How To/Model with CAD
WHAT YOU NEED TO KNOW TO DRAW DIGITAL PLANS
What you need to know to draw digital plans
Computer-aided design (CAD) continues to increase in popularity, and over the years we have received plenty of reader mail asking about what’s involved in its use for designing RC models. Die-cut kits are all but extinct now, and many RC airplane kit manufacturers are now using CAD to produce construction plans and kit parts. Laser cutters, CNC routers and milling machines, and even high-pressure water cutters (for metal parts) start with CAD programs to produce their tool path files. Here are some of the basics for developing a set of model plans using a CAD program.
WHERE TO START
First of all, you can’t just buy a CAD program and expect to start drawing plans overnight. Check out the various CAD programs available online, and pick one that matches your budget and computer requirements. There are many programs to choose from, and there are even some free downloadable CAD programs you can play with to get the hang of digital drawing. Once you’ve installed it on your PC, explore the features it has and get familiar with it to the point where you can draw basic shapes and line types. From there, you’ll learn how to develop various geometric shapes. Most programs today come with detailed manuals and helpful tutorials for the basic functions of the program. If you really want to get into CAD, you can also go to a local technical high school or community college and enroll in a basic CAD class or two to speed your learning process. Another great place to check out the use of CAD programs is on YouTube, where companies like Ashlar-Vellum and AutoCAD have uploaded tutorial videos to help you learn their various programs and functions.
After you’ve mastered the basics, you can then start developing your own model airplane plans. For this, there are two directions you can take:
You can design your own models and develop unique sport flier planes, or you can develop and enlarge scale 3-view drawings and reverse-engineer them into workable model plans.
BE A COPYCAT
Before you start using the CAD program, learn about basic airplane construction by first studying other people’s plans. See how they laid their plans out and how they solved basic engineering and structural issues. This will help you understand the proper wood sizes to use for specific purposes. Also, notice the placement and spacing used between fuselage formers and wing ribs and other such structures. Refer to drafting and drawing books so that you can develop a good sense of how things should look when drawn in top, side, and front views. A working knowledge of drafting is a basic requirement for using CAD, so don’t put your cart before the horse. Basic Drafting: A Manual for Beginning Drafters by Leland Scott (available from Amazon) is an excellent reference.
Next, find a good-quality, detailed 3-view drawing and scan it into a file that can be imported into your CAD program. Try to find drawings that also have a few crosssections and airfoils shown. With PC-based programs, this would be a bitmap (.bmp) format; for Mac users, a .pict file will do. You could use other formats, but these are the most popular. With the new Graphite platform, you can import several file formats, including text files (.txt), EPS, Metafiles, PNG, Claris CAD, .JPG, DWG (AutoCad), DXF, and IGES files. After your image is imported into your drawing file, you can begin tracing it with the various drawing tools your program makes available to you. And here is where many wannabe CAD users run into difficulty.
When I first started drawing plans, I began with simple airplane designs. Old WW I biplanes and home-builts have fewer curved lines and are relatively less complicated to draw than more modern aircraft. Start with the simple airplanes like the Curtiss Jenny, Fokker Eindecker, or Pietenpol Air Camper, then work up to curvier designs like Spitfires, Mustangs, and Warhawks. That being said, here are some basic rules to get the most out of your CAD program:
USE LAYERS. Import your image and assign it to a specific layer. With my Graphite program, I assign the image to Layer 1 and I rename it “3-view.” Place all the other drawing and details you add on their own separate layers. Think of layers as sheets of tracing paper laid on top of your imported 3-view. I typically trace the 3-view and then place structure (formers, ribs, and outlines
of other parts) on a separate layer named “Plan.” Details such as the engine, servos, and other radio gear go on the “Hardware” layer, and so on. I even add my dimension lines, arrows, and information on their own layers. In this way, you can look at specific items by turning layers on or off, or you can look at everything all at once. This helps keeps things straight in your head.
USE REFERENCES. Start all plans by drawing a datum line, centerline, or other reference line on the fuselage side and top views. From this, you can ensure that things like ribs and formers are drawn square with (or parallel to) one another. Vertical and horizontal reference lines also are important when you develop fuselage cross-sections and former shapes.
THINK SYMMETRICALLY. When it comes to things like wings and fuselages (in top view), draw only one half and then copy and paste a mirror image of it to complete the drawing. Do all your work on one side of the centerline, then duplicate it and flip it over to produce the other half. This ensures exact symmetry and cuts your drawing efforts in half!
SAVE DETAILS. One clear benefit of CAD is that you won’t ever need to draw anything twice. Once you draw something like an engine, electric motor, servo, receiver, or control horn, save it to a master “Hardware” file. After you have saved them, you can then copy and paste them into new plan drawings. You can also enlarge, shrink, or modify them to make new master details. (Drawing them in top, front, and side views also is a good way to hone your overall drawing skills.) USE THE TOOLS. The Tool palettes included with all CAD programs are a collection of several useful drawing tools and functions. It is always easier to use these tools than trying to draw freehand over your imported 3-view drawing. Geometric drawing tools for circles, squares, ellipses, and arcs are all easy to use, and using them will make your drawings look cleaner and more precise. Wingtips, engine cowls, and other parts are easily reproduced by combining segments of ellipses, curves, and straight lines.
WORK INWARD. After you’ve drawn your reference and centerlines, draw the outline of a wing half top view, fuselage side view, tail surface top views, and half of the fuselage top view. From here, you then establish the locations of the main formers, doublers, landing-gear mounts, wing spars, ribs, and so on.
AIRFOILS. Investigate the many sources of downloadable airfoil plots, or consider getting an airfoil generator program. These will save you hours of tedious airfoil layout and lofting work. Above all else, remember this is a hobby, so using CAD should be— above all else—fun!
DRAW FULL SCALE. Once you have your 3-view traced and cleaned up, enlarge the drawing to the size airplane you want to build before you add any internal structures or details. This way, when you draw an 1/8-inch-thick
former or 3/32-inch ribs with 1/2 x 1/4-inch spar notches, they will be the correct size. If you were to add all the details and then size the drawing for a specific wingspan, your details may not end up as common sizes.
Learning to use CAD for basic modeling is not all that difficult, and many experienced modelers have added this skill to their modeling résumés. For many, drawing plans with CAD can turn into another major part of the hobby. Before you know it, you’ll have a whole collection of CAD 3-views and drawings to show your friends. The hard part will be picking which ones you want to build and fly!
This is my newest CAD model: the 1/4-scale Sopwith Camel, which was published in the December 2015 issue of MAN.
These 3-view drawings were the basis of my plans. They are Wylam drawings that I cleaned up to eliminate unwanted details.
TYPICAL RC HARDWARE I think the best part of drawing with CAD is that it eliminates the need to draw anything twice. Once you’ve saved your files, you never have to draw those items again! Here are just a few RC hardware details I’ve drawn that I use over and over.
For some airplanes, like this Pietenpol Air Camper, you may not be able to find a good set of 3-views. This plane was based on one from years ago at the Old Rhinebeck Aerodrome. To produce my own 3-views, I measured the actual airplane and took a lot of photos.
This is an early version of sheet 1 of my Sopwith Camel plans (X1215A). As you can see, it includes several hardware details (in blue) that I had drawn earlier and simply copied and pasted into the drawing.
This scale 3-view of the Pietenpol Air Camper was the very first CAD project I drew, in 1998. I enlarged these drawings to 1/3 scale (116-inch span) and then developed the plans I used to build the model.