Going downwind faster than the wind

[Cybermortis]

Some couple questions to ponder:

1) How could it be faked?  (bicycle pulling it with monofilament fishing line).


--Mycroft

Why bother with towing it? Just put in on a slope and let gravity do all the hard work.

[RogueSpidor]

And if the wheels are connected to the propellors by a fixed gearing, and the vehicle moved faster as a result, which would in turn make the wheels turn faster, making the propellor turn faster, making the vehicle move faster and so on, up to a maximum velocity for the vehicle determined by a lot of other things...

...then at what point does the vehicle reach maximum velocity? For that matter, a mechanical linkage works both ways. In other words, the friction and work force done by the propellor will have a slowing effect on the wheels which, taking into account conservation of energy, will counter any increase in speed the vehicle might have surplus to the wind velocity.

Assuming it's not going downhill, or some other energy source aside from the wind is not at play here, and taking into account friction which will be stronger on the vehicle than the wind in the best of circumstances (solid of the vehicle acting against the fluid properties of the air creating friction against air, and that's only assuming it starts moving faster than the wind, not to mention ground contact and bearing friction), then I don't see how this is possible. It's more than counter-intuitive; it violates the laws of physics because it claims to use more energy than the amount put into it, similarly to a perpetual motion device. It's kind of like saying you can sit in a boat with a sail, point a fan at it, and make the boat move forward.

Don't bother trying it. That only works in cartoons.

[theSpecialist]

I think we've all seen sail-boarding --

They use what looks like a small paraglider's chute and a surfboard of some kind.  The parachute-like wing leans away with the wind from the surfer and at times looks like it gets to within parallel to the water.  Anyway, the surfer is propelled at great speeds with this rig in a tacking-like action, and I wonder if in fact he/she exceedes the 'ground speed' of the wind with this method?

Seems to me that this is a vector analysis problem?

[RoofingGuy]

Seems to me that this is a vector analysis problem?

That it is.  Another poster did provide a mathematical treatment over at the Other Place.

Basically if you accept that a sailboat's direct downwind component can exceed the wind when on a tack, if you accept that an iceboat's direct downwind component can exceed the wind when on a tack, and if you accept that sails can extract energy no matter what's moving and what's stationary, as long as there's a difference, then there's no reason to see this as being against all the rules of physics.

The major trick (or major hurdle), is grasping how the prop tips can be on a continual tack at an angle to the wind, while the cart moves parallel to the wind.

[Watcher56]

Over there, one of the test methods proposed was to place the unit on a treadmill with a backstop in a protected area (no wind) and start the treadmill.  At some point, the unit is supposed to leave the backstop and move forward on the treadmill.  It is supposed to move to the front of the treadmill and stay there. 

I just can't see this happening.  I can possibly see it momentarily bouncing off the backstop, but as soon as it leaves the backstop it starts loosing energy and will quickly return to the rear. 

Alternately, if the unit can accelerate downwind and then continue to accelerate upwind, that would imply it can start moving in a forward direction with either a tailwind or headwind.  Again, I just do not see that happening.

For the unit to do either of the above smacks to me of over unity.

Over there, they have been discussing building a *working* model for quite some time now.  So far, no model. 

[spork]

Wow, quite a bit of momentum seems to have developed here.  I'll try to respond one by one.

>>It would be better to provide information as to how this is meant to work from more than one source. From that Mythbusters should be able to design and build their own version without copying someone else's design.

Will do.  Not in this post, because I'm going to try and catch up.  But I will describe how it works through every analogy I can muster - as well as through the aero and mathematics as necessary.

>>The problem with using existing blueprints is that it could be construed as advertising


But there's nothing to advertise.  No one is selling anything - or even looking for recognition.  Nevertheless, your point is taken.  I will provide all the information necessary for the Mythbusters to develop any number of different designs.

>> I am not accusing you of attempting to advertise this in any shape or form, I'm simply noting what it could appear to be. Please do not take offence).

None taken.

>>(Tip; the 'quote' button is on the bottom of the two rows of white buttons...


Thanks.  Sometimes I use the quote feature, and sometimes I just do this.  If this is not approved, let me know.

>> Once you are out ahead of the wind coming up from behind, aren't you now subject to the relative wind you have coming at you from the front at this point?

When you've exceeded the wind speed, the cart itself will feel a direct headwind.  The prop will not however.  The tips of the prop will feel exactly the same forces that a sail would feel on a sailboat on a 45 degree downwind tack at twice the winds speed (as an example).  I'll go into this in much more detail as I describe in my next post how this works.

>> I'd assume that you'd get up to a higher than wind speed, then start to slow down until the wind catches you up and you start to speed up again.

Nope.  In a steady wind, you can beat the wind steady-state - all day long.

>> Has this ever been tested over any real distance, say a mile or more? Or has this only been done over relatively short distances?

I don't know how far Bauer went in his cart.  I do know Jack has put his on a treadmill and achieved steady-state operation faster than the wind.  I will show that it's possible in the coming posts.

>> Yeah, I could see it, if the wind suddenly died away that is.  That way you really would be going faster than the wind that sent you.

Nope.  It really works steady-state.

>> So, it is not supposed to outrun the wind in question - only obtain a faster rate -- regardless which direction it is going relative to the wind supplied, then.

Nope.  The cart will truly go directly downwind steady-state, faster than the wind.

>> It "clicked" for me when a sailboat comparison was made.

Yes, the sailboat analogy is a very good one - but not necessarily intuitive for a lot of folks.

>> There is one problem in your analogy - a road doesn't move, and therefore doesn't provide anything propulsive.

It's not the moving water that actually propels the boat in the analogy.  The boat extracts it's energy from the interface between the air and water.  As long as they move relative to one another there is energy to extract.  The same is true of the wind passing over the road.

>> Indeed a road surface would only provide friction to slow you down (Unless you go downhill anyway).

The road only need provide a constraint for the wheels to work against to extract the energry from the air that's moving relative to the road surface.  The road can be dead level, and the vehicle will go straight downwind faster than the wind.

>>I still have to question if this really would result in faster than wind speeds over any distance.

It can do so indefinitely.

>> Regarding how this thing works, you got me stumped

I'm really not trying to be coy.  Just trying to catch up with the posts. I will explain it in ways that will hopefully make it clear to all.

>> What's the principles involved and would it necessarily require the blades to be attached to the wheels?

Again, I'll try and describe all of this.  But basically, you need to have a clever way to extract the energy of the relative motion between road and wind.  Without this connection the craft would indeed see no force once it reached wind speed.

[spork]

Sorry for the continuation - and such a long post.  But I really do want to respond to all the concerns.  And I still haven't gone into much detail on how it works.  But I'll follow up this post with the beginnings of some better descriptions.



>>
And if the wheels are connected to the propellors by a fixed gearing, and the vehicle moved faster as a result, which would in turn make the wheels turn faster, making the propellor turn faster, making the vehicle move faster and so on, up to a maximum velocity for the vehicle determined by a lot of other things...
<<

That's a common criticism.  But there are plenty of things that have what appears to be positive feedback, but still have a "top speed" due primarily to non-linearities.  In my vector diagram you can see that an ice-yacht can make a VMG (velocity made good) directly downwind faster than the wind.  But it doesn't speed up out of control forever.

>>then I don't see how this is possible.

That's why I'm here.  If you'll follow my upcoming posts with an open mind, I'm confident I can describe how this works without violating a single law of physics (we take the laws of physics very seriously where I come from - even though I have diplomatic immunity).

>> It's more than counter-intuitive; it violates the laws of physics because it claims to use more energy than the amount put into it,

It makes no such claim.  It has innefficiencies like any other man made device.  But is has plenty of energy available to it to do exactly as it claims.

It's important to note that in it's wake it leaves a path of slower moving wind relative to the road.  You can't think of the vehicle as a closed system.

>> similarly to a perpetual motion device.

It's no more of a perpetual motion device than a sailboat is.  Stop the wind and the cart will coast to a stop as well.

>>  It's kind of like saying you can sit in a boat with a sail, point a fan at it, and make the boat move forward.

Believe it or not, you can.  That's how a thrust reverser on a jet engine works.  Of course it requires a sail of carefully defined shape.  But that's another thread entirely.

>> Don't bother trying it. That only works in cartoons.

This one works in real life.

>>
I think we've all seen sail-boarding --
They use what looks like a small paraglider's chute and a surfboard of some kind.
<<

I think you mean kiteboarding or kitesurfing.  I've been an avid kitesurfer for 7 years.  I've got hundreds of days on the water now.

>> Seems to me that this is a vector analysis problem?

It is - and a reasonably simple one at that.  My vector diagram on the first page describes exactly how the ice-yachts can tack downwind faster than the wind.  Perhaps I have to add some words to describe the vector plot.

>>
Over there, one of the test methods proposed was to place the unit on a treadmill with a backstop in a protected area (no wind) and start the treadmill.  At some point, the unit is supposed to leave the backstop and move forward on the treadmill.  It is supposed to move to the front of the treadmill and stay there. 
<<

Yup.  Jack Goodman has done exactly that.

>> I just can't see this happening.

If the Mythbusters take an interest in this you will.

>>For the unit to do either of the above smacks to me of over unity.

It sure sounds that way.  That's what makes it a great brainteaser and a great myth.  It seems impossible, but it's not.  It's not over-unity, and it violates no laws of physics.

>>Over there, they have been discussing building a *working* model for quite some time now.  So far, no model. 

I've offered very many times that as soon as the Mythbusters show an interest I'll be happy to build a model.  Jack Goodman has built a model.  And there's an open invitation for anyone that likes to see it in action.  Jack lives in the Tampa Bay area of FL.

Stay tuned for some better descriptions of the physics.

[spork]

O.K. I know we're not supposed to make several posts back to back, but I really wanted to respond to all questions and concerns.  Now I want to begin to offer some better descriptions.  Different people are likely to respond to different descriptions.  So please forgive the sheer magnitude of text.


Approach #1:

Consider a yo-yo sitting on a level table so that it can roll either direction.  We wind the string around its axle so that it exits below the axle at about half the radius of the yo-yo.  If I pull steadily on this string parallel to the table, the yo-yo will do something that's counter-intuitive to a lot of folks - it will wind up the string and hit my fingers.  Yes, it will go faster than the force that's pulling it.  So if I replace my fingers with a parachute, the yo-yo will go straight downwind faster than the wind - pulled by this parachute.  But you might say "that's cheating - the parachute goes downwind slower than the wind - and it all comes to an end when the yo-yo reaches it".  To this I'd say - you're right.  It's just an analogy to help get past one of the sticky points.  The reason this works, is that a rolling wheel doesn't rotate about it's axle, but rather about the contact patch on the ground.  The torque is exactly the opposite of what you might expect.  It's easy to say this is nonsense - but it's just as easy to try it on your kitchen table - as I have.

So now, what if I want to get around some of the shortcomings?  Let's consider a bicycle and to something very similar.  Grab one of the spokes directly below the hub - about mid-way between the hub and the ground.  Push that spoke toward the front of the bike.  You'll find the bike will move exactly twice as fast as your hand on the spoke moves - in the same direction. 

So now lets replace your hand with fan-fold sails on each of the spokes.  When the spoke is near the bottom, the sails deploy.  When the spoke begins to come around, the sail is retracted.  This way you'll always have the wind pushing on the middle of the bottom spoke.  And you'll have a vehicle that goes directly downwind faster than the wind.  I don't pretend this is a particularly practical approach. But it hopefully illustrates the principle.  This type of sail only acts as a bluff body (as with a sailboat running directly downwind "wing and wing").  On our cart the prop acts exactly like the sail of the ice-boat on a 45 degree downwind tack - in other words it acts as an efficient airfoil.  This is far more effective than a bluff body, and is one of the reasons the cart is more practical.

I'm sure there will be questions about this, and I'll be happy to address them.


Approch #2:

Put two ice boats inside a huge rectangular tube-steel frame such that they can always be on opposite tacks as they push the frame along.  The frame will move downwind faster than the wind, and the C.G. of the system will do the same.  If you doubt an ice-boat is capable of this I simply have to do a better job of describing my vector analysis - and provide examples that show ice-boat racers do it all the time.  I'll be happy to do both.

But I've typed too much now.  I'll be back to offer more explanation and a number of other approaches.

[RoofingGuy]

I'd just like to add, the downwind cart is no more "over unity" than a sailboat tacking into a relative wind.

If you're drifting along with the current, you can in fact tack into the relative wind the current is creating by moving the boat through the still air, and you can have your downcurrent vector exceed the velocity of the current itself.  Any (or at least most) sailing site will tell you this is fact, and some offer math to back it up.

But, if you want to take the "over unity" viewpoint, what you're really doing is using the current to create wind and then sailing using the created wind.  It is a "grab your ankles and pull yourself into the air"-type of concept, but it is also easily demonstrable.

If the cart tacking into the relative difference between the ground and the air is "over unity", then so is every keeled sailboat that's ever tacked to beat a current.

[Cybermortis]

>>(Tip; the 'quote' button is on the bottom of the two rows of white buttons...


Thanks.  Sometimes I use the quote feature, and sometimes I just do this.  If this is not approved, let me know.

Could you please use the quote feature. Its difficult to work out where the quotes end and you begin and therefore just adding to any confusion. It also doesn't look very professional - as I said before there are higher standards on here than on Disco.

>>It would be better to provide information as to how this is meant to work from more than one source. From that Mythbusters should be able to design and build their own version without copying someone else's design.

Will do.  Not in this post, because I'm going to try and catch up.  But I will describe how it works through every analogy I can muster - as well as through the aero and mathematics as necessary.

Links to websights that have the information would be helpful - although several have been given above. There is no problem with providing a link to the sight that deals with what is shown in the video, but this could do with extra information that at least deals with this phenomena or the physics behind it in some way. The MB researchers like to have as much information as possible from as many sources as possible.

Don't worry about using mathematical equations on here. Unlike Disco the members here tend on the whole to be smarter and better educated so fewer of them will have problems. (Of course the smart Disco members are here as well )  

>> I'd assume that you'd get up to a higher than wind speed, then start to slow down until the wind catches you up and you start to speed up again.

Nope.  In a steady wind, you can beat the wind steady-state - all day long.

>> Has this ever been tested over any real distance, say a mile or more? Or has this only been done over relatively short distances?

I don't know how far Bauer went in his cart.  I do know Jack has put his on a treadmill and achieved steady-state operation faster than the wind.  I will show that it's possible in the coming posts.

How can you say that it will be able to go faster than the wind all day long when you also say no one seems to have tried it over any distance?

Yes, you could argue that the maths say that, but there can be a big difference between theory and practice. A prime example is the oft quoted one about scientists 'proving' that Bees can't fly...only to realise that they did the maths wrong.

>> There is one problem in your analogy - a road doesn't move, and therefore doesn't provide anything propulsive.

It's not the moving water that actually propels the boat in the analogy.  The boat extracts it's energy from the interface between the air and water.  As long as they move relative to one another there is energy to extract.  The same is true of the wind passing over the road.

Both water and roads have one thing in common - they provide friction/resistance for anything travelling over them. The difference is that if the water is moving its friction can add or subtract to a ships speed.

Roads don't move, so while they will provide friction/resistance they will never be able to add to the speed of anything moving on them.

Your calculations seem to be missing that the road surface will be providing a constant resistance to movement. Over short distances this may be marginal, but over longer distances it could add up and slowly reduce the energy in the system - this also ignores that any device fitted to the wheels and using the wheels for energy will only add to the loss of energy. This is why I asked what distance this device had been tested over.

>>I still have to question if this really would result in faster than wind speeds over any distance.

It can do so indefinitely.

Again, you can't say that since it hasn't been tested over any significant distance. If MB can make one of these things and get it to travel for a couple of miles under its own power alone, and going faster than the wind speed over that distance then you can say it...

...I take that back. You can't say it could carry on indefinitely as you ourself noted that if the wind goes the device stops working (I have to be really annoying at least once a day)

[spork]

Could you please use the quote feature. Its difficult to work out where the quotes end and you begin...

As you wish.

Links to websights that have the information would be helpful - although several have been given above. There is no problem with providing a link to the sight that deals with what is shown in the video, but this could do with extra information that at least deals with this phenomena or the physics behind it in some way. The MB researchers like to have as much information as possible from as many sources as possible.

I'm not sure exactly which websites to link.  I don't know of any that are dedicated specifically to the concept of going directly downwind faster than the wind.  I can provide links that discuss ice-boats tacking downwind faster than the wind.  I can also give ample proof myself in the form of math, analogies, and physical models.  Given that I have an M.S. in Aero, I suppose I could also create my own website on the topic with as much authority as the next guy.

How can you say that it will be able to go faster than the wind all day long when you also say no one seems to have tried it over any distance?

Two reasons... first the vector analysis shows that it's easily possible;  second, it has been done steady state.  That means it can do it until it wears out or the wind stops.

Yes, you could argue that the maths say that, but there can be a big difference between theory and practice.

In aero, very frequently the math is based on measured quanitities (such as L/D or the "lift to drag" ratio).  In such cases the math predicts behavior as accurately as we're able to measure these parameters.

A prime example is the oft quoted one about scientists 'proving' that Bees can't fly...only to realise that they did the maths wrong.

This is one of the most common misconceptions/mis-quotes in science (right up there with the head of the patent office saying everything has already been invented).  In fact scientists, being reasonably keen observers, have always known bees could fly.  They were only pointing out that some of their models of lift did not successfully extend to the realm of the bumble-bee.  Given what they've since learned about the mechanics of the flight of bumble-bees, they can now describe the aerodynamics involved quite accurately.  As an aside, interpretted more literally, thier work proved that a bumble-bee can't glide - which is true.  Even a bumble-bee falling at terminal velocity can't produce enough lift with its non-flapping wings to support its weight.  It will cease to accelerate only due to the drag of its body.

Roads don't move, so while they will provide friction/resistance they will never be able to add to the speed of anything moving on them.

Roads move if they're part of a treadmill, and Einstein tells us that any inertial reference frame is identical to any other.  So a moving road in still air is identical to a still road with moving air.  In any event, energy is a tricky thing.  It is also entirely dependent on the reference frame in which it's measured.  In point of fact the vehicle has the effect of slowing the wind relative to the road.  The energy taken out of that interface is used to propel the cart.

Your calculations seem to be missing that the road surface will be providing a constant resistance to movement.

It may seem that way, but it's not the case.  There are two critical elements - the L/D (lift to drag) of the sail, and the L/D of the keel (or the wheel and transmision).  In either case the vector diagram shows that with even moderate L/D's the cart can go directly downwind faster than the wind.

Over short distances this may be marginal, but over longer distances it could add up and slowly reduce the energy in the system

Nope, steady state means nothing is changing.  The drag doesn't increase with time unless the bearings are wearing out.

...this also ignores that any device fitted to the wheels and using the wheels for energy will only add to the loss of energy. This is why I asked what distance this device had been tested over.

Yes, the wheels add drag, and this turns the prop, and there are frictional losses througout.  But there is plenty of energy available at the wind/road interface to overcome those losses.  As to how far it's been tested, it's a steady state device.  I will be happy to demonstrate this to anyone's satisfaction if there is any interest in doing a segment on this concept.

Again, you can't say that since it hasn't been tested over any significant distance. If MB can make one of these things and get it to travel for a couple of miles under its own power alone, and going faster than the wind speed over that distance then you can say it...

I can say it because the math, the analogies, and the vector analysis proves it, and because it's been shown to work steady-state on a treadmill.   I can also say it because I'm willing to prove it with a physical cart if the producers have any interest at all in it.

But let's first make sure we're debating the right thing.  Do you agree that an ice-boat can maintain a 45-degree downwind tack such that its downwind velocity component is greater than wind-speed?  If so, I think it'll be a relatively short hop to show how we can use this exact same principle with the prop-cart.  If not, it's my job to provide ample evidence that they do it all the time.

[RogueSpidor]

This isn't a thrust reverser, the fan has an equal and opposite reaction, you can't propel a sailboat with a fan that's mounted to the sailboat.

Sorry. After that, I just can't consider this any more. If you're right, more power to you. But you need more than "yes you can" to convince me.

[spork]

This isn't a thrust reverser, the fan has an equal and opposite reaction, you can't propel a sailboat with a fan that's mounted to the sailboat.

I can easily shape a sail so it will do exactly as a thrust reverser does.  If you don't think that any longer conforms to the definition of a sail, that's your choice.

Sorry. After that, I just can't consider this any more.

I don't follow your logic, but you're certainly not compelled to consider it.

...you need more than "yes you can" to convince me.

Fair enough.  How about the vector analysis?  How about the example of ice-boats tacking downwind faster than the wind all the time?  How about the yo-yo, the bicycle, or the description that any boat tacking into the wind is doing exactly the same thing seen from the inertial reference frame of the water?  That's already a fair bit better than "yes you can", and I'm willing to provide as much as it takes - for anyone that's stil interested.  If you're not interested, and don't want to understand the math, models, and analogies - so be it.

[RoofingGuy]

you can't propel a sailboat with a fan that's mounted to the sailboat

Sure you can.  It's even been demonstrated small-scale, AFAIK.

With basic physics, everyone can agree that the fan pushes the boat backwards while pushing the air forwards, and there is a big net zero there.

But the sail also deflects the forwards-moving air back backwards.  A change in velocity means the sail must be putting a rearwards force on the air.  Therefore the air is putting a net forwards force on the sail.

Now, in most cases, except for a specially designed sail to make the most of this effect, you'd be better off turning the fan around and using it to push the boat, but physics does, in fact, say (and experimentation backs it up) that a fan into a sail will propel the boat forwards - albeit slowly.  As sporks points out, this is the exact same principle as thrust reversers on a jet engine.  You re-direct the airflow, and get a net force.

[Watcher56]

Over there, one of the test methods proposed was to place the unit on a treadmill with a backstop in a protected area (no wind) and start the treadmill.  At some point, the unit is supposed to leave the backstop and move forward on the treadmill.  It is supposed to move to the front of the treadmill and stay there.

Yup.  Jack Goodman has done exactly that.

Do we have a a video of that test?  

I do find this 'myth' quite interesting and would like to see it done.  The math and examples would point to it working, but my intuition says it can't work and I must be missing something.