Kiteforum Wiki Recent Changes FAQ How To Gear Useful Tips Safety Rules Schools Shops

 All times are UTC + 1 hour

 Page 4 of 10 [ 93 posts ] Go to page Previous  1, 2, 3, 4, 5, 6, 7 ... 10  Next
 Print view Previous topic | Next topic
Author Message
 Post subject: Re: High Jumps kitePosted: Mon May 14, 2012 9:45 pm
 Rare Poster

Joined: Wed Feb 09, 2011 10:59 pm
Posts: 29
I believe the discussion was whether weight had anything to do with jump height. Same kite size, same conditions, lighter will jump higher. Right?

Change kite size to accommodate different weight riders and it seems that is an acknowledgement that weight is an issue.

I'm impressed with the scientific justifications forwarded in this thread, but weight makes a difference, IMHO...

Top
 Post subject: Re: High Jumps kitePosted: Mon May 14, 2012 9:54 pm
 Rare Poster

Joined: Mon Jun 20, 2011 8:15 pm
Posts: 17
IF the height to which a kiter can jump only came only from the kinetic energy due to the riders momentum then the riders weight would have little bearing on the height to which they could jump. In this case the maximum achievable horizontal speed would be converted to the maximum height vertically. A larger rider will have more kinetic energy for a given speed but it takes more energy to raise them to a given height, conversely, a lighter rider has less energy for a given speed but it takes less energy to loft them up.

However... the reason that this is not matching up with peoples experience is that this is only part of the story. The contribution of the kite to lifting the rider AFTER the launch is entirely missed in this model. It is this part of the equation that IS dependant on the mass of the rider. A kite is perfectly capable of continuing to provide lift while the rider is airborne and the kite is directed correctly. The effect of the lifting force on height very much depends on the mass of the rider....

Top
 Post subject: Re: High Jumps kitePosted: Mon May 14, 2012 10:03 pm
 Medium Poster

Joined: Fri Apr 01, 2011 10:31 pm
Posts: 73
It's not the kite....it's the kiter

Top
 Post subject: Re: High Jumps kitePosted: Mon May 14, 2012 10:22 pm
 Very Frequent Poster

Joined: Mon Feb 16, 2004 5:36 am
Posts: 7839
Location: Oahu
Kamikuza wrote:
@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!

I think fatty can also hold more power and that might cancel out the inertia. So where skinny goes on a 9 fatty takes the 12. Now lets also discriminate between fatty fat and fatty heavy on the muscle. The muscle fatty will hold more power given equal level technique.

Top
 Post subject: Re: High Jumps kitePosted: Tue May 15, 2012 12:12 am
 Frequent Poster

Joined: Tue Jan 02, 2007 11:50 pm
Posts: 243
OK, I think some of us were missing each other in the discussion. If the big guy has a bigger kite to compensate for weight, then yes, he may well jump higher. Look at speed records in kiting and windsurfing. They're not being set by small people.

And yes, the kiteR has the biggest effect of all!

Peter

Top
 Post subject: Re: High Jumps kitePosted: Tue May 15, 2012 1:31 am
 Very Frequent Poster

Joined: Fri Jan 09, 2009 4:49 am
Posts: 2774
Location: Japan
Ah, now I see - we're talking ideal systems where such thing as appropriate kite sizes cancels out between Fatty and Skinny.
"You're ignoring ..." No, I'm just ignorant of been many years since my high school physics classes ...
So maximum theoretical jump height is the same based on conservation of energy ...

TT - lets not forget fatty Fatty with bigger balls than Skinny

Top
 Post subject: Re: High Jumps kitePosted: Tue May 15, 2012 2:53 am
 Very Frequent Poster

Joined: Fri Jul 16, 2010 6:27 am
Posts: 1326
Location: Ford Lake, Michigan
jimovo wrote:
The contribution of the kite to lifting the rider AFTER the launch is entirely missed in this model. It is this part of the equation that IS dependant on the mass of the rider

i'll explain again. This particular model does not depend on the mass of the kiter.

there are two vectors.

1) kiter in relation to water

2) kiter in relation to wind

1) For the kiter in relation to the water, he pops. His vector is directed upward. It does not depend on his mass any more than a BMX biker doing a jump depends on his mass.

2) At the same time the kiter redirects the kite to boost.

The model in this case says that the kite has the same mechanical properties as a tack stuck in the wall with a lead weight swung from it. If the tack is big enough (if the kite is big enough), then the lead weight's direction will change from one direction to the other. It does not matter how big your lead weight is, unless you decide to put in a tiny little tack (under power the kite).

Think of it as sticking your kite in the air. You have already popped out of the water, and all of your horizontal motion relative to the water has been converted upwards. But the kite (and yourself) still have a lot of speed relative to the speed of the air, and you want to convert this to vertical height too. You use your kite in the air as a pivot to change your horizontal motion into potential energy. Then both you and the kite rise up in the air.

Now I'm not saying this model is perfect. But it does say, if you have enough kite, it doesn't matter how much more kite you won't boost higher.

So for instance suppose you go out and it's 25 knots and you are lit on your 10 meter.

Then you switch to your 20 meter, and you get worked up and down. You go to jump anyway.

Do you go twice as high on your 20 meter as you did your 10 meter?

Or did the 20 meter just let you drift down a lot slower?

Anyway, I'm not defending this model, just trying to explain it properly so you understand why your objections are not proper objections, they stem from a misunderstanding of this particular model. Even though vectors 1 and 2 come from different places you can just add them together for 1/2mv^2 = gmh, where v is velocity relative to wind, that's how vectors work. Notice the 'm's the mass cancels out here.

Top
 Post subject: Re: High Jumps kitePosted: Tue May 15, 2012 4:59 am
 Rare Poster

Joined: Mon Jun 20, 2011 8:15 pm
Posts: 17
edt wrote:
i'll explain again...

You needn't bother - If you read my previous posts then you will see that I have agreed that in a model such as you describe, the riders mass is irrelevant. Your equations are indeed correct. The reason that you are encountering resistance from kiters trying to relate what you are saying to their real world experience is because the model itself is not an adequate description of what you are applying it to. Why keep attempting to explain it when it obviously misses a large part of what really goes on? As I have said previously, your model is the equivalent of detaching from the kite the instant you are moving vertically - it completely ignores any contribution of the kite in gaining height due to lift generated while you are in the air. Is there anyone here at all that believes that their kite can not continue to pull them in an upwards direction while they are in the air?

Top
 Post subject: Re: High Jumps kitePosted: Tue May 15, 2012 5:17 am
 Very Frequent Poster

Joined: Fri Jul 16, 2010 6:27 am
Posts: 1326
Location: Ford Lake, Michigan
jimovo wrote:
edt wrote:
i'll explain again...

You needn't bother

I think you're right. I described the kite's contribution a couple of times, should have quit after the first time.

Top
 Post subject: Re: High Jumps kitePosted: Tue May 15, 2012 6:00 am
 Very Frequent Poster

Joined: Wed Aug 03, 2005 6:25 pm
Posts: 758
Location: Vancouver
jimovo wrote:
Is there anyone here at all that believes that their kite can not continue to pull them in an upwards direction while they are in the air?

Yes.

Also, ask hang gliders, paragliders and glider pilots why their craft don't glide upwards (barring localized vertical air currents).

Top
 Display posts from previous: All posts1 day7 days2 weeks1 month3 months6 months1 year Sort by AuthorPost timeSubject AscendingDescending
 Page 4 of 10 [ 93 posts ] Go to page Previous  1, 2, 3, 4, 5, 6, 7 ... 10  Next

 All times are UTC + 1 hour

#### Who is online

Users browsing this forum: Google [Bot], Google Feedfetcher, jwoodunlv, The Captain and 27 guests

 You cannot post new topics in this forumYou cannot reply to topics in this forumYou cannot edit your posts in this forumYou cannot delete your posts in this forumYou cannot post attachments in this forum

Search for:
 Jump to:  Select a forum ------------------ Sports    Kitesurfing    Snow / Land Info    Special Offers    User News    Reviews    Safety    Info    Second Hand    Lost & Found    Events & Demos    Kids Club    New Members    Tricks and Moves    Polls    PDF Files    Weather    Contests    Help Me Board Styles    Kiteracer    Airstyle    Wakestyle    Wave Kite Groups    Board Builders    FKA, RickI column    Women's Forum    Flamer's Paradise Travel    Travel Instructors    Instructor Discussions    Africa       South Africa Instructors    North America       Florida Instructors Europe    Portugal    Spain    Germany    France South America    South America    Argentina Middle East    Israel