Kiteboard Theory - Thread 7: The Biggest Jumps

[James]

It takes energy to jump. Where does that energy come from?

The answer is kinetic energy: specifically that resulting from your forward motion through the air. Not your motion over the water, and not relative to the earth. Just relative to the air.

Through the manipulation of aerodynamic forces, your kite can be used to convert kinetic energy (an exponential function of airspeed) into potential energy (a linear function of height). In a particular set of conditions, 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.

I should mention that there are a few other sources of energy that can augment the height of a jump. They are:
1. Kinetic energy stored in a wave as you ride up the face.
2. Electrochemical energy stored in your muscles that can be converted by contracting your muscles to accelerate your mass upwards against the vertical force of the water against the board.
3. Kinetic energy related to vertical air currents as described in "Kiteboard Theory - Thread 3: Lift, Looping and Hangtime".
These sources of energy are usually minor and I won't dwell on them any more in this thread.

Now, back to the conversion of kinetic energy to potential energy:
To achieve the highest jumps, you must start with lots of kinetic energy, as a function of airspeed. There are two obvious factors that are relevant to doing this:
1. Ride in strong wind.
2. Maximize your apparent windspeed at the instant you take off. This means riding fast in a relatively upwind direction. That's why carving hard upwind without losing too much speed is vital to good jumps.

Assuming that all of your basic technique is good, your highest jumps will result from efficiently converting as much of your kinetic energy as possible into potential energy. Ideally, at the apex of your jump, you will have no kinetic energy at all, which means that your horizontal airspeed will be zero, which means that you will be swinging below your kite downwind at exactly the same speed as the wind. This scenario can generally be achieved with the right timing.

Here are the fundamental factors to putting it all together:
1. The windier the better.
2. Just before jumping, ride fast and carve hard upwind to maximize your apparent windspeed.
3. Send your kite back aggressively to take off. The best technique is a matter of trial and error.
4. While jumping, your kite should be trimmed for "minimum sink rate", which might be different than the trim for maximum L/D ratio. That probably means your bar should be trimmed as tight as possible without inducing any aerodyanamic stall. You can probably only know for sure that you have ideal trim if you use telltales (which I'll go over in another thread).
5. Keep your kite back just enough that your pendulum motion matches as closely as possible to the windspeed and direction when you are at the apex of your jump. Again, trial and error. If you actually get it right, and you happen to be smoking at the apex, you could blow gentle concentric smoke rings. (After the apex, don't forget to dive your kite aggressively forward, or you'll do a nasty backflop.)

Here are a couple more thoughts about energy conversion and conservation:
1. No matter what, you'll never convert all of your kinetic energy into potential energy because there will always be some losses relating to aerodynamic drag, which effectively energizes the air by way of turbulence.
2. As I mentioned before, kinetic energy is an exponential function of airspeed, but potential energy is a linear function of jump height. That means that if you double the airspeed at lift off, your theoretical best jump height will be four times as high. Or another example: If you have the right equipment, nerve and skill, you should be able to jump nine times as high in 45 knots apparent wind as you could in 15 knots apparent wind (17-18 mph on iKitesurf), which could be pretty !@#$% high.

Cheers,
James

[cglazier]

Excellent description again James. This is consistent with what I understand are the 3 factors contributing to really big jumps:
1. go fast
2. send the kite back hard
3. briefly edge very hard (progressive edging)

One point.

.. Maximize your apparent windspeed at the instant you take off.

You certainly want to maximize your apparent windspeed before you take off, but in the very last second while you are edging, you are actually slowing down (horizontally) and converting this energy into loading up the kite (creating a pressure on the kite lines which will then pull you vertically). You may actually have little forward speed at the instant you leave the water, but the strong kite pull which you have created will squirt you upward.

Your kinetic energy (from your forward speed) is briefly converted into stored energy in the kite (we often call 'loading it up'), and then reconverted into vertical kinetic energy.

Anther part of what is happening is that by edging very hard, you create a tiny ramp, and the pull of the kite will shoot you up this ramp.


CG

[James]

You certainly want to maximize your apparent windspeed before you take off, but in the very last second while you are edging, you are actually slowing down (horizontally)...

Slowing down is an unfortunate consequence of edging or carving upwind; it definitely isn't the desired result. The degree to which you should carve upwind, balanced against the speed that you lose in doing so, is a matter of trial and error. But I can't emphasize enough, the reason for carving upwind is to maximize your apparent wind at the instant you take off. Your board speed alone is irrelevant. All that matters is your apparent wind!

...and converting this energy into loading up the kite (creating a pressure on the kite lines which will then pull you vertically).

Chris, I think it's been too long since you retired from engineering! Energy and kite-line tension are two completely different concepts which can't be interchanged (not to mention that pressure is a function of area).

The only way that the lines or the kite can store any energy at all is by stretching, and the amount that they do stretch in this context is totally insignificant. (If you used bungee cord for lines it would be a different story.) What is actually happening when you "load" your lines by heading up toward the wind is that you are increasing your apparent wind, which creates more lift from your kite, and consequently more tension, or force, in your lines.

Here's an analogy: Think of a gantry crane used for lifting containers. If you anchored the lifting hook to the ground and then pushed the "up" control, within an instant there would be tens of thousands of pounds of tension, but almost no stored energy. Then, if the hook was released from the ground while you held onto it, you'd only get "squirted" upward a few inches (if that), due to the very limited stored elastic energy in the lifting cables. (But you could get shot hundreds of feet into the air by a very long crane-mounted bungee cord, even if the tension was only a couple of hundred pounds.) Again, force and energy are totally different from each other, as is power.

You may actually have little forward speed at the instant you leave the water...

I agree, but again, what's relevant is apparent windspeed. If you stand still in a 60 mph wind, then you'll have 60 mph of apparent wind (or airspeed). That results in a lot of kinetic energy, which can result in one helluva jump (lofting) if you happen to be attached to a big kite.

Cheers,
James

[Mike]

Interesting and though provoking article, James.

One point, however:

Ideally, at the apex of your jump, you will have no kinetic energy at all, which means that your horizontal airspeed will be zero, which means that you will be swinging below your kite downwind at exactly the same speed as the wind. This scenario can generally be achieved with the right timing.

I think I've done a few ot those, but it was by no means ideal . If you have zero airspeed at the apex of your jump, your kite will be stalled, probably hindengurging, and you will be dropping like a stone. Pulling your front line won't do any good as your kite will no longer be flying. Plus you'll be travelling "downwind" with relation to the water at whatever the windspeed is, prior to hitting it (ouch).

At the apex of your jump you still want enough airspeed at your kite to keep it flying and in control.

I'm a mechanical engineer as well as a pilot with 4500hrs in fixed and rotary wing aircraft, and yes its been a long time since I was actively working these areas, but I'm pretty sure you don't want zero airspeed at the apex.

Cheers,

Mike

[cglazier]

Good points James. I have been waiting for your response.

We agree that there is virtually no elasticity in the kite or lines. And certainly energy and force are different ..I was just describing how a kiters forward kinetic energy is eventually converted into what the kiter feels as upward force.

But let's discuss this.

.. the reason for carving upwind is to maximize your apparent wind at the instant you take off. Your board speed alone is irrelevant. All that matters is your apparent wind! ..What is actually happening when you "load" your lines by heading up toward the wind is that you are increasing your apparent wind, which creates more lift from your kite, and consequently more tension, or force, in your lines.

Your post has got me thinking and I have another way of describing what happens. Take for example a pop type of jump where the kiter does not move the kite at all, just loads up and pops off the water. His apparent wind is not directly increased because his kite doesn't move. So how is he loading up the kite? I have been thinking and looking for some elasticity somewhere (which we agree is not in the lines or kite), so I think it is in the air which is very elastic when compressed.

When a kiter briefly heads upwind, he builds up air pressure upwind of the kite. This as a region of slightly compressed air which is highly elastic. This is promptly converted to airflow over the kite or apparent wind as you say. And this apparent wind creates the extra lift which pulls the kiter hard. We agree on the resultant effect.


CG

[cglazier]

Mike
You are quite right. This is undesirable as I have found out myself many times.

If you have zero airspeed at the apex of your jump, your kite will be stalled, probably hindengurging, and you will be dropping like a stone. Pulling your front line won't do any good as your kite will no longer be flying.

I think Jame's point was simply that for maximum height you want to use all your energy going upward. But in practice you want to save a little to keep your kite moving. Kites do not make good parachutes, they need forward airflow to work.


CG

[James]

If you have zero airspeed at the apex of your jump, your kite will be stalled, probably hindengurging, and you will be dropping like a stone...

At the apex of your jump you still want enough airspeed at your kite to keep it flying and in control.

You're right, but you're confusing the kite with the rider. Read my post again:

Ideally, at the apex of your jump, you will have no kinetic energy at all, which means that your horizontal airspeed will be zero, which means that you will be swinging below your kite downwind at exactly the same speed as the wind.

(and)

5. Keep your kite back just enough that your pendulum motion matches as closely as possible to the windspeed and direction when you are at the apex of your jump...

I didn't say your kite should have no airspeed. I said you should.

At any instant, your kite's airspeed is a function of its trim and the force in the lines. As long as you are hanging below your kite, and you maintain reasonable trim, your kite will keep flying through the air at a roughly constant speed. But regardless of what your kite is doing, you are about a hundred feet away and can swing independently below it. And the vast majority of energy stored in your kite/rider/board/gear system, whether kinetic or potential, is related to you, your board, and your gear, because it has many times the mass (weight) of your kite.

So again, if your airspeed reaches exactly zero at the apex of your jump, you will have converted every possible bit of kinetic energy to potential (height-related) energy. That means, you will be swinging downwind, at the same speed as the wind, as your kite flies through the air over you. Daoud has seen me jump this way, and commented that he thought I was going to crash badly. But by redirecting the kite at the last possible instant, I can get it back downwind and ahead of me where I need it for a decent (but slow) landing.

I could create a huge paper to represent the dynamics and geometries at play during such a jump, but the net result would simply support the law of energy conservation, and represent exactly the same conclusion.

Cheers,
James

[James]

Take for example a pop type of jump where the kiter does not move the kite at all, just loads up and pops off the water. His apparent wind is not directly increased because his kite doesn't move. So how is he loading up the kite?

Any kite that is flying is "loaded" to some degree. As long as there is any vertical component of lift, the rider's kite will convert some kinetic energy to potential during his leg-or-wave-assisted wake-style jump. And more so if he carves a little upwind to increase his apparent windspeed.

I have been thinking and looking for some elasticity somewhere (which we agree is not in the lines or kite), so I think it is in the air which is very elastic when compressed.

When a kiter briefly heads upwind, he builds up air pressure upwind of the kite...

Sorry Chris, but I'd say that's all hocus pocus. With the degree of aerodynamic loading on our kites, elastic energy stored in the air is utterly negligeable. Furthermore, when the kite is flying through the air at 20-50 feet every second, it would outfly any stored elasticity in the blink of an eye, even if it did exist.

Chris, when you develop your explanation further, I think you will find that it exactly parallels what I have just described. I do see where you're going with it, I think.

Cheers,
James

[cglazier]

Chris, when you develop your explanation further, I think you will find that it exactly parallels what I have just described. I do see where you're going with it, I think.

Interesting topic James. I agree that we agree , we are just discussing how to describe the same phenomena.

When a kiter loads up for a jump by edging hard, he exerts extra force or pull on the kite lines. This cannot directly increase the apparent wind speed because it is not in the right direction. All it can do is increase air pressure under the kite. This pressure can only be released by increased airflow over the kite.

The increased pressure area does move along with the kite. Think of the space shuttle returning to our atmosphere. It creates a high pressure zone in front of its nose which travels with it for many minutes until it slows down. Similarly the kite will not outrun the increased pressure until the kiter is in the air and being pulled downwind toward the kite ..and therefore no longer pulling hard on the kite lines.

So as you say James, loading up the kite results in increased apparent wind. This is certainly the ultimate result. My point is that loading the kite converts the kiters forward kinetic energy first into increased pressure (potential energy) and then back into kinetic energy shooting the kiter upward. If you did not have this increased pressure or energy storage, the kiter would instantly begin losing power when he left the water. But as we all have felt, a kiter continues to be accelerated during the early part of his jump. This is the pressure being relieved under the kite.

Do we agree James?

And Mike, please jump in here anytime. I'd be interested in how you describe what's happening when you "load up for a jump".


CG

[James]

Do we agree James?

Not even close, I'm afraid.

When a kiter loads up for a jump by edging hard, he exerts extra force or pull on the kite lines. This cannot directly increase the apparent wind speed because it is not in the right direction.

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. Anything else that you do to influence the kite will do so by manipulating one or the other of these two parameters.

If two guys jump out of a hot air balloon, each flying identical, identically-trimmed kites, the heavier rider will glide faster. With the same L/D ratio, they'll follow the same glide angle, but the heavy dude will go proportionately faster, horizontally and vertically. The only reason that his kite knows to fly faster is because it has more tension in the lines (due to the heavy rider). Same thing on the water.

The increased pressure area does move along with the kite. Think of the space shuttle returning to our atmosphere. It creates a high pressure zone in front of its nose which travels with it for many minutes until it slows down.

Yes, but you're confusing issues. Zones of varying pressure do follow any foil or shape as it moves through air, or any other fluid (including incompressible fluids, I might add), but that has nothing whatsoever to do with stored elastic energy that is relevant to this discussion.

My point is that loading the kite converts the kiters forward kinetic energy first into increased pressure (potential energy)...

When you say "pressure" I think you mean "elastic energy", but any such phemonenon probably isn't worth even one inch of jump height, literally. I think you want to believe that there is some huge "air spring" contained somewhere up there, with enough stored energy to hoist you substantially, but it just ain't so.

If you did not have this increased pressure or energy storage, the kiter would instantly begin losing power when he left the water.

I think you mean losing "energy"; not "power". Totally different concepts. And again, the kiter doesn't so much lose energy as convert it from kinetic to potential energy. Once it's all converted, you just can't go any higher. If you could, you would have singlehandedly violated the laws of physics and solved all of the world's energy supply challenges.

Bonus points:
1. Even if the rider makes a perfect jump, with optimal energy conversion, some energy will still be liberated to the atmosphere as kinetic energy by way of turbulence, and the extent is dependent on the aerodynamic efficiency (L/D ratio) of the kite, lines, rider and equipment as a system.
2. If the kite/rider/board/gear system had any sustainable "power" during a jump, which it doesn't, it would keep going up, which it doesn't. But that's a whole other discussion. Klaus and I covered that one on Kiteforum about a month ago.

Cheers,
James

[cglazier]

Geez you can sure tell it's a no wind day for all of us . Or even worse, it may have been kiteable at Squamish and we missed it.

Anyway, I tried to emphasize that a kite cannot directly convert pull on the kites lines to apparent windspeed. But certainly it eventually does. We are just arguing semantics. For pull on the lines to result in movement of the kite at right angles to this pull, there must be another force (which is air pressure). The kite line pull creates air pressure which results in airflow over the kite (increases the apparent wind). When you pull the kite toward you, and it goes sideways there has to be another force involved. That's all I'm saying.

I'm going to think about the "huge air spring". I think you are right Your point about incompressible fluids makes some sense. And I realize that wakeboarders manage to pop off the water without this effect.


CG

[James]

Anyway, I tried to emphasize that a kite cannot directly convert pull on the kites lines to apparent windspeed. But certainly it eventually does. We are just arguing semantics. For pull on the lines to result in movement of the kite at right angles to this pull, there must be another force (which is air pressure). The kite line pull creates air pressure which results in airflow over the kite (increases the apparent wind). When you pull the kite toward you, and it goes sideways there has to be another force involved. That's all I'm saying.

Agreed.

Is Topanga Cafe closed on Sundays or Mondays?

[Mike]

And Mike, please jump in here anytime. I'd be interested in how you describe what's happening when you "load up for a jump".

CG, my head is starting to hurt...I hope there is no final exam on this .

I think the load and pop does convert some energy to potential (spring) energy - a combination of both the lines (there is some elasticity there) tand the spring in the board, and maybe even the surface tension on the water(?). I think this spring energy gives more than a few inches of jump height, but probably not much more than a few feet.

The carve upwind (load and pop) does also slightly increase your apparent wind speed as James says, but probably the biggest effect it has by tensioning the lines is to allow you and the kite to more efficiently convert your kinetic energy to vertical potential energy (height).

That's it - time for a beer .

Cheers,

Mike

[James]

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 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

[Nobben]

Hi James,

I read all your threads and founded them very interesting. Scientifically very sounded too, which I like. I remain however a bit frustrated with #7 which doesn't put so much emphasis on edging nor on moving the kite quickly in the window.

In deed the boat experience would work without (edging so much nor) moving the kite quickly in the window. That doesn't fit with what I see in kite jump movies nor with what I feel from my first jumps.

I am hopefully still a bit better in physics than in kitesurf and I am wondering if the answer could not be at the very end of your #2 :

- edging is a little bit like standing on the ground with fixed feet. It allows you to send the kite into the window, to get a high kite speed (L/D) and a stronger lift (exponential) ;

- whereas not edging would be like being on the ground with roller skates. When you send the kite into the window, the lift might increase a little bit but not a lot as you immediately go downwind and lose your apparent wind speed ;

- in the former case this strong lift could give you some energy from the moment you release the edge until the moment you leave the water (F.dx), couldn't it ?. Afterwards nothing to add energy in the wind referential (the boat image).

Possible ?

Bruno (from France - excuse my English)