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

[rcgldr]

As for the cart running down wind, for some reasons nobody has provided me with a radar gun so I can measure it directly.

I've read that a few people are planning to make and test an outdoor device at the wiki forums.

Why don't you consider the Brennan torpedo as a working outdoor example?

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

Also I tried to eliminate the frame of reference issue by restating the claim mathematically:

within a reasonable range of

|wind speed - ground speed|

a DDWFTTW cart can achieve

|cart speed - ground speed| > |wind speed - ground speed|

 

[rcgldr]

"Apparent crosswind"? Can you link me to some site that explains "apparent crosswind"? I googled but I found no definitions, just mostly posts by you here and on other forums.

Think of that term as my invention, then. I simply separated the apparent wind into components perpendicular to the path of the sailcraft (apparent crosswind) and in the direction of the path of the sailcraft (apparent headwind). In aerodynamics, the aerodynamic forces on a wing are also separated into components perpendicular to the path of the wing (lift) and in the direction of the wing (drag) so I decided to follow this precedent.

Note that Beta, defined as the angle of the apparent wind = atan(apparent crosswind / apparent headwind), so mathematically apparent crosswind and apparent headwind have their proper place in sailcraft discussions.

I don't understand why you aren't speaking of everything in terms of plain "apparent wind". Why "apparent crosswind" and also "apparent headwind?

Because an airfoil doesn't generate thrust from an "apparent headwind". Thrust meaning the aerodynamic force in the direction of a sailcraft. Thrust can only result from a diverted apparent crosswind.

This search for slowed wind sounds very idiosyncratic and seems misleadingly incomplete. It suggests that the only operative force here is Newton's Third Law. It strikes me as essential to include Bernoulli. We aren't only applying force to the back of the sail we are also removing opposition to it from the front.

It doesn't matter how the wind is slowed down. It's just as valid to slow the wind from low pressure downwind of the airfoil as it is to slow the wind from high pressure upwind of the airfoil. The point here is that when air is accelerated in one direction, a reactive force coexists in the opposite direction, a combination of Newton's 2nd and 3rd laws.

All wind powered devices have to slow down the air in order to extract power from the air.

Because he thought to affix a tell tale, this video is the most apparently impressive I've seen:

http://www.youtube.com/watch?v=aJpdWHFqHm0&fmt=18

However, if you go to the linked address for the expanded look at the cart he specifically says it's designed to be brought up to speed as a windmill: the fan is configured to receive power from the wind from behind and transmit it to the wheels, in which case the fan blades are pitched to resist acceleration into the HH, not encourage it.

Look at the start of that video, the propeller is clearly pitched to generate thrust as it rotates counter-clockwise. Jack Goodmans' description would be better worded to state that the propeller initially acts as a bluff body, slowing the wind down due to aerodynamic drag more than by slowing the wind down due to the propeller generating thrust.

Jack's cart wasn't able to self start in that video, but that wasn't his goal. He later did treadmill tests with the cart constrained by strain gauges to confirm that the cart generated forward force against the strain gauges while on a treadmill.

 

[zoobyshoe]

Exactly. That's why it only works if air and ground have different speeds:
- The ground puts in a small backwards force over a long distance.
- The air puts in a large forward force over a small distance.
-> The net force points forward

The large forward-force applied to the top gear by the ruler (thrust force) does two things:
a) Push the cart forward
b) Create a torque on the gear which is transmitted to the ground, where it causes a small backwards force (=breaking force).

Here is another simple analogy of DDWFTTW:
Imagine you are facing a blue picket fence which is moving very slowly to the left. Close behind it there is a brown picket fence moving also to the left but much faster than the blue one. If you now hold a stick and put it trough both fences simultaneously it will accelerate you to the right.

So you are using two things which both are moving left to accelerate yourself to the right. Thats what the DDWFTTW-cart also does with the ground (brown fence) and air (blue fence):

I've already understood and accepted all this. The cart can receive energy from the difference in speed of the surrounding media regardless of it's own speed. In principle. My objection is not to this. My objection is that, in practice, there can arise a situation where, even though there is a speed difference, it doesn't represent enough energy to do what the cart wants it to do. For example: If you strap yourself to a tree the two fences will not be able to move you.

When the cart reaches downwind speed the energy difference between the two media has been reduced to an amount equal to the energy represented by the total momentum of the cart. This is what it boils down too. The budget is unbelievably tight here and being spent fast with no deposits to make up for the withdrawals.

 

[rcgldr]

When the cart reaches downwind speed the energy difference between the two media has been reduced.

Energy is tricky here, because it includes a velocity component, making it relative to a frame of reference. Note the speed differential between wind and ground is independent of the cart speed (and frame of reference).

Note also that the Brennan torpedo could reach a downstream speed of 31 mph in water, simply by pulling a pair of wires upstream to supply the power to drive the propellers. This was a real world working example of a DSFTTS (S = stream) vehicle.

 

[zoobyshoe]

Think of that term as my invention, then. I simply separated the apparent wind into components perpendicular to the path of the sailcraft (apparent crosswind) and in the direction of the path of the sailcraft (apparent headwind). In aerodynamics, the aerodynamic forces on a wing are also separated into components perpendicular to the path of the wing (lift) and in the direction of the wing (drag) so I decided to follow this precedent.

Note that Beta, defined as the angle of the apparent wind = atan(apparent crosswind / apparent headwind), so mathematically apparent crosswind and apparent headwind have their proper place in sailcraft discussions.

Because an airfoil doesn't generate thrust from an "apparent headwind". Thrust meaning the aerodynamic force in the direction of a sailcraft. Thrust can only result from a diverted apparent crosswind.

It doesn't matter how the wind is slowed down. It's just as valid to slow the wind from low pressure downwind of the airfoil as it is to slow the wind from high pressure upwind of the airfoil. The point here is that when air is accelerated in one direction, a reactive force coexists in the opposite direction, a combination of Newton's 2nd and 3rd laws.

All wind powered devices have to slow down the air in order to extract power from the air.

So, what I hear you saying is that there is no particular advantage to analyzing it your way, it's just your taste.

Look at the start of that video, the propeller is clearly pitched to generate thrust as it rotates counter-clockwise. Jack Goodmans' description would be better worded to state that the propeller initially acts as a bluff body, slowing the wind down due to aerodynamic drag more than by slowing the wind down due to the propeller generating thrust.

You're quite right. He misspoke.

 

[zoobyshoe]

Energy is tricky here, because it includes a velocity component, making it relative to a frame of reference. Note the speed differential between wind and ground is independent of the cart speed (and frame of reference).

You're right, I misspoke. Should have said "speed difference", not "energy difference."

 

[rcgldr]

I simply separated the apparent wind into components perpendicular to the path of the sailcraft (apparent crosswind) and in the direction of the path of the sailcraft (apparent headwind). ... an airfoil doesn't generate thrust from an "apparent headwind".

So, what I hear you saying is that there is no particular advantage to analyzing it your way.

The advantage to my analysis is to point out the fact that an apparent headwind isn't converted into thrust, only the apparent crosswind. It also gives a clearer picture as to the meaning of Beta, which is atan(apparent crosswind / apparent headwind). Also it may not be clear that apparent crosswind is constant (for a given wind and heading), independent of the sailcraft's speed.

 

[A.T.]

Here is another simple analogy of DDWFTTW:
Imagine you are facing a blue picket fence which is moving very slowly to the left. Close behind it there is a brown picket fence moving also to the left but much faster than the blue one. If you now hold a stick and put it trough both fences simultaneously it will accelerate you to the right.

For example: If you strap yourself to a tree the two fences will not be able to move you.

But there are no trees, just you with the stick(=~ cart) and two fences moving at different speeds(=~ air, ground).

When the cart reaches downwind speed the energy difference between the two media has been reduced to an amount equal to the energy represented by the total momentum of the cart.

I misspoke. Should have said "speed difference", not "energy difference."

Sorry, still doesn't make sense to me. The speed difference between the two media is constant, and is not being reduced by the speed of the cart. It seems that you are confusing physical quantities like energy, speed and momentum.

 

[Subductionzon]

schroder I think I see your problem. You are still mixing frames of reference and not seeing the experiment properly to start with. The Galilean equivalence of the cart on the treadmill is that of the cart already going downwind with the speed of the wind. If you notice in the various spork videos they hold it to get it up to speed on the treadmill. Again that would be the equivalent of using some sort of outside force (non-wind) to get the cart up to wind speed outside and then releasing it. If the cart moves up the treadmill then its speed would be its starting speed relative to the treadmill and whatever speed it generates on that short trip. In your last example you were mixing frames of reference left and right and measured the speed of the cart with respect to the wrong one. Think of it this way a line painted across the treadmill has a speed of zero with respect to it. The cart obviously is not staying with the line, it is passing it every second or so. So if it advances after release it is going faster than the wind. The treadmill is just a nice tool to see if the cart can pass that critical point of wind speed.

 

[zoobyshoe]

But there are no trees, just you with the stick(=~ cart) and two fences moving at different speeds(=~ air, ground).

The notion of moving a tree, here, represents some task the cart wants to perform that requires more energy than the fences can deliver.

Sorry, still doesn't make sense to me. The speed difference between the two media is constant, and is not being reduced by the speed of the cart. It seems that you are confusing physical quantities like energy, speed and momentum.

Did you get yourself up to speed on the TH, HH analysis?

 

[zoobyshoe]

The advantage to my analysis is to point out the fact that an apparent headwind isn't converted into thrust, only the apparent crosswind. It also gives a clearer picture as to the meaning of Beta, which is atan(apparent crosswind / apparent headwind). Also it may not be clear that apparent crosswind is constant (for a given wind and heading), independent of the sailcraft's speed.

It is not clear to me why we don't just stick with the concept of apparent wind. Can't thrust be calculated from that?

 

[swerdna]

That is a major part of the problem. An honest and straight forward evaluation of this cart does not require a “physicist” who deals in abstractions. I have the utmost respect for physicists, I am only saying this problem is not complicated enough to require the services of one. It only requires a qualified test engineer with a great deal of practical experience in this sort of thing. That’s where I come in. I am now going to run a test for you and demonstrate that this cart has never run faster than the wind, cannot run faster than the wind and never will run faster than the wind.
As test director, I choose the reference frame and I choose it to be the most logical frame which can act as a reference for both the treadmill test and the outdoor test. I choose the ground as my frame of reference. It is not an “absolute” frame, so it does not violate Galilean relativity but it is easily shown to be stationary “relative” to both the moving treadmill, the cart on the treadmill, the wind which is blowing, and the cart which is moving with the wind. It is a relatively stationary reference to all the moving components in the test. I am setting up this test so the treadmill test and the outdoor test can be run simultaneously and observed by me or any other witness by standing in one position on the common ground reference. The test is conducted outdoors on a large flat surface (the “ground”). The treadmill is sitting on the ground and is enclosed by Plexiglas so as to shield it from the wind. The wind is cooperating by blowing at a nice steady velocity and is constant at 10 m/sec and is being constantly monitored by calibrated anemometers. The treadmill has a setting which has been carefully calibrated to run at exactly 10 m/sec exactly the same as the wind and both velocities referenced to the ground reference. The re is no need to read the cart’s actual velocity on the treadmill, only to demonstrate that it is either greater or lesser than the tread velocity all velocities referenced to the ground. By video analysis, it is clearly demonstrated that the cart is moving at much less than the tread velocity. I have estimated it to be at about 30% of tread velocity at most. The actual number is not important. As for the cart running down wind, for some reasons nobody has provided me with a radar gun so I can measure it directly. I don’t know why my request has been turned down. I find this very strange but I will make do. All I need is to see if there is any evidence at all if the cart is going faster than the wind or slower, all velocities measured in respect to the ground reference. As the cart is running down wind a video is recorded and on playback it clearly shows that some debris which is also being blown by the wind passes the cart at a much higher velocity than the cart is moving. All comparisons are made with respect to the ground reference. From the evidence I conclude: 1) The cart on the treadmill, being driven by the treadmill, is moving much slower than the treadmill. 2) The cart in the wind, being driven by the wind is moving much slower than the wind. All of my comparisons are with respect to one common reference and can be considered as one reference frame. I can only conclude that the evidence is overwhelmingly against the claim of DDWFTTW. In fact, I now consider the claim to be so ridiculous that I will not participate in any more discussion or debate on this matter. My findings are here presented and open to inspection to all. I consider the matter as closed.

Hi Schroder (if you’re still following this thread). I understand your sceptical position because that’s where I’ve come from. I now believe DDWFTTW is possible and proven however and that there is a very good working example of this in the Brennan Torpedo. If the carts in the videos don’t work how does the Brennan Torpedo work? Isn’t it obvious to you that they both work on essentially the same principle?

I never quite trusted that the treadmill tests were correctly representing the principle so went to the trouble of designing and building my turntable and cart to do my own testing. I believe a turntable is better for testing because it doesn’t have a limited length. Every suspicion I had about what might be falsely represented by the treadmill tests have been disproved by the turntable tests. That the cart might be storing energy for instance. I have run my cart on the turntable for over 10 minutes with no apparent loss of speed so the what the cart achieves is definitely indefinitely sustainable.

If myself or someone else built a cart and provided a video of it clearly beating bubbles floating in an outside wind would you accept this as proof? If not, what you accept as proof?

 

[zoobyshoe]

Hi Schroder (if you’re still following this thread). I understand
If myself or someone else built a cart and provided a video of it clearly beating bubbles floating in an outside wind would you accept this as proof? If not, what you accept as proof?

If a thing works as claimed then it can be analyzed and accounted for by the proper physics and math. None of the adherents of the device have been able to show on paper that such a device is actually possible. Support for the theoretical possibility is from other devices which aren't doing the quite the same thing. I am pretty sure I've shown it's not possible in principle: the geometry of the directly downwind situation automatically leads to a point where the cart switches from extracting energy from the relative motion of the surrounding media to being a supplier of energy to the surrounding media and it must eventually be bankrupted. And, if it can't be what it looks like then it must be something else.

My best guess is that what all the apparently successful non-motorized demonstrations show is that the stored momentum lasts longer than we guestimate it should. (No one's actually calculated.) It could be collecting and storing enough angular momentum in the rotor and wheel to maintain itself into the HH for a whole minute, or even two minutes, but it won't be sustained and will eventually slow back down till the tailwind hits it again.

 

[A.T.]

The notion of moving a tree, here, represents some task the cart wants to perform that requires more energy than the fences can deliver.

The fences have plenty of energy, just like the wind. They can accelerate any mass in the opposite direction of their movement, if the stick and the fences are strong enough.

It could be collecting and storing enough angular momentum in the rotor and wheel to maintain itself into the HH for a whole minute, or even two minutes, but it won't be sustained and will eventually slow back down till the tailwind hits it again.

How did the Brennan Torpedo maintain itself in this situation for 2000 yards? Stored momentum? Under water?

 

[swerdna]

If a thing works as claimed then it can be analyzed and accounted for by the proper physics and math. None of the adherents of the device have been able to show on paper that such a device is actually possible. Support for the theoretical possibility is from other devices which aren't doing the quite the same thing. I am pretty sure I've shown it's not possible in principle: the geometry of the directly downwind situation automatically leads to a point where the cart switches from extracting energy from the relative motion of the surrounding media to being a supplier of energy to the surrounding media and it must eventually be bankrupted. And, if it can't be what it looks like then it must be something else.

My best guess is that what all the apparently successful non-motorized demonstrations show is that the stored momentum lasts longer than we guestimate it should. (No one's actually calculated.) It could be collecting and storing enough angular momentum in the rotor and wheel to maintain itself into the HH for a whole minute, or even two minutes, but it won't be sustained and will eventually slow back down till the tailwind hits it again.

If you think that theory is more robust evidence than an actual, practical physical demonstration then I respectfully suggest that perhaps you should get out into the real world more often.

As I said earlier, stored energy was my main concern before I did testing for over 10 minutes on my turntable 10 minutes is a lot longer than the one or two you suggest. I also did a turntable test to show how the kinetic energy of a marble lasts a long time on a turntable - http://nz.youtube.com/watch?v=kc88SrMG5fA

I repeat A.T’s question - “How did the Brennan Torpedo maintain itself in this situation for 2000 yards? Stored momentum? Under water?”

And ask you the same as I asked Schroder - “If myself or someone else built a cart and provided a video of it clearly beating bubbles floating in an outside wind would you accept this as proof? If not, what you accept as proof?”

 

[Phrak]

We must be talking about the same thing in different ways.

I've seen multiple definitions for induced drag, so I'm not sure on what you mean by induced drag.

Again, there are probably many ways to explain things. The way I understand it, to gain lift, a reactionary force, from an airfoil the momentum of the air must be given a downward component. The difference in momentum vectors, arriving and leaving, results in a difference vector that is aimed, not directly downward but somewhat foreward--just simple vector subtraction. dp/dt is the force exerted by the foil on the air. The reactionary force is equal and opposite. This is ignoring frictional drag.

I've always read that the helical flow off a propeller is related to total drag, not just induced drag.

I was ignoring frictional drag.

 

[rcgldr]

The advantage to my analysis is to point out the fact that an apparent headwind isn't converted into thrust, only the apparent crosswind. It also gives a clearer picture as to the meaning of Beta, which is atan(apparent crosswind / apparent headwind). Also it may not be clear that apparent crosswind is constant (for a given wind and heading), independent of the sailcraft's speed.

It is not clear to me why we don't just stick with the concept of apparent wind. Can't thrust be calculated from that?

Thrust can be calculated if the apparent wind magnitude and direction (relative to the path of the sail craft) are known. Thrust is a function of apparent crosswind which equals the apparent wind x sin(angle of apparent wind (relative to sail craft path)).

It's not clear in those articles that apparent wind is a vector, especially considering the article you linked falsely claimed that as the sailcraft's speed increased, it could turn even more downwind, get even more speed, then turn down even more downwind, get even more speed, ... , which simply isn't true.

Maximum VMG downwind (the component of speed in the direction of wind) is obtained when the heading of the sailcraft is offset (45 degrees - (Beta/2)) from true downwind. Maximum speed (regardless of direction) is obtained when the heading of the sailcraft is offset (90 degrees - Beta) from true downwind.

In either case the sailcraft can self start on the target heading and without ever changing the heading angle, eventually reach it's maximum speed. Over time as the sailcraft accelerates, the apparent crosswind remains constant, but the apparent headwind increases. Eventually the sailcraft stops accelerating when the apparent headwind increases to the speed where atan(apparent crosswind / apparent headwind) = Beta. Note that Beta depends on the sailcraft paremeters (lift versus drag of sail, "lift" versus drag of ground interface). and the true wind speed.

Thrust is a function of apparent crosswind which equals the apparent wind x sin(angle of apparent wind (relative to sail craft path)). The headwind component of the apparent wind does not generate any thrust and this is the misleading part of those artilces that refer to apparent wind without making it clear that it's a vector, or that imply that increasing apparent wind increases thrust (only an increase in apparent crosswind increases thrust).

 

[swerdna]

I’ve designed a “Brennan Boat” that uses the same principal as his torpedo except it uses the flow of water as the power source instead of pulling the cables. Or to put it more correctly, the water pulls against the cables. The cables (red) are simply anchored to a fixed point and as the boat moves away from that point the cables unwind from the spools and spin the prop. One cable feeds from the top of it’s spool, the other from the bottom of it‘s spool so they both turn in the same direction. The spools can be either directly attached to the prop shaft or via gearing if required. Hopefully the images make it clear how it works. The same setup could be used for wind tests.

http://www.accommodationz.co.nz/images/brennanboat.bmp

 

[rcgldr]

My issue with this is the idea of an incompressable fluid.

I think the main flaw here is that negative (below zero) pressures can be involved with formulas that deal with incompressible flows.

Again, there are probably many ways to explain things. The way I understand it, to gain lift, a reactionary force, from an airfoil the momentum of the air must be given a downward component. The difference in momentum vectors, arriving and leaving, results in a difference vector that is aimed, not directly downward but somewhat foreward--just simple vector subtraction. dp/dt is the force exerted by the foil on the air. The reactionary force is equal and opposite.

There are some complications due to the fact that the air also gets angular velocities and corresponding pressure changes due to turbulence (eddies with axis parallel to wing span, vortices with axis in the direction of travel at the wing tips), but this is essentually correct.

I've seen multiple definitions for induced drag, so I'm not sure on what you mean by induced drag.

This is ignoring frictional drag.

You're missing form drag. Frictional drag is minimal. Form drag is the net aerodynamic force that occurs when any solid travels through a fluid or gas (minus the tiny amount of skin friction related drag). Induced drag is an attempt to separate form drag of an airfoil producing lift into two components. You could consider induced drag as the theoretical limit as the minimal form drag required to produce lift with an idealized perfect airfoil. The actual definitions of induced drag vary depending on what article you're reading.

 

[rcgldr]

Hopefully the images make it clear how it works. Can also be used for wind tests.

Link was broken, it's fixed now.

 

[swerdna]

[/itex] doesn&#039;t work here. Link to image, but it&#039;s not working either:<br /> <br /> [b][url]http://forums.randi.org/picture.php?albumid=116&amp;pictureid=558[/url][/b][/QUOTE]<br /> <br /> Thanks Jeff. Works fine on my browser. I don&#039;t know how to load images to this forim so hot-Linked to the image fom a similar thread I&#039;m running on the JREF forum.

 

[rcgldr]

Thanks Jeff. Works fine on my browser. I don't know how to load images to this forim so hot-Linked to the image fom a similar thread I'm running on the JREF forum.

I'm guessing it's working for you because you're signed into that forum. Apparently that forum doesn't allow hot linking to it's threads.

 

[swerdna]

I'm guessing it's working for you because you're signed into that forum. Apparently that forum doesn't allow hot linking to it's threads.

Thanks, Have hot-linked to some of my own webspace so should work now. How do I upload images to this forum?

 

[rcgldr]

Thanks, Have hot-linked to some of my own webspace so should work now. How do I upload images to this forum?

I don't know, but your previous post is working now. The img does work, just requires approval from an administator, your's is already working. I assume that there's an offset between boat and prop? How will you steer the boat and/or keep the wire tension equal?

 

[swerdna]

I don't know, but your previous post is working now. The img does work, just requires approval from an administator, your's is already working.

Thanks. I'm tempted to build a wind cart with this design.

 

[rcgldr]

Thanks. I'm tempted to build a wind cart with this design.

You'd need an open framework so that the prop would be exposed with minimal frame drag.

 

[swerdna]

You'd need an open framework so that the prop would be exposed with minimal frame drag.

I like that the prop isn't driven via the wheels so there can be no wheel-slip or bounce that some people think makes other carts work. I wonder if the results of testing with this design would be acceptable to those that are still sceptical?

 

[rcgldr]

I like that the prop isn't driven via the wheels so there can be no wheel-slip or bounce that some people think makes other carts work. I wonder if the results of testing with this design would be acceptable to those that are still sceptical?

There could be issues with momentum (flywheel) effects since the wind speed varies. Plus some will claim those wires are driving the motors hidden inside the spools.

 

[swerdna]

There could be issues with momentum (flywheel) effects since the wind speed varies. Plus some will claim those wires are driving the motors hidden inside the spools.

It’s easier to get a constant flow of water than air but if it was demonstrated in water some would say “but that’s water not air”. I don’t need to do any more testing for myself so I have to ask myself if I want to continue spending time and money on this. It is a fun project though.

ETA - Couldn’t this be tested just using gravity? Let the cart run down a long slop without a prop and time it. Repeat the test with a prop fitted and see if it is faster. I realize the prop wouldn’t be thrusting against gravity as it would against a wind but the prop still would have thrust against air that should make it quicker.

 

[rcgldr]

it was demonstrated in water

The issue with the water would be keeping the wires in sync as they unwound off the spools.

Couldn’t this be tested just using gravity?

The cart needs a tailwind (wind speed different than ground speed). Using the numbers from your cart as an example, when the cart is moving at 10.5 mph, the prop advance rate thorugh the air is 6 mph, 4.5 mph slower than the cart, so without a tailwind, the prop ends up slowing down the cart.