Contact   Imprint   Advertising   Guidelines

High Pressure kite by Bruno Legaignoux

forum for kitesurfers


kitexpert
Very Frequent Poster
Posts: 619
Joined: Sun Aug 09, 2015 11:20 pm
Kiting since: 2003
Gear: many kites, also diy
Brand Affiliation: None
Has thanked: 0
Been thanked: 1 time

Re: High Pressure kite by Bruno Legaignoux

Postby kitexpert » Mon Dec 17, 2018 9:03 pm

edt wrote:
kitexpert wrote:
Mon Dec 17, 2018 6:53 pm


More. In general if diameter of a beam is halved stiffness is reduced to 1/2^3 = 1/8 of original.
where are you getting that cube from? the axial stress is proportional to the square of the radius right?
I learned/thought it like this: Bigger diameter structure (tube) has a benefit for rigidity for two reasons: 1) material of it is further from where forces affect ("leverage" benefit) 2) there is more material there (larger diameter=larger circumference=larger area).

Assuming tube has same wall thickness I think this is valid and explains why stiffness increases in cube of diameter.

User avatar
edt
Very Frequent Poster
Posts: 5190
Joined: Fri Jul 16, 2010 6:27 am
Kiting since: 2010
Local Beach: ford lake Michigan
Gear: slingshot liquid force wainman pansh naish north cwb burton ronix rpm ozone
Has thanked: 0
Been thanked: 0

Re: High Pressure kite by Bruno Legaignoux

Postby edt » Mon Dec 17, 2018 9:18 pm

kiteexpert I'm not sure if that makes sense? Every mechanical action and strength including physical strength muscle strength, tire strength, all increase directly proportional to the cross section. Rope strength too. So when you double the size of the diameter of a rope, let's choose some sampson asmsteel ropes for example, the 1/8th inch rope (3mm) is 1,100 pounds, while the 1/4 inch rope is 3,900 pounds. double the diameter means 4 times the strength not 8 times. Now that's not exact, it should be 4,400 since 2^2 = 4,400, but it's quite close. For cylinders you have to assume it's a thin wall. Then you integrate the pressure out, and I am pretty sure you get the same sort of result that the axial stiffness is proportional to the cross section. The hoop stress by the way is proportional to the radius, not the radius squared so you can gain some stiffness by making the cylinder bigger without enormously increasing the weight. Now if you have something like fiberglass rods (I designed a few kites with collapsible fiberglass rods, just to try to understand why we are inflating kites instead of using hard rods in our kites), then when you start increasing the size of the cylinder you decrease the wall thickness until eventually you can't make the tube any bigger without making it too fragile and the kite would just explode when it hit, or you would just have to make it too heavy. From my experiments I think I came up with an estimate of something like a 1 inch fiberglass rod being about as strong axially as a tube kite but the weight ended up being something like twice as much. Maybe it could work with carbon fiber? But then the expense is so much. I have temporarily given up on it just because sourcing the carbon rods to make a kite like this would be too expensive for a hobbyist like myself. I can definitely see this for a race kite tho, with a 20-30mm carbon rod, and incredibly small drag but it would be murderously expensive. I think your reasoning seems fine from a qualitative point of view, but I don't see where that cube law is coming from. Just seems like it should be a square law to me. I could be wrong of course, it's easy to calculate things wrong for stuff like this, I just don't see how it could be a cube.

User avatar
tilmann
Frequent Poster
Posts: 379
Joined: Thu Dec 29, 2005 10:02 am
Kiting since: 0
Brand Affiliation: None
Location: Kiel, Germany
Has thanked: 0
Been thanked: 0

Re: High Pressure kite by Bruno Legaignoux

Postby tilmann » Wed Dec 19, 2018 12:46 pm

:idea: wonder if the evolution would go faster if we all don´t buy low pressure kites anymore from now on :wink:
No statement from the industrie so far. Bruno, what´s going on behind the scenes ? :?:

evan
Frequent Poster
Posts: 364
Joined: Sat Jan 31, 2004 5:48 pm
Kiting since: 2002
Local Beach: Brouwersdam
Style: Hydrofoil - Big Air
Gear: Moses Comet 111 2018
Peter Lynn Aero
Brand Affiliation: Infexion
Peter Lynn
Moses
Has thanked: 0
Been thanked: 3 times
Contact:

Re: High Pressure kite by Bruno Legaignoux

Postby evan » Wed Dec 19, 2018 3:30 pm

Why the focus on a Thin LE and single skin for high performance?

We need to aim for a clean as possible proper profile shape, just like aeroplane and hydrofoil wings. That's the reason hanggliders can go much faster and are more effecient compared to paragliders. Single skin won't come even close, no matter how thin you make the LE.

Problem with the current twinskin racekites is the bulging of the cells, all the wrinkles and bridle.

User avatar
edt
Very Frequent Poster
Posts: 5190
Joined: Fri Jul 16, 2010 6:27 am
Kiting since: 2010
Local Beach: ford lake Michigan
Gear: slingshot liquid force wainman pansh naish north cwb burton ronix rpm ozone
Has thanked: 0
Been thanked: 0

Re: High Pressure kite by Bruno Legaignoux

Postby edt » Wed Dec 19, 2018 3:33 pm

evan wrote:
Wed Dec 19, 2018 3:30 pm
Why the focus on a Thin LE and single skin for high performance?

We need to aim for a clean as possible proper profile shape, just like aeroplane and hydrofoil wings. That's the reason hanggliders can go much faster and are more effecient compared to paragliders. Single skin won't come even close, no matter how thin you make the LE.

Problem with the current twinskin racekites is the bulging of the cells, all the wrinkles and bridle.
Hang gliders are in fact thin LE single skins aren't they? Correct me if I'm wrong.

kitexpert
Very Frequent Poster
Posts: 619
Joined: Sun Aug 09, 2015 11:20 pm
Kiting since: 2003
Gear: many kites, also diy
Brand Affiliation: None
Has thanked: 0
Been thanked: 1 time

Re: High Pressure kite by Bruno Legaignoux

Postby kitexpert » Wed Dec 19, 2018 3:36 pm

edt wrote: So when you double the size of the diameter of a rope, let's choose some sampson asmsteel ropes for example, the 1/8th inch rope (3mm) is 1,100 pounds, while the 1/4 inch rope is 3,900 pounds. double the diameter means 4 times the strength not 8 times.
But that is strength for pull not stiffness. Then it is irrelevant where material under stress is and strength for sure increases linearly to cross section. But when there is bending stress it is of course beneficial to get material under stress further from the bending axis, then it resists bending moment more efficiently.

kitexpert
Very Frequent Poster
Posts: 619
Joined: Sun Aug 09, 2015 11:20 pm
Kiting since: 2003
Gear: many kites, also diy
Brand Affiliation: None
Has thanked: 0
Been thanked: 1 time

Re: High Pressure kite by Bruno Legaignoux

Postby kitexpert » Wed Dec 19, 2018 3:48 pm

evan wrote:
Wed Dec 19, 2018 3:30 pm
Why the focus on a Thin LE and single skin for high performance?

We need to aim for a clean as possible proper profile shape, just like aeroplane and hydrofoil wings. That's the reason hanggliders can go much faster and are more effecient compared to paragliders. Single skin won't come even close, no matter how thin you make the LE.

Problem with the current twinskin racekites is the bulging of the cells, all the wrinkles and bridle.
Good questions.

LEI's have actually quite smooth LE and also airfoil shape because LE is pressurized and canopy is tensioned under load. Thick round LE is not optimal however and it is difficult to have as high PA than foil kites and single skins have.

Single skins have very much bridle lines and concept itself is not that smooth. Latest hybrids are improved however significantly.

Normal twinskin foilkites suffer also from inertia, which make them slower to react than other kite types. Increasing cell count is the only way to get them smoother but this means more expensive and more complicated kites.

When there was LEI race kites they used to have 5 to 7 struts or even more. With high pressure technique these struts perhaps could be made lighter and cheaper than before.

evan
Frequent Poster
Posts: 364
Joined: Sat Jan 31, 2004 5:48 pm
Kiting since: 2002
Local Beach: Brouwersdam
Style: Hydrofoil - Big Air
Gear: Moses Comet 111 2018
Peter Lynn Aero
Brand Affiliation: Infexion
Peter Lynn
Moses
Has thanked: 0
Been thanked: 3 times
Contact:

Re: High Pressure kite by Bruno Legaignoux

Postby evan » Wed Dec 19, 2018 3:53 pm

edt wrote:
Wed Dec 19, 2018 3:33 pm
There are recreational singe skin hangliders indeed, but the high-performance ones have a full smooth double sided wing profile.

And about the resistance against bending, please search for "bending moment".

For round rods the bending moment goes to the radius cubed and if we talk about pressurized tube with the same wall thickness and the same hoop-stress the bending moment goes to the radius squared because the internal higher pressure you can put in a smaller tube gives it extra stiffness

User avatar
edt
Very Frequent Poster
Posts: 5190
Joined: Fri Jul 16, 2010 6:27 am
Kiting since: 2010
Local Beach: ford lake Michigan
Gear: slingshot liquid force wainman pansh naish north cwb burton ronix rpm ozone
Has thanked: 0
Been thanked: 0

Re: High Pressure kite by Bruno Legaignoux

Postby edt » Wed Dec 19, 2018 5:04 pm

thanks evan I'll look up bending moment. I'm just thinking about the expense of a double skin high pressure tube kite. Come on kite racers you love throwing money at equipment you can do it! Then maybe someday cost can come down for us regular joes.

User avatar
Gigi;)
Very Frequent Poster
Posts: 502
Joined: Wed Jan 12, 2005 7:23 pm
Kiting since: 0
Location: drtuba.eu, kite-stuff.org, eXtremist.si, kajtar.si
Has thanked: 0
Been thanked: 0
Contact:

Re: High Pressure kite by Bruno Legaignoux

Postby Gigi;) » Thu Dec 20, 2018 8:56 am

Hi,

I think it is worth of checking this post:

viewtopic.php?p=685975#p685975

I think it was ahead of the time back then.

:bye:


Return to “Kitesurfing”

Who is online

Users browsing this forum: GregK, tautologies, Yahoo [Bot] and 18 guests

cron