope, a bottle of dope thinner, and a good-quality wide brush (1/4” wide or more). Read the warnings on the dope bottles and heed them!  Butyrate dope fumes are not only an intoxicant; they are somewhat caustic and will burn out your sense of smell over time. Use plenty of ventilation, send the family and pets to the movies, and if possible, wear a filter mask rated for paint fumes. If you must do this without a filter mask, take frequent breaks to get some fresh air and drink plenty of water.

            Paintbrushes carry surprisingly few safety warnings.

            Mark Bundick recommends using a mixture of half dope and half thinner for the entire procedure. I’ve used just plain dope with good results, and a surprise bonus that I’ll explain in a moment. I’d recommend working with the half-and-half mixture until you become comfortable with the process; it gives you a much longer working time than straight dope does.

 

ONE STEP AT A TIME…

 

1. Build the glider wing and set the dihedral (or polyhedral, if you’re into that). Do not glue it to the rest of the glider yet.

 

2. Give the wing a final sanding with 400-grit sandpaper.

 

3. Brush on a good coat of the dope mixture and let it dry—two hours for half-and-half mix, half an hour or less for full-strength—and sand it smooth with 320 grit sandpaper. Apply a second, slightly lighter coat, let dry, and sand lightly. The wood should be slightly shiny when you hold it up to the light.

 

4. Plan out your color scheme. Anywhere there is a joint between two colors you lose strength, so you might want to skip the checkerboard patterns in favor of a simple pattern. Ideally you want to cover the wing with a single, unbroken piece of tissue.

 

5. Cut the tissue out with about ½” overlap. Look at the tissue, and notice that one side is rather dull while the other is kind of shiny. You want to attach the dull side to the wood, with the nice low-drag shiny side up.

 

6. Start with the top surface. Brush dope mixture over the dihedral joint and place the center of the tissue piece down onto the dope. Smooth out any wrinkles by brushing over them or tugging gently on the free edges of the tissue. You can also use a balsa burnishing stick if you promise to be very careful; wet tissue is still wet tissue, even if it’s Japanese. If the wrinkles are severe, you can lift the tissue up and try again.

 

7. Once you have the tissue adhering nicely over the dihedral joint (adjust this procedure for polyhedrals), gently lift up the tissue edges, apply another band of dope, lay the tissue down, and work out the wrinkles again. Repeat this process from the center of the wing out to the tips, and from the center of the wing panels to the leading and trailing edges. Make certain that the tissue is adhering fully at the edges of the wing. A good way to do this is to pull gently downward on the tissue overlap, forcing it tight against the wing edge.

 

8. Let dry completely. Trim off the excess by pulling the overlap around the edge of the wing and sanding across the edge lightly with 400-grit sandpaper. It takes very little abrasion to sand through the tissue. Make sure the edges are stuck down thoroughly, using a bit more dope mix and brush (or a gloved finger) to stick down any renegade pieces.

 

9. Repeat this process for the bottom of the wing. You may also do the stabs and rudder in tissue, if you like.

 

 

FINAL ASSEMBLY…

 

            Since glue does not stick well to doped tissue, you will need to use a hobby knife to cut the tissue away from glue joint areas. Sanding lightly will improve adhesion. Use double glue joints.

            If you want to get fancy, you can add stripes and other such decorations. One technique I use is to add a wide stripe of the bright upper-surface color to the underside, in case the glider flips over on landing. Mark Bundick occasionally cuts his NAR number out of tissue and dopes it to the wing.

            Whatever you decide to do, dope the decorations down just as you did with the main tissue pieces. When dry, give them a light over-coat of dope.

 

 

AN ALTERNATE METHOD…

 

            I prefer to use full-strength dope for tissuing, and instead of constantly lifting up the sheet and doping under it, after anchoring it I pull it tight across the wing and brush the dope directly over the top surface. The dope soaks through the tissue and sticks it down quite nicely.

            My use of full-strength dope and the soak-through technique helped me to make a rather interesting discovery while tissuing my B R/G wing on the field at NARAM-41. While chatting with other competitors, I got distracted and mistakenly switched from using dope to using dope thinner. To my surprise, the tissue adhered quite well. I finally realized that because there were two good coats of dope on the balsa, the thinner was soaking through the tissue and slightly softening the top coat. As the thinner evaporated the dope underneath dried again, adhering the tissue firmly. Once the whole wing was done, I gave it an all-over light coat of dope to seal the surface and sanded the wing with 600-grit paper for smoothness.