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Old 14th October 2010, 10:57 AM   #3241
quarky
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Or, is my math wrong?
Any reply appreciated.
I hate to walk around with a delusion in my head.
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Old 14th October 2010, 02:03 PM   #3242
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Originally Posted by quarky View Post
Or, is my math wrong?
Any reply appreciated.
I hate to walk around with a delusion in my head.
There was no math in that post. It was in your head, so it is somewhat difficult to decide how correct it was.

As for very high speeds..

You get more air resistance (proportional to the square of the speed of the cart through the air.), so my WAG would be that the process would be less efficient, and you would end up with a lower multiple of the wind speed.

That had a little math in it.
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Old 14th October 2010, 05:51 PM   #3243
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Originally Posted by Ririon View Post
There was no math in that post. It was in your head, so it is somewhat difficult to decide how correct it was.

As for very high speeds..

You get more air resistance (proportional to the square of the speed of the cart through the air.), so my WAG would be that the process would be less efficient, and you would end up with a lower multiple of the wind speed.

That had a little math in it.
Thanks.

My unspoken math was the power/speed ratio (cubic), against the squared losses.
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Old 19th October 2010, 06:24 AM   #3244
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Still not sure if its wrong-think. I'm missing something.
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Old 19th October 2010, 11:15 AM   #3245
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Originally Posted by quarky View Post
Still not sure if its wrong-think. I'm missing something.
If I read you correctly, you are saying that the efficiency of the propeller is (roughly) proportional to the cube of the speed. We seem to agree that the losses to air resistance is (roughly) proportional to the square of the speed. (Not getting into the finer points of which speeds we are talking about right now.) I'd like to see an explanation for the cube factor. I think that is where the dog i buried. (Is that an expression in English?)
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Old 19th October 2010, 01:21 PM   #3246
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The question asked is rather an interesting one, and I don't have the answer.

What can be said is that a craft speed between 0 and wind ground speed you actually gain energy, by the wind/ground speed difference. This is because the craft has a tail wind up to that speed, with a wheel coupled to the prop so the effective power doesn't fall off as the craft approaches wind ground speed. The effective tail wind even increases with speed up to ground wind speed, because the prop air speed is counter to the wind.

Once the craft exceeds wind ground speed, it begins expending that energy differential against a head wind. It doesn't expend it linearly because the increased speed also increases the prop exhaust speed. Presuming a simplistic conservation effect I presume this is why it exceeds twice wind speed. So I don't know what a maximum craft speed would look graphed against variations in ground wind speed, at various cross sectional wind resistance efficiencies. Even the efficiency of the prop varies with speed.

It's far from as simple as it looks from squaring wind velocities. To many feedback and variable efficiency variances. My best guess is that, if you changed gearing ratios to maximize for a given wind ground speed, the higher the wind speed the greater multiple of that speed you could exceed. Just a guess based on the prop continuing to increase wind output speeds even during the power use phase, i.e., at or above ground wind speed.

Also note that, without drag, any small force over enough time can accelerate any large object to any speed respecting Special Relativity.
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Old 19th October 2010, 04:34 PM   #3247
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Originally Posted by my_wan View Post
The question asked is rather an interesting one, and I don't have the answer.

What can be said is that a craft speed between 0 and wind ground speed you actually gain energy, by the wind/ground speed difference. This is because the craft has a tail wind up to that speed, with a wheel coupled to the prop so the effective power doesn't fall off as the craft approaches wind ground speed. The effective tail wind even increases with speed up to ground wind speed, because the prop air speed is counter to the wind.

Once the craft exceeds wind ground speed, it begins expending that energy differential against a head wind. It doesn't expend it linearly because the increased speed also increases the prop exhaust speed. Presuming a simplistic conservation effect I presume this is why it exceeds twice wind speed. So I don't know what a maximum craft speed would look graphed against variations in ground wind speed, at various cross sectional wind resistance efficiencies. Even the efficiency of the prop varies with speed.

It's far from as simple as it looks from squaring wind velocities. To many feedback and variable efficiency variances. My best guess is that, if you changed gearing ratios to maximize for a given wind ground speed, the higher the wind speed the greater multiple of that speed you could exceed. Just a guess based on the prop continuing to increase wind output speeds even during the power use phase, i.e., at or above ground wind speed.

Also note that, without drag, any small force over enough time can accelerate any large object to any speed respecting Special Relativity.


Much thanks for the replies, and apologies for my confusion and lack of coherent question.

The power in the wind is cubed with the speed of the wind.
Catching it is another matter entirely, which is why I was careful to mention the 'hypothetical' aspect of my query.

The losses , in theory, don't accelerate as fast as the gains.
To take this game to its logical conclusion, the cart is suspended on a cable. No passenger or steering is required. Weight is reduced substantially.
Friction reduction is a bug-a-boo in my head, because the cart operates by pushing off the Earth.

A cable that had grip, like a chain, might be the ticket. The dusty flats are too primitive to accommodate elegant concepts. As are the roads.

Human powered vehicles (bikes) are hobbled by the road ways they were forced to adapt to.
I sense that with the right 'roads', hpv's will routinely average 50 mph; that they will be faster than the alternative transportation; free exercise, etc.

I'm jaded from having built and ridden and 2 person railroad-track bike-thingy.
It was "where it's at". Suspended from cable puts the tracks to shame.
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Old 19th October 2010, 08:33 PM   #3248
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Originally Posted by my_wan View Post
The question asked is rather an interesting one, and I don't have the answer.

What can be said is that a craft speed between 0 and wind ground speed you actually gain energy, by the wind/ground speed difference. This is because the craft has a tail wind up to that speed, with a wheel coupled to the prop so the effective power doesn't fall off as the craft approaches wind ground speed. The effective tail wind even increases with speed up to ground wind speed, because the prop air speed is counter to the wind.

Once the craft exceeds wind ground speed, it begins expending that energy differential against a head wind. It doesn't expend it linearly because the increased speed also increases the prop exhaust speed. Presuming a simplistic conservation effect I presume this is why it exceeds twice wind speed. So I don't know what a maximum craft speed would look graphed against variations in ground wind speed, at various cross sectional wind resistance efficiencies. Even the efficiency of the prop varies with speed.

It's far from as simple as it looks from squaring wind velocities. To many feedback and variable efficiency variances. My best guess is that, if you changed gearing ratios to maximize for a given wind ground speed, the higher the wind speed the greater multiple of that speed you could exceed. Just a guess based on the prop continuing to increase wind output speeds even during the power use phase, i.e., at or above ground wind speed.

Also note that, without drag, any small force over enough time can accelerate any large object to any speed respecting Special Relativity.
My Wan,

There is no change of "Mode" between less than wind speed and greater than windspeed.

You correctly state that below windspeed, the cart takes energy from the differential between the speed the wheels go over the ground and the speed the propellor goes through the air.

That differential still exisit above wind-speed. At all times, the cart's groundspeed is greater than it's airspeed, and that is where, by the gearing between the wheels and the advance ratio of the prop, it gets the energy to provide thrust.

Above windspeed the only thing that changes is drag, both from the increased friction of the mechanism, and the aerodynamic drag of the now present headwind.
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Old 20th October 2010, 11:36 AM   #3249
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Originally Posted by RossFW View Post
My Wan,

There is no change of "Mode" between less than wind speed and greater than windspeed.
Yes, you are correct. The "mode" change was merely a useful definition, not any sort of change in the mechanics of the system.

As a power graph to speed ratio, it merely marked the point were the difference in power used to power available was at a maximum. Which corresponds to a speed at which wind ground speed is (closely) matched. Merely a useful fiction to call it a change of mode, since it's in some ways easier to model if the math model starts at that point.
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Old 21st October 2010, 10:25 AM   #3250
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I remain uncertain, but I feel a little better about it.
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Old 21st October 2010, 10:54 AM   #3251
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Originally Posted by quarky View Post
I remain uncertain, but I feel a little better about it.
Listen to some Depeche Mode, it may help.
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