Wind powered cart traveling directly down wind faster than the wind

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Jim Logajan

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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.
 
This is the one that "blows" my mind ... how can it be? ;)

 
I'm an engineer and this post hurts my brain.
 
What happens when you put the device on a treadmill?
 
kyleb said:
I'm an engineer and this post hurts my brain.
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I sat here watching that video last night, sipping a crown and dr pepper, trying to make sense of it. I eventually came to the conclusion that I was going to hurt my brain - so I went to bed.

Now I'm watching it again, sipping a coffee, and I've came to the same conclusion...
 
The concept actually seems simple to me ( and that scares me) I'd suspect the propeller on the wind car drives gears that turn the wheels? much like riding a bike. When you shift gears you can transfer more or less energy or use more or less energy to travel at different speeds so it would not seem that shocking.

As for the guy with the ruler well all I can say is he gives me the hebee Jeebes. A bit scary really.
 
I had the same insight as Adam - it's a question of mechanical advantage, and a wind-driven propeller is NOT a sail.
 
AdamZ said:
The concept actually seems simple to me ( and that scares me) I'd suspect the propeller on the wind car drives gears that turn the wheels? much like riding a bike. When you shift gears you can transfer more or less energy or use more or less energy to travel at different speeds so it would not seem that shocking.

As for the guy with the ruler well all I can say is he gives me the hebee Jeebes. A bit scary really.
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The problem is (at least the way my mind conceptualizes it) that, once the vehicle is moving at wind speed -i.e. airspeed = 0, the prop can't harness any energy from the wind to drive the wheels.
 
kyleb said:
The problem is (at least the way my mind conceptualizes it) that, once the vehicle is moving at wind speed -i.e. airspeed = 0, the prop can't harness any energy from the wind to drive the wheels.
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kyleb said:
The problem is (at least the way my mind conceptualizes it) that, once the vehicle is moving at wind speed -i.e. airspeed = 0, the prop can't harness any energy from the wind to drive the wheels.
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Look at the blade angle on the propellor- I think you'll find they are at the "wrong" angle to act as a windmill and deliver energy to the wheels (initially).

OTH, the wind does initially push the rig downwind by air resistance. The wheels get the propellor spinning. After a certain speed is reached, the propellor blades are spinning fast enough that the apparent wind over the blades isn't coming from astern anymore. The apparent wind angle is like that of the sails of a boat on a reach. On a good sail boat, you need to trim the sails until it seems you are sailing upwind because of the change in apparent wind.

Initially, the wheels drive the propellor. I think that after a 'critical' velocity with respect to the actual wind is reached, the propellor drives the cart. EDIT: The wheels are still working with the propellor at this point, but I think something as boat's keel, or as an ice-boat.

This part of the original post explains part of it:
Jim Logajan said:
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.
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I don't know if this would work because of the losses with the propellors in the water but I'm not ruling it out. I think sailboats already go around 3x the true wind on a reach. Google "sailing faster than the wind" for a bunch of explanations.
 
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Cap'n Jack said:
Look at the blade angle on the propellor- I think you'll find they are at the "wrong" angle to act as a windmill and deliver energy to the wheels (initially).
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Agreed, the prop appears to be spinning in the wrong direction to be wind driven from the rear.

But I guess...maybe..this is the same as the big wheel moving the wrong direction under the ruler?
 
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timwinters said:
Agreed, the prop appears to be spinning in the wrong direction to be wind driven from the rear.

But I guess...maybe..this is the same as the big wheel moving the wrong direction under the ruler?
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Could be. I was playing with the vectors in my head.
 
Palmpilot said:
Do you have a link to the plans?
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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.
 
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.
 
Palmpilot said:
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.
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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 said:
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!
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The only way the relative motion between the ground and the cart can drive the propeller shaft is through force transmitted between the ground and the wheels, but that force is in such a direction as to decelerate the cart, i.e., a drag force.

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.)
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You also need to subtract the drag force that the ground is applying to the wheels.

That's why a qualitative argument is not sufficient. In order to show how the cart can continue to accelerate beyond the zero relative-wind point, it is necessary to provide a quantitative argument, because it is necessary to show that the thrust from the propeller exceeds the sum of the drag forces.

The rest of your explanation suffers from the same omission.

By the way, one necessary test is whether others are able to repeat the experiment and get the same results.
 
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Jim Logajan said:
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.
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Ok, now i get it!
 
OK, I get it.

Is there a treadmill involved?
 
Palmpilot said:
That's why a qualitative argument is not sufficient.
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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.
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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 said:
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.
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How do we know that the predicted thrust from the propeller exceeds the sum of the predicted drag forces?

Until that question is answered, we don't know whether the claimed results are consistent with the known laws of physics, and the only way to come up with those predictions is by doing the math.

If I get time, I will look at your links.
 
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All it takes is common sense.

When the car is sitting still the wind is blowing at the back of it. The quack science says that the wind blows the car, not the propeller.

That's silly. The wind has to blow the propeller and the car at the same time. There are no two ways about it.

The net result is that the wind will want to blow the car forward and the propeller in backwards direction from what is show in the video. At best the car will creep backwards into the wind or sit still like the log bump that it is.

The car moves forward in the video because it is a hoax. I don't how they do it. Photoshoppy, battery power and electric motor, tow car, . . . I dunno. But I am confident that it's a hoax.
 
ebacon said:
All it takes is common sense.

When the car is sitting still the wind is blowing at the back of it. The quack science says that the wind blows the car, not the propeller.

That's silly. The wind has to blow the propeller and the car at the same time. There are no two ways about it.

The net result is that the wind will want to blow the car forward and the propeller in backwards direction from what is show in the video. At best the car will creep backwards into the wind or sit still like the log bump that it is.

The car moves forward in the video because it is a hoax. I don't how they do it. Photoshoppy, battery power and electric motor, tow car, . . . I dunno. But I am confident that it's a hoax.
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Sorry- not a hoax. Some of the on-line references cited by other are quite good and are not given to hoaxes. As an earlier post links to some plans, you can build a model yourself and see if it is a hoax.
 
Cap'n Jack said:
Sorry- not a hoax. Some of the on-line references cited by other are quite good and are not given to hoaxes. As an earlier post links to some plans, you can build a model yourself and see if it is a hoax.
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It's a hoax. I have enough faith in my engineering experience, sailing experience, and common sense to forego building a model or prototype.

If you think the garbage works then build a car yerself, tell us when it's done, and I'll fly over and check it out.
 
Couldn't get the plans PDF to open, I'd like to build one myself if just for the waste of time!
 
ebacon said:
It's a hoax. I have enough faith in my engineering experience, sailing experience, and common sense to forego building a model or prototype.

If you think the garbage works then build a car yerself, tell us when it's done, and I'll fly over and check it out.
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What kind of plane do you fly? When you fly over, I'd like a ride.
 
I had some words here, but lost them due to getting timed out. For now, the pic will have to do. Its not a hoax, just look at the FBD.
 

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It's a hoax.

Dudes. It a car powered by a fan. If this was real science kids would have been building and rolling these fan-cars around schoolyards for recorded history. Like windmills, and sailing ships, and boat propellers, and . . .

But they didn't, and don't, because . . .

It's a hoax.
 
ebacon said:
It's a hoax.

Dudes. It a car powered by a fan. If this was real science kids would have been building and rolling these fan-cars around schoolyards for recorded history. Like windmills, and sailing ships, and boat propellers, and . . .

But they didn't, and don't, because . . .

It's a hoax.
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What kind of plane do you fly? you said if I make it work you'd fly over to see it.
 
Even the video people that built models didn't show the stupid contraption working in the wind like it's supposed to. Instead they had to demonstrate it on a treadmill because . . .

It's a hoax.
 
ebacon said:
It's a hoax.

Dudes. It a car powered by a fan. If this was real science kids would have been building and rolling these fan-cars around schoolyards for recorded history. Like windmills, and sailing ships, and boat propellers, and . . .

But they didn't, and don't, because . . .

It's a hoax.
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did you look at the FBD?
 
B&TPilot said:
did you look at the FBD?
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I did. It's flat out wrong.

The drag vector (yellow) self converts to thrust as if by magic. That can't happen. If it did then the car would go infinitely fast. But it doesn't because . . .

It's a hoax.

Here is a similar car but it goes into the wind and at angles to the wind. That makes perfect sense.

http://www.popsci.com/node/32069
 
May I offer this analogy:

It is like a microphone and amplifier in a feedback loop. The increasing intensity of the feedback squeal is due to the fact that the amplifier is plugged into the wall and is "amplifying" the sound as it loops mic to speaker to mic to speaker...

The wind is the equivalent to the plugged in amplifier. The wind pushes the vehicle, the motion of the wheels sets the prop into the proper direction of rotation, the feedback loop begins.
 
B&TPilot said:
I had some words here, but lost them due to getting timed out. For now, the pic will have to do. Its not a hoax, just look at the FBD.
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Unfortunately, those figures don't really explain anything. In particular, Figure 4 doesn't explain why the red arrow next to the propeller is drawn pointing to the left (thrust) instead of to the right (drag). I also don't understand why the relative wind is still stated as zero knots when the cart is alleged to be traveling faster than the wind.
 
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Jim Logajan said:
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
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I've corrected your first link so that it works. However, it's for a boat with propellers both above and below the water, and until someone posts a rigorous justification for why that would be equivalent to the land vehicle, I will confine my attention to the other link, which does discuss the land vehicle. Fortunately, the document gives enough information about the design so that we can check his analysis.

I'm troubled by the conclusion of these two paragraphs:

For the purpose of this analysis we won’t consider the issue of accelerating to speed, but rather the cart’s ability to maintain faster-than-the-wind speed and further accelerate from that point. Thus we’ll tow the vehicle up to a speed of 20 mph in a 15 mph tail-wind and then let it loose. In this situation the vehicle will experience a relative head-wind of 5 mph. We’ll adjust the generator output such that it produces 20 lbs of retarding force at the wheels. This tells us the wheels will be putting power into the generator at a rate of 20 mph x 20 lbs (400 mph-lbs). But the generator will only produce 340 mph-lbs due to its 85% efficiency.

We deliver that power to the electric motor. But we get only 289 mph-lbs at the motor’s shaft due to the motor’s 85% efficiency. This power is working to spin the propeller, but the propeller does only 245 mph-lbs of work on the air due to its 85% efficiency. Given the vehicle’s relative airspeed of 5 mph, we can see that the prop will be producing 49 lbs of thrust.
​

That last sentence is inadequately justified, IMO.

The problem here is that he's calculating work using two different inertial (non-accelerating) reference frames in the same analysis. You can use any inertial reference frame you want, but if you CHANGE reference frames in the middle of the analysis, that changes the relationship between force and work. He starts the analysis using the ground as his reference frame, and he finishes using the air mass as his reference frame.

Consequently, he takes the work being done by the propeller per unit of time, which is 245 mph-lbs, divides it by the relative wind of five miles per hour, and claims that the thrust is therefore 49 pounds. But if he had used the original reference frame, which was the ground, he would have divided the work per unit time by the ground speed of 20 mph, for a thrust of only 12.25 pounds, which is not enough to counteract the retarding force from the wheels.

The analysis could be done using a reference frame attached to the air mass, or one attached to the ground, but not both in the same analysis.

So I'm still waiting for an explanation of how the claimed results are consistent with the known laws of physics.
 
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ebacon said:
It's a hoax.
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You can repeat it million times and still you won't succeed turning it into a hoax.
I recall the same nonsensical statements when folks when debating this airplane on a conveyor belt.
 
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