Written by Tim Lampe
Cutting Holes in Fiberglass
As seen in the October 2021 issue of Model Aviation.
Modelers’ Tips and Tricks
In the spirit of Model Aviation’s annual "Build Month," we asked several contributors and columnists for some of their favorite tips and tricks. Tim Lampe, Joe Vermillion, Sal Calvagna, Stan Alexander, Mark Snowden, and Pat Tritle offer advice for building, finishing, and troubleshooting. We hope you’ll find this information helpful!
Carbide cutters, needle files, and homemade sanding sticks with sandpaper glued to them all aid in cutting neat, precise holes in fiberglass.
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Turn the speed all the way up on your rotary tool when using cutters.
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03. Finish to perfection with needle files and sandpaper.
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04. A perfectly cut hole in fiberglass is one way to make your ARF look the best it can.
Many of today’s model aircraft kits include at least one or two fiberglass parts such as wheel pants or a cowling. Some of the aircraft in my favorite category, RC Pylon Racing, are made entirely of fiberglass and other composite materials.
These parts usually require some cutting to open holes/passages or trimming them to fit. The skill of cutting fiberglass has become a requirement for today’s modeler. The images in this article depict cutting a simple, circular hole in the side of a composite fuselage, but the tools and techniques are the same for all kinds of fiberglass parts.
The single, most important tool for cutting fiberglass is a Dremel #569 1/16-inch carbide grout removal bit. This skinny bit cuts precisely with a minimal kerf to keep dust down. If the hole or shape you are going to cut isn’t obvious with molded-in cut lines, it helps to first mark your own guideline with a fine-point, felt-tip pen.
Always wear personal protective equipment. Turn your rotary tool up to the maximum rpm. This prevents the bit from skipping or jumping out of control. Punch through the piece then guide the bit gradually to make an initial cut. Follow up as needed with a Dremel #570 1/8-inch carbide grout removal bit and/or drum sanders, needle files, and sandpaper, as needed, until you get the cut to the desired shape/size and the edges are smooth and even.
Use denatured alcohol to wipe away any residual ink from marking the cutout. After the opening is perfected, I follow up with 400-grit sandpaper to smooth the edges to the touch.
One of the questions that I often get is how to hinge and set up the control surfaces on a model. This relatively simple process can be confusing when we start talking about things such as the hinge line, deflection, differential, etc. I felt that this would be a good chance to discuss the basics to hopefully help you understand and be more successful as you build.
Let’s look at a few of the items and tools that I use to help me along the way to having a great-flying model. I have just a few simple things, some of which will be easy to find. You should keep an eye open for others at swap meets and grab them when you see them. The easily accessible items would include the Du-Bro Hinge Slotter kit, the Robart SDI Gauge, Q-tips, and petroleum jelly.
The harder-to-find items would be the Great Planes Slot Machine and the Great Planes Precision Hinge Marking Tool. These are swap meet finds that you need to keep an eye out for and scoop up when you see them. They are worth their weight in gold for sure!
The first thing you need to do to have a control surface that deflects evenly is to ensure that you have the hinges on the centerline. This is where the marking tool comes in handy. You simply place the tool on the control surface, twist it until both shoes contact both sides, then just run it down the control surface. This will give you a perfectly drawn scoreline down the entire surface. You can achieve this, of course, without the tool, but it makes it simpler and only takes a second to score the line.
Next is the hinge choice. There are a ton of different options out there for hinges in the hobby. There are nylon hinges, pins, CA adhesive hinges, and more. I do not recommend CA hinges for airplanes greater than .40 size. There are a few options that would give you not only longer-lasting hinges, but also something you can rely on season after season. That noted, these tips can be used on any hinge.
One of the things that I was taught when I started in aeromodeling was to use a little bit of petroleum jelly on the hinge line of the hinge that you choose. This will keep any epoxy you use from getting into the hinge line and fouling your hinge. The last thing you want is a frozen hinge when your model is almost ready to take to the sky. It can be a real bear to remove them for repair.
On that note, I recommend epoxy for installing hinges to your models. I stick with 12-30 epoxy to give me a good amount of working time if I need to make some adjustments to the setup.
Now that the hinges are installed, let’s discuss setup. Each model you complete has a recommended throw setup. I suggest trying to stick with what’s recommended by the manufacturer, at least until you have a few flights under your belt. After that, if you need to make a few adjustments to the setup, it’s easy to program.
I like to use the Robart SDI Gauge for this step. It’s easy to use and it gives you an accurate reading on your deflections without a lot of fuss. Use some lowtack tape to secure it to the top of the wing and use the gauge to set your throws. Simple!
The last thing I want to consider is differential. Differential is added into the aileron control surfaces to make for a better-flying model. To explain it, an improperly set-up model can experience what’s known as adverse yaw. This is the model’s tendency to have its nose climb or drop during turns. As we all know, using a generous amount of rudder input during coordinated turns can correct a lot of this, but why not make it a little better with some differential?
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To achieve this, all that is needed is a little bit of setup and you’ll notice a big difference. Set up the control surface so that you have less throw on the bottom than you have on the top. It can be as simple as programming differential in your computer radio or, in the case of a single servo set up on the ailerons, you could use a circular control horn to achieve the same results manually. In most cases, only a little is needed to make a huge difference. Most of the recommended settings can be found in the manuals and/or directly from the manufacturers.
I hope this information helps you to set up your models for fun and exciting flights!
Dealing With Heat-Related Issues
By Sal Calvagna | [email protected]
Scale model building is a time-consuming but rewarding segment of our hobby. There are many facets involved and some issues to overcome. One such issue is dealing with heat. Whether it is turbine or piston powered, wet-fuel powerplants produce lots of heat.
To be clear, this subject is not about how to keep your turbine or engine within a proper heat range. What I’m discussing is how the heat from these engines affects the model. Turbines can produce more than 500° of heat—especially the exhaust. A piston engine can reach roughly 300°. This amount of heat can wreak havoc on fiberglass, plastic, or wood.
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Modelers have been using different products for years to reduce the transfer of heat. One such product is Heat Shield, which is available from Bob Violett Models (BVM). Here’s what the BVM website has to say:
"High temperature exhaust gases from both turbine and piston engines are sometimes unavoidably close to parts of the airframe. Tailpipes and mufflers contain the flame, but the convective heat can be too much for the surrounding balsa, plastic, or fiberglass structures.
"Three brushed-on coats of this water-based, ceramic material, ‘Heat Shield,’ will protect those exposed parts of your model.
"Simply treat the inside of the engine cowling (piston engines) or the inside of the aft fuselage (turbine), and the surface temperature of the model will remain cool. In the unusual case of a turbine tailpipe fire on start up, you will have several seconds longer to extinguish it before any damage to the model occurs.
"Heat Shield comes in an 8-ounce can and is easily applied with a brush and cleans up with soapy water. For extra protection, the thickness can be built up with additional layers of fiberglass cloth and Heat Shield."
Another product that I have found comes from outside the hobby industry. It’s called Reflect-A-Cool and is manufactured by Design Engineering, Inc. (DEI). As you can see in the photo, it is advertised to offer high heat reflectivity to protect temperature-sensitive components and materials against heat damage and hot spots.
It is made of aluminized reflective foil and backed with a base layer of fiberglass cloth, making it able to withstand continuous temperatures of 400° and radiant heat protection up to 2,000°.
It is very thin, super easy to install with a self-adhesive backing, and is available in different sizes.
If you are experiencing heat-related problems, try one or both of these products. They definitely work.
Hinges and Hinging
By Stan Alexander | [email protected]
Sometimes, the smallest parts of a model are the most critical in its performance. Most of us have seen a model airplane at the field, and as it makes a pass in front of everyone, you hear a telltale sound called flutter. Sometimes you don’t even hear the flutter before the control surface fails. It could be the ailerons, flaps, or the death pop of the elevator letting go. All are evidence of a control surface that was improperly installed or a hinge gap that was too large.
There are all types of hinges for scale and sport models or any other kind. Now we have so many choices for modeling materials, including foam, that we can ponder the different types of parts to put into the model.
There are still several parts that manufacturers who produce hinges, hinge guides, and materials use to make hinges. Sig Manufacturing Co., with late designer Mike Gretz, made the first CA hinges, which work well on most 20- to 60-size models. Of course, there are many other types that are used including Robart Hinge Points or others that often replicate the full-scale hinges on flaps, ailerons, elevators, or rudders. They are easy to use and can be sealed in place with epoxy using a little petroleum jelly to keep the center hinge joint free.
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05. These are some of the hinging tools that Stan Alexander uses on different models and a variety of hinges for control surfaces. These can also be adapted for other uses, such as hatches or access doors.
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06. The Aerobroach hinging cutters are shown as they come out of the package. It is the best thing Stan has used to cut through balsa. Handles are purchased separately.
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07. For centering the hinge on this horizontal stabilizer, Stan uses the Du-Bro centering tool, as well as the Du-Bro punch for hinging. The switch to the blade cutter keeps everything straight.
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08. Work the Aerobroach CA blade (the thinnest of the three) back and forth into the balsa, being careful anytime you handle anything sharp! This will give those CA hinges an excellent slot.
CA hinges are probably the most common today. Many sport models, as well as others, use CA hinges on every control surface on the model. The method of application is simple, and I always keep a box of tissues handy to wipe off any excess CA glue from the adjoining flight surface. I also allow these to fully dry then perform a pull test on them to make sure that everything has cured and that the hinge will do its job.
Having the proper hinge-slotting tools is a huge help in making good tight hinges and reducing the hinge gaps on models. These can be purchased from suppliers such as Aerobroach LLC or Du-Bro. I’ve found the ones from Aerobroach LLC to be very accurate and work well.
Start out with the thinnest slotting blade for CA hinges and mark the centerline on the back of the surface into which you plan to cut the slot. Carefully cut the slot into the wood at a f lat angle.
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09. The horizontal stabilizer of this Old School Model Works Kaos is shown with the slots cut and the CA hinges stuck into the slots to make sure that they all fit. Next, cover the back edge of the stabilizer, cut through the MonoKote covering, reinsert the hinges, then apply thin CA to the slots. Keep a tissue handy to wipe up any excess CA glue.
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10. Hinges are added to the vertical stabilizer and rudder. Make sure the slots align and the hinges fit into the slots. Next, pull all of this out, cover the parts, reinsert the hinges, and CA in place.
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11. Du-Bro Electric Flyer Hinge Tape is added to seal the gap for high-speed aerobatics. Over time, it will have to be replaced. You can also use MonoKote-type coverings.
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12. Aileron hinges are installed, checked, and glued in with Zap Thin CA adhesive. Always pull on the hinges to make sure that they are secure after the glue dries.
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13. All of the CA hinges have been added to this Kaos, checked, and secured. Make sure that there is no binding on the control surfaces that will stress the servos.
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14. Robart-style pin hinges are useful in applications such as the flaps on this Top Flite P-40. Always use a test piece of wood to make sure that you drill the correct size hole to give the barbs on the hinge/glue something to bite on.
Hinge Tape and Sealing Hinge Gaps
There are several types of hinge tape available or materials that you can use to seal those hinge gaps. Sometimes it’s built into an ARF, other times someone—maybe not you, but whomever built the model—didn’t do a great job installing the hinges, but those gaps can be sealed shut. Materials such as MonoKote, UltraCote, fabric, or for many electric models, Du-Bro Hinge Tape can be used.
Be sure to move the control surface to its maximum deflection and hold it there before you start adding the hinge tape. This will allow the hinge to work properly for any movement in flight. Apply the hinge tape for the entire length of the control surface, whether it’s for the ailerons or the elevator.
Take your time and always test-fit the CA hinges and others. Make sure your fit and gap are tight and minimized.
Fair skies and tailwinds.
There is a real satisfaction in making your own balsa cowling. For me, that moment came when I heard the sound of the rare-earth magnets clicking the cowling seating firmly against metal plates on the firewall.
It all began when I tried to update a hatch on a 1/5-scale Ford Flivver. There was also the P-51B cowling that all but threw itself together. Probably my hardest decision was trying to fill in the hole on a 25-year-old propeller jet left by a .91 SuperTigre engine. I realized that I needed a completed but very narrow cowling. It was sawing off what became a cowling build that made me realize that balsa builders needed to master their cowling skills.
There are many options when it comes to making a wooden cowling. Plan on it taking roughly 10 hours of bench time. However, you’ll likely use materials that you already have on hand, so technically you’ll save on time and materials.
It is important to have your electric motor on hand to get the measurements and centering correct. I used a Great Planes mount and a RimFire .46 motor.
Use or make your own F1 and F2 formers. Your front F1 former will be suspended by its side rails. You’ll want one of your F2 formers as the firewall. The second replica of the F2 might be balsa and be the rear of the cowling.
Make two balsa supports that will eventually be cut from the formers when you’re finished. Each support should be the exact distance that your cowling is long. Be sure to subtract the width of your F1 and F2 formers. Make a slit halfway down each and insert them into themselves so that they form an "X" that is perpendicular (90°) to each other.
Tack them in place with a small drop of CA just to hold them in place. Make sure that the center of the X is exactly in the middle of the propeller. In other words, the middle of these supports should be the propeller-to-tail line. Again, when finished, these formers need to be cut up and removed through the rear hole, which is big enough to fit in your motor and mounting hardware.
Add balsa sticks to the outsides of F1 to F2. These work best when glued into square recesses in F1 and F2. These will likely be on an angle because F1 is near the propeller and will be smaller than F2 near the firewall.
Plank your cowling, starting on the top and working down both sides. The planks should be at least 1/16-inch thick and probably 3/8- to 1/4-inch wide. These can be cut on a bevel so that they butt against each other. Cut them to fit then fill any gaps with lightweight spackling.
After the formers are covered, destroy the X supports inside of the cowling. Pull them through the larger hole in the F2 former in the rear.
Rare-earth magnets do a great job when they are epoxied to the cowling. On the P-51B, I used six placements: top, bottom, and two spread out on the sides. On my Carl Goldberg Electra glider, I only needed three. One version that I saw online bolted the cowling to the front, cut out a cowling hatch, and used it for the LiPo batteries. With mine going on an electric motor, there shouldn’t be vibration or fuel to worry about.
To save weight, the magnets in the cowling can seat on flat 1/4-inch square pieces of metal. I use metal snapped off from safety razor blades. Epoxy the metal plates to the firewall.
Mount your motor to your firewall then mount your cowling. It should align perfectly. I measured wrong and had to extend my motor on the mount by 3/8 of an inch. It did fine, but I’m glad I had some wiggle room with the motor mount. Make sure the motor comes through the front hole correctly or you’ll have some sanding to do.
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The process gets easier with each cowling that you make. Yes, there are many options that you can purchase, but if you’re a builder, half the fun is making something you can enjoy.
I’ve been a fan of early World War I and Pioneer-era aircraft for too many years to count, and have modeled many of those early designs, from the 1903 Wright Flyer and on. One major challenge in building these types of models is duplicating the natural linen coloring that was so common in the day. Those old airplanes were usually covered with muslin linen, which had a slightly off-white or a light tan tint. Except for the Wright Flyer, the linen was sealed with either shellac or varnish, which, over time, took on a brownish or amber tone.
Where the challenge comes in is getting that look without losing the slight translucence of the overall finish. In the sport-scale realm, tan- or cream-colored iron-on coverings served the purpose pretty well, but duplicating the actual finish for a good scalelike look has been a challenge for many years.
The best solution that I’ve stumbled across was done many years ago when I built the 1903 Wright Flyer, 1914 Ingram-Foster, and the 1910 Barnwell Monoplane. At the time, I was working with white silk covering, and as luck would have it, a friend gave me a jug of 30-year-old nitrate dope that had taken on the color of 90-weight gear lube. There was enough dope to cover the three models with silk and all turned out beautifully, but when the dope was gone, that was the end of that.
From there, I tried several ways to tint the dope, not the least of which was soaking tea bags and coffee grounds in dope thinner, which produced little success. I even thought about running dope thinner through the coffee maker, but I never could get my wife to go along with the idea.
Fast-forwarding to more recent times, Polyspan has become one of my favorite covering materials and, after a bit of experimentation, it looks like it’s going to be a terrific base point for natural linen finishes.
The major challenge with Polyspan is that it’s a synthetic material, so it does not accept dye well. In fact, I tried dying the material using Rit All-Purpose Dye, which ultimately had little effect on the color of the material. The next step was to find a way to tint the dope similar to the color of the aged nitrate from years past.
Tools and Materials
The beauty of this process is that it’s no different than doing a simple, unpainted cover and seal, except that the last three or four coats of dope are done using the dope/Rit Dye mixture. The materials that are required are minimal and include nothing more than nitrate dope and thinner, the appropriate color of Rit All-Purpose Dye, and an applicator brush.
The dye I used to get the results shown here was Rit tan. Another color that I haven’t tried yet is camel, which leans a little more toward the reddish side where the tan is a bit more yellow. I’ll give the camel dye a try at some point to see how it goes, but for the time being, I’m happy with the results using tan. There’s always room for further experimentation!
I discovered two things along the way: When the process is started, only small quantities of dyed dope should be mixed at any given time. I ultimately set up the mix at roughly 80/20 to achieve the right shade. When I mixed it, the dope in the jar looked black, but don’t let that scare you. As you can see from the results, the finish is definitely not black.
After the mixture had been in the jar for three or four days, the dye began to separate and solidify and left strings and globs of slimy residue at the bottom that would not remix with the dope. I poured the remaining mixture through a paint filter to clean it out to finish the samples, but only time will tell if it continues to solidify.
The other thing is that when the dyed dope was applied after the residue had been removed, it had more of a more yellowish tint than when first applied, but after a couple of days, it did darken a bit to a slightly darker, less yellow shade.
Covering and Tinting
The components to be dyed were covered with Polyspan in the typical fashion, using nitrate dope to attach the cover. The reason is because is not only that because that’s the way I’ve done it very successfully for years, but also because I didn’t want to take a chance on some other type of adhesive reacting differently to the dying process.
Again, more experimentation will tell the tale. After the components were covered and the Polyspan shrunk with a hot iron, three coats of clear nitrate dope, thinned to a good brushing consistency, were applied. From there, three additional coats of dyed nitrate were applied.
The finish looked good, but a fourth coat was applied just to see what might happen. In the end, there was not a significant difference from three coats to four, so for the weight conscious among us, three coats of dyed dope look great and are all that’s really needed.
To apply the dyed dope, I used a 1-inch soft bristle brush. The dope was spread on chordwise because that’s typically how the varnish was applied on the full-scale airplanes.
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15. The only items needed to apply the dyed natural linen finish are nitrate dope, nitrate thinner, the appropriate color of Rit All-Purpose Dye, and a soft-bristle brush.
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16. The parts to be dyed were covered with Polyspan using nitrate dope to attach the covering to the frame, followed by three brushed coats of thinned nitrate dope.
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17. The first coat of dyed dope was applied and allowed to dry completely. It will only take two or three additional coats of dyed dope to achieve the desired color and texture.
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18. Pat Tritle also tried dying silkspan using the same techniques. The elevators (bottom) are covered with Polyspan, and the stabilizer (top) is covered with light silkspan. Both are acceptable after three or four coats of dyed dope, although there is a notable difference in the texture of the finished items.
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19. The two rudders in the photo show the difference between the dyed and un-dyed Polyspan. The small jar contains the dyed nitrate dope, which appears to be black, but as the results show, it becomes tan when applied.
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20. The original method used to create a natural linen finish was done using silk and 30-year-old nitrate dope that had darkened to the shade of 90-weight gear lube.
During the application process, you’ll see a few brush strokes and it will look slightly blotchy, but that’s okay; the full-scale airplanes came out the same way. I also did a sample using light silkspan. The results were much the same as Polyspan, although the silkspan is slightly more translucent using the same three coats of clear, plus three or four coats of dyed dope.
Personally, I think the silkspan came out a bit too "yellow." Polyspan has a much more scalelike appearance overall, so that will be my first choice for future projects.
That’s all there is to it. Staining the covering is no different than the typical clear dope application that we’ve been doing for years. The only thing to keep in mind is to cut down on waste and only mix up as much dyed dope as you’ll need to finish the job at hand. And above all, don’t be afraid to experiment with different colors, or even other products to tint the dope. If the color will dissolve in the dope, it should work.
