Prior Work – Lancair IV-P Custom Wingtips
This is the first entry of many in my effort to go back though some of my previous work and document it. In October of 2009 I bought a FARO Silver Arm off Ebay for $5,500. It was bought as-is with no guarantee of working, and it was one of those auctions where we all bid at the very last second, and I beat out the second highest bidder only by virtue of the fact that I entered my bid about 2 seconds before he did. In a rare occurrence, we both had the same exact max bid, and so the winner was the one who entered his bid first, me. A week later I was staring at this big hard case and wondering just what the heck I had gotten myself into. While I knew that it would come in handy, I had no current work to book with it, and no idea whatsoever how to use it. So when I was asked if I could make a set of custom wingtips for the Lancair IV-P, I jumped all over it, knowing it would be a great way to figure out how to use the FARO arm. The general idea was to make something very much like the ones that Lancair sells, which look like this and are commonly referred to as sheared wingtips:
I had no stand for the FARO arm, but I knew enough to know that it had to be very stable, and the wing did as well. I also figured that it would help greatly in the surface modeling if the points I gathered using the FARO were planar sections. So I mounted the base of the FARO onto some sturdy wood boxes and crammed some bags of salt under the wing to keep it steady. I got some metal straight edges and clamped two sets of two together so that I could get planar airfoil sections that were aligned on the top and bottom of the wing. If you look closely you can see I put some Bondo between the straight edges along the leading edge of the wing, and then trimmed it with a razor blade so that I could digitize nicely around the leading edge. I can’t remember why I kept the outboard straight edge so far from the end of the wing, I certainly wouldn’t do it that way now.
Then for the very first time, I digitized something into Rhino, which was quite a thrill! It was so cool to see the wing in front of me show up in Rhino as a collection of points. Also a huge relief that I could get this thing to work and I hadn’t blown $4k on ebay on the world’s biggest paper weight.
Through a fortunate coincidence, I was taking a Rhino Level II class at the same time I was doing this project. The class was given online by James Carruthers of Hydraulic Design. I would highly recommend the class, and James, to anyone who is interested in refining their Rhino skills. The Level II class dives pretty deep into surface quality, and really helped my later transition to T-Splines. The really nice part about the class was that I was the only student enrolled, and so we spent about half the time doing the coursework, and the other half of the time working on the surface model for the wingtips. Since these were a custom one off job, I was able to fit the landing light hard point exactly to the light used, which was an Aveoflash LED. Here’s how the surface model shaped up:
This project represented the very beginning of my efforts to create CNC machined molds out of low cost foam. I will go more in depth on future posts into the state of that process as it now exists, but let’s just say I did a bunch of stuff the hard way on this project! One of my experiments involved using low temperature iron on model airplane film to serve as a finishing layer for the mold. It was time consuming and very difficult to keep from dinging the foam with the iron, but it certainly did work. However, when I de-molded the parts, they pulled the iron on covering with it, and often some foam as well, so these effectively became single use molds.
That was fine for this project, but not good if I wanted to make CNC molds and get multiple parts out of them, and so I knew that while I had made it through this project, I needed to work more on a process that would allow multiple pulls from the mold. The layups were 3-core-3 with 7781 glass, Aeropoxy resin and 1/4″ urethane foam cores. Fiberglass was used instead of carbon fiber because of the various antennas in the tips. Here’s some construction pics:
The final tips came out quite nice, and have since flown with no problems: