by Pete Hatchett
It doesn’t always compute, Scotty.
In the late 60’s early 70’s the world’s major computer manufacturers had a favourite mantra "Computers will release mankind from drudgery and free his mind to concentrate on creative endeavours". Tell this to someone struggling with the latest incarnation of Seattle’s finest! There is a tendency to become slaves to "the PC", it exists in it’s own right and we spend hours sating its appetite for our time. This is bad enough at work where at least we get paid for our efforts, but when it takes a hold at home, well! Computers often become the "raison d’etre" in organisations. Have you ever wondered why the highest paid jobs in engineering companies are often the IT support team? To avoid this syndrome at home careful selection of what we use our PCs for is required.
By now you are probably asking, "what’s all this got to do with model gliders"? So I’ll come to the point. I decided to design a scale glider from scratch, the Slingsby Skylark II in fact. My initial concept was to do the whole thing using a CAD package. Not having an A0 plotter all drawings had to be viewed on the screen. After a few evenings the result was an outline drawing that I could either view at some large scale without any detail, or bit by bit in zoom mode. Either way I found it unsatisfactory. So bin that idea, get out the A0 board, pencils and paper and do it the old fashioned way. Of course I needed to scale up the A4 drawings of the prototype. After a few stabs at the keys on the CASIO Scientific Calculator, the idea of a spreadsheet dawned on me. A simple spreadsheet was set up to do the conversions and the task left was to feed in the data. The added bonus was that the results were stored on the PC for all time. After a couple of hours I had a ¼ scale layout of the fuselage that I could stand back and view. It was easy to make small adjustments to frame positions, sketch in servo mounts, linkages etc.
Now I needed to detail out the formers, stringers and longerons. The Skylark has an elegant fuselage due to its elliptical cross section. Any of you draughtsmen out there will know that drawing ellipsis is a pain in the what's it, involving pins and string and major and minor axis. Double trouble of course as each former has an open inner elliptical section. Treble trouble as the section is asymmetric about the centre line. Back to the CAD program. All I had to do was feed in the major/minor axis data (already calculated in the spreadsheet) and press the appropriate key. The inner profile was easily drawn using the "repeat at delta x" function. The stringer and longeron sections were set up as symbols that could be positioned on the formers using "step and repeat". Again all the profiles are stored and each could be printed down full size. The prints were stuck to the ply/balsa stock using Copydex glue. This gave an extremely accurate cutting guide for the fretsaw.
Above - example of a former plotted using AutoSketch as described in the text.
The wing was going to use the "Quabeck" section favoured by Chris Williams. How do you get the co-ordinates? Do I really want the pain of plotting them out? Both questions answered by using a computer package which has all the co-ordinates, scales them to any cord including taper, positions spars etc and plots the results to correct scale. Simple.
So in summary its horses for courses. Computers excel (no pun intended) in doing repetitive work, doing complex calculations and storing data. They are useless at being creative and pretty limited (due to screen sizes etc) at giving a good perspective on large and complex designs. If your goal is to design models and get on with building them be selective over what you automate and what you do by hand. If your goal is to be a PC wizard and fly models on the side, Bill Gates awaits you with open arms. Keep repeating the mantra.
For information I was using Autosketch, MS Excel, CompuFoil and a 2B pencil.
The handraulic plan and formers cut from plots of the PC generated profiles.
A Dave Camp Photo