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At the recent Euro Champs there was a lot of discussion about line drag for F1A. Trends have seen lines gets smaller in diameter (less than 0.5mm) but CFD analyses (validated gainst test data) have shown that the drag was higher than thicker lines in some case. I’m told that the secret is in the braid pattern to reduce VIV (vortex induced vibration) which is a well known efect in my industry. So line thickness are slightly increasing but the new braid patterns give an overall drag reduction leading to better launches ! Not sure of availability as yet, the users were being rather coy.
Hello CHE
I have been comparing some Spectra 130 lb test line with the 1mm Russian rod.
My initial impressions are that the 130 lb line, diameter less than .5mm, made up of twisted braided bundles of very fine strands, somewhere about .001″ in diameter, is not as good as the 1mm polyester Russian rod.
I think the difference is that the 1mm polyester can stretch and store alot of energy for the release, where the small diameter braided line does not store much if anything, and does not stretch hardly at all.
The Spectra is OK for F1H models or smaller where weight becomes an issue. It is also outstanding for making snarly knots in the weeds, and improves dexterity for fly fishing!
Another option is to use 10 feet of silicone tubing, the type they use for high starts, and you will get an additional 10 feet of stretch at 40 lbs pull to use for a sling shot effect.
I forgot to mention my other opinion that the line drag is probably much less of a factor than being able to store a bunch of energy in the line for launch.
I am sure that the scientific types can come up with some equations to prove me right.
> can stretch and store alot of energy for the release … // … Another option is to use 10 feet of silicone tubing, the type they use for high starts, and you will get an additional 10 feet of stretch at 40 lbs pull to use for a sling shot effect.
Funny thing… all the years I’ve been flying gliders … maybe 60 …. I always thought that towlines were supposed to be inextensible ….. ho hum …
There is a pull test of 5 kg, about 11 pounds. I think it should be higher because a good launch is pulling 40 lbs.
I have pulled some monofilament, and it stretches a bunch at 11 pounds, and keeps right on stretching as the force increases. It does not return to the original length. So you would probably not be legal if the line was close after a few flights. It also does not seem to store any energy like polyester, just stretches.
But with a bungee cord material, you could meet the 5kg pull, and stretch several feet past the 50m at a 40 lb.launch , and still come back and meet the pull test.
Well, to me ‘inextensible’ means just that, no matter whether the pull is 11 pounds or 40 pounds.
If, as a CD, I see or hear of a towline with an obvious rubber insert then I’d refuse it, no matter what claims were made about pull tests.
I don’t know of any type of lines that do not stretch with load. I also do not find any definition of towline material other than the “launching cable”. (Inextensible??? where did that come from? I don’t find that term in current or 2004 FAI rules)
There are no characteristics described other than it must be less than 50 meters under a 5 kg load. I have measured stretch with different towline materials, but can’t remember where I filed the data. As mentioned previously, even with 150 lb monofilament, it stretches a bunch, and tends to reduce in diameter and lengthen permanently.
Some people use composite lines, kevlar and monofilament, spectra and Russian rod, etc. I don’t see anything outlawing a bungee chord for pulling back with 100 lbs of force on the ground and catapulting it at the launch.
I have been satisfied with the 1mm polyester (Russian rod) for several years, and it has not changed in length over several contest seasons. I use a loop of vinly tubing at the end for gripping during launch.
[Maximum length of launching cable loaded by 5 kg….. 50 m ]
Well, it’s in MAAC rules and it surely used to be in SMAE rules back in the past, though that may have changed since.
MAAC quotes: “Inextensible as herein applied may be taken to refer to such material as may be classified “nominally inextensible”, and for simplicity should be applied to eliminate the use of lines so obviously elastic as to give an unfair advantage in their use, the ultimate purpose of the towline being to launch the aircraft from a maximum of 164 feet above the contestant “
Now that ultimate purpose seems, to me, to be the objective of all models commencing flight from a common altitude. It’s likely that when other rules merely established a 5kg pull test without such accompanying provisos it was never thought that towline gliders would exceed that figure, the lack of which removes ‘unfair advantage’ from the concept and now leads to a practice that infers that 164 feet is no longer a maximum launch height.
“Inextensible ” has an engineering conotation meaning that, er it is inextensible ie doesn’t stretch. No material has this atribute as it infers an infinite tensile modulus which is impossible. This is why the line pull test came in I guess. Not sure the word appears in the Sporting Code or not.
“Honogenious” appears in many definitions of towlines. This means (in context) made of a single material so anyone who flyies to rules with that phrase and uses two materials tied together is, er, technically a cheat. Of course I wouldn’t use the ‘c’ word myself and suggest no one else does unless they want a knock on the door from a lawer.
@CHE wrote:
“Honogenious” appears in many definitions of towlines. This means (in context) made of a single material so anyone who flyies to rules with that phrase and uses two materials tied together is, er, technically a cheat. Of course I wouldn’t use the ‘c’ word myself and suggest no one else does unless they want a knock on the door from a lawer.
I am quite sure some years ago I asked Ian Kaynes about joining two
[or more]materials to make up towlines.
As I recall, he said this had been addressed, accepted and appears in
CIAM meeting minutes.
Maybe Sporting Code was never amended thusly.Ciao,
Lee
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