How do stunt pilots refer to line length? Eyelet to eyelet, as I just saw in another thread, or handle to prop nut center? In combat, it is handle to prop center.
Almost always eyelet-to-eyelet. That's because the airplanes vary a lot, and I know that David's 64' lines are the same as my 64' lines and Ted's 64' lines. As long as you don't go over the max, the handle-to-centerline measurement is irrelevant (unlike a lot of the other events). In any case, most people aren't that close to 70 and going over is a non-issue. It doesn't really matter as long as you specify the measurement type you use when talking to other people.
As noted, *beware* pre-made lines, and be sure and measure them. Poorly made end "crimps" are very common, and I have seen Sullivan/Pylon "60 foot" lines that were supposed to be 58' (which always seemed like a poor way to do it, since they have no idea what airplane it might be going on), but were instead 54 feet or 64 feet, or 74 feet.
At some WAM contest a long time ago, someone took off with their brand-new Sullivan ".015x60" lines, we set up our chairs dutifully outside the circle upwind. He takes off, and after about a quarter of a lap and it is obvious that the airplane is sort of coming at us. A little while longer, and hey, this doesn't look right, a little while longer and "HIT THE DECK!". That airplane probably went 15 feet *behind our heads* with us diving onto the ground. Afterwards, we had the lines checked and there were something like 85'. Both the pilot, and the judges, were a little less than pleased with Sullivan that day, the pilot because they had caused him to get a 0/DQ on a flight, and the judges, who take things like almost getting decapitated rather personally.
I am not sure how they do the measurement in production - probably something like count the turns of the reel required to get the necessary length. Which is a little different from OCD stunt guys out there pre-stretching them to set the breaid, and letting the lines sit on the blacktop for 1/2 hour so they will warm up, so they can get it within 1/64" including thermal expansion.
Brett