## Sailing downwind faster than the wind: resolved?

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.

 Quote by mender 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
 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.
 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.
 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.

 Quote by 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.
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.

 Quote by mender 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
 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.

 Quote by yoavraz 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

 Quote by 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, 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?

 Quote by mender 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.

 Quote by mender 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.

 Quote by 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?
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.
 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?

 Quote by yoavraz 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.

 Quote by mender 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.