Turn 3-view into plans

Model Airplane News, Jun 1998 by Ryan, Jim

OVER THE COURSE of the last few columns, we've discussed several practical approaches to importing a scale 3-view into the CAD environment as a basis for an original scale design, and we've looked at some general tips for reducing the structural weight of a model through careful selection of materials and stock thicknesses. Now, we're ready to put all this to use by turning the 3-view into a set of construction plans.

In this column, I have tried to avoid recommending one brand of software over another. There are many good CAD programs on the market, and which one you select is largely a matter of your computer platform, budget and personal preference. But there are a few features that are so important for using CAD to design a model that I would classify them as essential.

DRAWING LAYERS

Drawing layers makes it possible to keep all the information associated with a particular design in a single file without the plans becoming hopelessly confused. In practice, each layer is assigned a different color so it's obvious which entities are on which layer. Layers are a lot like drawing the plans on clear Mylar film and then stacking the sheets of film together. You can turn individual layers on or off just as you might remove a sheet from the stack, and the current drawing layer can be thought of as the top sheet. This is a great capability because the hardware can all be drawn on a single layer to make sure it fits inside the airframe, but then that layer can be turned off while you're printing formers so that the templates aren't cluttered with unnecessary detail. By the same token, dimensions and construction notes can be placed on a layer so they, too, can be turned off when not needed. You might draw a dense grid of construction guidelines on a layer and then turn that layer on or off, depending on whether you need it. The uses of layers are really only limited by your imagination.

THE OFFSET COMMAND

One of my favorite AutoCAD commands is called "Offset." This handy command allows you to produce a second line of any contour a given distance from an existing line. I use this command for all sorts of things, like setting the station points on the drawing, but it really shines when it comes to drawing formers and other similar tasks. You see, when you produced the 3-view, you drew fuselage sections, and as long as they're in appropriate locations, those sections make a natural basis for the actual formers. If your CAD system supports the "Offset" command (or something equivalent), all you need to do is offset the polyline that makes up the cross-section to allow for the thickness of your fuselage sheeting and add cutouts for any stringers or other reinforcement. Your former is complete (Figure 5). Isn't that easy?

Once you've drawn the formers, you can also offset the outline on the top and side views to allow for the skin thickness. As you get used to it, you'll see why "Offset" is my favorite command for filling in the internal structure in a CAD 3-view.

BLOCK INSERT

OR COPY VECTOR COMMANDS

Another important command is variously described as "Block insert," "Copy/paste vectors," or simply, "Copy." This family of commands makes it possible to import a drawing entity from one file to another. One way this capability is useful is for arranging the hardware like the servos, engine and fuel tank within the airframe. Rather than laboriously drawing these components time after time, you can draw them once and store them in an archival drawing file called "Hardware" (Figure 2) and then import them into your new design as needed (Figure 3). My "hardware" file is a monster. It has grown to contain multiple views of every servo, receiver, motor and cell that I use in my models, along with generic servo mounts, templates for common foil sections and hardware such as hinges, control horns and clevises. Because the blocks or entities can be easily dragged and rotated this way and that, a few minutes of work now can save hours of headscratching at your workbench later; you'll know where all the hardware fits before you cut the first piece of wood.

PUTTING IT ALL TOGETHER

Once I have the completed 3-view, I begin drawing the actual plans by drawing the wing ribs and formers (Figures 4 and 5). I generally import the foil sections from my airfoil plotting program as DXF files. A good foil plotting program will plot all the individual ribs for a built-up wing, and some even allow you to insert the slots for the spars, leading edges, etc. Note that when you get to designing things like retract mounts, CAD allows you to rotate the strut and wheel through their range of motion and look for signs of interference. Even when I don't need to, I do this because it looks so neat (Figure 6).

Drawing the stringer slots on the formers can be labor-intensive but it's a lot easier than doing it with a pencil. Note that you can save the cross-section of a stringer as a block and insert it wherever you need it. For a circular fuse with stringers all the way around it, the "Array" command will draw evenly spaced stringers, all at the proper rotation, clear around the former (Figure 7).