Power Kite Forum

Kite line springiness and gust response

pbc - 18-8-2015 at 07:13 PM

We've had a lively discussion in my Scorpion post about line strength. It's got me thinking it's time to share some thoughts I have had this year about line types and gust response.

Somewhere along the way I picked up this idea that less line stretch is better and that skinner line is also better. It's easier to parlay that reasoning into buying a new Q Powerline lineset. Please don't get me wrong--I love Q-Powerline. I think it's is some freaky cool stuff, but I think we must ask my favorite question of late: What are you optimizing for?

If you've ever made your own linesets you've experienced the elasticity of flying line. They stretch. Some of them stretch a lot more than others. If you compare a length of Q-Pro to a similar length and strength of hollow-core woven Spectra you will find the hollow core stretches dramatically more Q-Pro.

So if we value gust response, why would we want to pick a line that is less elastic? Should we instead pursue lines with a lower modulus of elasticity?

What if we took that further and picked lines that provide more stretch in the bottom lines than the top lines? This could allow the kite to automatically trim out in response to a gust as the back lines got momentarily longer than the top lines. Perhaps a lineset of Q Pro on the top and hollow core Spectra on the bottom would provide a useful elasticity in the bottom lines.

The downside of this could be a slower response to steering input. The stretch of the bottom line might also compromise the useful throw of the bar. I'm not sure what else might go awry because I haven't tested this...yet.

Has anyone experimented with this? Please share. I can't believe I'm the first person to think about this, but I have never heard a peep about it. Please save me from my ignorance.

Philip

indigo_wolf - 18-8-2015 at 07:24 PM

Once lines bed in, any subsequent stretch tends to happen very slowly over time, which would be kind of useless for guest management.

Kind of think asking for gust management from linesets is a bit overly hopeful.

As various arcs are characterized as "gust munchers" and to a lesser degree a limited number of foils, I think it would be more reasonable to look for gust tolerance in the actual design of the kite.

ATB,
Sam

BeamerBob - 18-8-2015 at 09:50 PM

I wouldn't want my steering lines to be springy. Sometimes in less than stellar winds, the kite jumps and changes direction and I have to make small but quick adjustments to steady the kite while running at speed. I want those steering lines to be responsive.

I'm much more likely to soak up some wind speed with the direction of the buggy if I hear rocks start to hit the side of my helmet and feel them on my arms and legs. If it's smooth enough, I'll turn back into it and see where it takes me.

I remember on my second visit to Ivanpah before moving here, I was running with Mark and the wind was less than fully powering my kite. Seems like I had a 10m Synergy flying and maybe Mark had the same thing but I know it was a similarly sized kite. He was able to trot right past me at a good 5 mph faster. I asked him later how he did that. We were going in the same direction and not much was different other than maybe I was heavier than he and his buggy. His answer even back 4 years ago was that he ran a lighter lineset. I've never broken a stock set of lines even in extreme conditions and flying it like I stole it, so I'm confident I could play with lineweight and gain some performance.

rtz - 18-8-2015 at 09:58 PM

"We recommend replacing race flying lines every 100 hours or season, whichever comes first."

http://ozonekites.net/proddetail.asp?prod=ozone_race_lines


A good video about "line stretch".

bigkid - 18-8-2015 at 10:26 PM

I did a line study a few years ago and found some very disturbing information and to top it off we have been mislead by the line manufactures. To make things worse the average kite flyer has no idea of what they are talking about when they speak about their favorite line set.
I am not looking to start a fight and get into a pissing match with anyone over what is the best line set or that I have no clue what Im talking about.
To start off you ask an interesting question and it has been talked about and discussed to the determining point of $$$$ or no $$$$.
Manufacturers have one thing on the corporate mind and its not anything that has to do with kites, how they fly or even why. Its all about $$$$.
I have spoke to a few kite manufactures, (ozone, PKD, and Pansh) and it was the biggest eye opener I could have imagined. I also talked to a company here in Washington State that makes "Amsteal" for the entire world, even went and took a tour of the plant.

I set up 40 differant makes of line material from different manufacturers and put them through the same tests at the same time at the same spot, every line was treated the same as all the rest.
The tests were to prove to myself and my team riders why these lines broke and those didnt. Along with a few other issues that came up, that in the end was costing me money for "the best" that were not.
A few of the test were,
stretch- add 40lb to the lines and see which ones moved.
add 140lb to the same lines and see how they move, more or less.
add water, add salt water, 150lb drop test at 5 feet, 200lb drop test at 4 feet, 40lb test for 1 week, 2 weeks, 3 weeks, and a few other tests that simulated flying kites.

When you buy a line set that is marked 400/200 it is not the same as another brand marked 400/200. Color does make a difference in performance, size does not reflect the strength as you would guess. Its all about the bottom dollar and if they are able to make a few more bucks for less line strength. Change the color, put another name on it, change the package, all of these and more are about making joe kiter spend more money and pick a brand that isnt anything to brag about.
Lines are produced and then tested to see where the breaking point is. If they break at 500lb they are classified as 300lb lines. The general rule is to include a 150% safety factor to the number you are sold. Here is the part that pissed me the most.
One kite company sold the line as 400/200 and the manufacture sold it to the kite company as 200/100. It was stated by a few here on this forum as the thinnest line set on the market and was the best set to buy. Not everyone fly kites down the beach at break neck speeds of 20mph, some go as fast as 80mph. Some go from 45mph to zero to 45mph in a few feet while making a 180 degree turn with the main goal of breaking the line set. I have broken more lines than I can remember, thus the reason why I wanted to find out whats up.

At the end of the day, we all have to go to bed with the idea that we accomplished something and it was good. Thinking about how we could change the way lines behave is a noble gesture but not worth the money to make it happen. But that never stopped anyone from customizing anything and everything .

I personally like a line set that reacts the same on both sides of the kite. No color but white. is really rated at what they print on the package. I buy my line material on the spool and make my own line sets. Life is good.:D

shortlineflyer - 19-8-2015 at 05:22 AM

I had no idea that color made a difference. does it have to do with the chemical they use to dye the fibers?

on your drop test which line did best?
how did the Q power rank in the 40?

BeamerBob - 19-8-2015 at 05:51 AM

So Jeff, what brand and weight of line to you make your linesets out of after all you learned?

Feyd - 19-8-2015 at 06:00 AM

I've never snapped a line. At least not without damaging it first, usually by accidental exposure to a ski edge.

Colored lines in my kiting environment are mandatory. White lines in the snow are a total headache.

With the wide ranging variety of lines, manufacturers and resulting quality based on "standards" that aren't really standards at all, I choose to limit myself to line sets that have proven to be reliable. Consistency is critical IMO in choosing your lines. Even with the wild dependencies in ratings if you are using the same set from the same manufacturer you at least know what you have to work with.

I'm lucky in that I don't have to deal with UV, salt or sand with our kites and lines. Stuff lasts a lot longer even under the heavy hours that some of our gear gets. I see what just a summer of ocean riding does to a bar and lines and I cringe.

Windstruck - 19-8-2015 at 06:04 AM

Chris (feyd) - what have you done when (if?) you need new line sets for your Peak 2's? I haven't broken or damaged the lines of my 6 or 12m P2s (yet!) but I suspect the day will come. I rubbed a line off of a soccer goal post on my NS3 set up and broke one line that way. Does Flysurfer have replacement sets that they sell?

PHREERIDER - 19-8-2015 at 08:26 AM

they all break....and yes they DO JUST break.(contrary to jeff in the vid)


the only lines that have not broken(yet), in my experience, SS comp stick lineset (the pigtails have) and a Q-line set. these are rated as an index around 3-400kg.

everything on the bar including the bar breaks, leaders, loops ,plastic this and that, metal rings whatever. on every type of kite tube,arc, FS ,NPW

i would not want "springy" in the equation,

then gee what does failure look like? you ask...

something like this https://vimeo.com/45808707

rtz - 19-8-2015 at 01:27 PM

Some more info: https://wetestkites.wordpress.com/2015/07/01/comparing-line-...

bigkid - 19-8-2015 at 02:58 PM

Wow, got to admit the kite boarders have a giant head start on the line set issue. It is still apples and oranges with fb kites and depowers.

pbc - 19-8-2015 at 08:12 PM

I'm glad I could provoke a lively discussion. I especially like the link to the We Test Kites page. Thank you, RTZ for that one. This is a critical piece of the puzzle.

My favorite item on the WeTestKites page was the elongation data for various lines. Every solid material has elastic properties. Kiteline is no exception. So maybe you say you don't want elastic lines, but you have them whether you like it or not. You might choose a less elastic line set, but it is still elastic. Every line under load will stretch.

The area of interest to me is whether the top and bottom lines stretch by different amounts on a depower rig. If the top line stretches more than the bottom, the stretch you will give positive feedback that will increase the angle of attack and with it the line tension. If the bottom line stretches more than the top, you will get a negative feedback loop that will reduce the angle of attack and the line tension. I'll accept that the effect might be small (more on that in a bit), but it is there. This is happening whether you think it's worth worrying about or not.

So how much is the effect? Certainly the answer is "it depends". We'll have to make some assumptions before we can get a measure of the effect. The kite itself will be an important factor. Some kites are well-balanced and minimize bar pressure as they are trimmed in. Others have painful amounts of bar pressure. The fore-aft load distribution provided by the kite will be a multiplier of the relative elasticity in the lines. In elastic deformation of solids, strain (aka stretch) is linearly proportional to stress (the load or tension). A cursory review of http://www.eurocord.nl/kitelines/ds shows this is pretty much the case for Dyneema. Double the tension on the line and you will double the percent stretch.

Flying style will also have a great effect on the relative stretch by changing the fore-aft load distribution. Flying with the bar just a millimeter shy of stall shifts vastly more load to the rear lines than flying with the bar all the way out. That said, I think the useful area of study part is when the bar is positioned such that the kite is near stall. If this is happening while the bar is against the stopper, proper gust response might keep you from going airborne when you didn't want to. A highly depowered kite on a long throw might not experience enough top line stretch to change the trim of the kite. That end of the analysis might not be useful.

The last effect on the relative stretch is the line choice itself. As you can see in the Dyneema data I linked to above, a lighter flying line will stretch more than a heavier weight one at a given load. If you want the bottom line to stretch more, pick a thinner line. This is where I think we could excersize some control over the relative stretch of the top and bottom lines.

I'll dive into the math in my next post. Full disclosure: I haven't done the math yet. The effect might yet turn out to be a gnat's fart, but my gut feeling is this is something we can control. I expect to find we can make choices that benefit us or place at greater risk.

If you want to join me in the research, review "elastic deformation" and "Young's Modulus". Engineering: It's like science except you get to make cool stuff.

Philip

BeamerBob - 19-8-2015 at 10:39 PM

When I said I didn't want springy lines, I meant I wouldn't choose lines that were light enough to be springy enough to offer gust response without my input. IE. More springy than is normally common.

TEDWESLEY - 22-8-2015 at 06:42 AM

Keep in mind that the breaking strength of a line is measured on a machine that provides a steady smooth pull until
failure. This does not replicate the dynamic loading that the line is actually subjected to. PHREE's video demonstrates what happens with
a high dynamic loading, he becomes a sea anchor just as the kite is powering up in a kite loop. All four kite lines break, but I'm willing
to bet that if a high speed camera were present, it would be evident that the lines broke sequentially as the weakest went first and
the rest followed in rapid succession as the load was transferred. A line that fails at 400lbs test will fail a drop test of a much lighter weight.
The lower the stretch factor the more profound this becomes as the energy is dissipated over a shorter distance.

bigkid - 22-8-2015 at 07:06 AM

I am so happy to hear some common sense from a few of the individuals here on this forum. It is nice to know that it doesn't take a rocket science major to figure out the simplest terms in the basic dynamics of kite line.
One of the issues that I found when talking to the manufacture of Amsteel was the inconsistency of the product on the smaller diameter lines. There is a gray area when getting down to the smaller diameter is due to the unimportance of quality. I was told that every line that comes off the machine has a tolerance of 50% more or less of what was designed to create. Not wanting to get into the nitty-gritty and all the specific numbers I understood what the guys were saying. And it's no different from the 400 pound as to the 200 pound test lines the bigger ones will not stretch as much as the thinner ones. But because the powerlines are larger than the brake lines the powerlines are pulled more and are under a higher pressure down the brake lines. Thus creating more stretch on the Powerlines then on the brake lines.
The downside to all of this once you reach the point where the lines are capable of doing what your design to do is now the kite starts to fail after using higher strength lines I started breaking bridal parts and tearing the tabs off the kite where the bridal attaches. So you could say it's a win-win situation or it's damned if you do.
As for the video of Chad breaking his four lines all at once, Ted was correct one goes and then the others cascade after that at least that's the way that they have broken for me. I have broken two powerlines and one Breakline before the handles were pulled out of my hands. There's no way that the bar would be removed in order not to break all the lines. Just glad that Chad was OK afterwards.

Windstruck - 22-8-2015 at 08:02 AM

Yikes! Finally got around to watching your video Chad. Looks as if you were in the water when and the kite looped on you. I've never flown an LEI or kite boarded, but I imagine the loop of an LEI is akin to a downturn force wise. Glad you weren't hurt (thank goodness for force distribution though a wide harness); I bet for an instant there was a lot of load on you!:o

kiteballoon - 25-8-2015 at 07:55 AM

Reading this discussion makes me thing of conversations from the desert around a kite that would auto-adjust to conditions. In simple terms, we all fly manual kites. What if there was such a thing as an automatic? It would be safer and easier for a beginner. Would you fly it? Those seeking the edge or old-school factors would not. What about the rest?

Naturally the discussion centered around how you would construct such a kite instead of if or why you would fly it, but I can't help but feel pbc is searching for something akin to this. If we can sort of include / encourage / obtain better kite behavior automagically, isn't this a good thing? There is definitely something to feeling the wind and those who fly extremely simple setups with handles or straps can attest to this. But we are not as fast as the wind, nor as perfect as we think and hope we are.

Surely a well designed AQR is better than a leash, even with a quick release. And surely I would rather fly newer and more forgiving kites in most scenarios, than the earlier models that influenced them. In the same vein, I welcome improvements into how I setup my kite. Despite all the innovation in other areas of buggying, I think we could do more around how we fly and control the kite itself. How we attach a kite and fly it still remains the biggest liability for a kite flyer, particularly with power kiting. By that I mean it represents the greatest amount of risk for which we must mitigate. We need open areas, strong lines, a means of disconnecting ourselves from the kite at a moments notice, etc.

I think I would fly a so called automatic kite. I would trust the cost would be worth the improved safety margins, while allowing me to retain everything I like. Think an automatic rally car with paddle shifters as opposed to the family sedan.

Windstruck - 25-8-2015 at 08:36 AM

@kiteballon - an interesting idea to ponder. I enjoy thinking about the geometry and physics of flying, kite and DP system set up, etc. I look forward to watching things evolve over time, maybe into areas like you are suggesting? A similar evolution has crept into high-end racing bikes (the pedal kind, not motorized). I'm not into that sport at all like I used to be (found traction kiting!) but I know folks have been working towards auto shifters, etc.

Keep up the good thinking!! :D

pbc - 26-8-2015 at 06:53 PM

NicS-P-A-M-L-I-N-K-s post got me thinking about machines that have been specially designed or redesigned because they had special dynamic requirements. The dynamics of a system matter. If we think about our own experience we can think of cars that ride well and those that ride poorly. I remember driving my Mom's Econoline van loaded two feet deep with wet sawdust.

Helicopters, for instance cannot have springy landing gear. Any motion the landing gear allow has to be heavily damped or the dynamics of the rotors can cause a destructive rolling called ground resonance

Ralph Nader's book, Unsafe at Any Speed documented the problems with the Chevy Corvair's swing axle suspension in the the first year of production. For want of an anti-sway bar, the suspension was quite dangerous compared to its contemporaries.

In our own world, kite buggies also need an anti-sway bar for rear suspension to be useful. Without them the near constant side load consumes suspension travel on both the upwind and downwind wheels. Meanwhile the buggy tips towards the kite placing the pilot at greater risk of spilling out of the seat.

The dynamics of a system matter. The better we understand the dynamics the greater hope we have of improving them.

Philip

rtz - 26-8-2015 at 07:14 PM

I found this video to be mind blowing. This car totally makes standard/normal automotive suspension systems inferior as it is so superior. I had to draw out the rear suspension on paper to wrap my brain around how exactly it works. Look at how those narrow front tires handle so well. This video explains the car out very well:

BeamerBob - 26-8-2015 at 08:41 PM

Very interesting. It's like they reinvented the wheel and did a much better job this time.

volock - 27-8-2015 at 05:27 PM

Quote: Originally posted by BeamerBob  
Very interesting. It's like they reinvented the wheel and did a much better job this time.


I think this is reinventing the wheel

Windstruck - 27-8-2015 at 06:04 PM

@ RTZ - just watched the delta wing car vid. Good stuff! Thanks for the post. Super cool to see folks pushing the envelope like this! :thumbup:

pbc - 27-8-2015 at 06:54 PM

After much thought and a lot of fiddling, I now have a spreadsheet you can use to estimate line stretch and relative line stretch. I've shared it as a read-only Google Spreadsheet for anyone that would like to use it. See https://docs.google.com/spreadsheets/d/12bTvQJvCOPNX6VTJ9GSz... To make revisions you will need to either download it and open it in Excel, or download it as Excel and then re-upload it to Google Drive and convert it back to a Google Sheet.

The most critical, most subjective input on the sheet is the estimate for "% of load on bottom line". My guess is 30%. It's just a guess. I haven't measured it so please feel free to dispute. I'd love to see some data on this.

Keep in mind the "% of load on bottom line" metric is for high load conditions. I assert that those are the conditions where any potential benefits or liabilities of line stretch will be maximized. When the buggy is at full speed and bar pressure is high, the way the kite responds to a gust will matter a lot. We want the effect of the gust to be diminished so we stay in the seat and stay in control.

A line set of DS-110/DS-55 and 30% load on the bar will allow the bottom lines to stretch slightly more than the top lines. The ratio of top line to bottom line stretch with distributed load is 0.89. If you change the line set to DS-110/DS-110, the now stiffer bottom line stretches substantially less. The ratio of top line to bottom line stretch with distributed load would become 2.33. This means the top line would stretch twice as far as the bottom.

I assert that the symmetric DS-110/DS-110 lineset would have an unfavorable gust response compared to the DS-110/DS-55 lineset. The former would increase the angle of attack at the worst possible moment while the latter would reduce the angle of attack at the moment it might do some good.

Please experiment and let me know what you think.

Philip

kiteballoon - 28-8-2015 at 06:53 AM

Quote: Originally posted by pbc  
After much thought and a lot of fiddling, I now have a spreadsheet you can use to estimate line stretch and relative line stretch. I've shared it as a read-only Google Spreadsheet for anyone that would like to use it. See https://docs.google.com/spreadsheets/d/12bTvQJvCOPNX6VTJ9GSz... To make revisions you will need to either download it and open it in Excel, or download it as Excel and then re-upload it to Google Drive and convert it back to a Google Sheet.


Click File, Make a copy to make a nice editable copy right on google docs without having to download and upload anything.

Time to crunch some numbers.

PHREERIDER - 28-8-2015 at 08:40 AM

balance, control and use of forces, well beyond grip and power absolute. love that. you have to harvest ALL!

better balance and less power, hmmm sounds familiar , electric will be(is) even faster

pbc - 28-8-2015 at 11:38 AM

I'd kill for elongation data from other line manufacturers.

Anyone? Anyone? Bueller?

pbc - 28-8-2015 at 11:43 AM

Quote: Originally posted by kiteballoon  

Click File, Make a copy to make a nice editable copy right on google docs without having to download and upload anything.

Time to crunch some numbers.


Right! Instructions revised to embrace the copy-and-edit method within Google drive.

Philip