I was looking at different F-16 spi shapes in photos and measuring their basic geometry last night. It seems like there is a noted difference between sailmakers. The Ullman spis look like they are cut with a straight or even hollow leech (which I like, makes the spi easier to fly properly in weak winds), whereas others look like they have a quite rounded leech. What is surprising is the amount of round in the luff, and how slack they are when sailed.
I made a list of photos from the F-16 galleries, which show how different spis are set up:
http://www.formula16.org/gallery2/main.p...alNumber=1&http://www.formula16.org/gallery2/main.p...alNumber=2&http://www.formula16.org/gallery2/main.p...alNumber=2&http://www.formula16.org/gallery2/main.p...alNumber=2&http://www.formula16.org/gallery2/main.p...alNumber=1&http://www.formula16.org/gallery2/main.p...alNumber=2&http://www.formula16.org/gallery2/main.p...alNumber=2&http://www.formula16.org/gallery2/main.p...alNumber=2&http://www.formula16.org/gallery2/main.p...alNumber=2&http://www.formula16.org/gallery2/main.p...alNumber=2&http://www.formula16.org/gallery2/main.p...alNumber=2&http://www.formula16.org/gallery2/main.p...alNumber=1&http://www.glasersails.com/F16Spin.htmSome small comments to the different spis. Looking at the last two links above, a Glaser spi and a Taipan spi (dont know the brand), the leech round is easy to see.
Having a rounded luff like in the pictures makes the luff longer, which is good. But it also means that you can not go as high (build as much apparent wind) as you could with a straighter luff. However, becouse the 75%mid girth rule is in effect for F-16 spis, there is a minimum permitted chord.
Have anybody measured the geometry of their spis, luff, leech, foot length and mid girth?
Some of those spis look like they are down to minimum mid girth (I measured some photos to have less than 75% mid girth, but photo analyzis is never correct!).
Are the newer spis buildt to their max size, 17.5m2?
Have anybody bothered to measure their spi luff length and compared it to the distance from the turning block on the spi-pole and the halyard block?
I am working on my own design, and feedback on the above would help a lot. If anybody want to download Sailcut and comment directly on the design, I'll post the design file.
