How high is it possible to jump with a kite ?

[peterbjarke]

I can assure you kite constructors also use theory.

Mor Nille kan ikke flyve.

OK
Lets make it simple: How high is it possible to jump without anything but a human body. This seems to be interesting far beyond the kiting community. It has been for thousands of years. If you look at the human body as perfect you can jump to March. Physics has limits. This should at least be known among people engaged in a sport depending on fluid mechanics. OK with theories. It's facts that counts. Ask any kiteconstructor.

Mor Nille er en Sten.

[vindsportknut]

It's due to my experience a question about what theories that makes sense, and how they influence on each other which is the issue. Not the simplification.

Hvorfor drikker Jeppe?

[KiteSurfingKen]

I agree with what Peet is proposing. it accounts for all of the forces that would have a considerable impact on the system (after simplification).

The AoA and L/D ratio determine the "Lift" of the kite based on it's size and also on it's design (aerodynamic efficiency).
The Lift of the kite is offset by the kiter's weight.

The challenge here is determining the Lift of the kite. While researching aerodynamic efficiency, I found that airplane manufacturers usually provide this type of data for each model they produce. We are not so lucky.

In theory we could roughly determine the lift of our own kites through a series of tests on the beach.

1. Stand with the kite overhead at 12 o'clock. Slowly power the kite up to 100%. If the kite does not lift you off the ground, you can assume the lift is less than your weight for the current wind speed.

2. Stand with the kite overhead again. This time quickly power the kite from 0 to 100%. This will determine the affect of a sudden AoA change on the kites lift at the current wind speed. Often this will lift you off the ground slightly, meaning the lift is slightly greater than your body weight.

3. Next if you put the kite at 3 or 9 o'clock and fly the kite quickly to 12 o'clock, you can determine the affect of speed on Lift. When doing this test measure the height you reach during the jump.

To calculate the lift is quite a bit more complicated in this case.
You need to determine the time it takes for the kite to travel this distance and the length of your lines.

The circumference of the circle is 2Pi*r, where R = equals the line length. Since you are traveling from 9 to 12, the distance traveled is 1/4 of the calculated value. If you divide this by the time it takes to travel, you can determine the speed in m/s.
To determine the Lift for situation 3 we could use Peet's forumala in reverse since we know all of the other values.

Using a video helps to determine the time and height reached more exactly.

As Peet noted, his model is assuming Lift to be constant. This would only be true if the kite maintained a constant speed and angle of attack throughout the jump.
I think this is an area that additional height could be gained or lost due to kite handling skills.

As you can see from the test above, we can approximate the Lift produced by various actions with the kite.
When I kite to Go Big, I like to be well powered, to the point that Lift in situation 1 is somewhere around 80% of my weight.

Good Winds (and height),
Ken

[JS]

Respectfully, your misinterpretation of the relevant dynamics ought to be evident here...

...Theoretically, once you use a kite big enough (or winds strong enough) for the effect of gravity to be completely nullified by the pull of the kite, you'd keep on going up for ever, until the wind dropped off ...

...if it's not evident, then clarification is probably well beyond the scope of this thread. Peter Frank's representation of maximum jump height ought only make sense to those with more than a basic understanding of classical mechanics and aerodynamics. "A little knowledge is a dangerous thing".

Here are some more threads that might be useful (but I caution that they're likely to be misinterpreted with only a basic or rusty physics background):
http://www.kiteboardbc.com/index.php?pr=Apparent_Wind
http://www.kiteboardbc.com/index.php?pr=Lift_Drag_Ratio
http://www.kiteboardbc.com/index.php?pr ... ,_Hangtime
http://www.kiteboardbc.com/index.php?pr=Biggest_Jumps
http://www.kiteboardbc.com/index.php?pr=Force_Energy
http://www.kiteboardbc.com/index.php?pr ... celeration

Best regards,
James

[KiteSurfingKen]

I have reviewed a number of those threads and I still agree with Peet.
If the Lift generated by the kite at a given windspeed is greater than the kiter's weight,the kiter will rise as they have overcome the gravitational force. This is what happens in a lofting event.

With high depower kite's we are typically able to reduce the lift to less than that of our body weight by adjusting the Angle of Attack.

Actually, that's exactly what it does. And the kite's airspeed is dependent on the line tension. If you increase the line tension, the kite will fly faster, which will increase lift, which will counteract the line tension, which brings us full circle. Without line tension, the kite won't fly at all.

A kite's speed is a function of 1) its L/D ratio, and 2) the tension in the lines. That's it. Nothing else.

http://kiteboardbc.com/phpBB3/viewtopic.php?f=1&t=175

The above is one point I fundamentally disagree with.
Line Tension is an affect of increased speed, not a cause.
The control of the kite's speed is managed by it's Angle of Attack relative to the wind.
The AoA is manged by the Difference in Line Tension between the front and rear lines.
Carving upwind, without adjusting the bar position, will not change the AoA.
It will however store potential energy in the board's edge that is used to break free of the surface tension when initiating the jump.

When a Wakestyle rider loads and pops, the kite's speed does not change. You can see this in many videos which show both kite and kiter.

http://kitesurfingschool.org/kite.htm
This site provides a great explanation of the different kite design elements that affect the ability to generate lift. Lift is what allows us to convert the kinetic energy (speed) to potential energy (height).
Without Lift, the height of the jump would be limited to the energy stored in our legs and the edge of the board (wakestyle load and pop jumps).

Good Winds,
Ken

[peet]

Respectfully, your misinterpretation of the relevant dynamics ought to be evident here...

...Theoretically, once you use a kite big enough (or winds strong enough) for the effect of gravity to be completely nullified by the pull of the kite, you'd keep on going up for ever, until the wind dropped off ...

...if it's not evident, then clarification is probably well beyond the scope of this thread. Peter Frank's representation of maximum jump height ought only make sense to those with more than a basic understanding of classical mechanics and aerodynamics. "A little knowledge is a dangerous thing".

Looking at your link http://www.kiteboardbc.com/index.php?pr=Biggest_Jumps

This line is just plain wrong: " there is a maximum possible theoretical height of jump that can be achieved, due to the physical law that energy must be conserved. You can't create energy from nothing. With an ideal kite, and ideal technique, this theoretical height may be approached, but it can never be exceeded."

This is assuming that energy is conserved - that basically your incoming kinetic energy is converted to potential energy at the highest point of the ideal jump. This is ignoring the fact that through the jump, the wind is continuing to blow on your kite (and generate additional lift forces) the whole time - hence feeding more energy into the system.

The statement would only be true if when you jumped the wind dropped to zero for the entire duration of the jump - then it would be a simple case of conservation of energy.

[peterbjarke]

Yes a little knowledge ca be a dangerous thing. I think I have enough basic understandings of mechanics to know that I don't know enough. That is why I am asking you again: Where did you get the correlation of apparent wind and kinetic energy from? Most of the pages you refer to describe simple facts and are cut and paste. The page with substance is the maximum height page but with no explanation of the key in these discussions: Deriving the kinetic energy from the apparent wind.

Cheers,

Peter

Respectfully, your misinterpretation of the relevant dynamics ought to be evident here...

...Theoretically, once you use a kite big enough (or winds strong enough) for the effect of gravity to be completely nullified by the pull of the kite, you'd keep on going up for ever, until the wind dropped off ...

...if it's not evident, then clarification is probably well beyond the scope of this thread. Peter Frank's representation of maximum jump height ought only make sense to those with more than a basic understanding of classical mechanics and aerodynamics. "A little knowledge is a dangerous thing".

Here are some more threads that might be useful (but I caution that they're likely to be misinterpreted with only a basic or rusty physics background):
http://www.kiteboardbc.com/index.php?pr=Apparent_Wind
http://www.kiteboardbc.com/index.php?pr=Lift_Drag_Ratio
http://www.kiteboardbc.com/index.php?pr ... ,_Hangtime
http://www.kiteboardbc.com/index.php?pr=Biggest_Jumps
http://www.kiteboardbc.com/index.php?pr=Force_Energy
http://www.kiteboardbc.com/index.php?pr ... celeration

Best regards,
James

[JS]

Yes a little knowledge ca be a dangerous thing. I think I have enough basic understandings of mechanics to know that I don't know enough. That is why I am asking you again: Where did you get the correlation of apparent wind and kinetic energy from?

Following is a hint copied from a link I posted earlier in this thread, in case you didn't read it. If this doesn't help, I think I might be out of time on this one. BTW I'm no stranger to concepts that, although seemingly straightforward, will never be grasped by all members of this forum. A good case in point was the straight-downwind-faster-than-the-wind concept which brought ridicule upon Rick (spork), Klaus and myself... until Rick built the real thing and NALSA ratified its speed at almost triple the windspeed. Anyway, here's the copied bit:

To make the dynamics of jumping a little clearer, consider this:

You are sitting on the stern of a powerboat going 20 mph on a calm day with absolutely no wind. You are flying a big kite at the 12:00 position. Your friends are in the boat holding your harness so you don't lift off. Then, you trim your kite for maximum lift [including a hard turn if necessary] just as your friends let go of you. As you lift off, forget about the boat and the water below you. All that is relevant is the motionless airmass that you and your kite are flying through.

At first, the only source of energy that you have is kinetic energy. But just like a glider that has been catapulted down a runway, you can trade that kinetic energy for potential energy by climbing. As you climb, you'll slow down. How high can you go?

Assuming perfect aerodynamics (infinite L/D ratio) and perfect technique, your maximum height will be v^2/19.6m/s^2, which is about 13.5 feet.

If you do the same thing at 40 mph, your maximum possible height would be about 54 feet. Or, at 80 mph (70 knots), 216 feet would be the limit, if you don't die.

As I said before, for maximum height, you have to convert as much of your kinetic energy as possible to potential energy. That means you (not necessarily your kite) should arc upwards and slow to a near standstill at the apex of your jump. As you do, your kite will overfly you. Unless your line lengths are totally screwed, you won't be able to stop it, and it will keep on flying at full speed as long as you are hanging from it. What you do after the apex is up to you, but I'd suggest redirecting your kite back the other way to avoid a rapid descent and backflop.

What you have to do in this example is resist the temptation to think in terms of upwind or downwind, because there is no wind. Remember?

Now, you can apply the same examples to kiteboarding jumps, lifting off into apparent wind of 20 and 40 mph respectively. As soon as you lift off, forget about the water below you. Pretend it doesn't exist. All that matters is your flight path through the airmass, and that flight path should match that of the boat-launch example, including your maximum possible heights. The only time that the water will become relevant is when you land, and you discover that the water is moving at 20 or 40 mph relative to the air.

James

PS edit: As I've also referenced earlier in this thread and related links, electrochemical energy from your legs can slightly contribute to your jump height, elastic energy stored in your lines might add a few inches, and wave energy from advancing waves can also add limited height. Receding waves will do the opposite (sorry, can't be bothered to explain this - Peter Frank, I think I see between the lines that you understand it). In reference to this thread's topic, however, all of these factors will be trumped in most cases by aerodynamic losses.

I write on this and other forums to provoke thought and understanding, but I acknowledge that I haven't remotely the time or patience to replace a proper education in classical mechanics or fluid dynamics.

Very best regards,
James

[JS]

This line is just plain wrong: " there is a maximum possible theoretical height of jump that can be achieved, due to the physical law that energy must be conserved. You can't create energy from nothing. With an ideal kite, and ideal technique, this theoretical height may be approached, but it can never be exceeded."

Just plain wrong? Although you appear confident in your assertion, I respectfully suggest that you work through your logic again.

...This is ignoring the fact that through the jump, the wind is continuing to blow on your kite (and generate additional lift forces) the whole time - hence feeding more energy into the system.

You are correct, but only in the case of a kiteboarder that's tethered to something stationary, or to something that is moving relative to the airmass. Typically, that's called parasailing.

[KiteSurfingKen]

James you come across as very condescending.
You propose Theories without evidence or mathematical proofs to support those theories.
This makes your statements conjecture and not fact as you claim.

One of the first and most important lessons they teach in science class is "Do my results make sense" and "Do my results answer the question being asked".

This is what struck me as I looked at the Tables in graphs in Peter_Franks post, the numbers simply did not make sense in practical terms.
We all know that we can jump more than 3m with a 14m kite.
So either underlying assumptions were wrong or the basis for the formula did not address all apsects of the system.

As Peet pointed out, Lift is continuing to be generated throughout the jump by the kite. We are not dealing with a static system here. The speed of the wind over the kite canopy is what controls the amount of lift generated. Even in the absence of apparent wind generated by the riders speed, there is still actual wind speed feeding into the system as he noted.
In this case it is the airmass that is moving relative to the kite's position, we call this wind.
This is the same principles as paragliding,except unlike the foils they use, our kites normally provide slightly less lift than our body weight, causing us to return to the water after the jump.

Your comparison to a boat proves Peet's point, as you are stating in this case there is no actual wind, only the apparent wind.

Of course all forum readers can understand that a kite can not fly without wind (all to well I am sure), so using a power boat to prove your point does not apply to the world in which we fly our kites and is therefor does little to support your theory.

I would be interested to see some mathematical proofs to support your theory, as Peet has provided. Otherwise this Peer is inclined to review your work as incomplete and unsupported.