14 Building and Construction Tips
Can you imagine what a model airplane would look like without sandpaper? The image is frightening. Sandpaper is the single most important element in providing a good finish to a model. For those of you who cannot afford new sandpaper all of the time, try vacuuming it with your shop vac to revitalize its cutting power.
Sticky back sandpaper can be purchased in bulk at just about any store that sells power tools to contractors. You can buy so much sandpaper for a few dollars more than what you paid for a much smaller quantity in the hobby shop that you will never be afraid to use sticky back sandpaper again. For sanding either ribbed or fully sheeted wings, two-foot long sanding blocks with grit all the way out to 400 is necessary. I typically use 600 grit sandpaper for the final finish on balsa wood prior to covering, and on fiberglass prior to painting.
Small razor block planes are necessary to provide quick and accurate shaping of leading edges and for quickly rounding fuselages. The plastic block plane is hardly worth wasting your money on. There are several aluminum block planes that are of a much higher quality, for just a few extra bucks. It's so much faster than sanding and a lot easier to clean-up.
There are several ways to reinforce the trailing edges to make them truly ding resistant. Roughed-up 14 mil mylar (with 80 grit sandpaper) sandwiched between two vacuum bag wing skins works pretty well. Fiberglass cloth works even better (2 to 3 oz. variety). I personally use .007 x 1/2 inch carbon fiber tape. I pre-laminate the carbon fiber tape to the lower wing skin, then place a layer of two-inch wide 1.4 ounce fiber glass cloth from the middle of the carbon fiber tape towards the center of the wing. Next use a light spray of 3M77 or other spray on contact cement to hold the fiberglass cloth in place. Once cured, just sand the TE until you see the black carbon fiber on both sides. This provides very light, extremely ding resistant trailing edges. If you vacuum bag wing skins of either obeechi or balsa wood, make sure you put the sheeted wings back in the beds with weights on top of them until you're ready to cover the surface. Once the sheeted wing is removed from the vacuum bag, it may change shape as the wood skins absorb moisture from the surrounding air. It becomes critically important to place the wings back in the beds if you use a filler compound that is water based, such as vinyl spackle.
No matter how hard you try, sometimes the trailing edges on a fully sheeted foam core wing just are not straight and true. Believe it or not, waves, twists and sometimes other deformations along the trailing edge can be removed from a fully sheeted foam core wing. Try misting the area of the wing skin that is out of shape and then place the entire wing back in the cores with weights on a flat surface until that area has fully dried. This is a surprisingly easy way to fix some wavy trailing edges on a foam core wing sheeted in balsa or obeechi.
Isopropyl alcohol can be used to thin or "reduce" epoxy glue. Upon the evaporation of the alcohol, the epoxy will attain nearly all its original strength. For clean-up, acetone is typically used as a "solvent" which cuts the epoxy and eliminates its gluing action. Acetone, however, is toxic and has the tendency to build up in your system. So get rid of that acetone. It's toxic, and it just plain stinks. Especially if you're doing any building activities inside the house. In its place, try using plain old table vinegar. Not only is vinegar it non-toxic but you can put in your salad! I know this may be hard to believe but vinegar works better than acetone!
The use of very thin epoxy such as West's 5 to 1 epoxy system when used in vacuum bagging obechi or a 1/32 inch of balsa skin on foam cores can lead to bleed through. If the epoxy bleeds through the skin, the sanding process can be difficult. Also, there are several new fluorescent covering films that will discolor when exposed to CA or epoxy glue. I have read several articles concerning the adding of microbaloons to the epoxy. This results in a thicker mix and no further bleed-through. Also, it leads to significantly weaker epoxy. Instead, try using Colloidal Silica. The Colloidal Silica reacts on a molecular level by aligning the epoxy molecules. Bleed-through is eliminated with no loss in strength.
We often spend hours preparing a fiberglass fuselage prior to painting. The second the paint hits the fiberglass, the fish-eyes begin. One, then two, then four, then six. Pretty soon there are so many fish-eyes we can't wait to throw the fuselage in the garbage. Several of us spent many hours discussing why we couldn't get rid of the cause of the fish-eye. The most accepted theory of the cause of the fish-eye is that the mold release agent remains deep within a pin hole. Try the following: (1) Wipe down the fuselage with alcohol and then with wax remover - usually available at your local ski store or an autobody paint store - to remove any releasing agent. (2) Let the fuselage soak in the tub for a couple of days with dishwashing detergent or other type of soap in an effort to dissolve any remaining mold release. (3) Fill in the obvious pinholes or craters in the fuselage with Red Devil or autobody spackling compound. 4.) Sand the fuselage smooth (down to 4/600 grit). 5.) Paint the fuselage with a light coat of gray or white primer. 6.) Now for the critical part, literally rub the primer ( with your finger )into each and every one of the pinholes that you can see. Keep applying fresh primer until all of the pinholes are sealed. 7.) Then sand the primer with 4/600 grit before final paint. 8.) Then use the final paint of your choice. For those of you that are willing to spend the money, try taking the fuselage to a local autobody paint shop for the best of all possible finishes. Gloss paint is much heavier than flat paint. Always use a primer or base coat ( flat paint ) that is very close to your final coat for the lightest paint job possible. When a primer is used, evenly coat the entire area. Partially primed fuselages ( a common result after the above pin hole filling exercise ) will require additional gloss paint to fully cover color changes in the base. Keep it light. A thin primer or base coat with only the final coat in gloss will result in a much lighter model.
Fiberglass hatch covers are nice, but slip-on/off nose cones are the ultimate. (You can always paint on a fake canopy if you wish.) The issue becomes how to create a joint between the slip-off nose cones and the fuselage which is so tight that you can't even slip a piece of toilet paper between them. Sand the edge of the slip-off nose cone until you have a straight, even edge. Rough up the fuselage in the area of the slip-off nose cone with 100 grit sand paper. Create a special mixture of epoxy, microballoons and 1/32 inch milled fiberglass until you have a mixture with the consistency of peanut butter. You could also try using West System's (Microlite) Faring Compound to achieve a similar consistency. Apply a thick coat of the special mixture to the fuselage in the area of the slip-off nose cone joint. Coat the inside and the outside of the slip-on nose cone in the area of the joint with a water soluble jelly. (It is important to use a water soluble jelly so it can be easily removed with soap and water prior to painting.) Then firmly press the slip-off nose cone into place. Allow the epoxy to nearly cure. Then carefully pull off the slip-off nose cone. Once it is cured, place the nose cone back into place, cut off excess amounts of epoxy with an exacto knife, and then sand the joints smooth. You are going to be amazed at the quality of the joint. Do not paint the fuselage underneath the nose cone in the area of the joint. The paint alone will cause a thickness that will disrupt the joint. Finally, on hard nosed first landings, it's possible to jam the nose cone past the joint. (you may not be able to get it off!) This can easily be prevented. Make a special epoxy mix similar to the one identified above, apply a large glob of it to the inside of the tip of the nose cone. Then coat the end of the internal nose with water soluble jelly. Then, press the nose cone into place and let dry.
We all know how easily and quickly the threads in soft balsawood can be pulled out or striped. Thin CA Glue to the rescue. Try placing a small drop of very thin CA Glue into the area with the damaged threads. (push it down in there with a straight pin) Re-tap the threads or re-set the screw. I have used this method to lock the threads into the wood wing joiner block in my Alycone. I used a stainless steel, one quarter-twenty bolt and the CA threads have lasted for over a year with no apparent damage.
All of the heat shrink coverings have different temperature requirements. Try purchasing the iron thermometer from Black Baron. It will make your life a whole lot easier. One area where we could all use improvement is eliminating wrinkles around those wing tips. First, make sure that your iron is set for the proper temperature for the type of covering material used. Second, make sure the wing is fairly well fixed to the table top with the wing tip hanging over the edge. I use one towel on the table top and four or five beach towels on top. Third, make sure there is plenty of material hanging past the end of the wing. Allow at least four to six inches. Then, as you heat the material you have plenty to hang onto. It's important to physically stretch the material around the wing tip as it's heated. With Many of the newer films such as Ultra Coat or the 21st Century films, you can wrap the material from the top all the way around to the bottom without wrinkles. Remember, heat and stretch, heat and stretch, just a little at a time.
Sand all balsa prior to covering with 600 grit sandpaper. If you're really concerned about wood grain showing through the material, try misting it lightly with water to raise the grain. Let it dry with the grain raised. Then sand it again. Any evidence of grain in the balsa will virtually disappear. Prior to covering, use 600 grit or finer everywhere. Then carefully vacuum everything (the airplane, the balsa both inside and out, the table top, and if you have time, the rest of your shop). Immediately prior to covering the balsa wood, wipe it with tac cloth and then wipe it firmly with the palm of your hand (after you have removed your rings).
This is a nasty subject for many a model builder. However, its actually easier to align a T-Tail than a mid or standard stabilizer. First, align the wing with the fuselage so they are square and the fin (rudder) perpendicular. Now were ready. On a flat surface turn the model upside down. Block the wing (I use two foam blocks of the same size.) and fuselage parallel to the flat work surface. Assemble the T-Tail in place and allow it to rest on the work surface. Tac glue it in place. The T-Tail is now level with the wing. If you need help to align the stab so that it is square with the fuselage, try some graph paper. (Make your own using butcher paper) Place the graph paper on the flat work surface before you turn the model upside down. Align the wing and the fuselage with the graph paper. Then align the T-Tail ( while resting on the work surface ) to also match the graph paper. Tac glue the T-Tail or the brass bearing tube in place. Turn right side up and finish glue.
A sagging T-Tail really stands out. It just looks bad. Try building them Upside down. Most T-Tails I've built have a swept leading and
trailing edge and are foam core. The foam cores are thinner at the tip. When you build the joiner boxes in the core, turn the foam upside down on a flat
surface. Now the top of the stab will be flat from tip to tip. The bottom will have a very slight diehridal. Visually, your T-Tail will never sag again.
I hope you found some of these construction techniques helpful. I want to thank the members of the Seattle Area Soaring Society who have shown me their construction tips over the past few years which contributed greatly to the content of this article. Good Luck! -- by Sherman L. Knight