A popular topic that keeps popping up in the old mail bag is Computer-Aided Design (CAD) and its use in designing big models. Today, many kits have been developed using CAD, and these files are then used to produce kit parts. Laser cutters, CNC-driven routers, milling machines and even high-pressure water cutters (for metal parts) can be driven using CAD programs. For this column, I thought it would be fun to go through some of the basics for developing a set of model plans using a CAD program!

First, 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. Once you've installed it on your PC, play with it until you can draw all the basic shapes and line types, and from there, you'll learn how to develop various geometric shapes. Several programs come with helpful tutorials on the basic functions of the program. If you really want to get into CAD, you can take classes to speed the learning process. Once you've mastered the basics, you can start developing your own model airplane plans.

There are two directions to take: you can design your own models and develop unique sport planes, or you can develop scale 3-view drawings and reverse-engineer them into workable model plans. The latter is what I discuss in this column, and here are some words of advice: before you start using a CAD program, study other people's plans to see how they laid them out and how they solved basic engineering and structural issues. This will help you understand the proper size of wood to use for specific purposes, the proper spacing of formers and ribs, and other such topics. Refer to drafting and drawing books, too, so 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 the cart before the horse.

Find good-quality, detailed, 3-view drawings and scan them into a file that can be imported into your CAD program. Try to find a drawing with a few cross-sections shown. With PC-based programs, this would be a bit map (.bmp) format, and for you Mac users, a .pict file will do. You could use other formats, but these are the most popular. With the image imported into your drawing file, you can begin tracing it with the various drawing tools at your disposal. And it is here where many wannabe CADsters run into difficulty. When I started drawing plans, I began with simple airplane designs. Old WW I biplanes and homebuilts have fewer curved lines and are relatively less complicated to draw than more modern aircraft. Start simply, and work up to the curvier designs.

Here are my rules for drawing with CAD:

* USE LAYERS. Import your image and assign it to a specific layer. With my umwingBoard 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. I typically trace the 3-view and then place the structure (formers, ribs and outlines of other parts) on a separate layer named "Plan." Details such as the engine, servos and such go on a "Hardware" layer and so on. In this way, you can look at specific items, or you can look at them all at the same time. Using layers really helps keep things straight in your head!

* USE REFERENCES. Start all plans with a centerline or a reference line. From this, you can ensure that things like ribs or formers are drawn square with or parallel to one another. Vertical and horizontal reference lines also are important when you develop fuselage 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 to it to complete the drawing. Do all the work on one side of the centerline, and then duplicate it and flip it over to produce the other half. This ensures exact symmetry and cuts the work in half!

* SAVE DETAILS. Never draw anything twice! Draw things like engines, electric motors, servos, receivers, control horns, etc. once, and then save them into a master file. Drawing these things in top, front and side views is also a great way to hone your drawing skills. After you have saved them, you can copy and paste them into new drawings. You can also enlarge or shrink them to make new master details.

* USE THE TOOLS. The palettes included with all CAD programs have many useful drawing tools and functions. It is always easier to use these tools than to draw freehand over your imported 3-view drawing. Geometric drawing tools for circles, squares, ellipses and arcs are all easy to use, and they'll make your drawings cleaner and more precise. Wingtips, engine cowls and other parts are easily reproduced by combining segments of ellipses 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 surfaces top views and half of the fuselage top view. From here, you then establish the locations of main formers, doublers, landing-gear mounts, wing spars, ribs and so on.