Power Kite Forum - Wind speed vs. your speed

Wind speed vs. your speed

power - 5-10-2009 at 08:07 PM

Is it possible to go faster than the wind speed in any condition, on any board and on any surface?

macboy - 5-10-2009 at 08:26 PM

Theoretically yes. The understanding I have is that if everything is perfect (and the moon is in line with Jupiter) you *should* be able to travel at 3 times the current wind speed. I've not found this state of zen myself but am certain I've doubled it. Coastal Wind Sports has a good scientific explanation of this if I recall. I know you're about to get more credible answers after mine so take mine with a grain of salt ; )

Kamikuza - 5-10-2009 at 08:36 PM

I think 3x is a bit optomistic - I've read 2.2x IIRC but that's splitting hairs :lol:

the problem is loses to friction, rolling resistance etc that mean realistically, you could expect maybe 1.5x in reality ... but isn't that enough?

over-unity dude, free energy man, power to the people brother don't bogart the joint

Maven454 - 6-10-2009 at 03:04 AM

We had someone hit ~50 mph at Wildwood, and the wind was only in the low 20s.

Kamikuza - 6-10-2009 at 03:50 AM

Ooh maybe I got it back-to-front then - maybe it was 2.2x practical ... my memory is pretty bad - I'm getting old and I'm medicated is my excuse :D
I have a book called "Stunt Kites 2" that discusses all that wind speed, apparent wind, something-or-other made good ... actually I'm not even sure of the name of the book :lol:

BeamerBob - 6-10-2009 at 03:56 AM

My understanding is that the multiple of wind speed goes down as the wind speed goes up and the kite size goes down. I've personally gotten 4 times the wind speed with Angus' 10.8 Reactor in 5-5.5 mph winds and went just over 22 mph. I've regularly hit 3 times the wind speed with my 8.3m Reactor. Both of these were on a flat beach with relatively hard sand. People breaking personal best speeds typically don't quite hit 2 times the speed of the wind because they are in high winds using around a 2m kite. I understand the dry lake beds are harder and faster than tidal beach sand and therefore could offer even higher ratios than a hard sand beach. I have to find that out for myself though. :wee:

WELDNGOD - 6-10-2009 at 05:24 AM

google the Bernoulli priciple. under optimum conditions you can easily go 3x the wind speed.

acampbell - 6-10-2009 at 05:52 AM

I have routinely done 4x the speed of the wind with the big Reactors. Larger kites seem to be more efficient, as less of a percentage of their area is given up to tip losses and other forms of drag. They just have a smaller usable wind range, at least with my un-weighted 70 lb Bigfoot. So speeds of 15-20 mph are not exactly blistering but very satisfying with winds of 4-5 mph. Even more satisfying is ghosting past buggy camp under full sail and seeing my winds sock and streamers hanging near-still.

And yes, I have duplicated this several times and in both directions in a given session.

Having the new Weatherflow wind sensor on Kite Beach on Jekyll Island this year has been a real treat for experimenting. I can read it in near real time from my iPhone via iKitesurf.com and verify what the wind was just doing, so I can know if there was an errant gust during a session. When I get a true off-shore breeze, it is like having my own giant wind tunnel.

Kamikuza - 6-10-2009 at 06:15 AM

jeez how old is my book? :lol:

Houston AirHead - 6-10-2009 at 06:40 AM

when ever i go out on my 12m and its an on shore wind around 10mph, i consistantly hit 20mph and above

acampbell - 6-10-2009 at 07:00 AM

Quote:

jeez how old is my book? :lol:

Old but useful in its way. Stunt Kites II by Servaas van der Horst and Nop Vellthuizen (c) 1994, 1995 and now out of print. It is the only text I could find with any kind of polar curves to plot theoretical speeds at various points of sail and different L/D's (lift/ drag ratios). However, the kites used then (like the 2-line Sputniks) were slugs compared to what we have today, and the max L/D they used was a value of 3. According to the authors...

"The highest buggy speed of 2.3 times the speed of the wind is possible on courses between 100 and 110 deg. from the wind."

According to a recent text from Peter Lynn about the Reactor, "An L/D of 6 is at the high end of what currently available kites can attain..." They suggest that this value would allow a kite to get just closer than 10 degrees from the edge of the window (or just over 80 deg. from downwind) I do not know how that figure of 6 was arrived at, but those in the industry that I have asked surmised that it was derived from design software calculations rather than empirical measurements.

I once came across references to a grad. paper where someone was going to calculated kite L/D by plotting kite speed (using radar) against wind speed, but I have lost that thread.

flyboy15 - 6-10-2009 at 08:59 AM

Think of it like a water skier behind a boat. When following the boat directly, you are matching the speed of the boat. This is the same as going directly downwind with a kite

If you start to cut back and forth behind that boat, or swing out of the wake around turns, your speed increases dramatically. The boats speed hasnt changed, but yours has because your getting closer to moving perpendicualr to the boats direction.

In theory, with everything 100% efficient, the more you hold your direction perpendicular to the wind's direction, your speed will actually increase to infinity :evil: lol. But of course, we have nothing that is 100% efficient, and we have all kinds of board, surface, kite, line, rider drag, so our speed only increases "quite a bit" lol instead of infinitely.

The numbers discussed are what really happens haha.

Good question!

power - 6-10-2009 at 09:31 AM

Thats what I figured. I didn't think it was uncommon to go faster than the wind speed. But think about. If a flier has a kite out and the winds are 15 mph and the flier is standing still, the air is hitting the kite at a speed of 15 mph. If the flier gets in a buggy and starts moving, if the buggy gets to a speed of 10 mph, the air is only hitting the kite at speed of 5 mph. Once the buggier gets to 15 mph, the kite is moving exactly with the wind and the speed of the wind is 0 mph relative to the kite. And if the kite buggier goes fastert than the speed of the wind, there would be a wind coming into the back of the kite. So how is it possible to go faster than the wind speed?

Maven454 - 6-10-2009 at 09:35 AM

Because you're not moving in the same direction as the wind. You're moving at an angle to it. So the wind at 15mph is always hitting at 15mph. And then you have to add in the apparent wind caused by your buggy moving. The big problem is that as you move, the usable wind (apparent + actual) direction gradually starts to move behind you, causing your kite to stop pulling you forward. Check these instructions by Angus --> http://www.coastalwindsports.com/BetterBuggyBasics.html#10

BeamerBob - 6-10-2009 at 09:35 AM

Quote:

Originally posted by power
Thats what I figured. I didn't think it was uncommon to go faster than the wind speed. But think about. If a flier has a kite out and the winds are 15 mph and the flier is standing still, the air is hitting the kite at a speed of 15 mph. If the flier gets in a buggy and starts moving, if the buggy gets to a speed of 10 mph, the air is only hitting the kite at speed of 5 mph. Once the buggier gets to 15 mph, the kite is moving exactly with the wind and the speed of the wind is 0 mph relative to the kite. And if the kite buggier goes fastert than the speed of the wind, there would be a wind coming into the back of the kite. So how is it possible to go faster than the wind speed?

You almost never go directly downwind (for long) in a buggy or any other wind powered vehicle. Your direction of travel will be sideways to the wind with the wind hitting your upwind shoulder instead of your back. Now your kite is not only getting the 15 mph wind from nature, but also the wind that you are creating by rolling along in the buggy. As your buggy speed increases the kite moves more and more to your side and you cease to accelerate. Read this over and over and you will be able to visualize what is going on with the wind, kite and buggy.

Coastal Wind Sports buggy basics

ragden - 6-10-2009 at 09:39 AM

Quote:

You are assuming direct exposure to the wind. When you go across the wind, it changes everything... That and the kite develops apparent wind.

You'll have to read the more technical write-ups. They do a very good job at explaining it. Used to be one on the forums.... Might need to search for it though...

edit: Responded too late... read the link that was posted, it explains it very very well...

flyboy15 - 6-10-2009 at 10:05 AM

Power the concept your referring to is called apparent airspeed. How you stated it is correct only if your going directly downwind - in which case you yes as the rider picks up speed the airspeed over the kite decreases.

However: the idea is to ride on a line perpendicular to the wind direction. In this case (also like you stated) the rider starts with ex 15 mph of wind over their kite. But contrary to what you said, as the rider begins to move, that motion is ADDED to the kites windspeed. So you will actually get more than 15kts of wind over the kite.

There are some technical terms in this one, but this is why some depower kites will backstall if the bar is pulled and the rider stationary. But when you get moving, the kite pulls more when the bar is pulled. The kite needed that extra airspeed to prevent a stall and will increase the coefficient of lift as the angle of attack gets increased.

More speed = more lift, less speed = less lift, and more stall

krumly - 6-10-2009 at 04:10 PM

It is entirely plausible that a high aspect ratio kite with a low drag profile could develop a lift-to-drag ratio of 6:1 or better. Competition pargaliders are in the 10:1 range now. Competition sailplanes hit 45 or 50:1. I bet my Rev Supersonic is close to 6:1 and my C-Quads and Cabrinha bows might be around 5:1.

Think of it like this: If your kite was a glider with 6:1 L/D, it would move forward 6' for every 1' of vertical drop. The forward windspeed the glider/kite would see is 6x the windspeed it would see from vertical sink. Now transpose the kite to zooming horizontally across the center of the wind window. The wind the kite sees from ahead is 6x the true wind blowing 'against it's bottom' so to speak.

That doesn't mean you can travel 5 or 6 times the true windspeed, but the kite has the potential to hit those speeds at certain points across the wind window - nearer the center than at the edges. The avergae speed will certainly be lower, cuz you have to turn which means the kite must decelerate and accelerate.

Your speed in a vehicle will depend on the efficiencies and drag angle of your craft, and you'll never hit the max speed or L/D potential of the kite. Buggies tires in loose sand have much drag. Skate blades on ice have low drag. That affects the total system efficiency.

Same thing with a sailboat on water. If that sailboat can come up on foils or plane to minimize drag and has efficient underwater foils for lift generation, it can get closer to the max L/D potential of the sail rig.

krumly

Kamikuza - 6-10-2009 at 06:20 PM

Phew I was starting to worry that I'd just imagined the book too - couldn't find it on my shelf :lol:

6:1 now eh, up from 3:1 like 10 years ago ... 12:1 in 2019 then? :eek:

william_rx7 - 6-10-2009 at 07:05 PM

markite - 6-10-2009 at 08:04 PM

"Are there any sort of speed records kept for buggies?
Where are the records set?"

Of course there are - Wasaga Beach!!

(okay that was a local joke for william_rx7)

WIllardTheGrey - 6-10-2009 at 08:19 PM

Quote:

Are there any sort of speed records kept for buggies?
Where are the records set?

yes Arjen van der Tol set the record 124.00kmh / 77.05mph at NABX last year I believe. Popeye the Welder is kind enough to host a list PTWs site

william_rx7 - 11-10-2009 at 07:08 PM