I've been poking around the forum, and seen a couple posts that seem to suggest that a land speed on a board or buggy greater than the maximum wind speed can be achieved. This doesn't seem to make sense to me, but perhaps there is some factor I am neglecting to consider. It would seem that *even* if you are riding straight downwind, the friction of the wheels in the sand would give you enough drag to keep you from going faster than the wind speed. It would also seem that if your kite were moving close to or at (let alone faster than) the speed of the wind, it would have a hard time staying inflated having a relative wind speed approaching 0 (whereas going faster than the wind would assumably give your kite a negative relative wind speed, actually producing drag).

Maybe faster than the *average* wind speed over a certain period is believable. If the wind varied between 10 and 20 mph, then the the avgerage would be 15 mph, but the buggy/board could have gotten close to the maximum of 20 at some point.

Is there some sort of manuvering that can actually get you going faster than maximum wind speed?

Thoughts?

Thanks.

Hello burninheretic,
You are asking a very interesting question that will trigger some very specific
replies from those of us who are very much into and math and other things
related to physics.
If I do recall properly a wind power vehicle can "theorically" go 3.7 times
the speed of the wind that is "propelling" it.
The wind "by itself" is almost irrelevant to the fact that the kite or sail, in
other terms "the engine" is the one single element that does the "work".

The wind is merely a "tool". Such kite (or sail) act in fact as an airfoil where
the displacement of air going “above” and “below” such airfoil creates 2 actions.
First you do have to understand what an airfoil is, it is a form where the upper
surface length is longer than the under surface length, the leading edge being
the "front" while the back is called the trailing edge.
When your draw a line from the leading edge to the trailing edge it is called
the cord. You can now see that an airfoil has a longer upper surface than the
lower. The air going over the upper surface will have to travel a longer
distance than the air going by the lower surface, thus the air going on the upper
surface will have to accelerate in order to meet by the trailing edge at the same
time with the air traveling on the lower surface.
This acceleration creates a "vacuum" or "negative pressure" while on the
bottom the air will be slower and creates a "positive pressure".
The faster the air over an airfoil, the greater the difference in pressure between
top and bottom, the greater the lift = the greater and faster the displacement /
speed of the airfoil motion.

In fact a kite or sail act in the same way as an airplane wind or a helicopter
rotor blade, except that in order to achieve result instead of the air going
over such airfoil ........... the airfoil is going thru the air.
Many members will be pleased to explain this to you in a more tech fashion,
I just hope this initial information will help you understand and wait for more
complex / additional info.
Do a research on the WindJet; I am sure that reading thru it will allow you to
gain additional knowledge.
Every one of us will be able to tell you that during the course of our hobby,
it is very common to go across the desert at 2.5 to 3 times the speed of
wind.
See you there.
:singing::singing::singing:

Very good answer RMV, Im going to try and elaborate in some simpler terms.

First off, when you fly a kite, as you notice, most kites will travel faster forward than the wind speed. In 10mph winds, your kite may be traveling 20-30 mph in a straight line.


When kiting in a buggy, you very rarely go straight downwind. What happens then is that you catch up with the kite and it does crumple up and fall out of the sky.

To go fast, you have to use leverage. Here is the best example I can think of. You have a model train on a track some few feet in front of you. Its turned so the train will go across you instead of toward you. Its turned ever so slightly toward you just a bit instead of exactly across. You tie a string to the train on the track (the train will not come off the track). Now you pull the string straight toward you. You notice that you only pull slowly straight back about 6 inches, but the train has moved much faster than your hand, and moved 6 feet.

You do the same with a buggy or board. You go across the wind. You use "almost" lateral pull to convert into forward speed. Your going directly across the wind, your kite is pulling not directly to the side, but slightly forward of the side. Now you turn this side (lateral) pull into forward momentum. Now something amazing happens, as you travel forward, you start to build apparent wind. The wind blowing is not as much as what the kite sees. The kite is in wind thats stronger than what is blowing. Now you can really start to pick up speed.

Many things affect the speed you can achieve with a kite. Mainly rolling resistance, and kite choice. Some kites fly faster and fly closer to the edge of the window. If your buggying on soft sand versus had pack dry lakebed, you can see the obvious advantages.

If you do have to go directly downwind, you have to zig-zag the buggy while zig-zagging the kite the opposite direction. Pretty complicated for new guys (like me ).

I hope this sheds some light on how this works.

Scoop

Scoopy and RMV, thank you both for your replies. The train example made the point. This is an over simplficication I am sure, but I see now that if you move cross wind, that the wind moves past the kite at a perpendicular angle and the relative speed of the wind on the kite remains virtually unaffected by the lateral movement of the kite and rider.

I'll look into windjet as RMV suggests, the more I understand the dynamics involved, the better I'll ride. Thanks again.

Quote:

Originally posted by RMV First you do have to understand what an airfoil is, it is a form where the upper surface length is longer than the under surface length, the leading edge being the "front" while the back is called the trailing edge. When your draw a line from the leading edge to the trailing edge it is called the cord. You can now see that an airfoil has a longer upper surface than the lower. The air going over the upper surface will have to travel a longer distance than the air going by the lower surface, thus the air going on the upper surface will have to accelerate in order to meet by the trailing edge at the same time with the air traveling on the lower surface.

RMV, while doing further research, I came across something called "Equal transit-time fallacy" which you might interest you. Wikipedia has a good article on lift here http://en.wikipedia.org/wiki/Lift_%28force%29

"Equal transit-time fallacy"

Thanks for that link, I've always wonder how bees were able to fly.

I am sure this explaination is unnecessary, but here is how I thought of it: It is al about angles.
Looking at a sailboat, think of the boat going 90 degrees to the wind, and the sail angle is 45 degrees to the wind. For every foot of wind that goes past the sail (wing or kite) it propels the boat an equal foot. Now if you sheet out, making the sail closer to inline with the wind, you will move forward less for every foot of wind travel. Now bring the sail more inline with the boat, in other words, closer to perpindicular to the wind, and the boat will move more than 1 foot forward for every foot of wind travel.

Now think about a kite. Most are fixed bridle, they are like a permanantly sheeted in sail. In other words, they will always move faster than the wind that is causing them to move. There are many advanced things that go into the math, but for a general concepts, this helped me.

Whatever the theory and math that goes into it, KITING IS A RUSH!!!

Sean

Thank you burninheretic for posting the scientific version as a more elaborate
reply to the original question, this will for sure provide the rest of us a detailed
info as to how things fly (except for the bees).
Scoop............ as always

Howdy,
I'm sure this won't help, but I've never let that stop me before...

"Sail" and "Wing".
Not just two different four-letter words.

Sails work by resistance.
They hold back the wind pressure.
The best they can be (most efficient) is when they resemble the top surface of a wing (fully-battened and close-hauled).

Wings are dual-surface and generate power through the pressure difference explained by RMV.

The faster a foil flies, the more power it can generate.
"Zooming" the kite vertically out near the edge creates a faster airflow over the foil. Result is more power. A good rider turns that into speed.
Now the foil is in a faster airstream and when you "zoom" the kite again, you go faster, again. Called "velocity-made-good" or vmg.

Moving a sail about does add some power, but not on the order of a foil.
Looping a sail creates more power, but mostly through the position change in the wind-window - As the kite loops it moves into the downwind area.

The last post confuses sails and foils in the general term of "kite".
Different technologies.
Different circumstances and requirements.
Understanding what is happening requires clear analysis.

I hope I haven't muddied the waters too much.


Buggy On! Safen Up!

"Often wrong... Never in doubt"

the coreylama

Well, the sail analogy was a simple illustration of the way in which either a sail or kite can move faster than the wind speed. I did not intend to mislead or muddy anybodies waters!!

Looking fwd to NABX!!

Sean