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Author Topic: Ground Level Wind vs. Actual Wind on the Kite  (Read 1672 times)
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Kitemac
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« on: September 14, 2010, 12:56 PM »

Not sure there is a clear answer on this but the other day when my meter was showing 12 mph at ground level my Hypnotist frame was showing quite a bit of flex going through the center of the wind window.  When this happens the dual kites are put away and the Rev comes out. 

When the wind comes off the water I do not see a big difference in wind speed (based on frame flex) but from the land it looks like there could be a pretty big difference. 

Any ideas on how high the wind speed is on the kite vs. ground speed?  Is the manufacturer spec based on wind speed on the kite or the ground?

Is frame flex a good indication that the wind is too high for a particular kite?



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sealcove
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« Reply #1 on: September 14, 2010, 01:11 PM »

I would think this would be specific to local topography, and could vary wildly from point to point.  Depending on the situation you could easily have higher or lower speeds above you, and it could constantly be changing as well if you are dealing with orographic turbulence.  This was certainly my experience from my pilot days.

« Last Edit: September 14, 2010, 01:16 PM by sealcove » Logged
tpatter
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« Reply #2 on: September 14, 2010, 02:33 PM »

Flex is generally not good.  A bit of steady bend (much like a Rev does) doesn't both me, but if it is wobbling around, it is in danger of breaking.

It the wind is bumpy and high, thats bad - as you go across the window it will be difficult for both you and the kite to have the wind go instantly from 6 to 18 again and back again.  Trying to do slack line tricks in this sort of wind can be very challenging - one second the kite is powered up, the next it is just floating.  Most kites handle a steady high wind much better than severe fluctuations.

Where I fly there is often a big difference in the wind at ground level vs at 7 feet vs at 60 feet.  The bigger the difference, the more challenging the flying.  Ocean wind is almost always 99% better, but I imagine it depends on the local geography and certainly the weather.  I love flying at the coast - it seems consistent from 1 foot to 120.

I would hope that the manufacturers specs are based on smooth wind, but they are generally not too accurate anyhow - exaggerating both the "real" low and high wind range for the kite.  The boutique kites estimates seem pretty accurate, however (at least in my experience).
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6 kite tom
Allen Carter
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« Reply #3 on: September 14, 2010, 02:48 PM »

When I was new to this (but not so new I hadn't bought a high end kite with expensive delicate sticks) I had a traumatic ground wind experience.

I set up on a feild that had a low hill and trees 50-100 yards upwind. Normally these obstructions hadn't been a problem, but the wind was coming in about 90 degrees westerly than normal. The (ground) wind was very light so I put on 75' x 50# lines. I knew from expedrience I'd have to pump the kite to get some altitude...

So I yank the kite into the sky and as it went above about 20' a whole lot of things happened at once. It felt like a big had had reached down and yanked on the kite. It looked like a goose hit by a shotgun at close range. The kite sort of exploded and crumpled at the same time. One line broke. The the ball of poly, mylar and carbon spun to the ground on the remaining line.

Spreader, spine and LLE were broken. This was a kite known for being hard on sticks, it had stiff thin walled tubes curved under a lot of tension.

I've thought about the incedent a lot over the years (it was in 1999) and still haven't seen anything quite like it. Did the line break cause the implosion? Did other way around? Just how hard was it blowing?

It must have been a gust of over 15mph right as I was hauling the kite up in a power launch but the steady wind was less than 5mph on the ground and probably 10 at treetop level.

I look to trees for wind info. A big tree will give you a ground level and a high level perspective.
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Allen, AKA kitehead
indigo_wolf
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« Reply #4 on: September 14, 2010, 03:10 PM »

There's a lot to be said for side launching.

Hot launching a 9-10'  kite with a deep sail in seemingly moderate winds will prove how much more fragile your ego is than any kite part. Looking around on the fields for stand-offs, after the fact, gives you enough time to reflect and fully appreciate this. 

No seagulls were killed or maimed during the entire incident, but pretty sure a couple of them had a collective "Oh Crap!!!  Huh moment.

ATB,
Sam
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RonG
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« Reply #5 on: September 14, 2010, 04:06 PM »

I don't think any manufacturer has anything but "wind speed at the kite" in mind when they state wind ranges.  There is no consistent way to translate wind speed on the ground to wind speed at altitude - there are far too many variables.

You simply have to learn to recognize the times there are likely to be big differences between the two.  The most obvious examples are when large objects on the ground (trees, buildings, hills) are acting as significant wind breaks.  The beach at Ocean City MD comes to mind, with an offshore wind.  I've seen kites limp along in the wind shadow thrown by the buildings close behind, only to explode when they reach a gap between the buildings.
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Gardner
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« Reply #6 on: September 14, 2010, 05:40 PM »

If you want excitement, have a whirlwind, dirt devil or thermal catch you brand new two line kite in the air.  Ever see a 150-pound test swivel straightened out? Huh

Gardner
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Kitemac
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« Reply #7 on: September 15, 2010, 12:51 PM »

I didn't think there would be an easy answer but it does explain why when I pick a kite based on ground level wind speed only to find the wind too strong.  Fortunately no kites were damaged and I probably bring the kites down before they are being too stressed although some of the ULs did not look happy in flight.

Thanks for the replies.

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Ca Ike
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« Reply #8 on: September 24, 2010, 08:00 PM »

Well first off the Hypnotist frame is designed to have a lot of flex to be durable enough to handle the abuse beginners dole out to a kite.  It can also  handle 20 mph winds easily (25 if you set to max light wind on the bridle)

As far as wind speed at ground level vs at altitude there can be a big difference.  If i remember right you can add 2mph for every 20 feet of altitude and be pretty close to what actual measurements will be or you can just keep a 20 foot telescoping pole around and stick your meter on top Tongue  The best thing to do is find a tree thats taller than any obstruction up wind of you and watch how it sways and the difference in the leaves at different heights.  What kills most kites and breaks line sets is the jump from one speed to another as you get above those obstructions and thats my biggest reason for always launching 30 degrees off from center window.  Much easier (on your arms and the kite) to gauge that jump if your off to the side of the window.
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Albert
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« Reply #9 on: September 25, 2010, 01:22 PM »

Yup, happens all the time. In engineer speak, this comes from something called boundary layer theory. Basically, the friction on the surface (ground) reduces the velocity dramatically, and the further away the faster the velocity to max. By the way, the national weather service standard is for wind speed to be measured at a height of 10 meters.

Albert
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Ca Ike
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« Reply #10 on: September 25, 2010, 03:23 PM »

Ah I was off by 10 feet. Oh well I still say unless the wind is light and there are no obstruction up wind it's still a good habit to launch to the side of the window.
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