DDWFTTW Turntable Test: 5 Min Video - Is It Conclusive?

[cesiumfrog]

Swerdna, that turntable video is awesome.

Does anybody here disagree (with M Grandin) that a "passive" car between two rails (that move at some speed relative to each other) can use the relative motion to move itself faster still? ThinAirDesign? Omcheeto?

 

[zoobyshoe]

Swerdna, that turntable video is awesome.

Does anybody here disagree (with M Grandin) that a "passive" car between two rails (that move at some speed relative to each other) can use the relative motion to move itself faster still? ThinAirDesign? Omcheeto?

This seems to be the same situation as the "faster than the ruler" youtube.

 

[cesiumfrog]

This seems to be the same situation as the "faster than the ruler" youtube.

Thanks for directing me to that! Now, is it worth replacing the ruler (and corresponding wheel) with a movable tray of water (a fluid and a little paddle-wheel propeller)?

Or should we just give up on trying to change people's beliefs?

 

[ThinAirDesign]

Powered by the difference between wind speed and ground speed, not just the wind.

That's fine if you wish to make that your claim, but that's a re-wording that I'm unwilling to make on ours. If "powered by the wind" isn't good enough for our claim, it isn't good enough for sailboats or wind turbines or kites or, or, or. All those are wind powered just like the cart on the treadmill

JB

 

[ThinAirDesign]

Which video's? Like swerdna, I've seen lots of them. I've yet to see any evidence that the air alone can push a cart faster than itself.

Well, if you've watched swerdna's, let alone ours (search Spork33), you've seen plenty of not just "evidence", but it actually happening.

Now, I understand that you have been unable to grasp the application of Galilean relativity from the street to the treadmill, but that inability doesn't change what's really happening.

JB

 

[ThinAirDesign]

Does anybody here disagree (with M Grandin) that a "passive" car between two rails (that move at some speed relative to each other) can use the relative motion to move itself faster still? ThinAirDesign? Omcheeto?

Anytime there are two mediums with relative motion to each other it's possible to extract energy from that motion. That energy can be used for all sorts of things, including travel. Cleverly applied, travel faster than either medium can be accomplished.

Anyone who believes that moving through a current faster than the current isn't possible dare not get on a sailboat and sail downriver on a calm day (or upwind on a windy day) -- from the very valid perspective of the fish, a sailboat is going downcurrent faster than the current every time.

JB

 

[zoobyshoe]

I think I can hear PM's flying.

 

[ThinAirDesign]

Unlike yourself and Spork I certainly don’t think I know it all and will happily listen to others and carry out their tests in an attempt to understand their case.

There are many, many things I don't know -- how the DDWFTTW cart functions or how it will behave in any given testing scenario just happens not to be one of them.

The “Spork” cart is a direct copy of someone elses design ...

Not true -- though it certainly *is* quite similar -- and thus why we give credit.

... and this person also used a treadmill for testing.

That's certainly right -- there's no patent on Galilean relativity.

I really have no idea why you have bought your childish, unsubstantiated attacks against myself to this forum

Very strange Swerdna -- where's the "attack", and what exactly is "unsubstantiated"? While I was merely watching the thread and hadn't posted at all, you wonder why others (us) haven't built an outdoor version (in a manner belittling our use of our own free time) and yet you yourself chose not to build one, but to simply use the turntable design of another (Bauer) to test a cart design of another.

There's absolutely no shame in that as long as credit is given to those who have gone before. We give this credit ... in spite of your non-original work, you're not so free with such credit. If you wish to demonstrate that this is unsubstantiated, simply produce a quote (from any forum) where you give credit to those who first designed and tested your setup.

Again, I find no shame in building and testing what others have already done -- it's just not cool to do so without giving credit. We did our research -- we know as close as anyone who the originator of this design is and our primary video makes it crystal clear who's work we build on.

We in no way claim to be the first, I certainly hope you don't claim any sliver of it either -- even by omission.

JB

 

[ThinAirDesign]

I have posted the following numerous times when folks have difficulties with the treadmill as it relates to the creation of wind:

***************************
If you have a problem with the motor on the threadmill, you're only arguing about what creates the 'wind'.

There are two ways to create 'wind' -- move the air over the surface, or move the surface under the air.

Stand on the deck of an aircraft carrier in the dense fog -- stiff breeze flowing from bow to stern. Fly a kite. Set up a turbine powered generator. Listen to the repeated crack of the flag. Run a small land-yacht around on the deck. Test a DDWFTTW cart. Blow drifting bubbles with the kids. Hell, flood the deck and hold a tiny regatta with tiny sailboats.

Is there wind? Every wind powered device in the world says yes and behaves normally. How can you know?

Is the carrier:

A: sitting still in the ocean with a breeze blowing across it?
B: cruising on it's engines on a calm day?
C: floating engines off and drifting with a strong water current on a calm day?
D: sitting on a rollers with large electric motors winching it across the ground?
E: floating down a man made recirculating river where large electric pumps move the water?
F: inside a foggy wind tunnel where large electric fans create the wind?
G: I'm lying to you -- there is no carrier, just a giant treadmill with a carrier movie set on it.

It's a serious question and I'd love for those who think there is a difference between the street and the treadmill to answer it.

Let's invert that -- same deck, same fog, no wind. Flags don't flap. Bubbles don't drift. Wind turbine doesn't rotate. Is it calm, or is the carrier steaming with the wind? Or is the water drifting with the wind? Or is the wind tunnel turned off? Or is the carrier on rollers and being winched with the wind. You get the idea.

That's the entire point of Galilean relativity, something established without fail for four centuries now -- of two objects moving relative to each other, one can't tell which is 'moving' and which is 'still' without external reference -- reference wind carts and sailboats don't have.

JB

 

[rcgldr]

Anyone who believes that moving through a current faster than the current

Now, is it worth replacing the ruler (and corresponding wheel) with a movable tray of water (a fluid and a little paddle-wheel propeller)?

A DSFTTS (S = water stream) device was already covered in the posts about the Brennan torpedo, which able to achieve a speed of 31 mph forward in the water, with the power and steering coming from a pair wires being pulled backwards. This device was patented back in 1877 and later used by the military. It was a working example of a device that travels faster than the medium it flows through (water):

http://en.wikipedia.org/wiki/Brennan_Torpedo

Powered by the difference between wind speed and ground speed, not just the wind.

That's fine if you wish to make that your claim, but that's a re-wording that I'm unwilling to make on ours. If "powered by the wind" isn't good enough for our claim, it isn't good enough for sailboats or wind turbines or kites.

That's a common usage of the term "wind powered", all of those devices extract energy from the difference between wind speed and ground speed. However, gliders are also "wind powered", but there is no ground interface, except for a kite like launch. The wind power for gliders can be a simple updraft to oppose gravity, or it can be a wind shear boundary where the difference in wind speed across a thin shear boundary allows a radio control glider to achieve very high speed.

The simple updraft case:
http://www.youtube.com/watch?v=LUVRUMc7DP8&fmt=22

There are two ways to create 'wind' -- move the air over the surface, or move the surface under the air.

Or a third way, move air over air. The more complex wind shear case. In this video, the wind over the ridge is about 50 mph to 60 mph while the air behind and below the ridge is turbulent but around 0 mph. By circling across the shear boundary, the glider reaches a max radar speed of 333 mph. That same glider went 357 mph later in the day. The last I read the record was 365mph.

http://www.youtube.com/watch?v=Vi0hrjqU15I&fmt=18

My point is that wind powered doesn't always imply a ground interface. For gliders the ground interface is only used for kite like launches:

Kite like launch (full scale gliders can be similarly towed up by land vehicles):
http://www.youtube.com/watch?v=mkIIlO7Ckos&fmt=18

 

[ThinAirDesign]

A DSFTTS (S = water stream) device was already covered in the posts about the Brennan torpedo, which able to achieve a speed of 31 mph forward in the water, with the power and steering coming from a pair wires being pulled backwards. This device was patented back in 1877 and later used by the military. It was a working example of a device that travels faster than the medium it flows through (water):

http://en.wikipedia.org/wiki/Brennan_Torpedo

Absolutely correct Jeff, and a sailboat is another perfectly good example and has prior art going back a fair bit before 1877 :-)

My point is that wind powered doesn't always imply a ground interface.

Again correct, but "wind powered" certainly *includes* those applications involving a ground interface as demonstrated by numerous common examples.

As an aside, we soaring pilots have our own little physics brainteaser ... "is a sailplane wind powered or gravity powered?".

Just as a second aside, a glider could not qualify for a DDWFTTW claim as it doesn't travel DDW but always at an angle to it (by measure of sink rate).

JB

 

[OmCheeto]

Well, if you've watched swerdna's, let alone ours (search Spork33), you've seen plenty of not just "evidence", but it actually happening.

But not strictly powered by the wind. Perhaps I've missed an important video.

Are you talking about http://www.youtube.com/watch?v=1pSYALWQ-nI&feature=channel_page"?

It's by far the best, but I still think swerdna's is much better. But neither proves DDWFTTW to me.

Now, I understand that you have been unable to grasp the application of Galilean relativity from the street to the treadmill, but that inability doesn't change what's really happening.

hmmm... I don't remember the term "Galilean relativity" ever coming up in my dynamics or physics classes in university. Perhaps they thought we would become confused and mix it up with "Einsteins relativity", and used a different phrase.

 

[rcgldr]

Absolutely correct Jeff, and a sailboat is another perfectly good example and has prior art going back a fair bit before 1877.

True, but the sailcraft goes at an angle to the wind, while the torpedo and other similar devices like the DDWFTTW carts do not require a component of force perpendicular to the direction of travel.

As an aside, we soaring pilots have our own little physics brainteaser ... "is a sailplane wind powered or gravity powered?".

Gravity isn't needed in the case of wind shear based dynamic soaring. Otherwise it's a gravity powered device using the air to slow the rate of descent and to travel horizontallay.

Just as a second aside, a glider could not qualify for a DDWFTTW claim as it doesn't travel DDW but always at an angle to it (by measure of sink rate).

How would you describe the situation when the updraft equals sink rate (slope soaring)? Clearly gravity is applying a force on the glider, but since the glider travels horizontally (in an updraft), gravity peforms no work on the glider. (The updraft does perform work on the glider.).

 

[ThinAirDesign]

But not strictly powered by the wind.

Nope, strictly wind power.

Perhaps I've missed an important video.

I doubt it -- if you've seen any of swerdna's where the device rotates CCW, you've seen it. If you've seen any of ours where the cart advances towards the front of the treadmill, you've seen it. Strictly wind powered -- both.

Are you talking about http://www.youtube.com/watch?v=1pSYALWQ-nI&feature=channel_page"?

It's by far the best, but I still think swerdna's is much better. But neither proves DDWFTTW to me.

I certainly believe that is doesn't prove it to you -- because you don't have a grasp of Galilean relativity. What you are seeing in both swerdna's and ours is DDWFTTW -- strictly powered by the wind.

hmmm... I don't remember the term "Galilean relativity" ever coming up in my dynamics or physics classes in university. Perhaps they thought we would become confused and mix it up with "Einsteins relativity", and used a different phrase.

I can't speak for your university, but do I doubt that's what they thought. "Galilean relativity" predates Einsteins relativity" by 3 centuries.

http://en.wikipedia.org/wiki/Galilean_relativity

Exerpt:

Galilean invariance or Galilean relativity is a principle of relativity which states that the fundamental laws of physics are the same in all inertial frames. Galileo Galilei first described this principle in 1632 in his Dialogue Concerning the Two Chief World Systems using the example of a ship traveling at constant speed, without rocking, on a smooth sea; any observer doing experiments below the deck would not be able to tell whether the ship was moving or stationary.

JB

 

[rcgldr]

I still think swerdna's is much better. But neither proves DDWFTTW to me.

So how would you describe what is going on with swerdna's cart and how a cart similar to sporks would behave outdoors in a steady wind?

I don't remember the term "Galilean relativity" ever coming up in my dynamics or physics classes in university.

There have been a series of posts in this thread about the appropriate frame of reference and if the results of indoors and outdoor test would differ because of some frame of reference issue. I tried to eliminate the frame of reference issue by restating the DDWFTTW claim in this manner:

|v_cart - v_ground| > |v_wind - v_ground|

(where |...| means magintude of ...)

Also confusing are comments about work being done on an object without that object performing equal and opposite work against the source. Seems if you enclose the test environment so it's a closed system (air, cart, treadmill), its becomes more clear that momentum and energy are preserved. (The energy from the motor eventually becomes heat inside a closed system). There can be an initial acceleration of the center of mass, but the system eventually reaches a mechanically stable state.

 

[cesiumfrog]

With all the cross-arguing here it's confusing to figure out which position each person actually holds. (If only the mentors would permit some kind of https://www.physicsforums.com/showthread.php?t=274996".) Am I correct in saying that only OmCheeto and Schroder still think DDWFTTW hasn't been demonstrated or proven?

(Perhaps I spoke too soon of futility changing others' beliefs. By exclaiming "what would Dawkins do?" I brought one disbelieving PhD student here's attention to how a sailboat can produce VMG-DDWFTTW, which might be enough that he'll realize the rest himself.)

 

[vanesch]

But neither proves DDWFTTW to me.

Round 2. If DDWFTTW would be demonstrated with an outdoor cart, in the neighbourhood of London, would you consider it proven ?
Would you think it works the same way in the Mid-West of the United States ? Or would you claim that DWFTTW in the Mid West is not proven yet, and that the demonstration in London is awesome, but doesn't prove yet that it will behave DWFTTW in the Mid-West ?

 

[ThinAirDesign]

... it's a gravity powered device using the air to slow the rate of descent and to travel horizontallay.

The answer is ... like a sailboat can be wind or water powered, it all depends on the frame of reference.

How would you describe the situation when the updraft equals sink rate (slope soaring)?

Still can't qualify as DDW since a glider in sustained flight will always be moving at an angle to the surrounding air. Unless the glider is falling, you can't get away from the issue of that angle.

JB

 

[schroder]

Here is a little thought experiment:
Turn the tread until it is Vertical, while keeping the cart Horizontal. You will need to place a Horizontal support table under the cart and maybe add an extra set of wheels for support but the basic operating parameters are unchanged from the Horizontal tread situation.
Now when the tread runs, it will deliver exactly the same turning force to the drive wheels as before. Yes ( ) or No ( )
The cart will move in the Horizontal Direction as before with the same velocity as before. YES ( ) or NO ( )
(I realize that when the cart does move it will disconnect from the tread, but its initial velocity away from the tread will be exactly the same as before. Alternatively, you can consider the tread as tracking along with the cart if you find that easier)
Are not all the forces and velocities exactly the same in both the Horizontal and Vertical tread situations? YES ( ) or NO ( )
Does the relative velocity between the Vertical tread and the Horizontal cart provide a true indication of the cart’s velocity in the Horizontal plane in which the cart is moving? YES ( ) or NO ( )
Would it not make a LOT more sense, and lead to the CORRECT velocity of the cart, to take the relative velocity of the cart with respect to a point on the table on which it is running and NOT wrt the tread? YES ( ) or NO ( )
Multiple Choice:
If the Vertical tread is running at 10 m/s and the cart moves Horizontally at 2 m/ sec , what is the cart’s true velocity on the surface of the table?
a) 10 m/sec (Idiot)
b) 12 m/sec (Vanesch)
c) 2 m/sec (Schroder)
d) Impossible to determine by the present laws of physics need to calculate the prop pitch wrt the inexplicable air ground interface and then pray to God. (Jeff)

If you picked 2 m/sec you get a door prize for the CORRECT answer and this would also be the correct answer on the Horizontal treadmill running at 10 m/sec.

I notice that Thin Air Designs is very concerned about not getting CREDIT for this amazing “invention”. I wonder if he will also be willing to accept the BLAME for this SCAM and HOAX?
I also wonder what this PHYSICS FORUM will do to correct the record and start repairing the damage done to the PHYSICS Community.

Schroder,
Member of New York Academy of Science

 

[ThinAirDesign]

I notice that Thin Air Designs is very concerned about not getting CREDIT for this amazing “invention”.

You really don't pay very close attention ... the credit for this invention certainly doesn't belong to me and I have NEVER made any statement of such.

The earliest credit we can find is a submission from a Iowa graduate student applying for a job to work with Dr. Andrew Bauer and company a Mcdonnell Douglas in SoCal circa '60.

JB

 

[ThinAirDesign]

I can take this to the Academy but they already consider it to be nonsense.

Schroder, Member of New York Academy of Science

I don't suppose you could point us to something supporting your position that "the Academy already considers it to be nonsense"?

JB

 

[rcgldr]

Am I correct in saying that only OmCheeto and Schroder still think DDWFTTW hasn't been demonstrated or proven?

and Zoobieshoe.

How would you describe the situation when the updraft equals sink rate (slope soaring)? Clearly gravity is applying a force on the glider, but since the glider travels horizontally (in an updraft), gravity peforms no work on the glider. (The updraft does perform work on the glider.).

Still can't qualify as DDW since a glider in sustained flight will always be moving at an angle to the surrounding air. Unless the glider is falling, you can't get away from the issue of that angle.

That wasn't the analogy I was thinking of. Would a glider in an updraft moving horizontally be similar to a sailcraft? My concern is the diffrence in the downwards force from gravity that is perpendicular to the gliders path, versus the force from the ice perpendicular to the path of an ice boat? If the analogy is similar, the glider does extremely well, an updraft of 1 mph translates into a forward speed of 60 mph on a very high end glider (80+ foot wing span). This translates into a Beta of < 1 degree while iceboats have a minimum around 8 degrees.

 

[rcgldr]

Turn the tread until it is Vertical, while keeping the cart Horizontal.

Assuming the tread is moving downwards in front of the cart, the cart will apply a horizontal force to the tread in addition to the vertical force related to driving the prop, if the body of the cart is resting on a near fictionless surface (like roller wheels). There is similar angled ruler version of the ruler + wheel craft videos at youtube. However this would happen even if the cart was not capable of DDWFTTW. Here the question is if the cart can apply a forwards force against the downwards moving tread.

Since the vertical treadmill would interfere with horizontal wind flow replace it with a couple of wires, or go back to the horizontal treadmill.

I restated your multiple choice for a horiztonal tread since that is what we mostly have in the videos:

If the (horizontal) tread is running at -10 m/s and the cart moves horizontally at +2 m/ sec , what is the cart’s true velocity on the surface of the table?
a) 10 m/sec (Idiot)
b) +12 m/sec (Vanesch)
c) +2 m/sec (Schroder)
d) Impossible to determine. (Jeff)

c) + 2 m / sec.
Note the wind speed is 0 m / sec. Therefore:

|v_cart - v_tread| = |2 m/sec - (-10 m/sec)| = 12 m/sec
|v_wind - v_tread| = |0 m/sec - (-10 m/sec)| = 10 m/sec

and my claim:

|v_cart - v_tread| > |v_wind - v_tread|

holds true since 12 m /sec > 10 m / sec.

 

[atyy]

I don't suppose you could point us to something supporting your position that "the Academy already considers it to be nonsense"?

http://www.nyas.org/membership/main.asp

Was the cost of building your DDWFTTW demo less or greater than USD 108?

 

[ThinAirDesign]

That wasn't the analogy I was thinking of. Would a glider in an updraft moving horizontally be similar to a sailcraft?

Jeff, I'm certainly not trying to directly equate a glider and sailboat by that analogy. I apologize if it seemed I was trying to imply that you were.

My only point was one not directly relevant to this thread -- that a wind powered device without a 'ground' interface such as as soaring aircraft could not ever qualify for DDWFTTW because of the lack of DDW due to sink rate.


JB

 

[rcgldr]

Would a glider in an updraft moving horizontally be similar to a sailcraft?

I'm certainly not trying to directly equate a glider and sailboat by that analogy.

I didn't mean to imply that anyone here was. I was just asking a somewhat off topic question. If anyone is interested, it belongs in a separate thread.

 

[OmCheeto]

Round 2. If DDWFTTW would be demonstrated with an outdoor cart, in the neighbourhood of London, would you consider it proven ?
Would you think it works the same way in the Mid-West of the United States ? Or would you claim that DWFTTW in the Mid West is not proven yet, and that the demonstration in London is awesome, but doesn't prove yet that it will behave DWFTTW in the Mid-West ?

I hate to repeat myself, but

??

Could you point me to one of the 400 or so post's in this thread that might prove me wrong. I'm afraid the video's just don't cut it.

 

[vanesch]

Here is a little thought experiment:

Haha, he's back

Turn the tread until it is Vertical, while keeping the cart Horizontal. You will need to place a Horizontal support table under the cart and maybe add an extra set of wheels for support but the basic operating parameters are unchanged from the Horizontal tread situation.
Now when the tread runs, it will deliver exactly the same turning force to the drive wheels as before. Yes ( ) or No ( )

Yes, if we ignore that the aerodynamical situation has changed a bit. Acceptable.

The cart will move in the Horizontal Direction as before with the same velocity as before. YES ( ) or NO ( )

Yes.

(I realize that when the cart does move it will disconnect from the tread, but its initial velocity away from the tread will be exactly the same as before. Alternatively, you can consider the tread as tracking along with the cart if you find that easier)
Are not all the forces and velocities exactly the same in both the Horizontal and Vertical tread situations? YES ( ) or NO ( )

No, but almost. I'll tell you what has changed. What has changed is that this is not a setup that transforms into a DWFTTW situation. There's an extra force in the game, which is the force of gravity (which compensates the force of the upward drive of the treadmill if the cart is heavy enough).

Does the relative velocity between the Vertical tread and the Horizontal cart provide a true indication of the cart’s velocity in the Horizontal plane in which the cart is moving? YES ( ) or NO ( )

As long as you stick to "true indication of velocity" you are terribly in error. There is no more true velocity than any other. There is no "correct" and "erroneous" velocity. There are only velocities which correspond to each other in different situations but which can be linked with a Galilean transformation. Clearly the setup you describe is not equivalent with the DWFTTW "outdoor" setup (there is no Galilean transformation which can transform your setup in the "outdoor" situation), so there is no corresponding velocity to that case. As we now have genuinely different situations, you can talk about any velocity you like. It isn't related to a velocity of the DWFTTW cart.

Would it not make a LOT more sense, and lead to the CORRECT velocity of the cart, to take the relative velocity of the cart with respect to a point on the table on which it is running and NOT wrt the tread? YES ( ) or NO ( )

No. You can consider any velocity you like, but none will correspond to the velocities in the "true DWFTTW outdoor situation", as no Galilean transformation links to it.

Multiple Choice:
If the Vertical tread is running at 10 m/s and the cart moves Horizontally at 2 m/ sec , what is the cart’s true velocity on the surface of the table?
a) 10 m/sec (Idiot)
b) 12 m/sec (Vanesch)
c) 2 m/sec (Schroder)
d) Impossible to determine by the present laws of physics need to calculate the prop pitch wrt the inexplicable air ground interface and then pray to God. (Jeff)

The velocity wrt to the table is of course 2 m/sec. But as I said, this velocity has nothing to do with the DWFTTW setup. You actually constructed a cart with a motor and a propeller.

If you picked 2 m/sec you get a door prize for the CORRECT answer and this would also be the correct answer on the Horizontal treadmill running at 10 m/sec.

No, because 1) the two situations are NOT linked by a Galilean transformation and 2) there is no "correct" velocity or "wrong velocity" in the absolute sense.

Schroder,
Member of New York Academy of Science

Bwahaha ! You bought that ticket on a fancy fair ?

 

[zoobyshoe]

A propeller in air is somewhat "fuzzy" and invites to fuzzy arguing

This is my main problem: the propeller. The relative motion of two solids provides for excellent purchase by the cart. My sense is that it very much more difficult to get a grip on air.

There was (and may still be) a $30.000 dollar prize for the first human powered helicopter. Last time I heard, no one has yet been able to lift themselves off the ground and hover long enough to win the prize. Anyone can lift their own weight: you do it anytime you climb stairs, but lifting your own weight into the air is vastly more difficult because it is so difficult to get a firm purchase on the air.

 

[ThinAirDesign]

http://www.nyas.org/membership/main.asp

Non-responsive.

Was the cost of building your DDWFTTW demo less or greater than USD 108?

Using our freely published parts list and Youtube build videos, one can build their own for less than that, but I've spend quite a lot more than that on the numerous ones I've built and tested.

Still, I'm only interested in Schroder supporting his assertion regarding "the Academy" taking an official position on DDWFTTW. I'm truly hoping that he is correct -- they would be the perfect entity to have droning on regarding how it's "impossible" while the MythBusters whoops the balloons bum down the runway.

Sadly, I predict that Schroder won't be able to produce evidence for any such position -- I have greater faith in "the Academy" than that.

JB

 

[vanesch]

Could you point me to one of the 400 or so post's in this thread that might prove me wrong. I'm afraid the video's just don't cut it.

Jeff's and mines for instance

Especially #378.

And the force model.

There was a specific reason for my question, it was not something I just threw out.
So could you answer it (the London and Mid West thing) and argue your answer ?

 

[atyy]

Non-responsive.

Using our freely published parts list and Youtube build videos, one can build their own for less than that, but I've spend quite a lot more than that on the numerous ones I've built and tested.

Sorry about the link - it was just to show that anyone can join the academy for ~USD 108 (~ because slightly different prices for foreign members). For poor students and postdocs, it's only ~USD 36!

 

[ThinAirDesign]

My sense is that it very much more difficult to get a grip on air.

Zoo, you are absolutely correct -- it *is* more difficult -- but that doesn't mean it must defy any laws of physics. If a device can move faster than the medium that is powering it (like pulling on the yoyo string, or the 'faster than the ruler' video) then there are no laws of physics being broken and the issue with the prop becomes only one of efficiency.

If your argument is "it doesn't defy any laws of physics but I just think it can't be done in practice", so be it ... but that position is still just wrong and building your own cart for ~$50 can demonstrate just how wrong it is.

If your argument is "it defies the laws of physics", any number of demonstrations can and have been done to put that one to rest - a spool of thread will do just fine. There is no "conservation of speed" law.

JB

 

[rcgldr]

This is my main problem: the propeller. The relative motion of two solids provides for excellent purchase by the cart. My sense is that it very much more difficult to get a grip on air.

It is more difficult, and I previously questioned if the propeller related losses (induced wash, slip ratio, angular movement of air, tip vortices, ...) would exceed what is needed to accomplish DDWFTTW. The videos have convinced me that DDWFTTW with a propeller works.

Could you point me to one of the 400 or so post's in this thread that might prove me wrong. I'm afraid the video's just don't cut it.

You stated that my last explantion wasn't simplified enough.

Simpler still description:

Prop power input = force at wheels times ground speed at wheels
Prop power output = force at prop times air speed at prop

The power input is used to create the torque and angular velocity used to drive the prop. The prop generates a higher force but at a lower speed than the wheel + ground interface. A tailwind allows the prop to interact with air that is moving slower than the ground (using the cart as a frame of reference), so that the slower speed at the prop still results in a net upwind thrust.

The net upwind thrust opposes the tailwind, slowing the wind down significantly below cart speed, allowing the cart to operate DDWFTTW.

 

[ThinAirDesign]

Sorry about the link - it was just to show that anyone can join the academy for ~USD 108 (~ because slightly different prices for foreign members). For poor students and postdocs, it's only ~USD 36!

No problem atyy. I had already visited that site but was unable to find anything related to DDWFTTW -- I'm still hoping I've missed something and that Schroder can point me to it, but I'm betting that's a lost hope.

JB