Building a scale helicopter adds a new dimension to the hobby. Sport helis are strictly held together by screws, which affix aluminum, carbon and plastic components together. As soon as you enter the world of scale, you are confronted by a variety of materials you may not be familiar with … Balsa, Basswood, Fiberglass, Carbon, and Resin castings may all be part of your build.
Tempting as it may be to just apply some CA and kicker to everything—that could be a recipe for disaster! The mounting support in your fuselage for the heli mechanics is a great example.
This assembly of plywood is going to have to carry the helicopter’s fuselage through the air. It needs to be relatively light and massively strong and inflexible to do the job properly. In order to get the structural rigidity needed, we use two different epoxiesand fiberglass in order to do the job right.
TYPES OF EPOXY
Let’s look at epoxies, as they come in various types; mostly different formulations that determine the curing times. From five minute to 24 hour, the formulations generally follow the same rule; the longer the cure, the stronger the bond. The five minute varieties are made for emergency “on the flying field” fixes only. Generally, a wax is introduced into the mix to accelerate the cure time, and this prevents the epoxy from making an integral bond with the material under work. Basically, it is just a glob of hardened goo affixed to your helicopter, and will not stand time, sunlight, or vibration. Here, we primarily use two types of epoxies, West System 24 Hour and Hysol/Aeropoxy. West System is a two-part liquid, we use the 105 resin along with the 205 hardener. These are available in cans and dispensed through a pair of calibrated dispenser pumps. West Systems makes other hardeners, the 207 was chosen for its clarity after cure.
West System epoxy is used to manufacture and repair fiberglass boats that require decades of service life in saltwater and sun; it’s tough stuff indeed. Hysol, a Loctite product, is a two-part thixotropic adhesive with a ceramic base. Thixotropic is a fancy term for “it stays where it is put.” Squirt a glob on the ceiling, and it will not drip (although you will have a heck of a time chiseling it off the ceiling). Hysol is dispensed through an applicator gun, and disposable binary mixers. Binary mixers are those “squiggle tubes” you can pump liquids through to mix them. The name is derived from the action, where baffles split the flow of the stream, and rotate it before it is split again. A 20-divider binary mixer splits the stream 2^20th times (which is over a million times). Cured Hysol is amazingly
strong. You can drill a hole in Hysol, and tap it like metal. It can be cut or shaped, but expect to use diamond or carbide tools!
NOW FOR THE TECHNIQUE
We don’t want to just lay the epoxy on the surface of the joint; this will not give us ultimate strength. What we do is to mix up a batch of West System, which is not very thick to begin with, and thin it with acetone or automotive reducer. Using a paintbrush, we paint the entire plywood assembly, letting the acetone carry the epoxy deep into the wood. A number of coats are required, as the wood will drink up this thinned mixture quickly. The entire assembly gets a final coat of un-thinned West System.
Now we build up fillets where the parts join. One method is to use more West System epoxy, mixed with a filler to make it thicker. Typically, a filler like West System 404 high density filler is used. West System has a number of fillers with a variety of densities, and ease of finish available. Lately, we have been building the fillets with Hysol. The applicator makes it very simple to apply, its thixotropic properties prevent droop and sag, and the strength is astounding. After the fillets are laid down, I generally smooth the entire assembly over with a brush. Affixing the assembly to the fuselage is a bit more of the same, after preparing the interior of the fuselage with 80 grit and wiping it down with reducer (to remove any trace of mold release compound), you first lay down a layer of West System. Using small strips of this Fiberglass (I use multiple layers of .6 oz. fiberglass) you affix the mount to the bottom of the fuselage. Remember that the fiberglass needs to be completely wet with the epoxy in order to generate its strength. You do not want dry spots or bubbles, this will compromise the integrity.
If you are a fanatic like me, you can also lay down a small fillet of Hysol prior to the fiberglassing. One benefit to using 24 hour mixtures is that you have plenty of time to work. When mixing epoxies, remember that they need to mix on the molecular level. Incomplete mixing reduces the strength of the final bond. While you generally do not want to whip air into the mixture, unless you are going to brush the epoxy on, you do want to mix the epoxy as thoroughly as you can. You would be surprised to note how much faster the epoxy cures after thorough mixing. I have seen 24 hour blends thicken as fast as 90 minutes after a 10-minute mixing session.
Cyanoacrylate (or CA) adhesives have their place as well. When joining wood, the ability of CA to wick into the wood and create a bond is astonishing. When joining wood, make sure that you have a strong physical joint before applying. Do not attempt to use the CA as a bridge to fill a gap. CA works best when you have enough surface area in the components to provide you with a good
bond. CA works best from the Inside out. One trick which allows CA to bridge a gap is to fill a gap or hole with a filler compound (some people even use baking soda), then apply thin CA until all of the filler and the components to be joined are wet with CA. Be careful, CA is violently exothermic (it releases heat) when it kicks off, and people have been burned by the reaction – seeing smoke is not uncommon.
Whatever adhesive you use, surface preparation is important. Any grease, lubricants, or anything with an oily base can prevent the best adhesive from performing the way it was intended to. Wood is very forgiving, as the adhesive (CA or 24 hour) soaks into the grain of the wood. Carbon, and fiberglass are less forgiving, and should be cleaned thoroughly. Just imagine that the part you need to glue has been dropped on the floor of a public Men’s room, and you have to lick it. Enough said about cleaning. Your mixing materials should be clean as well, EZ-Mix sells boxes of disposable mixing bowls, and a box of 500 tongue depressors will last a very long time. Acid brushes are also available in 100’s.
WORKING WITH FILLERS
When working with fiberglass fuselages, body filler is often needed. Bondo is the first name that comes to mind, but Bondo is rather thick, and does not lend itself to the much smaller scale helicopter needs as well as Evercoat Metal Glaze. Evercoat works just like Bondo, except that it is a bit thinner, and has the same characteristic smell we all love so much. Neither one is a structural adhesive. You must have a strong, inflexible structure to use these body fillers on, or they will crack like Humpty-Dumpty’s head. Their purpose is to smooth out the gaps where body panels or components meet.
SPECIAL PURPOSE ADHESIVES
Finally, there are some special-purpose adhesives engineered for unique uses. Epoxy putties, which generally come in a Twinkie-like roll and when kneaded prior to use can come in handy to build stand-offs or to create really large fillets where needed. Adhesives like Shoe-Goo are often used when vibration isolation is important, or to secure items like those capacitors sticking out of your ESC, or to secure any additional lead weights needed in establishing your CG. J-B Weld has an amazing strength (they have used it to repair engine blocks), but it is tedious to mix. J-B Weld is also rather thixotropic in nature, and will stay where it is put unless you build up too much of it at one time.
When gluing styrene, the solvent- based glue for styrene actually melts the
plastic until it flashes off, allowing the modeler to create complex shapes that are really one piece when finished, although it is not well suited for long butt-welds. Finally, if you do need a fast epoxy for quick repairs, try Qpoxy from BVM Jets. It has a very high strength and level of adhesion for a quick-set epoxy, setting up in 5-10 minutes with a strength that exceeds many four hour formulations.
There is no doubt that taking on a scale heli project can be challenging, but using the right tools and products can make the process go much smoother with beautiful, long-lasting results. As you can tell from the materials that I described in this article there are many more options than your typical five minute epoxy and hobby CA. Do your research and you will be much happier with the end results.