I was able to sneak out at lunch today and take advantage of a "balmy" 40-degree day to make some more measurements, now adding the newly arrived 100# Maxcuatro into the mix. First I measured "virgin" line elongation, i.e. the line as it just comes from the factory spool, with no previous load applied. Then, using the knowledge gained from Dennis Nunes' experiments, I added the procedure I will refer to as "conditioning", which involves applying progressive load up to 30 lbs, and holding this load for 1 minute, repeated two times, which sets all the knots and stretches the lines. The lines do indeed stretch out some, in my case about 1.25". I did not want to apply more than 30lbs of conditioning tension because I expect that if lines are stressed to 80-90% of the rated breaking strength, two things can happen: knots will fail, and/or the line will relax somewhat, changing the elongation properties and rendering original results useless.
I also discovered a mistake in my earlier reported measurement, so the new set of corrected numbers is here:
Brass plated 4-strand (Ukrainian) 0.0165" elongation 0.5%
Stainless 7-strand 0.018" elongation 0.41%
Stainless 7-strand 0.015" elongation 0.63%
"Virgin" Maxcuatro 65# elongation 0.79%
"Conditioned" Maxcuatro 65# elongation 0.65%
"Virgin" Maxcuatro 100# elongation 0.55%
"Conditioned" Maxcuatro 100# elongation 0.45%
(While I haven't tested it, I expect 80# Maxcuatro to fall somewhere in between 65# and 100# results).
Preliminary conclusions:
- after "conditioning" and with properly terminated ends, Maxcuatro lines with ratings from 65# to 100# will comfortably pass 10G pull tests for planes in the 55 - 65 oz range, with a 2X-3X safety margin;
- after "conditioning", Maxcuatro 65# line is expected to give about the same stretch feel as a 7-strand stainless 0.015" cable, and has comparable diameter;
- after "conditioning", Maxcuatro 100# line is expected to give the stretch feel between a 7-strand stainless 0.018" cable and a 4-strand 0.0165" cable, and has comparable diameter to 0.018" cable;
So far, the only performance advantage over the steel lines is smaller weight of the Spectra lines; the rest of the advantages fall into the usability category: kink resistant, hydrofobic, easy to see, etc. The ability to control after 40 loops and come out of a line tangle without damage are not of interest to us for stunt application.
What remains unanswered is to what degree will Spectra-based lines maintain their length, and their elongation properties, over time as well as under different environmental conditions.
Specifically, here are the example scenarios that worry me:
- A user stretch-conditions his lines, performs a few trimming flights to dial in the neutral, and starts practicing; then a month later he goes to a contest, where a contest official performs a 10G line pull; somehow one of the lines stretches a little more than the other - but the user is unaware, until he goes into official flight, and is unable to deliver his best results due to the neutral being off;
- At a contest, a user passes the 10G pull test, performs the first flight of the day, and then puts down the lines on a hot tarmac to wait for the second round flight; the heat shrinks one of the two lines a little more than the other - once again, the neutral trim is lost, and a good flight cannot be performed.
The time and temperature study is difficult to do by using measurements: as Dennis N. noted, how do you accurately and repeatably measure a length of nearly 70' down to 1/64" accuracy (or better) over the course of days, weeks, and months? Of course this could be done by someone with a very keen sense of neutral, flying a plane with the same set of lines and same handle over a long period of time, but I'm not one of them - my neutral can be off by 1/8" and I'm still OK with it.
As of today, I'm still on the fence about adopting these lines.