Sailing downwind faster than the wind: resolved?

[Phrak]

where does the energy come from to overcome friction?

 

[swerdna]

Despite what I said in my last post the sceptic inside still has some unanswered questions and I must retract my acceptance of the claim that a vehicle can travel downwind faster than the wind solely in the direction of the wind.

The two main issues are . . .

Can the thrust if the propeller ever exceed the rolling resistance caused to create that thrust? I don’t see how it ever can. If it could the vehicle would be able to be self propelled by it’s own inertia. When the vehicle has reached the speed of the wind or is placed on the treadmill in no wind, then wind can’t be considered as a factor as there simply isn‘t any wind relative to the vehicle. When the vehicle is placed on the treadmill the thrust of the propeller is created solely by the motion of moving treadmill surface relative to the vehicle, and is being transferred to the propeller via the turning wheels and linkages. This is not free energy and there has to be a rolling resistance energy loss that is greater then the energy of the thrust developed by the propeller.

The second issue is whether two opposing winds have a compounding effect or whether they somehow cancel each other out. The wind is moving relative to the ground at 10mph and the vehicle is traveling with the wind at 10mph. It’s claimed that some of the speed of the vehicle is coming from the thrust of the propeller. If say 3mph of the vehicles speed is coming from the propeller then only 7mph of speed is coming from the 10mph wind. What happens to the remaining 3mph of wind? As the vehicle is traveling at the speed of the wind it can’t be going past the vehicle at 3mph.

If there’s something I’m missing what is it?

 

[Subductionzon]

swedna a quick point on rolling resistance, it is a constant. Once you overcome the initial static state any more speed has very little effect on rolling resistance. This is a point brought up in plane on a treadmill arguments.

minder you got the right answer but you could not see how that means when you are on a 10 mph treadmill you have in effect a 10 mph tail wind. The "tailwind" on the treadmill comes about if you stop, you will feel the "wind". A large enough treadmill is indistinguishable from having a wind on an open field. The "body" of the cart may not feel the wind, but the rotating prop does. The prop is coupled directly to the ground. And it picks up its energy from the difference in the wind speed and the ground speed. With the treadmill you can argue that the wind speed is actually zero, but then you have to admit that the ground speed is -10mph. And if you were standing on the ground that was going 10 mph backwards with regards to the wind you would call it a ten mph tailwind.

 

[uart]

I have done my part to try and keep this forum a respectable physics forum but as long as this type of nonsense if being pushed off as real physics I can no longer have any part of it. This is now a kiddie’s forum for toys and gadgets. Have fun children.

Actually I think that the only thing you've show us is a lack of understanding of physics and a tendency for dogma.

 

[schroder]

Despite what I said in my last post the sceptic inside still has some unanswered questions and I must retract my acceptance of the claim that a vehicle can travel downwind faster than the wind solely in the direction of the wind.

The two main issues are . . .

Can the thrust if the propeller ever exceed the rolling resistance caused to create that thrust? I don’t see how it ever can. If it could the vehicle would be able to be self propelled by it’s own inertia. When the vehicle has reached the speed of the wind or is placed on the treadmill in no wind, then wind can’t be considered as a factor as there simply isn‘t any wind relative to the vehicle. When the vehicle is placed on the treadmill the thrust of the propeller is created solely by the motion of moving treadmill surface relative to the vehicle, and is being transferred to the propeller via the turning wheels and linkages. This is not free energy and there has to be a rolling resistance energy loss that is greater then the energy of the thrust developed by the propeller.

The second issue is whether two opposing winds have a compounding effect or whether they somehow cancel each other out. The wind is moving relative to the ground at 10mph and the vehicle is traveling with the wind at 10mph. It’s claimed that some of the speed of the vehicle is coming from the thrust of the propeller. If say 3mph of the vehicles speed is coming from the propeller then only 7mph of speed is coming from the 10mph wind. What happens to the remaining 3mph of wind? As the vehicle is traveling at the speed of the wind it can’t be going past the vehicle at 3mph.

If there’s something I’m missing what is it?

Yes, swerdna, you have hit the nail on the head. The standard explanation put forth by the people who are promoting this nonsense on this forum is “the cart extracts its energy from the air-ground interface” as if that is an explanation. Exactly how does the cart do this? Let's remove the air-ground interface and have a frictionless cart that floats in mid-air. Now the wind blows and it moves with the wind at wind velocity. It cannot possibly go any faster than wind velocity under those circumstances (what is it going to work against?) Now introduce the magical air-ground interface. In order for the wheel to add any drive force against the ground it must have friction with the ground; no friction, no drive force. If it has friction with the ground, that is additional drag and the cart must slow down so it is now moving at less than wind velocity. It is that simple. Unless you believe that a wheel dragging on the ground, which requires friction to get it turning, can also provide a drive force at the same time it is being driven! Can we now put a stop to this travesty of physics?


Uart, perhaps you would like to be the first to answer the question I have posed here.

 

[mender]

Shroder, I can answer your last question: there is no travesty of physics going on.

It is a basic but not obvious use of surplus energy to generate surplus speed. Exchanging force and velocity. Leverage. Mechanical advantage. Gearing.

Please try this next demonstration if you can. Get a planetary gearset. Label each of the components in this way: outer ring is the ground, planet carrier is the air and the inner gear is the cart. Hold the "ground" with one hand and move the "air" with the other hand. The "cart" moves faster than the "air".

This time hold the "air" and move the "ground" backwards (this is necessary to keep the proper orientation between the air movement and the ground movement). Again the "cart" moves faster than the "air". Notice that the cart has a lower speed relative to what you see but the same speed relative to the "air".

Now have someone lightly hold the "cart" from moving while you repeat both experiments. If you can provide enough twisting force, the "cart" will move even though it now has a noticeable amount of drag. Because of the gear ratio, the amount of drag on the "cart" has to quite a bit lower than the twisting force that you provide on the other gears. You are trading effort for speed.

I hope you're following the analogy. When using the air as one of the components, there will be a lot of slippage. That can be simulated by using a light grip on the "air" during any of the experiments. Please indicate that you have read this far.

Swerdna, your question relates to the amount of energy that can be harnessed from the wind and whether that is greater than the energy needed to turn the propeller as well as the parasitic drag of the cart. Let's look at what is happening when the cart is moving along the ground with a 10 mph tailwind but let's replace the prop with a sail. If the cart reaches a speed of 6 mph, this means that it only needs the energy harnessed from a relative wind speed of 4 mph to overcome all the drag.

How do we get the cart to go faster? Put on a bigger sail. Let's double the sail size. How much wind speed do we need now? If my understanding is correct, we will need 2.83 mph of wind. For the sake of argument , let's ignore that slight increase in drag increasing the speed of the cart will cause and say that now the cart moves at 7.17 mph. By continuing this process, we can see that the cart will never reach wind speed no matter how large that sail is.

Now let's change the cart slightly. Let's keep it very light but mount the sail at the front of the cart on a track that moves from the front of the back straight to the back. Let's spring-load the sail and install a latch that we can trigger remotely. We set the cart in the wind and let the cart get to its highest speed of say 9 mph. We trigger the latch, the sail moves back on the track at 7 mph. What happens? The sail moves back, pushing against the wind. The chassis since it doesn't present much resistance moves forward relative to the sail. Let's say that it moves forward at an additional 4 mph (total of 13 mph) while the sail moves back at 3 mph. The amount of resistance that the sail now presents to the wind has increased considerably. Before the sail moved, the relative wind speed to the sail was 1 mph; now it is 3 mph more than that or 4 mph. How much extra energy does the sail extract from the wind during that time? Please post an answer to show that you are still following along.

The key, which I hope you see now, is that the wind is always working against a resistance. In this example, the sail resists the flow of air from behind. With the prop cart, the air being pushed back from the prop resists the flow of air from behind. In order to get more energy from the wind, the "sail" either has to be bigger or it has to have a higher speed difference between it and the wind. The prop never reaches or exceeds 100% efficiency; that would be perpetual motion. The drag of the cart and the drivetrain plus the energy needed to move the air back requires that a greater difference in speed between the wind and what it is acting on be present on order to extract more energy to overcome all the drag. When there is enough difference, the cart moves accordingly.

I don't know the exact numbers because I haven't build a cart and tested it yet. My take is that with a wind speed of 10 mph from the rear, that wind is slowed to say 7 mph by the air coming off the prop. The prop is geared to push air back at 1.75 times what its ground speed is or 7.5 mph relative to the cart. Since the prop efficiency and the "wind interface" efficiency is low, it achieves maybe 50% of that. It pushes the air back at 3.75 mph. Add that to the 7 mph that the wind from the rear is providing and the cart speed is 10.75 mph.

The wind sees a resistance to its travel. The speed difference is 3 mph. If the prop is increased in size, the amount of energy increases. If the speed difference is more, the amount of energy available from the wind increases. Harnessing the energy for the cart is still the same as a sail or a wind turbine. The power is there.

As I said, the numbers are speculative and I won't know the correct ones until I conduct a test. I hope I was able to get the point across though.

The more efficient the cart is, the lower the energy requirement, and the lower the speed difference between the air and the ground required for the cart to reach a specific speed. This was shown in the development of several carts. The most efficient requires a tailwind of only 2.7 mph to reach wind speed; others are 4 mph and 8.5 mph.

It's a simple energy balance. It's not so simple to see how it works.

 

[ThinAirDesign]

If it has friction with the ground, that is additional drag and the cart must slow down so it is now moving at less than wind velocity. It is that simple. Unless you believe that a wheel dragging on the ground, which requires friction to get it turning, can also provide a drive force at the same time it is being driven!

Swerdna, don't believe him. He's presenting a "something that produces drag can never be good" argument and it's incredibly simple to demonstrate the fallacy of schroder's above example -- we'll use a simple sail.

Like his cart, we'll hang a sail in mid-air where it cannot do anything but drift at the wind speed. Attach a drag inducing keel to the bottom and drop that into the water -- I don't need to explain the possibilities other than they include going faster than the wind speed.

The cart just uses gearing between the propsail and the wheels to produce a keel constraint (and of course drag).

Can we now put a stop to this travesty of physics?

A simple sail/keel combo is a rather mature technology schroder -- you might want to rethink your example.


JB

 

[Subductionzon]

schroder, I hope you do not deny that when on a sailboat you can sail faster than the velocity of the wind when sailing across the wind. Well that is exactly what the propeller blades are doing. They are moving across the wind. The sailboat works by using the difference between the speed of the wind and the body of water. If it only ran before the wind it could only run at some fraction of the winds speed. The boats keel allows it to extract this energy. A boat without a keel can only run before the wind, and not too rapidly. The propeller is the cart's sails and the wheels are its keel. Try not to look at the wind blowing on the cart, rather concentrate on the propeller which is acting like a tacking sail.

The small cart on the treadmill is a perfect proof. You refuse to see that in frame of reference it is exactly the same as being on an open field with a wind at your back. There was also a video of it outside if you check all of spork's (he's the one with the Hitler mustache) videos and you can see JB (Thin Air Design) had to run rather quickly to catch it. Now I can't say because I wasn't there but it looked like he had to run faster than the wind to catch it.

 

[ThinAirDesign]

Try this:

http://au.youtube.com/watch?v=IMEerIkOVZ0

The above proposed vehicle doesn't meet my personal criteria for a vehicle going DDWFTTW simply because neither it's CG nor pilot is going DDW. That is easy to fix by placing two of the tacking craft in the frame and placing the pilot on the frame rather than the boats.

Anyway, just a little thought expanding animation using perfectly well established sailing principles and no "travesty of physics".

(swerdna, notice the wheel acting as the keel on the boat)

JB

 

[Phrak]

swedna a quick point on rolling resistance, it is a constant. Once you overcome the initial static state any more speed has very little effect on rolling resistance.

It's technically called rolling friction. With fairly constant rolling friction, power loss increase directly with the velocity.

 

[Subductionzon]

It's technically called rolling friction. With fairly constant rolling friction, power loss increase directly with the velocity.

Yes, power will increase directly with velocity, but that is a direct outcome of W=F*D. With W being work, F the force of friction, and D the distance that you travel. Swedna seemed to have a problem with overcoming the force of friction which I pointed out is a constant. Once you overcome it you do not get more, but if you speed up more of your power goes into friction.

 

[swerdna]

The claim that the combination of the speed of the wind and the opposing thrust of the propeller causes the vehicle to travel faster than the wind is incorrect.

(1) All of the energy to move the vehicle comes from the wind.

(20 All of he energy to spin the propeller comes from the wind.

(3) The energy produced by the spinning propeller is less than the energy taken from the wind to create it because of friction.

(4) The combined energy of the wind and the propeller is therefore less than the speed of the wind.

Not only could this vehicle not exceed the speed of the wind, it couldn’t even achieve it. The same vehicle with a sail the same area as the sweep of the propeller would perform better as there would be less friction involved.

There’s no free lunch and there’s no free energy.

 

[mender]

So you didn't read my post.

The vehicle has demonstrated beyond a doubt that it moves DDWFTTW. I also took a lot of time to explain why there is no issue with energy. Ignoring the facts while making statements does not alter anything.

This doesn't use free energy.

 

[schroder]

Shroder, I can answer your last question: there is no travesty of physics going on.

It is a basic but not obvious use of surplus energy to generate surplus speed. Exchanging force and velocity. Leverage. Mechanical advantage. Gearing.




It's a simple energy balance. It's not so simple to see how it works.

Why don't you add VOODOO to your above list. The game is over. You must really have a vested interest in this to blow so much smoke. Were you planning on selling them as Christmas stocking stuffers? It's over, your hoax is exposed. No more free advertising on this forum.

 

[ThinAirDesign]

Why don't you add VOODOO to your above list. The game is over. You must really have a vested interest in this to blow so much smoke. Were you planning on selling them as Christmas stocking stuffers? It's over, your hoax is exposed. No more free advertising on this forum.

There's nothing to sell and thus nothing to advertise.

It's a physics brainteaser and a darn good one -- it elicits immediate, strong and emotional responses such as yours which are ultimately demonstrably wrong.

It's just good clean fun with physics.

To anyone who want's to build one of these 'VOOODOO' devices, I'm happy to provide a parts list and any build advice requested. I would only ask that in exchange for my input you promise to video and post the results *no matter the result*.

JB

PS: One good thing to remember is that the person who presents a brainteaser usually has inside information.

 

[mender]

Why don't you add VOODOO to your above list. The game is over. You must really have a vested interest in this to blow so much smoke. Were you planning on selling them as Christmas stocking stuffers? It's over, your hoax is exposed. No more free advertising on this forum.

No, I'm only trying to help you understand this. You don't want to. No game, no product, no haox, just physics. Report this thread if you feel the need. At this point I'm sure that the moderators are quite aware of the content.

Why don't you try reading my post and doing the experiments that I suggest? You might find something of interest.

 

[Subductionzon]

Why don't you add VOODOO to your above list. The game is over. You must really have a vested interest in this to blow so much smoke. Were you planning on selling them as Christmas stocking stuffers? It's over, your hoax is exposed. No more free advertising on this forum.

If this is a hoax how did they get the cart to move faster than the wind directly down the wind? Or don't you understand physics well enough to see that the treadmill is equivalent to a perfect tailwind.? If you cannot understand something as simple as frame of reference there is no point trying to explain the aerodynamics of sailing to you.

 

[rcgldr]

Can the thrust if the propeller ever exceed the rolling resistance caused to create that thrust?

Yes, I already explained this in earlier posts. Effective gearing between the driving wheels and the propeller muliptly the force and divide the speed. The reducion in speed works because the propeller is interacting with the slower (relative to the cart) moving air, while the wheels interact with the faster (relative to the cart) ground.

Vw = speed of wind
Vc = speed of cart
Vp = speed of induced wash from prop
Fp = force from prop
Fc = force from cart wheels
Fd = overall losses (drag) related to forward speed of cart

Speed of air through the prop = induced wash + relative air speed = Vp + (Vc-Vw)
The power input = Fc x Vc
The power output = Fp x (Vp + Vc - Vw)
The power loss = Fd x Vc
The net force on the cart = Fp - (Fc + Fd)

Vc can be > Vw, Fp can be > Fc, with power output still well below power input as long as Vw is greater than zero (a tailwind). The gearing factor (diameter ratios between wheels and prop, gearing at the differential, prop pitch), multiplies the force and divides the speed. The reduction in speed is relative to the ground though, but the prop is interacting with the air, so although its speed is a fraction of it's ground speed, if the gearing ratio is not excessive, the prop speed is still positive relative to the air it interacts with, Vp + (Vc-Vw) > 0.

A conservative setup might only try to achieve DDWFTTW by a ratio of 1.2. In a 10mph wind, the cart would move at 12mph, resulting in a 2 mph working air speed for the propeller. If the effective gearing for wheel speed to prop speed was 2:1, then the prop speed would be 6mph, and the induced wash speed, Vp - (Vc-Vw) = 4mph. If the gearing was 1.5:1, the prop speed would be 8mph, induced wash speed: = 6mph. The actual force from the prop depends on the change in air speed due to the prop times the mass flow, and this force propels the cart forwards. The force at the driving wheels (Fc) is the prop force minus all the losses in the system (rolling friction, aerodyanic drag of the cart, prop inefficiencies).

If someone gets some actual numbers for the mini-cart, and prop, here's a link for some prop math:

http://www.grc.nasa.gov/WWW/K-12/airplane/propanl.html

 

[schroder]

“It is a basic but not obvious use of surplus energy to generate surplus speed. Exchanging force and velocity. Leverage. Mechanical advantage. Gearing.”

That is Voodoo. That is your theory? This thread has been reported and Admin can do what they want with it. This is not worth any more of my time.

 

[ThinAirDesign]

Or don't you understand physics well enough to see that the treadmill is equivalent to a perfect tailwind.?

Sub, that one has clearly been asked and answered.

Schroder has got it in his head that it matters *how* the relative motion is created. It's a case of the watermellon seed being squeezed apparently by only one finger.

JB

 

[swerdna]

Gearing only moves things faster it doesn’t create extra or new energy. In fact because of friction gearing always reduces energy. Some clever guys worked all this out many years ago.

Conservation of energy is not just a good idea . . . it’s the law!

 

[Subductionzon]

Gearing only moves things faster it doesn’t create extra or new energy. In fact because of friction gearing always reduces energy. Some clever guys worked all this out many years ago.

Conservation of energy is not just a good idea . . . it’s the law!

Alright, I will ask you the same question I asked shrodeer. You saw the various videos of the carts working on the treadmill, how did they hoax them since they are obviously running faster than the wind? And for frame of reference an ant on the treadmill for the short time before it ran underneath could not tell the difference between being on the treadmill with a 10 mph wind blowing towards the front of it or being on an open field with a 10 mph wind. C'mon people the frame of reference part of this is the extremely easy.

One question, how many people who do not think the treadmill is a perfect representative of a 10 mph tailwind thought the plane would not take off? Be honest now.

 

[ThinAirDesign]

Gearing only moves things faster it doesn’t create extra or new energy. In fact because of friction gearing always reduces energy. Some clever guys worked all this out many years ago.

And we agree with that clever guy completely.

Conservation of energy is not just a good idea . . . it’s the law!

And we think it's a very good law.

Somehow folks who think this device must be over-unity forget the potential energy between the two moving mediums. This device slows that relative motion and uses the extracted energy to overcome a bit of prop drag, a very very small bit of rolling friction and bearing drag, and an even smaller bit of aero drag created by the chassis moving some tiny amount more than the wind.

We don't need very much and there is a lot available. This surplus is demonstrated easily by the 3-4x VMG that traditional sailing rigs can achieve. We after all are only looking for 1.01 to visibly prove the point.

JB

 

[ThinAirDesign]

This thread has been reported and Admin can do what they want with it.

As has been demontrated earlier in this thread, the moderators of this forum agree with us as to both the theoretical concept and the validity of treadmill testing.

This is not worth any more of my time.

Only you can determine that.

Best wishes

JB

 

[swerdna]

Alright, I will ask you the same question I asked shrodeer. You saw the various videos of the carts working on the treadmill, how did they hoax them since they are obviously running faster than the wind? And for frame of reference an ant on the treadmill for the short time before it ran underneath could not tell the difference between being on the treadmill with a 10 mph wind blowing towards the front of it or being on an open field with a 10 mph wind. C'mon people the frame of reference part of this is the extremely easy.

One question, how many people who do not think the treadmill is a perfect representative of a 10 mph tailwind thought the plane would not take off? Be honest now.

It was always obvious to me that the plane would take off (honestly). The plane gets all of it’s propeller energy from an onboard motor and not from the ground. The difference with the vehicle on the treadmill is that it does get all of it’s propeller energy from what is essentially the ground.

 

[Subductionzon]

It was always obvious to me that the plane would take off (honestly). The plane gets all of it’s propeller energy from an onboard motor and not from the ground. The difference with the vehicle on the treadmill is that it gets all of it’s propeller energy from what is essentially the ground.

Close, it gets its energy from the difference in speed between the moving treadmill and the still air (or the difference in speed between the ground and the wind if you are outside). It is a subtle difference but a very important one.

 

[mender]

I'd like to address the power issue. I'll likely need some help on this. I'll list where I'm getting info so that it can be checked for validity.

I'm going to use the wind power chart found on this site:

http://www.windpower.org/en/stat/unitsw.htm#anchor1345942

It lists the power of the wind at regular intervals. 10 mph converts to 4.4m/s. The wind power is 313.6 w/m2 at 8 m/s. Dividing 4.4 by 8 and cubing then multiplying by 313.6 shows that at 4.4 m/s there is 52.18 watts available.

I need some indication on how the wind power numbers were arrived at, specifically is the wind slowed to a percentage of the original wind speed when the wind turbine removes the energy? Also, I request that someone (like Jeff) looks at this site to see whether this calculator can be used:

http://members.jcom.home.ne.jp/4223215501/staticthrust.htm

Next, I need some more numbers on JB's cart. I need the force readings on a gram scale when the cart is on the treadmill and the treadmill is moving at 2.5, 5.0, 7.5, and 10.0 mph both with the prop connected and disconnected. Also, I need a better idea of the rpm of the prop at those speeds. As I mentioned, I counted about 20 revolutions in four seconds when the cart was at 2.7 mph, but it's hard to be sure over the internet.

Finally, a confirmation on these specs: prop is a 14 x 8 slow flight (part or model number?), overall cart weight is 169 grams, steady state is achieved on level treadmill is 2.7 mph, incline needed to resist advancing at 10 mph is 4.4 degrees. I'd like the incline for 5.0 and 7.5 mph as well if possible.

Please let me know if and when you can assist me in this. This may be another part of the puzzle that is needed. I know that an actual test is a better proof of the concept but this may help others accept what they see.

 

[molinaro]

Gearing only moves things faster it doesn’t create extra or new energy. In fact because of friction gearing always reduces energy. Some clever guys worked all this out many years ago.

Conservation of energy is not just a good idea . . . it’s the law!

It's one thing to be able to state a principle in physics. It is something entirely different to be able to use it when appropriate.

I do not agree with any of your points in this thread.

 

[mender]

Quick note, found that the wind is slowed by 2/3 of the original speed, and that Betz' law states that at most 59% of the energy can be harnessed. That reduces the wind speed to 3.3 mph when a maximum of 30.8 watts per square meter is harvested.

 

[ThinAirDesign]

Quick note, found that the wind is slowed by 2/3 of the original speed, and that Betz' law states that at most 59% of the energy can be harnessed. That reduces the wind speed to 3.3 mph when a maximum of 30.8 watts per square meter is harvested.

Hey Mender, you might wish to double check on that Betz' law deal. It's my understanding that it applies to turbines and not props. I'm pretty sure prop efficiency can be significantly higher than that.

Not suggesting what the efficiency of our prop is -- just that Betz' may not apply to our application.

JB

 

[mender]

JB, you may be right. I'll use the lower number for now since it is based on the difference in air speed before and after the wind turbine (2 cubed over 3 cubed or 16/27) and only references the speed, not the efficiency of the wind turbine itself. I suspect that the propeller efficiency will be part of the other side of the equation.

Here's a good site explaining some of this:

http://www.windturbine-analysis.netfirms.com/index-intro.htm

 

[rcgldr]

Gearing only moves things faster it doesn’t create extra or new energy.

Gearing also moves things slower. It's how your car operates. In 1st gear the overall gear ratio is high, about 10:1, the engine rotates 10 times every time the driven tires rotate once. The tire speed is reduced by a factor of 10, but the torque is multiplied by a factor of 10 and the power is identical if there are no losses. Assuming a realistic loss of about 15%, then the torque from the engine is multiplied by 8.5. If the car is in top gear, then the gear ratio is around 3:1, the speed is divided by 3, and with the 15% loss factor, the torque is multiplied by 2.55.

The point here is that gearing was used to multiply the torque output of the engine, but at a slower speed, and taking losses into account.

Getting back to the wind cart, the fact that the speed of the air that the prop interacts with is much slower than the speed of the ground that the wheels interact with, allows a gearing effect that reduces speed but increases force, even though power output is less than power input.

http://www.grc.nasa.gov/WWW/K-12/airplane/propanl.html

Assume the wind is 10mph, the cart is going downwind at 12mph, and the required additional induced wash to generate the force to overcome all the drag factors is 3mph. The air speed through the prop is 5mph, so an effective gear factor of 2:1 between wheel speed and prop air speed would be enough, and with a 30% loss, the force at the prop would be 1.4 times the force at the wheels.

 

[russ_watters]

Yes, swerdna, you have hit the nail on the head. The standard explanation put forth by the people who are promoting this nonsense on this forum is “the cart extracts its energy from the air-ground interface” as if that is an explanation. Exactly how does the cart do this? Let's remove the air-ground interface and have a frictionless cart that floats in mid-air. Now the wind blows and it moves with the wind at wind velocity. It cannot possibly go any faster than wind velocity under those circumstances (what is it going to work against?) Now introduce the magical air-ground interface. In order for the wheel to add any drive force against the ground it must have friction with the ground; no friction, no drive force. If it has friction with the ground, that is additional drag and the cart must slow down so it is now moving at less than wind velocity. It is that simple. Unless you believe that a wheel dragging on the ground, which requires friction to get it turning, can also provide a drive force at the same time it is being driven! Can we now put a stop to this travesty of physics?

You are describing the scenarios wrong. You motion vectors do not match what is being claimed/what is happening: you have it backwards. Since the craft is moving faster than the wind, the wind can't be powering the wheels. The wheels are powering the propeller to create thrust to move ahead of the wind. You need to draw a diagram. Or better yet: look at the one I already drew!

This thread is going nowhere and is therefore locked. I may reopen it if you respond to the query I made with the diagram a few pages ago. You can send me a PM.