- Jul 29, 2005
- Orange County, California
Thanks for the reply. As Hephaestus and others have noted if we need really accurate airfoil templates they maybe should be made via CNC and just skip the paper all together? Or better yet just cut the foam cores with CNC. For hand build wood or metal planes I've often wondered if we are too worried about plans accuracy? I was taught way back in the old days to never scale off the prints, only use the dimensions given. It is nice to have full size templates to glue to the metal for rough shaping but when it comes time to cut the holes they probably need to be done with blueing, scribes and measuring tools.
Some thoughts, roughly in the order of your questions:
- If you've got easy access to a laser cutter or CNC machine, sure. But remember that 90% of the full-depth foam composite airplanes out there were done with paper templates glued to masonite and cut out with a hand-saw, and then finish sanded to final shape. All those VariEZ's, LongEZE's, Quickies, Q2's, Cozy's and so on seem to be flying just fine. I wouldn't do a top-end competition sailplane that way, but then you're probably going to invest in molds for that sort of highly-laminar airplane anyway.
- "Are we too worried about plans accuracy?" Probably. To put more of a point on my statement in my last post, if you can't make it to 0.005" accuracy in your shop, you don't need a print to 0.005" accuracy, either. Just because you can design it to six decimal places in a CAD system doesn't mean it needs to be accurate to six decimal places in the real airplane. I'm not advocating for "sloppy", but you can spend more time and effort on needless precision, too, with little to show for it in the end.
- I was taught to only use the dimensions on a drawing, too. And dad also said that, if there was some dimension you couldn't read off the drawing, the drawing was badly dimensioned in the first place. He wasn't talking about things like airfoil hot-wire templates, but you get idea. Paper stretches, expands or shrinks with changes in humidity, and generally changes shape with age.
- One of the best lessons I ever got about working in almost any material, for complex shapes like airfoils, was to scribe or just draw oversize, then sand or grind down to finished size. Don't try to cut finished size initially. Plus, again, you can blue, scribe, and even rough-cut, grind, and then file to precisely the finished shape called out - and then have your press equipment not be capable of forming that into a rib without distorting and stretching the whole thing a little bit.
Once again, my favorite engineering cartoon, explicitly about manufacturing tolerances which, ultimately, are what we're talking about here: