Designing & Building Precision Scale Composite Models

When someone asks "How do you do plugs and molds?", there is no short answer. (except maybe "Buy a LOT of sandpaper!") That's why so many different pages exist on this site, covering each of the pertinent subjects, or directing you to links that have the information. What follows is the most compact description I could come up with, as a "get you started" page for composite building. If you're new to composites and/or scale building, this is the place to get a feel it. I've re-posted the construction article on the 1/3rd scale Berkut-Long EZ, which you can read here.

Any RC modeler who has built a half dozen kits could probably sit down and build a flyable model from scratch. Many of us who call ourselves "designers" begin that way. But, what you find in that process, if you're paying attention, is that doing some more research on the details can make a very big difference in the quality of your design work, fabrication, and flying. All of us have the goal of building something beautiful, that is lightweight, strong, and flies wonderfully. I hope some of these tips will help you do just that. Whether your design is all balsa. all composites, or a combination, these things are all important.

Here's my 6-step process for designing & building a scale composite model:

1. Research -

This is where I'm the most finicky, since no model will generally ever be better than the research. I get every scrap of information I can, through every source, and (since I don't want to hear about inaccuracies later) I compare everything I get. I then make the decision on which data to trust, based on two things that tie together... First, what scale documentation is the most accurate? Second, what scale documentation do I need for a judge at a contest, that will prove that my model is accurate? I have taken anywhere from a few weeks to an entire year to get all the "stuff" together. (Drawings, specs, photos, details)

There's another issue I take the time to do right, and that's airfoils. The King Air, for example, would not fly as well if I used scale airfoils on the main wing. So, I research this myself, contact "aerodynamic whiz" friends, and determine two things. First, what airfoils will enable the model to fly great? Second, are the changes visible enough to show, to an attentive scale judge? Most of the time, there are proven model-size airfoils that are better than the scale airfoils, and the differences in shape are too small to be noticed by a judge. This task can be tedious, but with a molded airframe, you have to do this math first! You wouldn't want to make plugs and molds, and THEN find out that the thing was a lousy flier. When I start cutting parts, I already know that the model will fly well.

Right from the start, I create a separate folder for the project on my computer, with sub-folders like "Research", "CAD", "Airfoils", "Paint Schemes", and so on. As you go along collecting more and more stuff, it will help keep it easy to search. This year, I've started to take the extra step of testing a flight model in the X-Plane flight simulator. I don't depend on the results, but it can at least give you a general idea of the flying qualities.

2. CAD Model -

I always throw together a basic 3D model right away, to help me visualize how I'll make the plugs, molds, and parts. It also lets me test-fit engines, radio gear, etc. This "thrown together" model has another benefit, which is in getting more documentation. When manufacturers see a nice 3D image of their plane, it helps convince them that you're serious, and sometimes helps to get them involved. As the model becomes accurate, the images can be used for advanced marketing, and the CAD files themselves might be useful in some parts of the production, such as CNC machining. It's worth pointing out that today, it's pretty easy to make a perfectly smooth, beautiful, shiny CAD model. It's another matter entirely, to create the same model you visualized in real life. Buy a lot of sandpaper.

All linkage details, retract operation, equipment access issues, etc., are solved using the CAD model, so there are no surprises later. If a part such as the fuselage is able to split in two, for example, I'll find a way to make the separation lines fit with panel and hatch lines on the actual aircraft. For these giant-scale kits, shipping can be expensive, so I even build a virtual shipping container, and test fit the parts, so we know what size boxes to order for the kits. Another benefit of the CAD model is that it's easily re-scaled to a different size, if necessary.

3. Drawings and Plug Parts -

I take some (isometric camera) front, side, and top views from my CAD program to a local printer, and have actual size prints made. These aren't used directly. I use vellum, and trace the CAD outlines, then do the internal details by hand. If your CAD program will output accurate cross-sections, that's an advantage. Otherwise, you will have to create those yourself.

Having precise outlines on paper lets me test-fit different retracts, for example. From these drawings, I produce a "kit" of parts used to build the plugs. The first part I generally make is the parting board. (I've been using 1/8" Lucite acrylic.) When I cut out the fuselage side view, I use the part I cut out as the actual keel on the fuselage plug. This insures a very tight fit with the plug in the parting board later. Also, the acrylic won't warp over time like wood, so if a project goes "on hold", time won't degrade the parts fit. (I never put a project "on hold" until it's sealed and fiberglassed, for the same reason. I want them stabilized!)

By this time, I've collected the airfoil data I need, and get some foam cores done. I have a Tekoa "Feathercut" machine, and can do foam cores myself, but lately I've started to investigate subcontracting this step, to save me some time. Some kits have simple balsa-sheeted foam cores. The King Air and "Avanti" will be entirely molded, including the wing and tail parts, so the airfoil research was critical.

4. Plug Making -

I've been making mostly the traditional glass layup, female molds, and the only thing that's changed is that now I vacuum-bag everything in the molds. So I make male plugs, and this is the toughest physical step in the process. The parts have to be perfect, and even so, there's a ton of sanding/filling/sanding/filling to do. Obviously, any small nick, ding, or other inaccuracy will show on every part you ever make in a mold, so I'm "crazy" at this point, to get everything right. The plugs have to have a mirror-like, smooth finish. The plugs are then detailed (panel lines, etc.) before making the molds. This reduces the added weight of scale detailing, and saves the builder a ton of work.

5. Mold Making -

This is actually one of the easiest steps. If the plugs are right, and the parting boards fit, it's possible to make the molds for an entire kit in just a few days. Although time-consuming, it's not very demanding. I use West Systems epoxy, mixed with their #423 Graphite Powder, for the mold interior finish. It's smooth, tough, and helps with the later release of the parts.

6. Final "Proof" -

With the molds done, I make the first parts and assemble the first kit. I generally don't spend much time on paint, etc.. I just assemble it, and go flying. I always do the flight tests the same way. First, gentle takeoff and climb to a safe altitude, some gentle turns, and some practice at slow speed, to see how the landing/stall performance is. Then I do high speed, but gentle maneuvers... rolls, big loops, and whatever else the plane is "advertised" to do. After all these are done, I then totally wring the airplane out. With ALL airplanes, including the King Air, I will snap roll, lomcevak, spin, tumble, high-speed dive and pull-up, etc., to feel confident that the model is stronger than it needs to be. Barring any problems with the test flights, that's the end.

Here are some helpful sources of information:

1. Airframe -

The aircraft manufacturer (if they're still in business) is the best place to start. Remember, if you're interested in building a scale kit, then you probably think it's beautiful. So...If you keep telling the manufacturer reps that their aircraft is "the most beautiful thing you've ever seen", and that "you can only do it justice if they'll help you document the details", they will often cooperate. If the manufacturer is no longer in business, then check museums, air shows, fly-ins, and of course books and magazines. The internet is a major resource for me. I usually start with a simple Google search, using both the "on the web" and "images" options, and collect everything I can. There's no such thing as too much documentation, at the beginning. Just remember that although you can download a lot of information and images for your own use, you may not be able to publish it in any way, due to copyright issues.

2. Airfoils -

So far, it's been my experience that the manufacturer can't help much. I first find out what the actual airfoils are, then go look at several sources, to see if there's published data for those airfoils, at model-size Reynolds numbers.

Great sources for aerodynamic information:

Langley Technical Report Server
NACA Technical Report Server
Selig Airfoil Database
Incomplete Guide to Airfoil Useage

I recommend computer-based airfoil and/or template creation software. On my Mac, I use "MacFoil", which can use the entire Selig Airfoil Database. (above) "CompuFoil" seems to be the most popular one for Windows systems, but I haven't personally used it.

3. Math -

I'm a Project Manager, not as an expert on all these things personally. When it comes technical issues I need information about, I go to expert sources. One really great reference is Andy Lennon's book, available through Model Airplane News, ( Air Age Publications), "The Basics of R/C Model Aircraft Design" It's also helpful is to create a library of templates for various "fixed" items, such as engines, servos, wheels, etc., along with a record of their weight. It'll save you headaches later. I recently found some sources on the net that can give you an idea of airfoil performance, such as this site.

4. Composites -

If you haven't built plugs and molds before, almost all the major composite companies have free, downloadable tutorials right on their web site, and also books, videos, technical brochures, and so on. It's my opinion that West Systems makes the best products, and they're also readily available just about everywhere, including Fibreglast Incorporated.

Unless otherwise noted, all content on this site is ©Copyright by Mike James - - All Rights Reserved
NOTE: I am not at liberty to redistribute any of the documentation used to build any of the 3D models on this site.