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how can we ride 40-50 mph in only 20-25 mph of wind?

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spork
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Postby spork » Fri Jun 09, 2006 7:47 pm

kitebored wrote:spork, where do you kite in the bay?
99% of the time I kite 3rd Ave. I do make the occassional trip to Berkeley or Sherman. How 'bout yourself?
enjoying your posts.... i agree with the yoyo, but it's a transient condition that runs out when the string runs out.
You're right of course. This one is interesting in that it demonstrates the concept fairly clearly, and can be done sitting at your desk. In order to avoid the transient aspect you have to do some fancy stuff. The wagon with deployable sails on the wheels is one (not so practical) way to use the same idea without the transient issue.
About the wagon spokes, as far as practicality, wouldn't you end up gearing a rigid sail(wing) to rotate through a series of angles with respect to the wheel's rotation? Seems possible, but not unless someone lays down their $100K.
I'm not sure that I could conceive of a way to actually build this one and beat the various losses of deploying sails etc. I've considered a number of things including rigid sails that rotate (as you describe), sails that accordian out from the spokes, and even a conveyor belt of sails that go all the way around the deck of the vehicle such that they're deployed on top and fold back on bottom (or vice versa). This belt would be geared to the wheels to have the same kinematics as a sail at the mid-spoke below the axle.
Every one of spork's concepts involves using the wind to create a force, and using gears or levers to cantilever that force into a weaker, but farther moving (hence faster) push. The stationary object (ground or water) provides the pivot for the cantilever system.
This is true - and it's really the crux of the issue. The energy we extract is that of the difference in velocity between the ground/water and the wind. It's a bit like dynamic soaring in that regard. Birds use this to fly as far as they like without ever flapping their wings by extracting the energy from wind gradients - even without any vertical wind components (mechanical or thermal).

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Postby kitebored » Fri Jun 09, 2006 7:52 pm

Image

I take no credit, I essentially ripped off Spork's ideas. But this is sort of a hybrid yoyo/cart idea.

Let's say the diameter of the wheel is twice the diameter of the axle to which the rope is attached.

Wind speed = W
Cart speed = V

As seen from the ground:

The wind pushes a sail, which is attached to a rope on the wheel of the cart, which pulls the sail backwards. From the frame of reference of the observer on the ground, the boat will move forward with speed V, but since the sail is being pushed back, it moves with speed V/2.

Since the maximum speed of the sail w/ reference to the ground is the windspeed (W), this is V/2 in the previous paragraph. Solve for V=2W, and the cart moves twice as fast as the windpeed.

As seen from the Boat:
Windspeed is W
Boat speed is V.

Apparent wind on the boat is (nonrelatvistically :) ) W-V. Apparent wind drops to zero when the boat is travelling downwind as fast as the wind.

But the sail is moving backwards with speed V/2 thanks to the rope/pulley system, so the apparent wind on the SAIL is W-(V/2). The apparent wind on the sail doesn't hit zero until the boat is moving at V=2W, or twice as fast as the wind.

The catch is that the total force on the boat is now half what it would be with just a stationary sail. By making the inner wheel larger, we can increase the max speed of the boat, but we're decreasing the total force of the wind, and thus the acceleration. As we reach the limit Vmax=infinity, Acceleration will go to zero.
I feel I'm preaching to the choir now that spork has managed to get everyone on his side, but i just wanted to add my thoughts.

Upon further thought, the "moving sail" concept is also very similar to the boats in a box and the propeller contraption.

Disbelievers, see the truth.

B.

EDIT:
dammit, too late, you already called this one the conveyor belt method.


I live in the city, and I mostly kite Sherman or Chrissy, but poke my head around the other spots too.
Last edited by kitebored on Fri Jun 09, 2006 8:15 pm, edited 2 times in total.

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Postby spork » Fri Jun 09, 2006 7:56 pm

gdorfman wrote:I did my empirical test on the spokes. I had visualized that top/bottom mattered for forward/reverse, but it’s actually front half/rear half. Anywhere you push on the spokes in front of the axel will make it go forward.
Actually, every point on the wheel has a forward component to it except for the contact point with the ground (which has 0 velocity). That is the definition of the rolling constraint. We imagine wheels rotate about their axle (as they would when on a mechanics lift). But a rolling wheel is actually spinning about the contact point on the ground (which is continuously moving).
We agree about the current. My point was that it’s very easy to make upwind progress faster than the current—but that does not show you can stay stationary or make progress downwind relative to ground
This is the one I said you have to kind of look at upside down and backwards. Both the wind and water are fluids. So I want to imagine the keel as the sail and vice versa. Because the air isn't moving and the water is, we have to stick our head under water and upside down and we'll see that the water is pushing the keel downstream, but the keel ends up going downstream faster than the water that's pusing it. In this case the sail that's sticking up into the air acts as our "keel".
I still think that the encapsulated, reframed ice boats is the most conceptually elegant of all these contraptions…
I like that one too. Unfortunately, there are plenty of people that claim no sailing vessel can make a VMG better than windspeed downwind. It can be done. Ice boats do it routinely, and I'd be happy to post the vector analysis if people care to go the that level of geekdom :D

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Postby spork » Fri Jun 09, 2006 8:03 pm

Kiteboard, I like your diagram, but I think there's one error. Assuming the wind is moving from left to right, the rope would have to attach to the opposite side of the hub so the rope is being wound up as the cart moves in the direction of the wind.

EDIT:

Yes, you're edit makes more sense to me now.
Last edited by spork on Sat Jun 10, 2006 10:37 pm, edited 1 time in total.

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Postby gdorfman » Fri Jun 09, 2006 8:14 pm

it will get wound up eventually anyways, just have a brief slackening period first... it should be able to glide through it
spork wrote:Kiteboard, I like your diagram, but I think there's one error. Assuming the wind is moving from left to right, the rope would have to attach to the opposite side of the hub so the rope is being wound up as the cart moves in the direction of the wind.

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Postby kitebored » Fri Jun 09, 2006 8:16 pm

fixed.

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Postby spork » Fri Jun 09, 2006 8:16 pm

Yes this scheme still winds up the rope. I imagine a conveyor belt carrying sails that fold up and down. Perhaps I can draw and post it. I guarantee the diagram will be ugly.

EDIT:


O.K. here we go:

Image

This requires a mechanism to retract the sails as they make their way to the upper deck.

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Postby JS » Fri Jun 09, 2006 11:38 pm

hein wrote:Want to go downwind faster than windspeed?

Point a skate board straight down wind.
Get a small fast kite and start looping it in
the power zone while you ride straight down
wind. I'll bet you can exceed windspeed.

You'll have losses from drag/friction so when
they balance out the pull of the kite then you
will stop accelerating.
I thought about this too, but I don't think it works. Here's why:
1. As soon as you speed up to match the windspeed, your apparent wind will be 0 and no amount of skill will keep the kite flying.
2. Assuming you somehow surpass the windspeed (still going straight downwind), the apparent wind will then be opposing your direction of travel, and any kite-flying (looping or otherwise) will have a backward component which will slow your rate of speed to match the windspeed or less.
(BTW I've read many of your posts, and I greatly respect your contributions.)
spork wrote:...etc...etc...
This is an interesting thread, and in my (somewhat educated) opinion, spork is 100% on the money with virtually every concept discussed.

Basically, to exceed the windspeed on a dead run, or to make progress directly into the wind, you need to get tricky with geometry and/or mechanical devices that provide a gearing effect, such as with the devices and techniques posted in this thread (including tacking upwind or downwind, which have theoretical gearing effects).

Cheers,
James

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Postby spork » Sat Jun 10, 2006 12:18 am

JS wrote: This is an interesting thread, and in my (somewhat educated) opinion, spork is 100% on the money with virtually every concept discussed.
This is extremely dissappointing. I was prepared for a really ugly name-calling adventure, which is where it went when this came up on the hang gliding forum years ago. This might explain why my wife REFUSES to hang out with any of my hang gliding friends, but enjoys my kiting pals. She's clearly brighter than me.

RC

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Postby fxeric » Sat Jun 10, 2006 6:20 am

would anybody like to invest in my ping pong ball motor powered car?
my ping pong ball motor consists of a container split in half. half of it filled with water half not. on the bottom of the conatiner where the two halfs meet there is a special valve which lets the ping pong balls though into the water but does not let the water out into the empty half of the container. once the ping pong balls are in the water they float to the top of the container while pushing a conyveyer belt like structure with scoops which is geared to the wheels of the car. then when the balls reach the top of the container they fall back into the empty side and run down to the bottom of the container where the process begins again.


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