Sailing downwind faster than the wind: resolved?

[mender]

I've been following this on another forum and am planning to build a non-propeller design to sidestep the sailing references. I accept that the vehicles in the videos linked in the other two threads presented on this forum are doing what it looks like they are doing without any trickery. I also feel that the treadmill test is a valid way to test and refine a design. However, it appeared that there was some disagreement about that before the previous thread here was locked.

My understanding is that general tone of the disagreement is what caused the thread to be locked, not the topic or actual disagreement, so I'm hoping that this thread won't be automatically shut down. I've read and agreed to the conditions of this forum and will abide by the guidelines.

I feel I have a pretty good understanding of what is happening but would like to make sure that I haven't overlooked something that could affect the outcome of my experiments with a non-propeller design. I want to observe and measure the various forces and interactions of this under controlled conditions to optimize a small device before attempting to scale this up.

Is the treadmill test a valid substitute for an outdoor test? If not, why not?

 

[rcgldr]

One way to explain why these carts work is to note that the power input is equal to the force at the driving wheels times the forwards speed of the car relative to the ground. After losses, the power output is equal to the thrust times the relative air flow through the prop, which is much slower than the ground speed. Via gearing, prop diameter, and prop pitch, the torque at the wheels is multiplied so that the prop generates more thrust than the force from the wheels, but at a much lower speed, so that power output remains well below power input. As long as the difference between wind speed and ground speed is large enough, (and perhaps not too large), the cart can go downwind faster than the wind, depending on the ratio of power output versus power input (efficiency factor), and the ratio of air flow speed through the prop versus ground speed.

 

[russ_watters]

Is the treadmill test a valid substitute for an outdoor test? If not, why not?

Yes, I think so. One thing to note: on the treadmill, you start with the wheels moving. With the videos of these devices, they need a push to get them to work, which is the same thing.

 

[mender]

I don't think it's required to get the cart started but is more dependent on the wind speed and the surface area of the prop. The Jack Goodman cart did start to roll on its own after the brake was released, but once the propeller is spinning it does seem to help noticeably.

I did notice that the smallest cart that was tested on the street in the wind caught enough of a gust that the wheels and prop were turning backwards briefly but that might have more to do with the traction of the drive wheels and the strength of the gust.

By the way, a nice long extended version of the the treadmill is available at some airports, enough to go from a stop relative to the moving surface without running out of room too quickly. I don't know if the speed of those walkways is sufficient though.

 

[yoavraz]

Mender:
Yes, this has been resolved. In order to get there you have to tack downwind. If the best "velocity made good" (VMG) is greater than wind speed, at a given wind, then you "get there before the wind." Otherwise, no. Also, even having faster VMG you cannot make it if you go straight downwind (wind direction 180 degrees). You find best VMG by finding the point where the tangent to the sailboat-speed Vs. real-wind-angle graph is parallel to the horizontal axis of the graph (prove this). A positive example is with the 18ft Skiff class that has best VMG of ~12kn at 10kn wind. Data are from such polar diagram for skiff in the book "The symmetry of sailing" 1996. I believe that by now other fast classes can make it too. It is done easily by ice and land sailboats.

 

[ThinAirDesign]

By the way, a nice long extended version of the the treadmill is available at some airports, enough to go from a stop relative to the moving surface without running out of room too quickly. I don't know if the speed of those walkways is sufficient though.

Our smallest device works in winds down to 2.7mph. I believe most moving walkways are above that, but not by much.

I'd love to find a moving walkway that had a surface suitable to our small light wheels. Most of those walkways are a myriad of slots.

JB

 

[yoavraz]

With regular sails or with other means the result is the same (my previous comment):

At 180 degrees wind (wind from behind), when reaching wind speed, the relative wind at the vehicle is 0. This is a fact that cannot be changed. With wind 0 no lift can be generated on the vehicle: either by sail, or wing, or wind turbine. If passing wind speed by some external means, e.g., push, immediately the relative wind is from front, and will apply a stopping force that will reduce speed, and so forth.

The only way to pass wind speed in the direction of the wind (180) is to tack (zigzag) downwind. If the vehicle has high lift and low drag, it is possible that the best VMG (velocity component in 180) is larger than wind speed (my previous comment), and the vehicle "gets to target before the wind." In this case, when zigzagging, always a side wind component, even small, must exist, i.e., wind direction is <180.

This applies to all videos with wind turbines that I have seen, with, or without treadmills. Neither magic nor unfamiliar physics.

 

[schroder]

With regular sails or with other means the result is the same (my previous comment):

At 180 degrees wind (wind from behind), when reaching wind speed, the relative wind at the vehicle is 0. This is a fact that cannot be changed. With wind 0 no lift can be generated on the vehicle: either by sail, or wing, or wind turbine. If passing wind speed by some external means, e.g., push, immediately the relative wind is from front, and will apply a stopping force that will reduce speed, and so forth.

The only way to pass wind speed in the direction of the wind (180) is to tack (zigzag) downwind. If the vehicle has high lift and low drag, it is possible that the best VMG (velocity component in 180) is larger than wind speed (my previous comment), and the vehicle "gets to target before the wind." In this case, when zigzagging, always a side wind component, even small, must exist, i.e., wind direction is <180.

This applies to all videos with wind turbines that I have seen, with, or without treadmills. Neither magic nor unfamiliar physics.

I agree with this 100%. Going Directly Downwind, Faster than the wind, (180 Deg) is not possible. This has been my stand all along. The best data I have seen was for an iceboat in a 35 mph wind doing 34.9 mph DDW. That is very impressive, but only proves my point that it is not possible. The treadmills are very confusing, in that merely advancing against the tread has convinced some people that this is equivalent to outrunning the wind. I was confused about this also. In retrospect, advancing against the tread by employing another medium, such as air, is no more amazing than a right-angled drill; just another way of redirecting force! I hope this issue is finally resolved.

 

[ThinAirDesign]

The best data I have seen was for an iceboat in a 35 mph wind doing 34.9 mph DDW.

Schroder, of course a traditionally equipped ice-boat can only do 34.9mph when going DDW in a 35mph wind. There's no lift involved when going DDW - it's always going to be Wind Speed minus Overall Drag with a traditional rig.

Now, let the ice-boat zig zag and their VMG (velocity made good, or the downwind component of their path) can exceed the speed of the wind dramatically

In land yachting, VMGs of 3x to 4x of true wind speed are common.

http://sports.groups.yahoo.com/group/2nalsa/message/161

That is very impressive, but only proves my point that it is not possible.

Unfortunately, your point that "it is not possible", doesn't hold up to much scrutiny.

The treadmills are very confusing,

Perhaps they are, but here on a physics forum, where the principles of 'the equivalency of inertial frames of reference' should rule, I'm surprised to find you stlll clinging to the notion that a treadmill running at 10mph relative to the air is somehow different than a street with 10mph of of relative air passing over it.

I was confused about this also.

And apparently, you still are.

The device in the videos is powered *only* by the relative motion between air and a rolling surface.



If the device advances on the treadmill running in a still air room, it is now running DDWFTTW (directly downwind faster than the wind).

If the device is hovering on the treadmill (that is no forward or rearward motion relative to the belt), it is running DDWTSSATW (directly downwind the same speed as the wind).

In retrospect, advancing against the tread by employing another medium, such as air, is no more amazing than a right-angled drill; just another way of redirecting force!

I'm not concerned about how "amazing" you do or don't find it. I'm am concerned when you say that it's not what it claims, which is DDWFTTW.

Again, a treadmill running at 10mph relative to the air is exactly the same as a street with 10mph of of relative air passing over it. I'm sticking with Galileo on this one.

I hope this issue is finally resolved.

I wish it was, but alas DDWFTTW is such a maddening brainteaser to some that it wouldn't be over if they were run over by a vehicle doing it.

Russ ... could you help our gentlemen friend Schroder here to a lesson on inertial frames of reference? He's not going to listen to me.

Thanks

JB

 

[Phrak]

Actually, you could sale downwind, 180 degrees, faster than the wind--just not for very long. I have a computer model with variable L/Ds and L_sale/L_keel options. I was very suprised to find that it could sail downwind, and when setup, would perpetually sale in circles with a net downwind drift each cycle.

 

[schroder]

To discuss this in unambiguous manner, we need to define some terms. Running in circles, is not running directly downwind, but is in fact, a form of tacking. There is no argument that by means of tacking or zigzagging, it is possible to outrun the wind. That is well established in yachting and especially in iceboating. So there is no need to argue that point any more. The question here is about a wind powered vehicle proceeding directly on a downwind course, outrunning the wind on a steady basis. There is no evidence that has ever been accomplished. I have cited the best case of an iceboat doing 34.9 mph DDW in a 35 mph wind and Thin Air Design has agreed with that. However, TAD seems to think that because his propeller driven cart can advance on a treadmill, that is equivalent to moving DDWFTTW. That is where the issue is. I can easily design and build a cart which is driven by a treadmill that will advance against that treadmill. If I place two wheels on the tread, which extract power from the tread, and two wheels on the stationary floor, which are driven by the wheels on the tread, it will advance. As I said, that is about as amazing as a right-angled drill. It is a simple redirection of force. Using a propeller instead of wheels is just a variation on this idea. To extrapolate from this example, that a cart can outrun the wind, while going DDW is a fallacy. Until Thin Air can Prove this assertion, which he has never done, this argument can go on forever. My final statement is: Prove it! Do it in the wind, get it certified and then the issue is over.

 

[ThinAirDesign]

There is no argument that by means of tacking or zigzagging, it is possible to outrun the wind. That is well established in yachting and especially in iceboating. So there is no need to argue that point any more.

Excellent. Glad to hear that. It's amazing how many folks will *not* concede that point even after 90+ years of it being physically demonstrated on ice and a couple decades on water.

The question here is about a wind powered vehicle proceeding directly on a downwind course, outrunning the wind on a steady basis.

For the record, I will state my two claims:

A: I have built and can demonstrate on demand a vehicle which will go directly downwind, faster than the wind, powered only by the wind, steady state.

B: Based on the principle of the equivalency of intertial frames of reference (IFOR), a treadmill in a still air room is a technically perfect environment to prove or disprove claim "A".

One can extract energy from the difference in speed between the air and the ground. One can use Galiliean relativity to prove that physics of the cart at fixed position on the treadmill are identical to the physics of the cart moving at wind speed on level ground. From that it follows that steady advancement against the rotation of the rolling surface constitutes a perfect and valid demonstration of claim "A".

There is no evidence that has ever been accomplished.

And yet I have a device that regularly accomplishes it sitting right here on my desk and have demonstrated it countless times now in front of all sorts of folk.

TAD seems to think that because his propeller driven cart can advance on a treadmill, that is equivalent to moving DDWFTTW. That is where the issue is.

That *is* where the issue is, and fortunately for me I have time tested principles of IFOR behind me. 10mph of relative airflow is just that "relative". I can create that relative movement anywhere I wish -- in the back of a truck, on the back of truck, on the highway, in a gymnasium, inside a space station, on the equator -- ad nauseam. The cart won't care and there isn't a single scientific test available to tell those apart.

I can easily design and build a cart which is driven by a treadmill that will advance against that treadmill. If I place two wheels on the tread, which extract power from the tread, and two wheels on the stationary floor, which are driven by the wheels on the tread, it will advance.

Excellent, you have now devised a mechanism which will successfully climb a treadmill using relative motion between to solid surfaces. Congrats. Next time someone asks me if that can be done I will tell them "yes", Schroder devised such a system quite some time ago. Unfortunately, to go DDWFTTW, we need a device which exploits the relative motion between a solid and a gas, not two solids.

As I said, that is about as amazing as a right-angled drill. It is a simple redirection of force. Using a propeller instead of wheels is just a variation on this idea. To extrapolate from this example, that a cart can outrun the wind, while going DDW is a fallacy.

I didn't "extrapolate", I did it.

Until Thin Air can Prove this assertion, which he has never done, this argument can go on forever. My final statement is: Prove it! Do it in the wind, get it certified and then the issue is over.

I did do it in the wind. 10mph of it to be exact.

JB

 

[M Grandin]

I also didn't understand why earlier thread on this issue was locked - the discussion climate was rather polite, I thought.

Regarding this downwind device, I also was a little duped and doubting at first - but after thinking a while I realized it must work. But I think it is not necessary compare with sailing, tacking and so on. Although it could be accomplished using vectorized aerodynamics.

The theory behind is rather simple: Assume vehicle velocity = V1, wind velocity = V2 and "propeller" backwards projected pushing velocity V3 at force F. Received power Pr =
F x V1 , picked up by rotating wheels. Consumed power Pc = F x (V3 - V2) at driving "propeller". Net power received is Pr - Pc = F x (V1 +V2-V3) . All velocities related to +Z direction. Net power > 0 as long as V3 < V1 + V2. Or relative velocity
V3-V2 < V1.

Some error may have occurred - but the core is that consumed power is lower than
received power because the propeller acts att lower relative velocity toward wind than the wheels against ground - at the same but opposite directed force F.

 

[mender]

It appears that the treadmill test is in question again. That was the original question that I asked to start this thread.

Is the treadmill test a valid substitute for an outdoor test? If not, why not? Please state your view and explain your reasoning. I am not addressing DDWFTTW.

For the record, I feel that the treadmill test is valid because of the replication of a moving ground/air interface as experienced in a wind. The difference in speed and direction between the two is identical. The energy available is a result of the relative motion between the ground and the air.

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.

Have I presented this correctly?

 

[ThinAirDesign]

It appears that the treadmill test is in question again. That was the original question that I asked to start this thread.

Is the treadmill test a valid substitute for an outdoor test? If not, why not? Please state your view and explain your reasoning. I am not addressing DDWFTTW.

Thanks for keeping the thread on track mender.

Like you I am interested to hear peoples arguments for and against the application of IFOR in this case.

JB

PS: M_Grandin, I'm also happy to discuss DDWFTTW if you wish to PM me. My above comments were not meant to insult your thoughtful post.

 

[ThinAirDesign]

mender, I certainly don't wish to patronize anyone but at the same time I don't wish to leave a stone unturned.

If you are not sure you understand the basics of IFOR -- the issue at the heart of the treadmill matter, I am happy to give you some good examples and explanations.

Your thread and your call.

JB

 

[M Grandin]

It appears that the treadmill test is in question again. That was the original question that I asked to start this thread.

Is the treadmill test a valid substitute for an outdoor test? If not, why not? Please state your view and explain your reasoning. I am not addressing DDWFTTW.

To me it is obviously the same thing - if you understand the "theory" behind the machine.
Imagine for instance the thread-mill moving at extremely high velocity. The slightest force
from propeller transferred to apparatus would generate a corresponding enormous power generated from apparatus wheeels. Power = Force x Velocity. While that slight force
would claim very small power extracted from wheel generator to rotate the propeller giving that slight force.

If vehicle is hold still the wheels are rotating att speed of thread-mill. Already a fraction of max power obtained that way may be sufficient running the propeller holding the vehicle still or accelerating passing the velocity of ambient wind = faster than downwind .

 

[mender]

I agree. The force that is available is from the relative movement between the surfaces. A method of harnessing and redirecting that force is all that is needed to provide movement of a device. It's a matter of gearing and total drag vs force as to how fast the device moves and in which direction.

Shroder provided an explanation of a similar interface. By using the relative motion between the treadmill and the stationary frame of the treadmill or the ground beside the treadmill, energy can be extracted and through the appropriate gearing cause the device to advance on the treadmill. The forces involved can be measured and the experiment is repeatable by anyone with the same equipment under the same conditions.

I accept this solid to solid or ground/ground interface as a valid test. If the device moved forward on the treadmill, it would be moving forward faster than both the treadmill surface and the treadmill frame or the supporting ground around it. It is an exchange of force in one direction for movement in another.

An intermediary step between ground/ground and ground/air would be ground/water. If a trough of water were to be placed around the treadmill and paddlewheels substituted for the wheels in shroder's device, the device would have a less direct link but would still move forward as long as the total drag from the device was less than the force generated.

To me, having a less obvious and more tenuous medium to work with does not negate the principles involved. Nor does interchanging which surface is moving relative to the observer, since the energy is extracted from the relative movement of the media. The ground/air interface works just like the ground/ground and ground/water but with less obvious interaction visible.

 

[ThinAirDesign]

Have I presented this correctly?

You have presented it perfectly.

Well done.

To me, having a less obvious and more tenuous medium to work with does not negate the principles involved. Nor does interchanging which surface is moving relative to the observer, since the energy is extracted from the relative movement.

Again perfectly put. Thanks.

JB

 

[yoavraz]

Amazing reactions.

All experiments I have seen on videos do not prove the 180 possibility, to my opinion.
It is impossible in such conditions to keep the wind at 180 all the time. Even small fluctuations generate deviation from 180, which makes it equivalent to tacking at <180.

Even a fully controlled wind tunnel experiment with a treadmill, with laminar stable flow in the exact direction of treadmill and vehicle wheels, will be hard to convince me that motion above wind speed is generated without wind direction (possibly only minor) fluctuations around treadmill and vehicle direction.

 

[mender]

Why would you need a wind tunnel with a treadmill? Either one or the other will provide a difference in speed between a solid surface and air. Having both is unnecessary unless you want to investigate combinations of speeds, like 5 mph ground speed and 5 mph air speed in the other direction. The result will be the same.

Your concern about the fluctuations is addressed by the treadmill test. If the test begins in a large enough room with still air, all the resultant air movement can be studied and accounted for. Even the air movement at the surface of the treadmill is duplicated in the test.

 

[mender]

I am discussing only the treadmill test and how it relates to an outdoor test. It seems to me that you are approaching this from the standpoint of wind turbines and arguing DDWFTTW. We can discuss that later if needed.

 

[yoavraz]

Why would you need a wind tunnel with a treadmill? Either one or the other will provide a difference in speed between a solid surface and air. Having both is unnecessary unless you want to investigate combinations of speeds, like 5 mph ground speed and 5 mph air speed in the other direction. The result will be the same.

Air flow relatively to solid surface is not sufficient. The question is how you move in a tunnel both ail and solid surface relatively to vehicle. you need vehicle motion relatively to both. Without a treadmill or equivalent you need to let the vehicle run down the tunnel (should be quite long, and harder to control and measure).

A wind tunnel with treadmill can control the exact 180 wind direction and laminar flow the best way I can think of.

 

[ThinAirDesign]

Why would you need a wind tunnel with a treadmill?

I get a kick out of that one as well mender.

People often say "test it in a wind tunnel", "test in a wind tunnel" -- we have. In this case, that's *EXACTLY* what the tread mill IS equivilent to ... both physics wise and practicality wise.

Think about it -- the reason we use wind tunnels is to take an experiment that would otherwise take say miles, and shorten it up until it fits into a short space. Don't want to move the car? ... move the air instead. Don't want to move the plane? ... move the air instead. In those cases we don't want the ground to 'move' , so we stop it and move the air.

In the downwind vehicle case, it's the air moving that makes the test take up sooo much darn space -- so we stop it and move the ground.

A wind tunnel is designed to be the most controllable and instrumentable environment for testing upwind vehicles. If you were going to devise the most controllable and instrumentable environment to test if a vehicle can go DDWFTTW, -- a treadmill would be it.

Traditional wind tunnels long enough (hundreds of feet +) to test a vehicle like this are not the domain of this sort of simple science -- the operators would say "what's wrong with a freakin' treadmill in a still room?"

JB

 

[mender]

Yoavraz, I think I see where you're going with this. Correct me if I'm wrong.

As a starting position, the rolling surface is stationary and the wind tunnel is generating a 10 mph wind. When/if the cart starts to move, the rolling surface starts moving in the opposite direction to keep the cart within the confines of the wind tunnel. To compensate for the forward movement of the rolling surface, the wind tunnel drops the air speed by the same amount that the rolling surface is moving to keep the "wind" (difference in speed between the ground and the air) the same.

If I am wrong about what you're saying, perhaps it would help if you could expand on this statement:

"A wind tunnel with treadmill can control the exact 180 wind direction and laminar flow the best way I can think of. "

Please give an example of what you would consider a valid test using the wind tunnel and a treadmill.

 

[ThinAirDesign]

Air flow relatively to solid surface is not sufficient. The question is how you move in a tunnel both ail and solid surface relatively to vehicle. you need vehicle motion relatively to both. Without a treadmill or equivalent you need to let the vehicle run down the tunnel (should be quite long, and harder to control and measure).

And in the above, yoavraz has like Schroder devised a method to prove IFOR wrong after centuries of attempts.

I present that his test fails and that given a large enough treadmill (say the size of the earth) and a big enough moving airmass (say the size of our atmosphere) even he might be convinced of his error.

JB

 

[yoavraz]

I am discussing only the treadmill test and how it relates to an outdoor test. It seems to me that you are approaching this from the standpoint of wind turbines and arguing DDWFTTW. We can discuss that later if needed.

Treadmill test can be made equivalent to any outdoor one.

 

[mender]

Yoavraz, can you please elaborate? Are you now saying that a simple treadmill test is a valid substitute for an outdoor test or are you saying that more needs to be done to make the treadmill test valid?

 

[yoavraz]

As a starting position, the rolling surface is stationary and the wind tunnel is generating a 10 mph wind. When/if the cart starts to move, the rolling surface starts moving in the opposite direction to keep the cart within the confines of the wind tunnel. To compensate for the forward movement of the rolling surface, the wind tunnel drops the air speed by the same amount that the rolling surface is moving to keep the "wind" (difference in speed between the ground and the air) the same.

This is unrealistic since when vehicle speed gets closer to airspeed, all movement stops, and you continue nowhere. In a realistic experiment the wheels are running all the time, which makes a difference, for example in friction force.

If I am wrong about what you're saying, perhaps it would help if you could expand on this statement:

"A wind tunnel with treadmill can control the exact 180 wind direction and laminar flow the best way I can think of. "

Please give an example of what you would consider a valid test using the wind tunnel and a treadmill.

I cannot expand beyond explaining each word.

 

[yoavraz]

Yoavraz, can you please elaborate? Are you now saying that a simple treadmill test is a valid substitute for an outdoor test or are you saying that more needs to be done to make the treadmill test valid?

Yes, valid. In principle you can get exactly the same physical effects, and the experiments are the same regarding the needed results. Having the vehicle stationary or close to this on a treadmill is much more convenient for controlling, observing, and measuring it.

 

[mender]

For the sake of accuracy, how would you conduct the experiment? So far, a treadmill that has been leveled and is in a room of still air has been proposed. Is there more that you would add/specify?

It sounds like you've had experience testing devices that interact with the wind. What variables should I consider in my design? I know that the power generated from wind will be dependent on the amount of air influenced (propeller disc for example), efficiency of the interface (design of blades in terms of surface area, pitch, profile), and the energy available to be harnessed (wind speed). Anything else?

 

[mender]

This is unrealistic since when vehicle speed gets closer to airspeed, all movement stops ...

All movement of what stops? The rolling surface and the air are controlled to keep the relative speed between the two exactly the same. If the cart starts to move, the wheels have to roll and the prop being geared to the wheels has to turn. All the drag is present. No movement stops that I can see.

In a test of a cart that has a simple bluff body mounted on top, this method should allow the measurement of the speeds and forces. A treadmill test alone would not work for that scenario (directly downwind slower than the wind), nor would it work for a vehicle that went DDWFTTW.

Are you referring to the possible outcome of the test rather than the test itself? I still want to clarify the test conditions first.

 

[yoavraz]

All movement of what stops? The rolling surface and the air are controlled to keep the relative speed between the two exactly the same. If the cart starts to move, the wheels have to roll and the prop being geared to the wheels has to turn. All the drag is present. No movement stops that I can see.

I'm sorry, I misread your text and misunderstood your scenario. What I said was an answer to a different scenario and incorrect regarding yours.

If we continue your scenario, air speed drops until it cannot overcome the wheels' friction with the treadmill, the wheels stop rolling (with air speed > 0) and the treadmill starts to move the vehicle backwards (remember: the treadmill is going backwards to keep the air-pushed vehicle stationary!). Now then air flow relatively to vehicle is getting faster (and Treadmill changing to slower and slower), its force increases, and it starts pushing the vehicle again forwards. This repeats in a cycle, loop, (or steady state) and we never get to wind speed!

(This is equivalent to what happens outdoor without a treadmill.)

The only possibility to get to wind speed and pass it is with a side-wind component (meaning that wind direction is <180) that does not change during the experiment. The side component continues to generate forward force on the vehicle (sail, wing, wind-torbine, does not matter), overcome the friction, and accelerate until friction (drag in general) equals the forward force. Then the vehicle continues at constant speed, that can be larger than wind speed. If VMG is greater than wind speed, then the vehicle's velocity component in the wind direction at constant speed can also be larger than wind speed.

 

[mender]

So to extrapolate what you're saying, there would be a point where the air speed is zero and the treadmill is at 10 mph. At that point (or well before that point by your consideration), the cart would have no air speed to work with and quickly run off the back of the treadmill if the various speeds were not adjusted to compensate.

So what is happening in the videos with the treadmill? The vehicle should not be able to advance or even maintain position on the treadmill according to your explanation, but it clearly does. Are you saying that the treadmill test is not valid now?

It appears to me that you are stating that the treadmill test is invalid solely on your belief that the cart is not doing what it looks like it is doing. I'm not trying to discuss DDWFTTW at this point but I find it hard to follow your logic unless I conclude that you are.

In your opinion, what is it about the treadmill test that needs to be changed to represent what would happen in an outdoor test? The results or the procedure?

 

[schroder]

I'm sorry, I misread your text and misunderstood your scenario. What I said was an answer to a different scenario and incorrect regarding yours.

If we continue your scenario, air speed drops until it cannot overcome the wheels' friction with the treadmill, the wheels stop rolling (with air speed > 0) and the treadmill starts to move the vehicle backwards (remember: the treadmill is going backwards to keep the air-pushed vehicle stationary!). Now then air flow relatively to vehicle is getting faster (and Treadmill changing to slower and slower), its force increases, and it starts pushing the vehicle again forwards. This repeats in a cycle, loop, (or steady state) and we never get to wind speed!

(This is equivalent to what happens outdoor without a treadmill.)

The only possibility to get to wind speed and pass it is with a side-wind component (meaning that wind direction is <180) that does not change during the experiment. The side component continues to generate forward force on the vehicle (sail, wing, wind-torbine, does not matter), overcome the friction, and accelerate until friction (drag in general) equals the forward force. Then the vehicle continues at constant speed, that can be larger than wind speed. If VMG is greater than wind speed, then the vehicle's velocity component in the wind direction at constant speed can also be larger than wind speed.

Yes. Finally another voice of reason! The cart will advance until it reaches treadmill velocity, or close to it, then it cannot advance any more. It will fall back, recover, and advance again but never achieve treadmill (wind) velocity. What is wrong with the videos is the treadmill is too short to allow the cart to get up to the speed of the tread before it runs to the end. Also, the operator may be speeding up the tread to achieve acceleration, but I am not sure about that. If the treadmill is running at 10 m/sec the Max possible velocity the cart can achieve is also 10 m/sec with reference to the floor, or 20 m/sec with reference to the moving tread. This is exactly equivalent to moving directly downwind in a 10 m/sec wind at 10 m/sec. That is something that has never been achieved, although the iceboats do come close to that. Incidentally, Thin Air Design, I notive this thread has so far been conducted very well but the only personal slights so far have all been coming from you.