(Off with the Old, on With the New)

Recovering Techniques for ARFs or Repairs

A shorter version of this article was originally published in Model Aviation magazine. Model Aviation is the monthly 180-page plus, full-color magazine published by the Academy of Model Aviation (AMA) for its members. As this is a very important How-To article, Sport Aviation is publishing this expanded version. The knowledge and techniques presented here will help both new and experienced pilots.

The Sport Aviator title implies that the techniques can only be use to refurbish an old model. They do work well for that purpose. But the same techniques can be used to personalize even a brand new ARF airframe as well as repairing a damaged airplane.

Even experienced pilots sometimes suffer from the dreaded “Dumb Thumb” syndrome and damage is the result. Imagine you’re out at the flying field at 10 AM on a bluebird-fine Saturday morning. Clear skies, no wind, and the happy buzz of motors fill the sky. Sipping hot coffee from a Thermos, your friend, Bill, looks on approvingly as you make “touch ‘n goes”  across the runway with your favorite sport airplane.

But, the fates are fickle today. On this “final” pass (Ed. Note: It is always that” final pass” that gets you.), your right wing tip stalls, digs in and your pretty little ARF cartwheels down the runway like a Rumanian gymnast. When it finally comes to rest, the structure is somewhat intact but the covering looks a lot worse for wear from its recent brush with the blacktop. As you bring the pieces of the model back to the bench, Bill asks, “So, what you gonna do now?”

Your answer may come down to a decision between your wallet and your willingness to learn new skills. You could go out and buy a replacement ARF; that’s your wallet speaking. Or, if willingness whispers in your ear, you could learn to recover your model.  

Recovering a model doesn’t always involve a crash. Wear, tear and time can be stimulations for recovering an airplane. The sharp metal molding on a car or the wooden corner of a work table can produce a dent here or a puncture there. That, plus the normal wear and handling from flying can necessitate a recovering job. Another reason for recovering a model is less about the flying qualities and more about not wanting your airplane to look like all the other ARFs at the field. Re-covering can let you express your individuality, resulting in a compliment like “I didn’t know that airplane came in that color scheme.”

Photo 1

Looks like a lot of tools are required. If you are an experienced modeler, you have these already. If not, even RTF assemblers need most of these items, especially the heat irons and heat gun used to tighten RTF and ARF coverings.

Of course, if this is you very first covering project, you might want to try borrowing some of the more expensive items. Borrowing a few tools can be good option: Ben Franklin said “neither a borrower nor a lender be”. But while we know that Ben flew kites, the odds are zero that he covered them with iron-on plastic coverings.

 Borrowing some tools and time from a flying buddy who knows model covering can be excellent “tools” to have in your shop. Covering is not hard to learn, but having a friend who knows how to do it can significantly reduce your learning curve. As for lending, on my first covering job I hardly knew what a heat gun was until my friend, Don, came over and lent me his. With a few passes from what I took to be an overgrown hair-dryer, Don tightened up my plastic covering, pulling out sags and dimples, and making my airplane look a lot more like a knowledgeable builder had covered it. So, the moral is, check out whether a friend or your club can loan you tools, especially more expensive ones like a covering iron, trim iron or heat gun. (Ed Note: Don’t ever, not ever, use a covering heat gun as a hair dryer. A model covering heat gun blows out air at temperatures above 450 degrees F. YES, above 450 degrees F. That will not only dry your hair, it will burn it off. Your scalp will melt, your brain can boil and you will definitely be hospitalized for some time. Then, who will be recovering your airplane?)

Table 1 explains tool usage and which ones are required and which are optional. In addition, make sure you have a large, clear work surface with good lighting, ideally from two sides. A large towel, laid under the model covering you are working on, helps prevent scratching of the film.

Table 1: Project Tools

If your project is a repair, evaluating damage determines the “recover or buy decision”: Your first step in recovering is to evaluate the pros and cons of recovering your airplane vs. buying a new one. To assess the condition of your model, disassemble it, removing the landing gear, motor, radio gear, and pushrods, followed by a visual inspection of the airframe. The structural damage on the Tiger 400 used in this exercise was minimal and repairable, so recovering rather than buying a new ARF was the better choice. Besides, when completed, the Tiger 400 will look different and better than new.

For structural repair techniques, read the Sport Aviator Article” ARF Wing Repairs” in the Flight-Tech Section. The same general repair principals can be used on a damaged fuselage as well. Remember, everything must be kept straight and aligned once repaired. If the damage is such that proper alignment is not possible, it is not truly repairable.

Photo 2

Removing All of the Old covering is Essential: Once the re-covering decision has been made, it’s time to remove all the old covering unless you plan on retaining the same color scheme or just recovering a broken or punctured part, such as a wing.

If you only cover one part, do some research to find out what brand of covering the model originally used. Matching the same brand will facilitate matching the original film color. On the Tiger 400, the kit instructions state clearly that the airplane was originally covered with Cub Yellow and White Oracover film.

Photo 3

To remove the old covering, first warm the old covering with heat gun to loosen the adhesive. Wear cotton gloves to protect your hands; at temperatures of up to 450°F, a heat gun can burn an uncovered hand. As you pull off the covering, go slowly and pull at an angle to minimize residual glue problems. A covering iron can also be used to loosen the adhesive if you do not own a heat gun.  

Photo 4

Once you have the covering off, remove any residual adhesive or stuck bits of covering with Acetone or a covering thinner like Coverite Ironex. Acetone and the toluene in Ironex are toxic, flammable, and can be absorbed through the skin so wear rubber gloves and have adequate ventilation.

Working outside on a folding work bench covered in plastic is a safer way to use these chemicals. A spot remover like K2R (if you can find it) can be used to remove and absorb fuel stains on a glow model. (Ed Note: Since K2R is extremely hard to find today, there is an even better way to remove oil from wood. That good ole` heat gun works wonders on oil-soaked wood. Keep the gloves on, hit the wood with full heat; close up. The oil will bubble out of the wood. Wipe away the oil that surfaces with a paper towel. Repeat the process until the wood is dry enough to cover. I have used this method for 20 years and some early airplanes recovered like this are still flying and the covering is still adhering well.)

As in painting or wallcovering, good surface preparation and surface damage repair makes the final covering appear professional: Now that the covering is gone, re-inspect the model for cracked ribs, broken spars, cracked sheet surfaces, loose engine and landing gear mountings. If the airplane needs repairs or additional re-gluing, now is the time to do it.  

Once the repairs are made, fill nicks and dings using light or vinyl spackle and a palette knife. Lightly sand the surface with 240-320 grit sandpaper. When you are done sanding, wipe it off with a tack rag, running your finger over the surface and holding it up to a light at an angle to a light to check for imperfections. If you can feel or see any imperfections now, you’ll see them later when it’s covered. Covering does not hide poor workmanship under it.

Photo 5

Careful pre-assembly saves time later: Pre-assembling the model, before recovering, ensures all surfaces fit together and all hardware is in place. During the trial assembly, be sure to check the following items if they are on your model:

Ø  Dowels: Remove these before covering if possible. Fit them, unglued, during the pre-assembly. If the dowels, usually rubber band hold-downs or forward wing mounts for bolt-on wings, can not be removed, it is easy to cover around them. Do not damage the airframe just to remove dowels.

Ø  Hinge slots: Cut slots and fit hinges in the wing, stabilizer, and control surfaces prior to covering. Make sure control surfaces flex easily. If using Mylar type hinges, read the Sport Aviator article “Installing Mylar Hinges” in the Flight-Tech Section. If the hinge control surfaces are still attached using Mylar type hinges, just cut the hinges with a sharp hobby razor knife. Remount with new hinges located adjacent to the ones cut.

Ø  Control Horns: Fit these prior to covering. I sometimes inset 1/32 or 1/16 plywood behind a nylon control horn on a sheeted surface to keep the control horn from flexing or worse, the screws pulling out. (Ed Note: a neat trick is to slice a centered hinge groove under the control horn and insert a Mylar hinge all the way inside. Use thin CAA to lock it in place. This strengthens the control horn mount as much as do plywood inserts but is faster.)

Ø  Pushrods and pushrod exit covers: Fit these before covering, making sure the pushrods line up with the control horns.

Ø  Structural improvements: On an ARF, check to make sure there are enough internal braces for the control push rods in the rear of the fuselage to keep the cable from flexing and causing control inconsistencies. If you want to change from a wing held in place using rubber bands, now is the time. For details read the Sport Aviator article “Not Your Stock Hobbistar Part One” in the Pri-Fly Section.

Ø  Air vents: On open surfaces, like wings and some stabs, drill small air vent holes in the ribs if there are none. (Ed Note: An Excellent Point. This allows the heated air under the heat gun to escape to a cooler area so the covering over the heated area can “lay down” properly.)

When you are done with the pre-assembly, disassemble the model to begin covering.