Going downwind faster than the wind

[spork]

So the myth I want to see the Mythbusters bust is that it's impossible to make a wind powered vehicle that can go directly downwind, faster than the wind, continuously.

I know it seems completely non-intuitive to many (most?) people, but by being clever, it is possible to extract the energy of the wind relative to the ground (or water) to make a craft that can beat the wind to it's destination. 

There are a number of ways to do this (some more practical, and others easier to describe).  Here's a video of such a vehicle in action:

http://www.youtube.com/watch?v=aJpdWHFqHm0


And no, this video is definitely not a hoax.  Jack Goodman is a very bright guy that I've discussed this vehicle with in detail.  It's easy enough to describe how this can work, but it generally leads to lengthy and heated debates nevertheless.  This is why the myth that it can't be done needs to be busted.  Well that and the fact that it's just way cool.

[Wyoming Cowboy]

Hello,

I watched the whole video.  I guess I don't understand.  I get the premise that the vehicle travels with the wind even faster than the wind is blowing.  This video however does not instill confidence nor do I feel proves anything.  It is just 3 minutes of this vehicle driving down the road.  It shows nothing of the vehicles mechanics and does nothing to prove to me that it is not being simply towed by another vehicle.

I know you said that this is hard to explain but I think you must try.  Also please do read about how to post a myth as this does not meet the requirements.  WE need some more to go on than just a video that really does not show much.  Also how you would think that the mythbusters should test such myths.

Brad

Addition, Here is some more INFO that I found.        http://www.boingboing.net/2007/02/06/video-can-a-vehicle-.html

"It's taken me a while to put my brain around how this works-- for me, the key point is that the propeller is a propeller, not a wind vane, and when the cart is rolling, the wheels are powering the propeller, not the other way around. With the right gearing, the propeller will always push backwards against the air, whether or not the air is moving forwards or backwards relative to the cart. The tailwind and the propeller action combine to make the wheels spin fast enough to keep the whole system rolling Faster Than The Wind. Definitely counter-intuitive (or a hoax I've fallen for)."

[spork]

>> I watched the whole video.  I guess I don't understand.  I get the premise that the vehicle travels with the wind even faster than the wind is blowing. 

If you get that, you do understand the only point I'm trying to make.  Most people think this is impossible.

>>This video however does not instill confidence nor do I feel proves anything. 

It's true that the video *could* have been faked.  I happen to know that it's not.  While that shouldn't instill any more confidence by itself, I'll try to do so with further explanation.

>>I know you said that this is hard to explain but I think you must try. 

Will do.

>>Also please do read about how to post a myth as this does not meet the requirements. 

Sorry, I did read about how to post a myth.  I figured this would just be the opening round of a longer discussion in which I'd describe many ways of accomplishing this.  My apologies if I didn't follow the normal format.

>> Also how you would think that the mythbusters should test such myths.

I think they could build a cart much like this one.  Jack has published a paper on it's design (which I'd be happy to provide).  They could put the cart on a treadmill, and watch it advance against the moving belt in still air - or they could take it to the Alameda air base on a windy day and do the same test shown in the video.  If they aren't wanting to build such a cart I'll be happy to build one for them.  In fact I'm nearly certain Jack would be happy to provide the very cart used in the video.

How can it work???

Consider an ice-boat on a downwind tack.  On a 45 degree downwind tack, the ice-boat can achieve a speed several times the wind speed.  This means that it's downwind velocity component can be well above the wind speed.  Thus by making two tacks, the ice boat can beat the wind to the goal line directly downwind of its starting point.  But that's cheating you say (to which I say "fair enough") since the boat is tacking, not going directly downwind.  So let's make a BIG frame inside of which we'll put two of these ice-yachts.  At any time each ice-yacht will be on opposite tacks.  Thus the frame (and even the C.G. of the entire system) will be going directly downwind faster than the wind.

That's basically what this vehicle does.  But it just wraps that 45 degree downwind tack into a continuous downwind spiral.  The tips of the blades follow the same downwind tack that the sail of our ice-yacht would follow on it's 45 degree downwind tack.  The transmission and wheels simply provide the same kinematic constraint that the keel of a sailboat provides, or that the skates on the ice-yacht would provide.  The prop is simply geared to the wheels such that it will turn through a prescribed amount of arc for a given amount of forward motion.

Here's a vector diagram I've done to describe how it's possible for a sailing vessel to manage a downwind tack such that it's direct downwind velocity component is faster than the wind.  But frankly that part isn't really in question.  Ice-yacht racers do it all the time.



Looks like you have to click on the thumbnail to get a full-size image - and then click on that image to get the *real* full-sized image.

[Wyoming Cowboy]

I also found this pic of a vehicle that was built for just such exploits back in the 50s     http://www.dcss.org/bauer_cart.jpg

Upon further research it sounds more like a known thing and not a myth.  Interesting though.  It makes perfect sense.  The video is grainy and describes NOTHING.  It actually makes it more hoax like.  After digging I found Jacks website---------------------- http://www.ayrs.org/DWFTTW_from_Catalyst_N23_Jan_2006.pdf

It describes it pretty well.  It is a little mind bending at first though.  I can definitely see how people would not believe such things are possible.  I'll kind of end my input and see what others write here.  Interesting reads though,

Brad 

[spork]

>> Upon further research it sounds more like a known thing and not a myth.

I would tend to agree, but I can point you to a number of threads on different forums where it has been *hotly* debated for 10's of pages.

I assumed I would find some folks here that find it hard to believe.  I figured the dynamic of this thread would require me to answer all their doubts with further analysis, analogies, etc.

On the Mythbuster's website (can I say that here), one detractor threatened to "beat me up".

[Cybermortis]

On the Mythbuster's website (can I say that here),

Yes, you can mention the Discovery websight here, if you look around you'll note that there are a couple of myths posted here that either started on there or which cross over. The Strafing tanks myth I started was done after reading two separate posts on the same myth over there. I started the topic here simply because the standards of this board are a LOT higher - both in what is expected and in regards to the replies you can expect from other members.

Of course you may have just noticed that

>> Also how you would think that the mythbusters should test such myths.

I think they could build a cart much like this one.  Jack has published a paper on it's design (which I'd be happy to provide).  They could put the cart on a treadmill, and watch it advance against the moving belt in still air - or they could take it to the Alameda air base on a windy day and do the same test shown in the video.  If they aren't wanting to build such a cart I'll be happy to build one for them.  In fact I'm nearly certain Jack would be happy to provide the very cart used in the video.

It would be better to provide information as to how this is meant to work from more than one source. From that Mythbusters should be able to design and build their own version without copying someone else's design.

The problem with using existing blueprints is that it could be construed as advertising - not least because you have stated that you know the person who made the video.

(Just to be clear, since such comments on message boards can be taken the wrong way. I am not accusing you of attempting to advertise this in any shape or form, I'm simply noting what it could appear to be. Please do not take offence).


Seeing MB building such a device for testing, both small and full scale, would also make for a much better show - as well as allowing them to test this when they can and without worrying about damaging it. Simply borrowing someone else's device would not make for an interesting show, require them to work around when the owner is free and possibly require working out transportation.


(Tip; the 'quote' button is on the bottom of the two rows of white buttons, one along from the end - the one that looks like a speech bubble. You can also use [ quote] [/ quote] (without the spaces) if you just want to type it out)

[theSpecialist]

Alright.

I've got a dumb question. (Figures, right? LOL)


Anyway,  Once you are out ahead of the wind coming up from behind, aren't you now subject to the relative wind you have coming at you from the front at this point?

[Cybermortis]

Most likely. I'd assume that you'd get up to a higher than wind speed, then start to slow down until the wind catches you up and you start to speed up again. Over any distance your total speed probably isn't faster (or that much faster) than the wind due to slowing down and speeding up.

*edit*

Has this ever been tested over any real distance, say a mile or more? Or has this only been done over relatively short distances?

[theSpecialist]

Yeah, I could see it, if the wind suddenly died away that is.  That way you really would be going faster than the wind that sent you.

[RoofingGuy]

Once you are out ahead of the wind coming up from behind, aren't you now subject to the relative wind you have coming at you from the front at this point?

I'll let spork explain better, but basically the cart would be tacking into this relative wind, and continuing to move at a steady-state.

[theSpecialist]

Ah.

So, it is not supposed to outrun the wind in question - only obtain a faster rate -- regardless which direction it is going relative to the wind supplied, then.

[RoofingGuy]

I didn't think there was anything fake behind the concept, I just took a long time to even give it a moment's thought.

It "clicked" for me when a sailboat comparison was made.

If a sailboat is sitting in still water with no wind, it goes nowhere.  If there's a current but no wind, then the boat can drift with the current, but can not exceed the current.  But if you tack into the relative wind once you're drifting with the current, you can exceed the current, and are being powered solely by your energy source (the current).  As long as there's a difference between the air and the water, a sail can extract energy from it.

Likewise, this cart can extract energy from the difference between the wind and the ground.  It doesn't matter if the wind is moving and the ground is still, like you'd expect, or if you put it on a treadmill in no wind -- as long as there's a difference to extract, the cart will have propulsion.



Edit to clarify:  In the sailboat analogy, obviously with a wind but no current, you can sail, but if the wind and current are equal, then you can only drift with the current... if you raise the sail, it will hang limp in the zero relative wind.  The sail needs a difference between the air and the water, but doesn't care (or know) which one is moving.  The difference between the two is the key.

[Cybermortis]

There is one problem in your analogy - a road doesn't move, and therefore doesn't provide anything propulsive. Indeed a road surface would only provide friction to slow you down (Unless you go downhill anyway).

I still have to question if this really would result in faster than wind speeds over any distance.

[RoofingGuy]

There is one problem in your analogy - a road doesn't move, and therefore doesn't provide anything propulsive.

I don't get the problem.

If the road is not moving, and the air is, then a sail can extract energy from the difference and provide propulsion.

If the cart were on a treadmill in still air, so that the road were moving but there was still a difference between the two, then the cart will propel itself forwards along the treadmill, against the motion of the belt.

A sail on its own (on a boat) also needs the boat to have a keel, to be able to extract useful energy for propulsion.  (Yes, you can catch wind without a keel, but to beat the wind and to effectively tack, the vessel needs a keel.)  Likewise, the prop on this cart is geared to the wheels so that there is a fixed relationship.  The fixing of the prop to the wheels is what allows the difference to be extracted.

[Mycroft]

Interesting video.  Recenty, the Mythbusters did solicit "viral videos" for upcoming episodes and I think this one definitely fits the bill.

Regarding how this thing works, you got me stumped but I would have to say that there have been many ingenious designes for boats regarding methods to extract energy from the wind.  Jacques Cousteau even had a ship that used a large rotating rigid "turbine" (looked like a giant cylinder) to propel the boat so you can't necessarily dismiss this idea just because it seems counter-intuitive.

Some couple questions to ponder:

1) How could it be faked?  (bicycle pulling it with monofilament fishing line).

2) What's the principles involved and would it necessarily require the blades to be attached to the wheels?

--Mycroft