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 Post subject: Re: High Jumps kitePosted: Mon May 14, 2012 4:11 am
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Joined: Sat Aug 29, 2009 11:25 pm
Posts: 1557
Location: The Naki
edt wrote:
plummet wrote:
i wonder what the formula is for maximum height?

board speed x kite speed x wind speed squared x lift ratio x projected area/ density and humidity?

peter frank worked this out a while ago, you are working out the classical kinetic energy problem of 1/2 mv^2 = mgh, with two additional factors, when you measure v it is not actual speed of the kiter, it is speed of the kiter relative to the air, and next, most kiters when they jump top out around 20 mph.

working from those ideas I worked out a bit of "maximum height" numbers from here https://www.kiteforum.com/viewtopic.php ... 6&start=40 "Re: any math/science people?"

mph jump height in feet
10 17
15 21
20 27
25 34
30 43
35 54
40 67
45 81

According to Frank the air density, kite size and board size do not matter here, the equation works whether or not you are in Denver or sea level, on a surfboard, or have two 100 pound weights strapped to your waist.

hmm... the maths don't seem to stack up compared to my life experience. 17 foot in 10mph?... i call no on that.
54foot in 35mph... i've a guy with my own eyes at least his lines length high 75foot in those winds...... though he was hitting off a monster wave lip which would give him some extra energy to use i guess.

also surely mass of the kiter plays a huge role. light guy surely goes higher in any give wind?!
i suspect we need to alter the maths.

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 Post subject: Re: High Jumps kitePosted: Mon May 14, 2012 4:31 am
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Joined: Fri Jul 16, 2010 6:27 am
Posts: 1323
Location: Ford Lake, Michigan
plummet wrote:
also surely mass of the kiter plays a huge role. light guy surely goes higher in any give wind?

no the mass of the kiter doesn't matter. Think about it.

When someone takes off from a huge wave, measure from the top of the wave, not sea level.

I'm not saying the model is perfect, but some of the things you think are important like mass of the kiter, size of the kicker, density of air, they don't matter even in a more sophisticated model.

17 feet in 10 knots is theoretically possible with someone catching air on a raceboard instead of a twintip. Usually when it is blasting 35 knots and you get huge air it's not because of your suddenly awesome technique but you felt some pressure and went for it. A gust of 10 knots in 35 knots gives you a boost of 30 extra feet so instead of 50 feet you boost 80. You know what I'm talking about.

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 Post subject: Re: High Jumps kitePosted: Mon May 14, 2012 5:34 am
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Joined: Fri Jan 09, 2009 4:49 am
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Location: Japan
edt wrote:
no the mass of the kiter doesn't matter. Think about it.

That seems too counter-intuitive - I need proof.

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 Post subject: Re: High Jumps kitePosted: Mon May 14, 2012 5:45 am
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Joined: Fri Jul 16, 2010 6:27 am
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Location: Ford Lake, Michigan
Kamikuza wrote:
edt wrote:
no the mass of the kiter doesn't matter. Think about it.

That seems too counter-intuitive - I need proof.

hint instead of kites, first imagine Evil Knievel on his motorcycle jumping over 100 buses but he can only do it if he reaches 100mph. Now imagine instead of Evil Knievel on that ramp there is a giant locomotive that goes 100mph. Will it only jump over half the buses because it is so much heavier?

if you can't figure it out with intuition try using math.

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 Post subject: Re: High Jumps kitePosted: Mon May 14, 2012 6:09 am
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Joined: Mon Jun 20, 2011 8:15 pm
Posts: 17
Quote:
you are working out the classical kinetic energy problem of 1/2 mv^2 = mgh

This is only part of the story and is equivalent to using your kite as a 'ramp' - ditching the kite as soon as you are moving vertically. It ignores any contribution of the kite while you are in the air. Any tension on your lines while you are in the air contributes to your max height/hang time (unless your kite is below you?!) What the kite is doing along with what the wind is doing while you are in the air play a big part in the equation. I doubt very much that you are going to get a simple equation to give you any sensible figures...

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 Post subject: Re: High Jumps kitePosted: Mon May 14, 2012 6:36 am
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Joined: Fri Jan 09, 2009 4:49 am
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Location: Japan
Math and physics not my strong points any more having reviewed the classical equations, I now get how mass cancels itself out.

But

still hard to believe that lifting Fatty or Skinny using the same kite and velocity will produce the same height ... drag has to come into it somewhere

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 Post subject: Re: High Jumps kitePosted: Mon May 14, 2012 8:01 am
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Location: The Naki
Nope i still don't believe. There is something wrong with your maths.

Sure i undstand evil kinevil hitting a ramp. but their are more factors at play.

If rider weight makes not difference then the following logic will not prevail.

get a 50kg bloke and a 100kg bloke. stand them on the beach each with a 15m kite. supply 30 knots and get them both to pull the bar in. Which bloke will be lofted the highest? The answer is the the 50kg bloke.

Why? the kite produces so much lift force at a given wind speed. . lets say its 86kg. the 50kg dude is lofted. the 100kg dude is not.

Try an tell be that has no relavent to how high you jump? your maths are wrong. After you get past the penduluming effect of rider speed and kite swing i say its all about lift forces the generates overcomming gravity and the weight of the dude.

please mathmatically prove me wrong then explain why the 50kg gets lofted high if rider weight makes no difference.

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 Post subject: Re: High Jumps kitePosted: Mon May 14, 2012 8:28 am
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Joined: Wed Aug 03, 2005 6:25 pm
Posts: 758
Location: Vancouver
Many of the questions and assumptions made in this thread are based on a misunderstanding of math and/or physics. Unfortunately, there's no shortcut to a solid understanding of the relevant principles. If you're truly interested, get educated. Otherwise, you'll lead yourself to all sorts of intuitive but incorrect 'flat earth' conclusions.

The following linked article is followed by considerable discussion and clarity of this issue, including some jump height calculations.
JS wrote:

Last edited by JS on Mon May 14, 2012 4:20 pm, edited 1 time in total.

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 Post subject: Re: High Jumps kitePosted: Mon May 14, 2012 8:50 am
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Joined: Mon Feb 16, 2004 5:36 am
Posts: 7838
Location: Oahu
Kamikuza wrote:
edt wrote:
no the mass of the kiter doesn't matter. Think about it.

That seems too counter-intuitive - I need proof.

Because force of gravity is constant (signed sir Isaac Newton..or was it Galileo). You would see a difference if one guy has a lot more wind resistance than others....so I guess the drop shaped people are at an advantage here compared to the really wide ones.

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 Post subject: Re: High Jumps kitePosted: Mon May 14, 2012 10:54 am
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Joined: Fri Jan 09, 2009 4:49 am
Posts: 2774
Location: Japan
@JS - I don't think it's a misunderstanding of the physics, just that in all our real-world experience, the distance a light thing can be thrown versus a heavy thing seems to indicate that Fatty wont jump as high as Skinny.

Here's the math/physics as I read and understood it ... a jump is converting kinetic energy into gravitational energy (rocking along then going UP) and energy is conserved so ...
KE = GPE
­½mV² = mgh

(­½mV²)/m = gh is ­½V² = gh ... ­so mass of rider has been taken out of the equation.

And so h=­(½V²)/g ... Evil Kineval, Hogwart's Flyer or The Fattest Kiteboarder in Japan will all jump the same

Which CAN'T be the final answer ... what about inertia?! It's going to take more energy to accelerate (change velocity vector) Fatty upwards than it will Skinny!

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