Floft:

FIGURE 1

Lofting Fuselages The Easy Way When I got my first computer in 1980, one of my maiden projects was to write a lofting program. It has evolved over the past ten years into FLOFT. This article describes what FLOFT does and

By PETER GARRISON

1613 Altivo Way Los Angeles, CA 90026

how it was used to help develop the fuselage of a particular airplane ... namely, John Roncz's DLR.

To loft a body is to define its shape. I believe the term dates from the days when boats were constructed on the main floor of a building, and full-size patterns for their frames were laid out in a spacious loft overhead. The challenge of lofting a boat, or any other streamlined body, is to develop crosssections that blend properly into one another. This is most easily done if the

CHIMERA FS 100.000 1

Scale I - 5.00* 6/17/90

curves used are all members of the same mathematical family. Various geometrical and computational procedures have been used to produce such families of curves; the classical one in the aircraft industry, which was first applied in a thoroughgoing way in the design of the P-51, is the method of second-degree conic curves. Although it has now been partly supplanted by other techniques in the manufacture of large airplanes, this method is still completely appropriate to small airplanes. Among its advantages

are its ease of use and the fact that the curves it generates are always (like the P-51's) pretty to look at. Until the advent of digital computers, loftsmen used straightedges to lay out swarms of lines - based on a geometrical demonstration by the French philosopher Pascal - which eventually yielded strings of intersections lying along the desired curves. With computers it's all much simpler and, of course, faster; you tell the computer a few parameters for a curve, and it immediately spits out either a picture of it, a list of coordinates along it, or a plot

FIGURE 2 SPORT AVIATION 53

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can smooth out the radii graphs, at the surface of the body. It can generate contour maps, vertical, horizontal and same time refining your data set. When you superimpose the corrected shape oblique cross-sections, measurements on the original one, you realize that the of surface area and volume, lines of intersection between this body and any curves that looked fair to you on the first go-around actually left considerable other lofted body, and graphs of the room for improvement. I think of this as radius of curvature of the various the digital equivalent of the patternholdlines along the length of the body. This last feature is a very useful one, maker's stroking and eyeballing process. On the first cut illustrated here, I debecause our first cut at a shape may liberately incorporated the canopy in the not be as geometrically smooth as we body loft for the sake of pointing out a would like. Remember, we simply drew pitfall of that approach. Figure 7 shows the airplane on a piece of paper and a contour map (top view) of the converted it into a computer model. windshield. Note its bluntness, which Each segment is smooth, but how well comes from the fact that it has the same do they blend into one another? Were K factor as the fuselage top ahead of it. the curves we drew really optimal, and Many airplanes are lofted this way, but did we break them into segments in the it's better to loft the canopy as a sepabest possible way? rate body, streamlined in the front and Figure 6 shows the moldline radii for TW, BW, and MB for our first cut. Do blending smoothly into the fuselage at the rear. Figure 8 shows the contour map you see how broken and scalloped the lines are? Scallops indicate long sur- of such a canopy, and Figure 9 shows face waves not yet apparent to the eye; how the radii graphs for the canopy steps indicate sudden discontinuities in merge into those of the fuselage. radius of curvature. Whether these imYou have now seen how FLOFT perfections would have significant aes- works, and you perhaps understand thetic or aerodynamic consequences why it is such a convenience. Part of its we don't know; but John and I both felt value is that it puts all the information intuitively that it is worthwhile to smooth you ever need for tooling and drafting the radius graphs. at your fingertips; in fact, it can produce FLOFT includes an EDIT command coordinate files that can be read by Auwhich enables you to break curve seg- toCAD and incorporated directly into ments into two or combine two into one, engineering drawings. change K factors, and change the locaAnother part of its value is that it gives tions, angles, and points of tangency of you a means of making changes to your frames. With a little experimenting you design by simply changing one or two

numbers in the data file. For example, when it turned out that the spark plugs on John's airplane were outside the cowling contours, reshaping the cowling was just a matter of changing two or three numbers in the first segment data for the Top K Factor. DLR has undergone some 16 revisions (plus several smoothing operations) since the first sketch as various requirements for clearance and aerodynamic fairness became apparent, and I think John would agree that the work was as nearly effortless as it could be. FLOFT runs on IBM PC's and compatibles with at least 256 kilobytes of memory. It can be used entirely in text mode, directing numerical output to a printer or to the screen; but it is much more powerful and more pleasant to use with a graphics display (which can be EGA, VGA, or Hercules, with resolutions from 640x350 to 800x600 pixels). It can drive plotters that understand Hewlett-Packard Graphics Language (HP-GL). FLOFT does not require a math coprocessor, but it must do huge numbers of calculations and so runs much faster with one than without. FLOFT, together with the support programs XING, POLATE, and SEEKK, sample data files and complete (though terse) documentation, costs $185 and can be ordered on 5.15- or 3.5-inch disks. My address is 1613 Altivo Way, Los Angeles, CA 90026 (phone 2137 665-1397).

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