Wind powered cart traveling directly down wind faster than the wind

[Jim Logajan]

The following interesting concept is likely to blow the mind of not only those who can't grok the plane on a treadmill problem, but many others. This video basically shows a ground vehicle that captures the wind using a prop that drives its wheels and accelerating to a speed directly down wind faster than the wind:

The initial reaction of most people is likely to suspect a fraud - after all - once the relative wind drops to zero, surely the force on the prop drops to zero and it can't travel down wind faster than the wind?

However, since this device isn't hard to build (there are plans and other videos on the net) and has been established, the next thing is to figure out where one's physical intuition has gone awry. Here is one video I think does a good job of first showing how a wind (represented by the ruler) can drive a prop (represented by the big wheel) can make a vehicle caught between two co-linear planes travel faster relative to one of the planes (the ruler) in the reference frame of the other plane (the table):

I think the concept is neat precisely because the vehicle is a simple mechanical device, yet its behavior appears contradictory and takes some time to comprehend what is happening.

P.S. A sailboat can't sail faster directly downwind than the wind - though it can tack across the wind and thereby sail faster than the driving wind speed - but if the sail was replaced with an air prop geared to a water prop, then the boat could be made to travel faster downwind than the wind.

 

[Jim Logajan]

Do you have a link to the plans?

For the purposes of searching the net (or video sites) for relevant hits, the phrase or keywords "directly down wind faster than the wind" (or "downwind" in place of "down wind") or even the abbreviation a lot of people now use: DDWFTTW.

I found the following plans but have not tried them:

http://www.rtfa.net.nyud.net/wp-content/uploads/2008/12/dwfttw_build_plans_147.pdf

The following web sites may be of interest:

http://www.fasterthanthewind.org/
http://thinairdesigns.com/
http://www.physorg.com/news194851568.html

The physorg link provides some insight, such as it is:

"Cavallaro explained the car is able to move faster than the wind because the propeller is not turned by the wind. The wind pushes the vehicle forward, and once moving the wheels turn the propeller. The propeller spins in the opposite direction to that expected, pushing the wind backwards, which in turn pushes the car forwards, turning the wheels, and thus turning the propeller faster still. The vehicle was built after over a year of trials. Building a transmission able to transfer power from the wheels to the propeller was the most difficult part of the design. The next stage in development will be to have trials confirmed by NALSA."

It is not a perpetual motion machine - it is extracting the kinetic energy in the relative motion of the air and the ground. Run enough of these babies and you'd eventually suck the wind out of the atmosphere, forever removing the need to worry about cross-wind landings. :wink2:

Also, per the fasterthanthewind.org web site, the concept appears to be several decades old. If I follow, first proposed back around 1949 in a student paper and first proved in 1969.

 

[Jim Logajan]

That explanation doesn't really explain how the cart can travel faster than the air around it on a sustained basis. Once the relative wind becomes negative, it should extract energy from the cart in the form of drag, rather than providing energy to drive the propeller.

You're right that a negative relative wind causes drag. Oddly, it is part of the key to understanding this if you look at things in the correct reference frame:

Consider that moment when the cart has accelerated to the point of moving at zero relative wind. In the frame of reference of the cart the wind is now dead calm. But the relative speed of the ground is driving the propeller! So the force of the propeller on the cart is greater than zero in the cart's reference frame (because the propeller is accelerating that dead calm air.) So the cart will continue to accelerate until the force of the propeller (which increases roughly linearly with speed) equals all the sources of drag (such as the "negative" wind, which increases with the square of the speed.)

Only at some speed above the zero relative wind speed can the force generated by the prop reach equilibrium with the force generated by air drag; they CAN'T reach equilibrium AT zero relative wind.

Yet another way to look at it is by noting what happens at some of the more notable speeds:

• At zero speed relative to the ground the wind force accelerates the cart.
• At zero speed relative to the air the ground-driven-prop force still accelerates the cart.
• At some speed above zero speed relative to the air the prop force equals the wind force as seen by the cart, at which point the cart reaches equilibrium speed.

Not sure if that helps, but it is what I came up with to satisfy my own understanding when I first saw this intriguing video a year or so ago.

 

[Jim Logajan]

That's why a qualitative argument is not sufficient.

My own belief is that there is no point getting into mathematical analysis if one first hasn't got a physical insight into the operation of a system.

A web search yields the following links containing quantitative analysis or pointers to such:

talkrational.org/attachment.php?attachmentid=1336&d=1263112012

http://blueplanettimes.com/?p=3922

By the way, one necessary test is whether others are able to repeat the experiment and get the same results.

True. That is why I posted net search terms earlier that yield hits showing experimental replication by others. I also posted a link to plans to allow anyone to attempt independent replication.

 

[Jim Logajan]

OK, I get it.

Is there a treadmill involved?

Shirley you're joking? But of course! See:

 

[Jim Logajan]

As such, claims of continuously sailing downwind faster than the wind are extremely dubious. I'm not sure they are making that claim though.

That is precisely the claim they made, and accomplished. This is from the preamble at the top of their web site:

"Can a vehicle be built which can go directly downwind, faster than the wind (DDWFTTW), powered only by the wind, steady state? Thin Air Designs, in collaboration with the San Jose State University Aero department, along with generous corporate sponsors set out to definitively answer this question. On 3 July 2010 we established a world record by going directly downwind at 2.8 times the true wind speed. "

http://www.fasterthanthewind.org/

 

[Jim Logajan]

Here is a frame from the beginning of the video. It ends up that there is a wind vane (tape) on a pole that projects from the nose of the car.

The wind tape clearly shows that the car is pointing into the wind before it starts moving. The claim that they are pointing downwind is a hoax.

Whatever that is, it is barely visible, even at 480p. When the video pans to the anemometer mounted on the pace car, it definitely shows the wind from the right at the height of the prop hub. You're stuck on believing it is all a vast conspiracy and looking for evidence to support that theory and basically disregarding everything else.

 

[Jim Logajan]

You're stuck on believing the video.

I understand the physics and mechanics and you simply do not. A pity for you. :wink2: (There is more than one video; here are a couple more

 

[Jim Logajan]

At least I can learn. You're ugly and there's no cure for that.

I look even uglier without the hat.

Seriously dude, if your best argument boils down to implying that your challenger is stupid then you're done. You're out of ammo and have hung yourself.

Dang! I hate when that happens!

All the purported data posted in this thread supports that this is a hoax:

* The wind tape on the car proves that the car is always pointing into the wind (this alone is conclusive)

Good point - but so conclusive that it proves other indicators - and all other replications - false?

* The person that made the video and designed the car holds on the order of twenty five patents in video editing and special effects

And that he has a BS and MS in aeronautical engineering. But clearly his patents make him a patent fraud. Good catch.

* The four free body diagrams magicly convert friction to thrust (this alone is a joke -- there is only one car so there is only one valid free body diagram, not two, three, or four)

I wish I knew what you were claiming here, but since it is all a fraud I guess it doesn't matter.

* The sanctioning body did not award a prize

Rats - another proof it is a fraud. NALSA merely proclaimed it a record - something (it seems) they do for everyone, fraud or not.

* No one has reproduced the results using wind power

True - so long as you ignore the reproduction about 1 minute 20 seconds into the first video of post 62 of this thread. And so long as we all accept that Galilean transformation is a fraud, all those reproductions using treadmills must also be considered fraud.

 

[Jim Logajan]

When the cart is sitting still and the wind is blowing across it then the cart has zero kinetic energy. After the cart gets moving it has kinetic energy that came from the wind.

The maximum kinetic energy is 1/2mv^2 where v is the wind speed if the cart is 100% efficient at capturing the wind energy. But it isn't 100% efficient. It's less than that and if it's more then it's magic.

What value do you think should be entered for "m" into that kinetic energy equation, and why?

(Hint: You're starting off on the wrong path in the analysis, which is why you've immediately run into a road block by starting where you did.)

 

[Jim Logajan]

m is the mass of the cart.

Where is the error?

The error is that the cart isn't the source of energy. After all, when the cart is stationary in the earth's reference frame, its kinetic energy in that frame is zero.

If you are going to claim a physical limit due to conservation of energy, then at the beginning (before the cart starts moving) the only thing moving in the earth's reference frame is the wind. What is the mass of the wind? Answer that and you have an upper limit on how much energy is available in the system.

 

[Jim Logajan]

The mass flow rate of working wind comes into the equation as an expression of force. For example assume two different carts, but each presents a different area to the wind. One more than the other.

The cart that presents more wind area could generate more force (F) because it interacts with a greater mass flow rate of the wind. If the masses of the two carts are equal then the bigger cart will accelerate faster (A=F/m) but their limit velocities and final kinetic energy (E.sub.k) will be the same (E.sub.k=1/2mv^2).

Another way of looking at the problem would be to race two identical carts, but one in a helium atmosphere and the other in a nitrogen atmosphere. The helium cart will accelerate slower than the nitrogen cart. That's because the force generated by the less dense helium will be less.

No?

No. The V and m in the kinetic energy equation must both describe the same object. You can't use m = mass of cart and V = velocity of wind and plug them into the kinetic energy equation and expect to get anything meaningful.

 

[Jim Logajan]

m = mass of the cart
v = velocity of the cart. It starts at 0 and accelerates (A), at best, up to the wind speed.

If I understand what you are saying then m is the mass of the wind. How do you arrive at that m?

Earlier you said V was the wind speed. To use wind speed meaningfully in the kinetic energy equation one has to plug in the mass of the wind for m, not the mass of the cart.

If V and m are now consistently describing the state of the cart, we can plug them into kinetic energy equation. But all that equation tells us is that the cart's kinetic energy increases with V. So too does its momentum. But they do not per se indicate any limit in the cart's speed.

I suspect your reference to the kinetic energy equation was to show that, once the relative wind on the blades drops to zero the available kinetic energy from it drops to zero? If so, then no disagreement. (There is just the, ahem, small matter now of what the wheels of the cart are doing....)

 

[Jim Logajan]

Jim,

I'm still not understanding what you mean by "mass of the wind". Wind has a mass flow rate, but not a static mass that's uniformly defined. The static mass would depend on the volume of air in the wind. If that mass described the total wind kinetic energy available then the acceleration of the cart would be dependent on how much air is in the volume, not how fast the wind is blowing, e.g. the mass flow rate. That's clearly erroneous.

Air has mass, therefore wind has mass, though you are correct that most fluid problems like this are best solved by using mass flow rates. I did not choose to use mass - you first brought it into the discussion with the equation for kinetic energy - I was merely trying to draw out the fact that your usage was ambiguous at best.

I only have a lowly BSc in physics - no experience in education - so I do not know what the best approach would be here. I think I'll let spork and others take it from here. I think it odd that you can consider finding the time and money to fly to see a demonstration rather than build and try a model device yourself, but I guess some pilots will invent any excuse to fly! That's understandable! :wink2:

 

[Jim Logajan]

So if the entire atmosphere were moving at 10 mph and all of the KE was transferred to a cart with 150kg mass, then the cart would travel 41000000 times the speed of light. AWESOME. (I really did work the numbers)

"Scotty - I need more wind power!"

"Captain, the propeller's at warp 9 - she canna take no more!"