Sailing downwind faster than the wind: resolved?

[swerdna]

I don’t see that a treadmill is comparable with a wind as an energy source in this experiment. A treadmill moving at 10mph is always moving at 10mph relative to the vehicle and the vehicle always receives a constant 10mph worth of energy. The vehicle traveling at 5mph with a 10kmph wind is only receiving 5mph of wind energy. 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.

If you think the vehicle on a treadmill is the same as a vehicle in a tailwind, put a flag on the treadmill vehicle and see if it flutters in the wind.

 

[swerdna]

As I understand it the claim is that the propeller is gaining it‘s energy from the rolling wheels as the vehicle moves over the ground. And that the movement of the vehicle over the ground gets it‘s energy from a combination of the thrust created by the propeller and the speed of the wind against that thrust. When the vehicle reaches the speed of the wind however only the thrust of the propeller remains as an energy source. How can the thrust of the propeller alone provide extra energy to the vehicle to move faster over the ground than the wind into a headwind?

 

[cesiumfrog]

What a long thread.

However, with a sailboat it is different, and with some side-wind component most land sailboats can go faster than the wind. Never in 180 wind though.

Think about what you are saying. First your sailboat tacks, goes at right angles to the wind, and builds its speed up to be much faster than the wind. Then it collapses its sail, and turns 90 degrees to coast parallel with the wind, but retains its speed and so will be overtaking the wind (provided that friction is small and this cycle is repeated, the average velocity of the sailboat can be larger than and in the same direction as the wind).

Perhaps we should poll whether anyone still thinks such a contraption isn't possible.

 

[ThinAirDesign]

What a long thread.

Think about what you are saying. First your sailboat tacks, goes at right angles to the wind, and builds its speed up to be much faster than the wind. Then it collapses its sail, and turns 90 degrees to coast parallel with the wind, but retains its speed and so will be overtaking the wind (provided that friction is small and this cycle is repeated, the average velocity of the sailboat can be larger than and in the same direction as the wind).

Perhaps we should poll whether anyone still thinks such a contraption isn't possible.

Your above scenario may or may not be possible, but it is not necessary for a sailboat to ever change directions to reach VMG of greater than 1.0.

Also, the above doesn't meet the requirements of our claim as it strays from DDW.

JB

 

[mender]

I have just realized that without treadmill, i.e., outdoor, you need the wind as an energy source. Wind is the only possible one on horizontal surface. On a treadmill the treadmill provides the energy. No wind exists. Vehicle speed depends on treadmill speed.

Right! Now you're got it! In the outdoor test, the wind provides the difference in speed between the air and the ground reference. In the room, the treadmill provides the difference in speed between the air and the ground reference. In Frame one the outside wind is the power source; in Frame two, the treadmill is the power source.

This brings us again to the question of faster than the wind. At wind speed the relative wind on the vehicle is 0, and no wind force exists to transfer energy. Above wind speed the wind force is backwards, which slows the vehicle (energy is being lost with no renewal) and drives it back to "slower than the wind." Thus it only temporarily can go faster than the wind, and so on, possibly in cycles. This with DWFTTW.

Wrong! Sorry! You're making another classic mistake. You're assuming that the cart outruns its power source. It doesn't; it is immersed in the air that is moving along the ground at 10 mph. All the cart needs to do is interact with that mass of air that is moving and it can always get energy from it. When the energy it takes from the air is the same as the total drag of the vehicle, it reaches a stable speed and stays there until something changes.

This works the same way as overdrive in a car. The propeller is geared to run faster than the ground speed. On Jack Goodman's cart, the ratio was 1.75:1. If the cart had no friction, the cart would always move 75% faster than the wind. Because of friction, it doesn't quite get there. In a 9 mph wind, the cart moved at about 13 mph for a ratio of 1.4:1. At slower wind speeds, there isn't as much energy to be harnessed so it can't achieve that high a ratio. At 4 mph it runs at 1:1 or 4 mph.

 

[Subductionzon]

Here when Thin Air Design and spork developed their cart I thought all controversy would disappear. I was definitely an anti before and have come to learn the errors of my ways. The treadmill to me was an obvious proof of concept. As I have said before make the treadmill arbitrarily large and you cannot tell the difference between being on a large treadmill and being in an open field. Now it is understandable to look at the cart an say "No way" but people here should understand frame of reference and how the treadmill perfectly emulates a tailwind.

 

[yoavraz]

With a "conclusion first" position, facts to fit will follow.

JB

Again: When the vehicle is stationary, in a room with stationary air, no air moves around the vehicle, no wind is felt by the vehicle, no force of wind is applied to the vehicle, and no wind energy is transferred to the vehicle. The vehicle gets its energy from the turning wheels, that are turning because of friction with the treadmill belt and the pulling propeller. The belt applies force on the wheels and transfers energy. The vehicle keeps its stationary position due to the thrust generated by the propeller.

I hope you understand this in spite of your sarcasm.

 

[mender]

I see we are posting past each other. It seems to me that threads get out of control when this happens. Let's slow things back down and let everyone catch up.

Yoavraz, you are correct, in Frame two the treadmill is providing the energy to turn the wheels and power the prop. Please read my last post and comment on that so I can see where you're at.

 

[ThinAirDesign]

Again: When the vehicle is stationary, in a room with stationary air, no air moves around the vehicle

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 vehicle,

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.

no force of wind is applied to the vehicle, and no wind energy is transferred to the vehicle.

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.

I hope you understand this in spite of your sarcasm.

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.

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

 

[cesiumfrog]

Your above scenario may [..] not be possible

Why?

[..and] doesn't meet the requirements of our claim as it strays from DDW.

The blades of the contraption's fan also stray from DDW.

 

[ThinAirDesign]

If you think the vehicle on a treadmill is the same as a vehicle in a tailwind, put a flag on the treadmill vehicle and see if it flutters in the wind.

How about we put a flag on the chassis *both* vehicles ... the one on the treadmill, and the identical on out on the street. We'll place the streamers on a small shaft far enough above and ahead of the propsail so as not to be influenced by it.

Let's cover the street one first:

10mph tailwind
Wheels rolling 10mph.
Chassis speed = 0 relative to surrounding air and thus the streamer hangs limp.

Next let's go to the treadmill in still air room:

10mph treadmill speed
Wheels rolling 10mph.
Chassis speed = 0 relative to surrounding air and thus the streamer hangs limp.

So now, for emphasis, I will repeat your above statement:

If you think the vehicle on a treadmill is the same as a vehicle in a tailwind, put a flag on the treadmill vehicle and see if it flutters in the wind

As you can see from the demonstration that you requested, *both* streamers show the *exact same behavior* and thus according to your own presented logic, a vehicle on a treadmill is the same as a vehicle in a tailwind.

It was an excellent test to suggest BTW and if you think carefully about the results it will help you understand why the treadmill test and the street test are the same.

JB

 

[ThinAirDesign]

The blades of the contraption's fan also stray from DDW.

Of course they do -- that is the secret to it's success after all.

If you plot the course of the indiviual parts of an airplane, ship, car etc, they go in all sorts of different directions ... doesn't mean that the plane, car and ship can't be going straight south.

The pistons in a Porsche after all go back and forth with a regular East/West component even while the driver, chassis and CG of the Porsche drive directly South on the Interstate. No one argues over the which direction the car is going.

As with the Porsche, both the Chassis and the CG of the cart are going DDW.

JB

PS: I suppose anyone can define the point that defines the direction a vehicle is going any number of ways. I believe a quite logical point to pick is the CG. It's hard to argue that if the vehicles center of mass is moving south, that the vehicle isn't.

 

[ThinAirDesign]

Me:

Your above scenario may [..] not be possible

Why?

Because the outcome depends on data not in evidence.

You say things like "provided that the friction is small", but you don't say how small.

You also say the cycle is to be repeated -- how long does it take to "collapse the sail" and then set it again?

May or may not be possible.

JB

 

[russ_watters]

No Russ. What you are in effect saying is that the work done by the propeller is more than the work done by the wheels which are powering the propeller. You and I both know that is impossible.

That isn't what is being claimed at all. Please read and respond to one at a time, the description of the parts of the scenario I described. Your error is in there somewhere, but I think the basic problem is you are not working through the problem - you are going with an instinct in a situation where instincts often fail.

Please forget all these hypothetical’s and address the argument I made earlier about the floor being the common reference frame.

There is no need for a "common reference frame".

 

[russ_watters]

There is no need for a "common reference frame".

The only way out of this is to look at a diagram and I asked you to draw one, but you haven't. So I have. Please excuse the crudeness.

We're going to take this slow. The cart is sitting on some surface, which is moving past the cart at 6mph as measured by a speedometer mounted on the cart. The air is moving past the cart at 1mph as measured by a wind speed indicator mounted on the cart.

Is the surface the cart is sitting on a treadmill or outside on a road in a 5mph wind? How can you tell?

Put another way: Is it possible for the sensors on the cart to measure those conditions on either or both scenarios?

 

[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