Sailing downwind faster than the wind: resolved?

[mender]

It appears that there are some who aren't understanding exactly what is being proposed regarding the treadmill test. At this point it may be beneficial to restate the problem clearly.

There have been several devices made specifically to demonstrate that it is possible to have a device powered only by the wind that reaches a higher speed than the wind traveling directly downwind. There have also been several videos made showing various tests. At this point, only one cart has been shown on an extended test outdoors in a wind. Many questions have arisen about the validity of that video. In order to address those questions, an indoor test that simulates an outdoor test has been used and the results of those tests recorded and posted.

The treadmill test is limited in scope compared to the outdoor test but has the advantage of a more controlled and repeatable environment. The treadmill test starts at a point that will either prove or disprove that a device can achieve Directly Downwind Faster Than The Wind (DDWFTTW). The test simulates the cart moving across the ground at the exact same speed as a 10 mph wind from behind. At that point, the cart is traveling across the ground at the same speed as the air, so the cart will experience zero air movement. The treadmill simulates the ground moving under the cart at 10 mph. By reading several different descriptions of Interchangeable Frames of Reference (IFOR), this can be seen as exactly the same as the cart moving at 10 mph along the ground with a 10 mph tailwind in an outdoor test.

A streamer that is attached to the cart will, in both cases, hang straight down. This is to show that the air movement around the cart is for all intents and purposes not moving relative to the cart.

For the cart to demonstrate DDWFTTW travel, it has to move forward relative to the still air. If it does not but holds steady during the test, it is only moving at the same speed as the air, i.e. 10 mph along the ground in a 10 mph tailwind. If it moves backwards on the treadmill, it is not achieving DDWFTTW travel but is performing in a manner that equates to being blown along the ground.

This is much easier to see on a treadmill than trying to guess what the wind speed is in an outdoor test or relying on a streamer on the cart as irrefutable proof of DDWFTTW. However, the treadmill test must be acceptable as a substitute for the outdoor test.

The small cart in the later videos moves forward on the treadmill. If the propeller is disconnected or removed, the cart will fall back on the treadmill because of the rolling resistance. This has not been shown but could be on a subsequent test.

 

[rcgldr]

The faster the vehicle travels with the wind the less energy the vehicle receives from the wind. If the vehicle reaches the speed of the wind it receives no energy from the wind at all.

Except in this case the propeller, which is driven by the wheels, accelerates the air upwind so that the air flow through the propeller opposes the wind. The propeller is moving downwind faster than the wind, but the air flow through the propeller isn't; the air flow through the propeller has a net upwind component. The DDWFTTW cart interacts with both air and ground, and the source of power is the difference in speed between air and ground, and this difference remains constant regardless of how fast the cart is moving.

I don’t see that a treadmill is comparable with a wind as an energy source in this experiment. A treadmill moving at 10 mph is always moving at 10 mph ... The vehicle traveling at 5mph.

The starting condition is different. The treadmill case is similar to pushing the mini-cart up to near wind speed outdoors then releasing it. The treadmill cases where the cart is pushed backwards would be similar to releasing the cart at say 8mph in a 10mph wind, where the cart then accelerates forwards.

 

[rcgldr]

alternative concept model - backwards moving bluff body cart

Start with a simple cart, no propeller. The wheels drive a long treadmill backwards at 1/3rd the speed of the cart. Attached to the front end of the treadmill is a large but light square sail, a bluff body, which the treadmill moves backwards at 1/3rd the speed of the cart.

The cart only runs until the sail reaches the back end of the treadmill. So to make it continuous, as the sail reaches the back end, it's collapsed and moved forward and under the treadmill, while at the same time, a second sail that was previously collapsed and under the treadmill now reaches the front end of the treadmill, is raised and it moves backwards along the treadmill. Obviously a lossy design but the concept should work.

If there were no losses, then a 10mph wind results in the sails moving at the same 10mph as the wind, and the cart moving forwards at 15mph, with the sails moving backwards at 5mph relative to the cart. OK, loss free isn't realistic, so assume the losses require a net 2mph on the sail in the 10mph wind case. The sails move downwind at 8mph, 2mph slower than the wind, while the cart moves forwards at 12mph (faster than the wind), with the sails moving backwards at 4mph relative to the cart. If the losses require a net 3mph on the sail in the 10mph wind case, the sails move downwind at 7mph, 3mph slower than the wind, and the cart move forward at 10.5mph (a bit faster than the wind), with the sails moving backwards at 3.5mph relative to the cart.

 

[ThinAirDesign]

The small cart in the later videos moves forward on the treadmill. If the propeller is disconnected or removed, the cart will fall back on the treadmill because of the rolling resistance. This has not been shown but could be on a subsequent test.

mender, one of our early videos does show the test with the prop removed.



Watch from ~1:00 forward.

JB

 

[yoavraz]

And this is your critical mistake. There is still plenty of wind apparent to the propsail as it is spinning.

No wind is felt by the *chassis of the cart* - there is plenty of wind felt by the propsail as it is spinning.

Put a fly on the tip of the prop and try to tell it there isn't any wind.

The prop creates the wind. Not the wind turns the prop. Critical difference.

The apparent wind of the propsail generates more than enough thrust to move the vehicle forward relative to the wind. This is demonstrated in both indoor and outdoor videos.

On treadmill - yes. Plenty of energy transfer. Outdoor - No. Past wind speed energy and speed in vehicle are lost until vehicle is below wind speed, and then speed increases again, may pass wind speed again, and so on.

There is no sarcasm whatsoever -- the mistake is yours and it's a common one. There is an entire posse of knowledgeable folks here attempting to demonstrate to you the error of your position, but you have "concluded" it is not possible and thus refuse to listen.

Show me the problem. I have not seen yet any problem in my logic, and until proven mistaken I hold to my opinions.

You continue to say there is "no wind" and yet relative motion between the air and the rolling surface exists in fact, and can be measured, documented and exploited.

JB

Relative wind between air and belt does not matter (yes, you warm the belt and air a little bit). Relative wind between air and vehicle is the ONLY one that matters, since if air speed (wind) is a factor in propulsion it has to induce force upon the vehicle and exchange momentum and energy. If relative to vehicle wind speed is 0, no wind force can exist and play role in vehicle propulsion.

To dispute this you have to answer in specifics (or give me a pointer), not to say "some wise people say that..."

 

[mender]

Thanks, JB, I hadn't looked at all the videos, and I missed that!

I tried to get a count of the propeller rpm during the 2.7 mph run and it seemed to be around 5 rps or 300 rpm. At 10 mph, that works out to about 1100 rpm - does that sounds about right? And you are using a 14 x 8 slow flight prop? The steady state incline at 10 mph was 4.4 degrees and the cart weight is 169 grams?

I know I've said it before but good job, guys!

 

[mender]

Yoavraz, you have an important point. For most people, this is where things don't make sense:

"On treadmill - yes. Plenty of energy transfer. Outdoor - No. Past wind speed energy and speed in vehicle are lost until vehicle is below wind speed, and then speed increases again, may pass wind speed again, and so on."

On a cart that is using a sail or a parachute, this would be true. But since the propeller is pushing air backwards against the wind, it can continue to accelerate past the speed of the wind. The speed of the air moving back from the propeller is added to the wind pushing from behind the cart. If the cart had no friction to deal with, the extra speed would be exactly the same ratio as the propeller is overdriven in relation to the ground speed. Since there is friction, the cart can't go quite that fast but a portion of the propeller air speed is added to the real wind speed and the cart moves accordingly.

 

[rcgldr]

The prop creates the wind. Not the wind turns the prop.

The prop creates a wind which is opposed by the tail wind. The combined velocity of upwind flow from the prop and downwind flow from the wind generate a forwards force onto the prop, which tranfers this forward force to the cart at it's axis. Most (but not all, since there are other drag losses) of the forwards force on the cart is applied to the driving wheels, and the ground exerts an equal an opposing backwards force onto the wheels. The opposing backwards force from the ground creates a torque on the driving wheels, which is used to drive the propeller. Effective gearing multiplies the effective force and divides the effective speed of the prop, but the prop is interacting with slower moving (relative to the cart) air (plus it's own induced wash). If the gearing reduction in speed is less than the difference between wind and ground, the cart works, depending on other losses.

wind speed relative to ground (or treadmill speed)

Note the speed factor here are relative to the medium the wheel and prop interacts with. The wheels interact with the faster moving ground and the prop interacts with the slower (relative to cart) moving air.

If the cart is moving at the same speed of the air, then the only speed at the prop is due to it's own induced wash, which is 1/2 the sum of the entry and exit (when air pressure returns back to ambient downwash of the prop) speeds, while at the wheels, the relative gound speed is equal and opposite to the wind speed (if the cart is going the same speed as the wind).

The power input is the force the ground applies to the wheels time the forwards speed of the cart. The power output is the force the prop applies to the air times the backwards speed of the air flow through the prop.

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

The net force on the cart = Fp - (Fc + Fd)
The power input = Fc x Vc
The power output = Fp x (Vp + Vc - Vw)
The power loss = Fd x Vc

The point here is that 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), because the cart uses effective gearing to multiply the force and divide the speed from the ground to the prop, which works because the prop interacts with the air (wind), as long as the reduction of speed by the gearing is less than the difference than the speed between the air (wind) and the ground, and the force is mutliplied enough to overcome any loss factors.

 

[ThinAirDesign]

Relative wind between air and belt does not matter. Relative wind between air and vehicle is the ONLY one that matters, since if air speed (wind) is a factor in propulsion it has to induce force upon the vehicle and exchange momentum and energy. If relative to vehicle wind speed is 0, no wind force can exist and play role in vehicle propulsion.

To dispute this you have to answer in specifics (or give me a pointer), not to say "some wise people say that..."

You've been *given the specifics repeatedly*. Some very wise people have given you the specifics and you simply ignore them.

Again, the key to your mistake is bolded: Separate the chassis of the vehicle from the propsail -- they are not rigidly attached to each other. From the time the vehicle first begins moving, to the time it's DDWFTTW, there is ALWAYS wind over the propsail. ALWAYS!

JB

 

[swerdna]

EUREKA! . . . and all that

I now believe that this type of vehicle can and does travel faster than the wind in the direction of the wind (no zigzagging required). What’s more important I believe I know and understand the why and how.

There are two important facts to consider that aren‘t intuitively apparent . . .

(1) There are two winds involved not one. The initial wind that moves the vehicle over the ground and a second wind created by the spinning propeller.

(2) The initial wind doesn’t push against any part of the vehicle (including the propeller) it pushes against the opposing propeller wind.

The total amount of potential energy available to move the vehicle is a combination of both winds as they are opposing each other. Of course it takes energy from the initial wind to create the propeller wind, but this loss is immediately replaced by a piece of “new” wind at full energy. The rolling resistance of the vehicle caused by the generation of the propeller wind means the vehicle will never reach the combined speed of both winds, but it can and will exceed the speed of the initial wind.

The treadmill demonstration proves this as it effectively recrates the conditions of the vehicle traveling at the speed of the initial wind. That the vehicle moves against the movement of the treadmill clearly demonstrates that the rolling resistance to create the propeller wind is less than the thrust of the propeller wind.

My slight disappointment at being previously wrong is more than compensated by the discovery of the truth. Thanks to those that were always right for your tolerance. Now I’m off to design a myriad faster than wind vehicles.

 

[mender]

Yes! I had the same feeling when I finally "got it". That's why I'm wanting to design and build a non-prop version to help demonstrate this unique concept.

It's always good to find out what turned on the light for others. Now, since you're a fresh convert, I'd like to ask you what your thoughts were initially about the treadmill test. Did you understand the equivalency of the test before or after you accepted the concept? And which explanation or combination helped you get it?

 

[yoavraz]

Yoavraz, you have an important point. For most people, this is where things don't make sense:

"On treadmill - yes. Plenty of energy transfer. Outdoor - No. Past wind speed energy and speed in vehicle are lost until vehicle is below wind speed, and then speed increases again, may pass wind speed again, and so on."

On a cart that is using a sail or a parachute, this would be true. but since the propeller is pushing air backwards against the wind, it can continue to accelerate past the speed of the wind. The speed of the air moving back from the propeller is added to the wind pushing from behind the cart. If the cart had no friction to deal with, the extra speed would be exactly the same ratio as the propeller is overdriven in relation to the ground speed. Since there is friction, the cart can't go quite that fast but a portion of the propeller air speed is added to the real wind speed and the cart moves accordingly.

Mender,

I have to retract on one central point: From the vehicle point of view a treadmill at 10mph is exactly the same as back wind of 10mph. Indeed when the vehicle on the treadmill is stationary, the relative wind is 0. Same with outdoor at 10. Energy and momentum transfers as well as forces are the same. Also if vehicle speed deviates from 10 (relatively to outdoor-ground or belt) by + or - it is still the same. Thus treadmill is a complete, accurate simulation, I believe. Assuming that simulated wind speed does not change (e.g., 10), I have no intuition how a vehicle that still accelerates when at 10 relatively to travel surface (outdoor ground or belt) will behave, and what max speed it can reach. Outdoor it is quite impossible to keep wind constant. On a treadmill it is impossible to let vehicle pass simulated wind speed beyond seconds (unless you use a fan with controlled wind speed to keep simulated real wind speed 10 when vehicle and treadmill speed goes beyond 10 for vehicle to remain stationary), even if you build a very long treadmill. Finding this by calculations translates to considering how vehicle speed translates to wheel speed, translates to prop speed, and translates to thrust. On the other hand, vehicle overall drag at given vehicle speed and given relative wind speed (real_wind_speed - vehicle_speed) need to be found. The diff gives the net force and acceleration. Then speed when diff=0 can be found. Interesting exercise.

What can be done experimentally is to measure the extra thrust when at wind speed by treadmill inclination. I.e, when turning the slop up until the vehicle is stationary, the extra forward force can be easily calculated from the angle of inclination. I have not checked, but given all the treadmill experiments that have been done, I'm sure that somebody already has done it.

No change from what I said before regarding sailboats.

 

[mender]

You are right, it has been done in one of the tests. The slope was set at 4.4 degrees, the treadmill speed was 10 mph and the weight of the cart is 169 grams. That is enough to accelerate the cart forward on a flat surface at 2.5 ft/second squared (don't know how to do notation) when released at a treadmill speed of 10 mph. You are also correct, the final speed of the cart can't be measured on the treadmill; the treadmill can only show that the cart is capable of exceeding the wind speed. Hard to say what the actual top speed would be without a few more numbers to play with. I think a close approximation can be made with a bit more data.

For comparison, Jack Goodman's cart needed 4 mph on the treadmill to hold station vs 2.7 mph for JB and spork's cart. During the outdoor test of Jack's cart, the wind speed was estimated at 9 mph and the cart was clocked at 13 mph. Given the difference in performance of the two carts, I feel it's safe to say that the little cart would exceed the wind speed by a larger margin than Jack's.

And thank you for sticking with this long enough to confirm the treadmill test. What was the turning point for you? Did you accept the treadmill test before or after DDWFTTW?

 

[schroder]

I have clearly established that the floor is a valid reference for both situations: 1) where an actual wind is moving against a stationary cart on a stationary tread and 2) where a moving tread is working against the stationary cart in stationary air. The only responses I have received are : “Why would you want to use the floor as a reference?” “And there is no need for a common reference.” These replies are not very technical, are they? Show me where the floor is Not a common reference in both frames, or accept that it is. And if it is, then the cart is not advancing against the floor at wind/tread velocity. As I pointed out, if I place a cardboard box on a treadmill and run it at speed, the relative wind resistance will overcome the static friction of the box on the tread, and push it backwards relative to the tread but not relative to the floor. Would you say that the box is going faster than the wind? I can even make a paper cup fly off the tread. Would you say I have invented an anti-gravity device? Amazing things, these treadmills! What you are doing with this device is no different than the box or the paper cup. You wind up the tread to speed, running up the wheels which turn the prop while holding the cart in place. The propeller is taking the energy from the wheels and redirecting that force into propeller thrust which can hold the cart in place and even advance against the tread, not unlike the cardboard box. The only difference is, the box is actually more efficient at this. But you claim there is no wind, as there is with the box. The fact is, the wheels that are driving the prop are equivalent to that relative wind and you cannot create this effect without first holding the cart in place against the tread. The only thing that is amazing about this entire scenario is how much play you have gotten out of it and how many people who should know better have been suckered into believing this nonsense. 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.

 

[mender]

Here are my replies to requests for explanations regarding the treadmill test:

"If you are standing in the room before the test starts, do you feel a wind? When the treadmill is running, do you feel a wind? If you are right about the floor being the proper reference, you should feel a 10 mph wind.

The correct answer is of course that you can't feel the wind unless you are on the treadmill and moving backwards at 10 mph. If you are on the treadmill, do you have to run at 20 mph to keep up with the treadmill?"
"Frame one: the cart is sitting on the stationary tread. It has a wind blowing from behind at 10 mph. The movement of the air relative to the ground is what we call wind. Therefore the cart feels that relative movement as wind. Just to make sure, the cart is tethered to hold it in position.

Extend this: tow the cart forward at 10 mph in the 10 mph wind. The cart no longer feels a relative wind even though the wind relative to the ground is still there. Correct? At that point the cart is moving at the same speed as the air.

Frame two: to duplicate the conditions in Frame one, a speed difference between the air and the ground needs to somehow be achieved so that the cart can feel a 10 mph wind from behind, just like Frame one. One way to do that would be to tow the cart backwards at 10 mph in still air. But that raises a problem. In Frame one, when the cart feels the 10 mph wind from behind, the wheels which are part of the drive system are stationary. In order for the conditions to match Frame one, the wheels need to stay stationary as the cart is towed backwards. Easy enough: we attach a long flat surface that can slide along the ground to the same tow line and pull both backwards at the same time. Now we have matched the conditions of Frame one exactly. The air flow relative to the cart is 10 mph from behind, The ground speed relative to the cart is zero. The cart is held in position.

An observer standing on the ground beside the cart in Frame one would feel the same 10 mph wind from behind as the cart. An observer standing on the surface being towed backwards with the cart in Frame two would also feel that wind. An observer in Frame two standing on the ground beside the surface being towed would not feel the same wind that the cart feels; therefore the ground reference in Frame two is invalid. The only valid reference in Frame two is the moving surface."

(Show me where the floor is Not a common reference in both frames, or accept that it is.)
I've carefully explained why the ground is not a common reference. Do you accept that it isn't?"Yoavraz, let's say we want to check a wind turbine to see how much power it can harness from a 10 mph wind. We mount the wind turbine on a trailer and drive around until we find a place that is experiencing a 10 mph wind. We stop, point the wind turbine into the wind and start taking our measurements. Everything is good, we get our measurements before the wind goes calm. We pack up but forget to secure the wind turbine. As we get to 10 mph in the still air, we notice that the wind turbine is turning. For the fun of it, we again hook up our test equipment and find that when the wind turbine is being towed through the air at 10 mph, it acts exactly the same as when it is sitting still in a 10 mph wind. Therefore, from now on we decide that instead of wasting time looking for the right wind, we generate our own wind by moving the ground reference at 10 mph.

The wind turbine doesn't know the difference. When the air is still and the ground is moving, the wind turbine behaves exactly the same way as when the ground is still and the air is moving. Those are equivalent frames of reference, and both are perfectly satisfactory for testing. According to the wind turbine and the output that is measured, the air is flowing past the wind turbine. To the wind turbine, that is wind. To the cart, air flowing past is also wind and generates the same results. You can call it fake wind (most would call it relative wind which is the correct term - look it up) but the results are the same as a wind moving across the ground.

The treadmill allows us to "tow" the cart along the ground at 10 mph. If we had a really long treadmill that was moving at 10 mph, we could step onto the treadmill surface holding our cart, set it down facing "downwind" and let it go.

What are the possible outcomes of this?
1: The cart doesn't move.
2: The cart moves and reaches a speed of less than 10 mph in reference to the treadmill surface
3: The cart moves and reaches 10 mph in reference to the treadmill surface.
4: The cart moves and reaches a speed above 10 mph in reference to the treadmill surface

Since we only have a short treadmill (I hope that you now accept that the treadmill is a valid substitute for a wind - if not read the frames of reference again and ask questions), we can only test a short portion of the full scale outdoor test. That segment is with the cart at 10 mph, with only a few feet either way to indicate a trend. If we place the cart on the treadmill surface moving at 10 mph, the outcomes that I listed above would look like this:

1: The cart would quickly move to the back of the treadmill when it is released
2: The cart would slowly move to the back of the treadmill when released (may be hard to see the difference between this and #1 but in either case this would be a failure)
3: The cart would stay in position on the treadmill (this would be downwind at wind speed)
4: The cart would move forward on the treadmill ( this would indicate moving directly downwind faster than the wind) "
"Shroder, your question about using the floor as the reference for the second case has been answered by several people. The answer is no. You are failing to accept that answer. Your reason for not accepting the answer is based on your belief that "Nothing powered by the wind can go faster than the wind going directly downwind."
"A test of this would be to interchange the observer's perspective. Increasing the scale of the treadmill test in a still room would allow the observer to ride on the treadmill surface and measure the velocity of the devices mentioned as well as the air flow relative to the observer.

What would the riding observer get for measurements? If the treadmill were running at 10 mph and the air in the room was still, the observer would measure a wind of 10 mph and a ground speed of zero when the observer is sitting on the treadmill surface. If the cart were to move at the same speed as the walls of the room, it would appear to be moving at the same speed as the wind since it would be stationary relative to the air. The cart's speed would be measured as 10 mph forward relative to the viewer on the treadmill surface.

Let's freeze this for a moment and add a second observer outside the room watching through the window. What would the second observer see when we unfreeze the scene? They'd see the first observer moving backwards at 10 mph and the cart holding station in front of them. If the cart starts to move relative to the second observer, that movement is either added or subtracted from the speed that the first observer would be measuring the cart's progress at."
"The treadmill test is limited in scope compared to the outdoor test but has the advantage of a more controlled and repeatable environment. The treadmill test starts at a point that will either prove or disprove that a device can achieve Directly Downwind Faster Than The Wind (DDWFTTW). The test simulates the cart moving across the ground at the exact same speed as a 10 mph wind from behind. At that point, the cart is traveling across the ground at the same speed as the air, so the cart will experience zero air movement. The treadmill simulates the ground moving under the cart at 10 mph. By reading several different descriptions of Interchangeable Frames of Reference (IFOR), this can be seen as exactly the same as the cart moving at 10 mph along the ground with a 10 mph tailwind in an outdoor test.

A streamer that is attached to the cart will, in both cases, hang straight down. This is to show that the air movement around the cart is for all intents and purposes not moving relative to the cart.

For the cart to demonstrate DDWFTTW travel, it has to move forward relative to the still air. If it does not but holds steady during the test, it is only moving at the same speed as the air, i.e. 10 mph along the ground in a 10 mph tailwind. If it moves backwards on the treadmill, it is not achieving DDWFTTW travel but is performing in a manner that equates to being blown along the ground.

This is much easier to see on a treadmill than trying to guess what the wind speed is in an outdoor test or relying on a streamer on the cart as irrefutable proof of DDWFTTW. However, the treadmill test must be acceptable as a substitute for the outdoor test."
To test your "Box", set it out in the wind. In a ten mph wind how fast does it move across the ground? Let's say 5 mph. Place the same box on one of those long walkways at the airport and crank the walkway up to 10 mph. The box will experience air moving past it from the rear at 10 mph, the same as in the outdoor test. If everything remains the same as the outdoor test, the box will move along the walkway at 5 mph but backwards at 5 mph in reference to the floor.

Place the cart alongside the box on the long treadmill and compare what happens. As shown in the treadmill test, when the surface under the cart is moving back at 10 mph, the cart holds station in reference to the floor (actually is trying to advance and only holds station against an incline of 4.4 degrees) while the box moves backwards at 5 mph and disappears into the distance. Do you see the difference between the two now?

I don't know how else to explain this unless you ask questions instead of make statements. These are not new responses to your statements except the last one, but apparently if you only remember the two responses that you mention, maybe having them all in one place will help.

 

[minder]

Speaking of points of reference I don't know if this has been asked before but what could go faster: A boat sailing down a stream moving ten mph with no wind, or a boat sailing down the same ten mph river with a ten mph tail wind? If you can answer this correctly you can see why the cart on the treadmill is the same as the cart sailing directly downwind faster than the wind.

The boat sailing down stream with no wind. It does so by tacking back and forth. Hopefully no one is claiming that a boat can't sail into the wind by tacking. Similarly, a boat can achieve a downwind VMG greater than wind speed by gybing downwind. Plenty of examples of that in the real world.

What is being said is that a boat can't sail directly downwind faster than the wind speed. Nor can it sail directly upwind in your river frame of reference... if we constrain the boat as we are constraining the cart to directly upwind or downwind then the 10 mph tailwind frame of reference wins.

 

[ThinAirDesign]

Schroder:

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.

This has been added to the list of quotes that are perfect for the next video.

Just to be clear, real physics apply to toys and gadgets as well as to the rest of the world and in fact can be excellent tools for learning.

JB

 

[minder]

It appears that there are some who aren't understanding exactly what is being proposed regarding the treadmill test. At this point it may be beneficial to restate the problem clearly.

There have been several devices made specifically to demonstrate that it is possible to have a device powered only by the wind that reaches a higher speed than the wind traveling directly downwind. There have also been several videos made showing various tests.

When in the treadmill video is the DDW cart "powered only by the wind"? I see them holding the cart while the treadmill drives the cart propeller until thrust = drag of the cart when placed on the treadmill. Then they place it on the treadmill.

I see the cart holding its own when released. Looks great. But at what point was the 10 mph "tailwind" acting on the prop? I see the treadmill acting on the wheels, driving the prop, producing thrust (when they are holding it, agreed?) and then is the thesis that when they release it onto the treadmill the frame of reference completely shifts, the wheels are no longer driving the prop and it is the 10 mph tailwind that provides the motive force?

Perhaps we should rethink this whole "frame of reference" exercise. We see this cart on the treadmill but we have never seen any real world examples of a DDWFTTW craft. (Forget Jack's video, if you believe everything you see on the Internet then let's talk about something else.) So if it works on the treadmill and it doesn't work in the real world, then what you might think is happening on the treadmill really isn't a good representation of the "real world".

 

[mender]

Minder, please read my last very long post. It explains, as do quite a few others elsewhere in this thread why a treadmill test is valid. If you still have a question after reading, please ask.

The thesis is that the cart behaves on the treadmill exactly like it would if someone was moving downwind at 10 mph (at the same speed as the 10 mph tailwind), set the cart down and released it. Because of the limitations of the treadmill size, it only represents what's happening when the cart is moving at 10 mph relative to the ground with a 10 mph tailwind. At the point of release the cart's speed relative to the air is zero in both cases. Jack's cart was tested both in the real world and on a treadmill. The treadmill test wasn't recorded but the data was. It correlates to what the real world test showed. By the way, don't dismiss everything you see on the internet just because of where you see it, you'll miss a lot of good stuff. I don't take your word as proof that it doesn't work just because you say it doesn't.

 

[ThinAirDesign]

So if it works on the treadmill and it doesn't work in the real world, then what you might think is happening on the treadmill really isn't a good representation of the "real world".

The treadmill IS the "real world".

The "real world" laws of physics are not suspended when a test is moved inside to a wind tunnel or placed on a treadmill.

JB

 

[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.