Howz this Mister:

Agree but more significant in the lower strength than higher strength spectrum of the wind speed

Very significant pull difference from: 5 to 6, then 6 to 7, then 7 to 8, then 8 to 9, then 9 to 10 knots

Pull of the kite difference from 30 to 31, 31 to 32 ,32 to 33 ,33 to 34 , 34 to 35 knots is negligible

Same small 5 knots increase in wind speed

At the lower end.....extremely high pull difference
At the upper end....extremely low pull difference

That is the reason that there is always more discussions and BS about low end wind speed claims

When do you ever see discussions and BS about high wind claims?

Single digit winds beside being difficult to measure.....
.....will always be #1 on people's mind til the end of BS time


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Thanks Peter, this table summarizes an important but often understood principle. I'm curious to know what board speed(s) you assumed, since I'm too lazy to calculate your assumptions from your results. I figure somewhere around 10 mph or 10 knots is a minimum threshold for reasonable board performance, depending on the board. Of course, board drag increases at high speeds and also at very low planing speeds.

Regarding force and power, I wrote an article about five years ago to help provide clarity. In case you're interested:
http://kiteboardbc.com/phpBB3/viewtopic.php?f=1&t=178

Cheers,
James

Interesting article James.

I wonder if you and/or some of the other engineering guys following this can comment on my questions below.

I have always wondered if there is any meaningful way to express a "power" rating for kites by doing wind tunnel tests or some other methods.

Are there ways that characteristics like turning speed and lift (or pulling force) can be shown while the kite is "parked" or being "sined"? And also at what angle does the kite fly relative to the "edge" of the window?

Maybe all those things are just nebulous numbers? What do you think? Is this information that kite designers/manufacturers could publish with any degree of validity or usefulness to the consumer?

thanks

No. Kites in isolation don't generate power, so there's nothing to measure. However, a rider with kite, lines, board and wind acts as a system that can generate power. (If you read my linked article carefully, you'll see that I went into that question.)

Power is a linear function of velocity and a linear function of the force (resistance) opposing the direction of travel. Most of the resistance is caused by the interface between the board and water.

For illustration, here are various ways that power can increase:

1. Trim your kite for more pull (force) and edge your board harder (or on a race board, load your fins harder) to go faster. The faster you go, the more power you'll be generating.

2. Bear off onto a faster reach. Again, the faster you go, the more power you'll be generating (except in specific cases where the board's new attitude reduces resistance proportionally more than speed increases).

3. Switch to a different kite. If the switch allows you to go faster on the same board, then you'll be generating more power. Depending on the conditions, that could mean switching to a bigger kite, a smaller kite, or just a more efficient kite.

4. Switch to a more efficient board. If it allows you to reach a higher speed (before resistance impedes further accleration), then you'll be generating more power than with a slower board. You could say a Spleene Session is usually more powerful than a typical TT, and a race board is more powerful again, and you'd be sort of right. But not literally, because a board in isolation doesn't generate any power.

5. Gain weight, and upsize your kite accordingly. If two riders, one heavy and one light, are going the same speed with different kites but matching boards, then the heavy rider will be overcoming more resistance and therefore generating more power.

6. If a gust of wind increases your speed, you'll be generating more power.

Basically, anything that makes your speed increase also increases power. Alternatively, if you manage to keep going the same speed while increasing resistance (dragging your ass in the water, for instance), then power will (must) also increase.


That is simply a function of the kite's L/D (lift/drag) ratio. Most optimally trimmed modern kites can probably achieve L/D ratios of about 7:1 or 8:1. Simple trigonometry indicates that a 7:1 kite will fly about 6.4 degrees deep (back) into the window and an 8:1 kite will fly about 5.6 degrees deep. In other words, they'll fly 6.4 degrees and 5.6 degrees respectively behind perpendicular to the apparent wind direction.

Cheers,
James

Thanks for the additional explanation JS.

I guess there is still nothing that really replaces flying it for yourself to get the true feel of a kite, which is sort of what I had thought. But it's still interesting to break it down, (if one is into understanding the physics of it).

I am not trying to start any sort of mass war here... but your quotation of the weight of the XR2LW made me want to weigh in.... literally.

I went and weighed the Fat Lady.....(name disregarded... she is actually likely the "leanest" kite on the market at less than .5lbs per meter.)

I just put the Fat Lady on the scale... at 8.25lbs... for a 17m... I don't have the mass of the 17m core... but 17/19 * 10.75= 9.6lbs? I don't have a Core 17 to weigh.. but is that about right? The XR2LW has 5 struts?

If you are saying that light wind "float" is related to the mass of your kite.... then wouldn't a 3 strut kite "float" better?

Squared apparent speed is what is meaningful for static flying (riding with kite parked), yes.

When the kite is sined, however, the force (lines tension) vector displacement over time will generate momentum that will definitely be felt by the rider through the lines as well. Given a fixed line lenght and apparent board speed, this momentum will be directly proportional to the kite's traveling speed across the perimeter of the wind window sphere.
The stronger the wind, the more reactive the kite is, so it takes less time for it to cover a certain arch of the window - in other words, this momentum increases with wind speed - but I wouldn't know how to estimate this function.

In an underpowered (windspeed lower than sweetspot) situation, I'd say neither charts (Peter's squared apparent speed and Hansen's cubed windspeed) are very representative of real life, because the less wind there is, the more the kite will be sined, so what should be compared is A - the lines tension with the kite parked in the sweet spot with B - the lines tension with the kite being sined in less wind. So the actual lines tension difference would be less than what both charts suggest (with the squared chart being obviously less far off than the cubed chart)

For windspeeds higher than the sweet spot, I'd say the squared speed chart is pretty close to the truth - since the more the kite is parked in an overpowered situation, the better. If we take into account the kite's depowering capabilities, however, we should realize that the chart seems much more dangerous than real life!

Kudos to everyone for the excellent analysis!

Edited:
No matter what you feel in your lines or how you fly your kite, the power available has a cubed relation with wind velocity. The point of my chart is the rapid decrease in available power at lower windspeeds.

I don't think so. The kinetic energy of a "given volume of air" is 1/2(mv^2).