CAD drawing: It starts with a 3-view

Model Airplane News, Dec 1997 by Ryan, Jim

LAST TIME, we talked about using CAD as a tool for designing scale models, and we addressed the problem of getting the paper 3-view into your CAD environment as a starting point for the actual construction plans. We showed how a flatbed scanner can create an image that can then be imported into your CAD program, and we discussed the minor problems with that approach.

This month, we'll look at a second method for achieving the same end, this one requiring nothing but a ruler and a set of calipers.

DIRECT MEASUREMENT METHOD

The second way of importing the 3view requires only the most basic of tools, and in all honesty, it's my favorite of the three for anything but the most complex projects. All that's needed is a photocopy of the 3-view, a pencil, a ruler and a set of calipers. Some triangles, circle and ellipse templates and other basic drafting tools are also helpful, as we'll see later on. A comment on the calipers: I own both dial and digital calipers, and the digital calipers are a lot easier to use. Not only is the digital display easier to interpret, but the calipers can be zeroed on any setting and will then measure the difference between two measurements. This can save time and reduce the chance of mistakes.

Let's get to work: First, draw a datum line through the side and top views of your 3-view. I generally draw this line on the engine thrust line, but since it's only a reference, use whatever is convenient. Obviously, the datum for the top view should be the centerline of the fuselage, unless perhaps you're building a Blohm und Voss BV-141. Next, draw a series of station lines perpendicular to the datum. Set the spacing close enough to capture the contour of the fuselage; the more station points, the more accurate your 3-view will be, but the more time it will take to produce. I frequently vary the spacing; closely spaced near the nose, where the fuselage is more sharply contoured, and farther apart near the tail, where the contours are very gradual. Draw an additional set of station lines on the top view for the wing and stabilizer. For a straight taper wing, you only need one at the leading and trailing edges of the root and tip. For an elliptical wing or stab, you'll need to draw complete datum and station lines just as you did for the fuselage.

Now for the fun part: I like to start with the top view; it's usually the easiest, especially since you can draw half of it and then use the "Mirror" command to make the other side. Measure the length of the datum line on the 3-view from the spinner backplate (or another convenient reference) to the tail, and generate a line that length on your CAD screen. Next, use either the "Offset" or "Copy" command to duplicate the spacing of the station lines you drew on the 3-view. This is the basic framework for your drawing. Incidentally, I put the datum and station lines on a layer by themselves so that I can turn them on and off when necessary and delete them when I've finished with them.

With the construction framework completed, I switch to a new layer to begin the actual 3-view. Starting at the first station line, use the digital calipers to measure the distance from the datum line to the edge of the fuselage (see Figure X). Draw a line this length in the corresponding location on your CAD screen. Repeat this process at each of the station lines all the way back to the tail, and then turn off the layer that has the construction lines to get them out of the way. Now, draw a polyline (this is an entity made up of multiple line segments that are edited together) that connects the endpoints of all the station lines. This line will have a faceted or jagged appearance because it's made up of short, straight, line segments instead of being a single smooth contour. We'll fix that in a moment. Use the "Polyline Edit" menu to edit the line into a "Spline" or "Fit" curve. Now you should have the smooth gradual curve you wanted. The curve should pass through, or very close to, the endpoint of each station line, but if it doesn't, you can edit the line or simply redo it to correct the fit. Since I usually use this method on sport-scale models, I don't worry about getting a perfect match.

Follow the same steps to draw the wing and stabilizer. Now, you'll recall that I suggested gathering a set of circle and ellipse templates, and this is one place they come in handy. Features like the wing and stab tips, cowl, etc., tend to be made up of circular or elliptical arc segments, and it's much easier to simply match the contour to one of the templates, use the "Circle" or "Ellipse" commands to create a matching object on your screen and then trim off the excess, than it is to try to match sharp contours like this with an edited polyline. With the overall outline complete, fill in details like the control surfaces, landing gear, cockpit and any other important features. Once you have half the top view completed, just use the "Mirror" command to make the matching half.

The side view is handled in much the same way as the top view. I find it easier to treat the top and bottom edges of the side view as two separate polylines, as this makes them easier to edit and adjust. Depending on its shape, the vertical fin can either be copied with edited polylines or with circular and elliptical arc segments.