How can an vehicle move faster than the wind that is powering it?

[rcgldr]

L/D of the skates can just as easily be lumped in with the L/D of the sail.

Not quite, the direction of the skates is different from the direction of the apparent wind relative to the sail. I'd be happy assuming zero drag from the skates and seperating the direction of lift from the skates into the aerodynamic lift and drag factors.

L/D is literally the lift-to-drag ratio.

I know about the aerodynamic part, and just assumed that skate drag was a function of force perpendicular to the direction the skates moved and possibly speed.

apparent wind vector

A bit different than the normal wing case because the "lift" force from the skate is not perpendicular to the apparent wind. This is one source of confusion. The closest analogy would be flying a glider in an updraft or downdraft, except the force from gravity is constant (in level flight), while the lift force from the skate opposes to the aerodynamic force component perpendicular to the skate, and is variable (as opposed to equal to the weight of a glider).

High performance sailplanes get 50:1.

60:1 for a high end gliders (80 foot wing spans or more, for example the Nimbus 4T at 59knots (68mph):

http://www.mandhsoaring.com/nimbus4T.html

In the case of wings, I prefer to use the air as a frame of reference, because it's easier to see what's going on by how the air is disturbed. Using the air as a frame of reference, I'm trying to figure out how it's being diverted when Vmg is greater than downwind speed. (The diagram isn't approved yet, so all I see is a red x at this point).

 

[Trond]

While the discussion about sailing is interesting, most are missing one vital point. The contraption it's referred to isn't powered by wind, but by the wheels, which is clearly described by the builder...

 

[spork]

Not quite, the direction of the skates is different from the direction of the apparent wind relative to the sail.

Agreed. I didn't mean to imply that they would add linearly, but you can easily model the combination of skate and sail L/D by simply degrading the sail L/D and assuming skate L/D is infinite. In any event, toward the end of my above analysis I describe how we can account for skate L/D correctly (the ratio of cross to forward component of the resultant force must be >= to the skate L/D).

I know about the aerodynamic part, and just assumed that skate drag was a function of force perpendicular to the direction the skates moved and possibly speed.

I don't know what the polar would look like for the skates. But in the end the only forces relevant are the "lift" (sideways force) and drag they produce. So it can be modeled as you say - even if the polar varies with any number of parameters.

A bit different than the normal wing case because the "lift" force from the skate is not perpendicular to the apparent wind.

The lift force from the skates (or keel in the case of a sailboat) would be perpendicular to the free-stream of the medium in which they operate. This is easiest to think of in the case of a sailboat because it operates between two fluids (air and water). It has one wing in the wind (the sail) and one in the water (the keel). This ends up being completely symmetrical (which is interesting for other parts of the analysis). For our purposes, the skates are equivalent to the keel. In both cases the lift and drag are measured relative to the vehicle's velocity.

This is one source of confusion. The closest analogy would be flying a glider in an updraft or downdraft, except the force from gravity is constant (in level flight)

In all the gliders I've flown I've found the force from gravity to be pretty constant even when not in level flight.

...while the lift force from the skate opposes to the aerodynamic force component perpendicular to the skate, and is variable (as opposed to equal to the weight of a glider).

I have to confess, I'm not quite following you here.

60:1 for a high end gliders (80 foot wing spans or more, for example the Nimbus 4T at 59knots (68mph):

True enough. I like to remain a bit conservative in all such discussions so as not to get into a tangential debate about things like sailplane performance. I was consider a Stemme when I typed that. The gliders I fly are down in the miserable 30's range (my HG is down in the 12 range, and my paraglider is probably 7 or worse )

(The diagram isn't approved yet, so all I see is a red x at this point).

A red x?

Would it help if I produced a diagram to scale? And included all the angles, magnitudes, and performance params assumed? It would be easy enough to do.

 

[spork]

While the discussion about sailing is interesting, most are missing one vital point. The contraption it's referred to isn't powered by wind, but by the wheels, which is clearly described by the builder...

I've had long discussions with the builder. Unfortunately, there are some definite subtleties to the question you raise. Energy is entirely measured relative to the inertial frame of reference in which it's measured. This vehicle uses exactly the same principles as an ice-boat on a continuous downwind tack. It's just that in the case of the cart, the sail (prop tips) are on a continuous spiraling downwind tack.

The wheels and belt drive perform exactly the same function as the keel or skate blades - namely providing the kinematic constraint that defines the path followed by the airfoil (sail or prop tips).

In the case of the cart, the wheels insure that the prop turns through a prescribed amount of arc for a given distance the cart moves forward - just as the keel insures the boat moves a prescribed amount forward for a given amount it moves downwind.

In the case of the cart, the prop applies a force to the cart through the thrust bearing, this causes the wheels to turn, and they in turn insure that the prop turns.

When analyzed form the perspective of the ground the cart is quite definitely wind powered, whereas it would be "ground" powered if analyzed with the steady wind as our base inertial reference frame.

 

[schroder]

Let me see if I understand what is being said here: You place this machine on a treadmill with a backstop so it stays in place. Next, you run the treadmill and let the moving tread do work on the wheels. The wheels then do work to turn a belt, which in turn does work to spin a propeller. Then a velocity is reached where the combination of lift generated by the propeller and feedback to the wheels allows the thing to move forward. Is that what the claim is? The claim is that now the machine is doing work on the treadmill, allowing the machine to move forward, when all the energy is being supplied by the treadmill? I need to get one of those!

 

[spork]

A bit different than the normal wing case because the "lift" force from the skate is not perpendicular to the apparent wind. This is one source of confusion.

I hope I dealt with that adequately above, but I think it's also worth noting that this is the secret to the whole thing. considering a sailboat, the keel is the wing in the water (fluid A) while the sail is the wing in the air (fluid b). This is the clever way the vessel exploits the energy available at the wind/water interface - while it can only exploit the energy of the wind/sail interface when running straight downwind.

If you consider a bird that is dynamically soaring, he's doing the same thing as the keel and sail, but his wings simply perform both functions, and his momentum is the "structure" that tie the two together so he can exploit the energy that comes from the interface of two airmasses (or more commonly, a wind gradient).

 

[spork]

Let me see if I understand what is being said here: You place this machine on a treadmill with a backstop so it stays in place. Next, you run the treadmill and let the moving tread do work on the wheels. The wheels then do work to turn a belt, which in turn does work to spin a propeller. Then a velocity is reached where the combination of lift generated by the propeller and feedback to the wheels allows the thing to move forward. Is that what the claim is?

Close, but you're off in a subtle but critical way. There is no feedback to the wheels in the sense that I think you mean. The wheels will alway be a source of drag on the treadmill. However the prop is pushing the car forward by an amount greater than the drag the wheels provide.

The claim is that now the machine is doing work on the treadmill

That would be nice - but no. The treadmill provides all the energy in this case (just as the wind provides the energy in the case of operating on a stationary surface).

I need to get one of those!

I think I can put you in a nice one that will fit your needs and budget.

 

[rcgldr]

While the discussion about sailing is interesting, most are missing one vital point. The contraption it's referred to isn't powered by wind, but by the wheels, which is clearly described by the builder...

One issue is that propellers operate in their own induced wash, which is significantly different than an ice boat. Helicopters consume more power in a hover than in forward flight for the same reason, while in a hover they operate in their own induced downwash.

If you consider a bird that is dynamically soaring.

This is different. Dynamic soaring takes advantage of differing wind speeds separated by a thin shear boundary, by flying through the shear boundary in and out those different regions of wind speed repeatedly. The bird doesn't experience both wind speeds at the same time. A sail craft does experience the difference between wind and ground or water speed at the same time.

Also it turns out that dynamic soaring birds are also getting slope lift from the upwash off the upwind side of waves (which explains why albatrosses routinely climb to 50 feet instead of just 10 feet or so). However dynamic soaring does work as evidenced by radio control gliders. The last time I checked the record was 365mph. There's a youtube video of a 333mph run.

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

By the way, your diagram attachment still shows as a red x, so apparently it's not been uploaded by physics forums yet. You might want to use an alternate host in the meantime.

On a bit of a side note, one of those sailing sites asked what would happen if a powered fan at the back of a boat directed the air towards a sail which then diverted the air flow backwards. The given answer was that the boat would move in forwards, but this ignores the fact that the fan is accelerating air from behind the fan forwards. What happens between fan and sail doesn't really mater because it's internal to the boat. The boat only moves forwards if the net backwash of air has more momentum change than the net forward wash of air drawn into the fan. This could happen if the pressure jump across the fan disc was high enough, and the sail was efficent enough. Jet engines can reverse thrust, but the pressure jump is huge because of the thermal jump from combustion.

 

[Trond]

It's just that in the case of the cart, the sail (prop tips) are on a continuous spiraling downwind tack.

Kinda like any propeller and I know this is your view of the problem spork. For me it's not necessary to repeat it. I still feel you have a way to complicated view of the problem.

You keep talking of L/D ratio for the propeller blades while I prefer Goodmans version where L/D is the ratio between thrust from the propeller and the sum of airdrag, wheeldrag and friction in the system. As long as this value is >1 you are good to go.

Goodman:

The key point is that the propeller is a propeller, not a wind vane

Which means you are not sailing.

And btw, the heading:

How can an vehicle move faster than the wind that is powering it

is not the problem of the cart. The ice yacht yes, but not the cart, as it's powered by the rolling wheels and not the wind, as per Goodman who is one of those that have built one.

 

[spork]

This is different. Dynamic soaring takes advantage of differing wind speeds separated by a thin shear boundary, by flying through the shear boundary in and out those different regions of wind speed repeatedly. The bird doesn't experience both wind speeds at the same time. A sail craft does experience the difference between wind and ground or water speed at the same time.

I agree. That's why I said the bird uses its momentum as the "structure" to allow it to exploit the energy that could be extracted from the differing speed of the two fluids. The boat sticks a wing in both fluids simultaneously, while the bird has to fly back and forth between them to get his wings in one then the other. His momentum is what ties it all together.

Also it turns out that dynamic soaring birds are also getting slope lift from the upwash off the upwind side of waves (which explains why albatrosses routinely climb to 50 feet instead of just 10 feet or so).

True that birds can exploit this second source of energy/lift - but that is not a requirement for dynamic soaring. It's possible for them to stay aloft for hours with no flapping and no vertical wind component. But... that comparison is not central to this situation. I'd love to discuss dynamic soaring too, but I'm eager to see if we can get on the same page with respect to this DWFTTW issue.

By the way, your diagram attachment still shows as a red x, so apparently it's not been uploaded by physics forums yet.

That's bizarre! I didn't realize what you meant initially about the red x. It's working fine in my browser. It's a .gif image. Is it possible that it's a problem with your browser? This is where it's supposed to be:

https://www.physicsforums.com/attachment.php?attachmentid=15991&stc=1&d=1224565850

On a bit of a side note, one of those sailing sites asked what would happen if a powered fan at the back of a boat directed the air towards a sail which then diverted the air flow backwards. The given answer was that the boat would move in forwards, but this ignores the fact that the fan is accelerating air from behind the fan forwards.

Yes. This is the same principle as a thrust reverser on a jet engine (but I'm sure you knew that). One of the guys on another forum actually made a little video where he did exactly that to test my claim that this would happen.

 

[spork]

Which means you are not sailing.

Two points here...

1) Goodman is a very sharp guy, but he's not an aerodynamicist.
2) When he and I have discussed this I got the definite impression that the point he is trying to make is that the transmission is not working in the way that many people assume it is. Clearly the wheels cause the prop to turn, just as the prop causes the cart to move forward - resulting in the wheels turning.

None of this implies that the cart is "not sailing" or is not powered by the wind. I think anyone that saw this vehicle moving downwind faster than the wind on a static road would agree it is being powered by the wind. But as I've said, that's strictly a matter of reference frame. If you're working from the reference frame of the wind you are perfectly justified to call this vehicle "road powered" if you like.

Finally, I know you think my explanation is overly complicated. But I assure you the operation of this cart is aerodynamically identical to the ice-boat on a downwind tack. I understand your desire to simplify it, but your approach simplifies it beyond the point of describing how it can actually work.

...as per Goodman who is one of those that have built one.

Initially, I independently concieved of this design, based entirely on the ice-boat analogy. You just wrap the ice-boat's downwind tack in a spiral and you realize this can be done directly downwind with a prop on a cart. True I was not the first to conceive of such a design. I think Bauer probably gets the credit for that. True Goodman built one - and I haven't. But I don't know whether he conceived of in independently or copied Bauer. Finally, I'm pretty confident that Goodman and I are on the same page since I've discussed this with him many times. Please don't be so sure he would agree with your simplifications. You're reading far too much into his words.

 

[spork]

By the way, your diagram attachment still shows as a red x, so apparently it's not been uploaded by physics forums yet. You might want to use an alternate host in the meantime.

Here you go:

http://www.putfile.com/pic/8419299

 

[ThinAirDesign]

While the discussion about sailing is interesting, most are missing one vital point. The contraption it's referred to isn't powered by wind, but by the wheels, which is clearly described by the builder...

Trond:

First, the sailing discussion simply serves as a starting point for DWFTTW (or DDWFTTW). Most people are like Topher -- they absolutely believe that a sailing craft would have to be over-unity to beat the balloon. Once the fallacy of this position can be demonstrated the discussions regarding the cart generally get much more interesting and productive. I'm not saying that everyone then believes the cart works -- far from it ... I'm saying that at least then they can look at the cart without saying "if it goes faster than the wind it's automatically over-unit".

Second, your statement saying the cart "isn't powered by the wind, but by the wheels, which is clearly described by the builder" is absolutely wrong and a misrepresentation/misunderstanding of anything Jack may have said. Jack built the cart *specifically* to prove that a device powered *only* by the wind could go, steady state, directly downwind faster than the wind.

Remember, Jack just built a smaller version of a long existing design (Baur cart) and like all of us, even a builder can fall prey to a use of words that is less than perfect. There is no question the cart is wind powered and no question that the builder knows this.

JB

 

[Trond]

How come this thing will advance on a threadmill without any wind present?

 

[schroder]

Close, but you're off in a subtle but critical way. There is no feedback to the wheels in the sense that I think you mean. The wheels will alway be a source of drag on the treadmill. However the prop is pushing the car forward by an amount greater than the drag the wheels provide.



That would be nice - but no. The treadmill provides all the energy in this case (just as the wind provides the energy in the case of operating on a stationary surface).





I think I can put you in a nice one that will fit your needs and budget.

I see. So the treadmill is exerting a force on the cart which is trying to accelerate it backwards. The restraint prevents this, so the turning wheels transmit force to the prop which then tries to accelerate the cart forward. And, the force of the prop on the cart is greater than the force of the treadmill on the wheels, so it moves forward. So what you are describing is a force multiplier. A simple lever is a force multiplier but the output force cannot be used to drive the input! What you are really describing here is a perpetual motion machine and it is an elaborate hoax. I am surprised that this thread has been allowed to continue for as long as it has.

 

[ThinAirDesign]

How come this thing will advance on a threadmill without any wind present?

To the sail (propellor), there's as much wind present on the treadmill as on the street.

This is only meant in a constructive manner Trond, but the fact that you asked the above question is a perfect demonstration of your lack of understanding of how the cart works.

You insist on attempting to explain it to others and you are critical of Sporks explanations, but in the end you will have little success if you continue to believe you understand something that by your own questions you demonstrate that you do not.

In operation, the spinning sail/propellor sees the *exact* same wind and *exact* same forces on the treadmill as on the street -- the entire cart can't differentiate between the two (frames of reference are like that after all).

JB

 

[Topher925]

I am surprised that this thread has been allowed to continue for as long as it has.

You and me both, schroder.

 

[Trond]

To the sail (propellor), there's as much wind present on the treadmill as on the street.

Please elaborate that. Are you saying that the wind the propeller feels because it's turned by the wheels is making the propeller turn faster?

 

[ThinAirDesign]

...so the turning wheels transmit force to the prop which then tries to accelerate the cart forward.

You've got it half right.

During initial startup, something *does* have to get the sail moving/spinning. On the street, it either the drag of the entire device slowly being 'blown' down with the wind or simple chassis push. On the treadmill, as you point out, it's the restraint 'pushing' the chassis while the wheels spin up the prop.

The half you are missing is that once the sail begins to 'see' apparent wind at the appropriate angle, it generates thrust (just as a sailboat sail does) and it is this thrust that pulls the cart off the treadmill restraint. Again, the gearing between the wheels and the sail/prop only serves to set the tack angle of the sails.

JB

 

[Topher925]

Also, I have constructed a vector diagram and analyzed it as I have asked spork to do several times. Its pretty clear just by looking at the diagram itself that the thing will never have a downwind component faster than the wind. If I have time today, which I probably wont, I'll graph the resulting equation you get from the graph to make the point a little more clear. I'll get it all posted by the end of the weekend at the latest.

 

[ThinAirDesign]

You and me both, schroder.

Topher -- what, you've got no response to the land sailors beating the cr** out of the balloon?

JB

 

[ThinAirDesign]

Please elaborate that. Are you saying that the wind the propeller feels because it's turned by the wheels is making the propeller turn faster?

Trond, I will get back to you soon -- beginning my commute.

JB

 

[spork]

How come this thing will advance on a threadmill without any wind present?

I've said many times now that it doesn't matter whether the road moves or the air moves. This is the whole business of inertial reference frames. Einstein tells us that they're identical. I've explained to you that an airplane and pilot cannot detect any difference between flying upwind and downwind. You really do need to gain an understanding of what an inertial frame is.

What you are really describing here is a perpetual motion machine and it is an elaborate hoax.

Seems like you should accept that $100K bet. Sounds like easy money since even I know a perpetual motion machine is impossible.

Also, I have constructed a vector diagram and analyzed it as I have asked spork to do several times.

I guess you haven't seen the diagram I posted on both this and the other forum. I guess you haven't seen my explanation detailing every aspect of the diagram.

Its pretty clear just by looking at the diagram itself that the thing will never have a downwind component faster than the wind.

Either your diagram is wrong, you're interpretting it wrong, or both.

If I have time today, which I probably wont, I'll graph the resulting equation you get from the graph to make the point a little more clear.

Forget the "resulting equation" - show us your diagram.

 

[Topher925]

Topher -- what, you've got no response to the land sailors beating the cr** out of the balloon?

JB

Response to what? All you have done is just made some claim with no real evidence or any type of analysis to support it. What am I suppose to respond to?

 

[spork]

Response to what? All you have done is just made some claim with no real evidence or any type of analysis to support it. What am I suppose to respond to?

I imagine he'd like to hear what you have to say to the land sailors that he quoted. The ones who say they actually do this, and that it's easy even for a relatively low performance land yacht.

By the way - you want analysis? I've posted a vector analysis showing exactly why this is possible and how it works. Given that I have an M.S in aero engineering I think that qualifies as "analysis". And I assure you it's absolutely correct.

Personally I find GPS data pretty compelling as well.

 

[ThinAirDesign]

Response to what? All you have done is just made some claim with no real evidence or any type of analysis to support it. What am I suppose to respond to?

Yeah, the President and multiple members of the national sanctioning body for land yacht races stating unanimously that they exceed 1.0 vmg by multiples -- that's "no real evidence".[sarcasm off]

These land sailors providing L/D and other performance data such as:

Vb = Vt * sin(gamma - beta) /sin(beta)
Vb: yacht speed
Vt: true wind speed
gamma: course sailed relative to true wind (gamma = 0 is straight
into the wind)
beta: apparent wind angle, measured between course over the ground
and the apparent wind vector

The closer the yacht can sail to the apparent wind (small beta), the
faster it goes. A typical beta for an efficient landyacht is on the
order of 14 degrees, which yields a maximum boat-speed ratio of 4
times the true wind, and a maximum Vmg downwind of 2.5 times the true
wind.

No analysis there is there.


Look, it's fine to disagree with these guys, but simply saying "there's nothing to respond to" rather than discuss it with me (or better yet, with the guys who do it over on their forum) is just a major copout.

You want to talk about "real evidence" ... How much more *real* does evidence get than guys in wind powered craft with VMGs of over 1.0?

Do you think these guys are lying to us or mistaken when they say that on a 45degree downwind tack and in a 15mph wind, they can achieve 22mph over the ground? This isn't some fuzzy line for them that they may or may not be crossing --- they claim to be achieving multiples of this. Kinda hard to confuse 22mph and 66mph.

So, are they lying? Can they not tell the difference between 22 and 66mph? Are they in on some sinister plot to keep this thread from being dumped to GD? Or perhaps could it be that you have just missed something?

Think about it, and yes ... a thoughful response would be nice. You, the OP asked a question. I managed to dig up some interesting evidence. Seems the perfect place for interaction.

JB

 

[ThinAirDesign]

Please elaborate that. Are you saying that the wind the propeller feels because it's turned by the wheels is making the propeller turn faster?

I'm saying that the wind the propeller feels creates *thrust* -- whether that thrust then makes the propeller turn faster depends on whether the total drag of the device at that moment is less than the generated thrust -- it of course can't spend more than it earns.

JB

 

[ThinAirDesign]

This confusion people have about the wheels "driving" the propeller really would make more sense if they understood the way the gearing is utilized -- exactly the same way that the skates on an ice-boat are utililized.

The skates on an ice-boat constrain the path of the sail to some angle relative to the true wind (say 45 degrees). If you are in the middle of a downwind 45 degree tack (or reach) and suddenly the skates become castors, the path of the sail reverts to straight downwind, relative wind is lost and thus thrust is lost. Soon, the vehicle slows to something less than the speed of the true wind.

The pullies and belts between the prop and the wheels provide the same constraint as the skates to -- they ensure that the sail (prop) moves through the air at the exact same angle as the ice-boat sail. Cut the belt and you get the same result as the castoring skates -- the path of the sail (prop) is no longer 45 degrees, there is no more relative wind and thus thrust is lost. Soon, the vehicle slows to something less than the speed of the true wind.

I know some of us keep saying almost the same thing over and over, but those that resist the sailing comparisons are losing out *precisely because* the cart is nothing more than two sailing devices (ice-boats or land-yachts or sailboat - take your pick) mounted on a chassis. It is only the chassis that is going *directly* downwind, while the two sailing devices continue merrily on their 45 degree, relative wind creating, thrust generating way.

JB

 

[Trond]

The half you are missing is that once the sail begins to 'see' apparent wind at the appropriate angle, it generates thrust (just as a sailboat sail does) and it is this thrust that pulls the cart off the treadmill restraint. Again, the gearing between the wheels and the sail/prop only serves to set the tack angle of the sails.

So the propeller turns solely by itself as soon as it starts turning, is that what you are saying? (Y/N?)

Let's keep with the cart on the treadmill here and let's assume it's held back perhaps by a tensiometer, no wind. What's making the propeller spin?
The apparent wind created by it's own rotation, is that what you are claiming? (Y/N)

 

[LURCH]

If it can go downwind faster than the wind, then it can sail when there is no wind. After all, that would just be going "faster" than the stationary air around it.

In fact, it must do exactly that at some point during the test run in the video. Somewhere along that run, the craft is stationary relative to the air around it, yet contiues to accelerate. This means that, as stated above, the craft can sail during a dead-calm. This amounts to a sailing craft that needs no energy input to provide propulsion.