Sailing downwind faster than the wind

[ThinAirDesigns]

Like you I am still wondering where the energy comes from. The explanations given haven't "hit home" just yet so I'm "sleeping" on them for a few days until they do.

Hi Mee_n_Mac, don't feel bad about have to stew over it -- it was quite a mind-bender for most of us.

In the meanwhile I'm back to looking at the forces involved and trying to understand how the wind plays into them. The drag on the body must be negligible and reduces to zero when traveling downwind @ windspeed. So pushstart the vehicle to this point (in your mind) and then release it. What happens then and what are the forces involved ?

Your "analyize at exactly windspeed" method is an excellent one and as you noted allows one to eliminate bluff drag from the chassis and consider only the path of the airfoils relative to the wind.

Here is an image that might help you do that -- it's a plot showing how the spinning airfoils is moving at an angle to the wind just like the sail of the reaching boat and by 'unwrapping' it's path it's easier for some to visualize.

If you want to think more about the direct relationship between the sail on the boat and the sail on the cart, consider this: Adjusting the gear ratio between the wheels and the propeller is the same as changing the course of the boat (it alters the angled path the airfoils takes through the air) and adjusting the pitch of the propeller is the same as adjusting the angle of the sail to the boat (changing it's angle of attack to the air).

JB

 

[Mee_n_Mac]

Like you I am still wondering where the energy comes from.
...
Why does this only work with some downwind and not zero (down)wind ?

As already stated: The energy comes from reducing the true wind speed (air vs. ground). If that true wind speed is zero, there is no energy to harvest. Seen from the cart, the air must move backwards slower than the ground. So the cart can apply a higher force to the air, than it applies to the ground. It's like a gear box:
- lower force & higher speed on one side (ground),
- higher force & lower speed on the other (air).

Thanks ! I think I understand. I've now got to revisit the MIT analyses. But to put it simply, if I have to turn a crank to turn the prop, I get more thrust from the prop (for a given input torque) when the wind is blowing "downwind" vs when there's no wind or it's blowing "upwind". This "additional thrust" is what pushes the balance in favor of the prop vs the retarding forces. Have I got that right ? If so then it's a simple experiment that anyone could do that's easily extendable to the vehicle dynamics.

 

[Mee_n_Mac]

Like you I am still wondering where the energy comes from. The explanations given haven't "hit home" just yet so I'm "sleeping" on them for a few days until they do.

Hi Mee_n_Mac, don't feel bad about have to stew over it -- it was quite a mind-bender for most of us.

JB

A little stewing is good for the brain.

If I've stated it correctly in my post above, then the next thing to understand is why the prop acts thusly and your diagram should help there. Like most people I saw the demo and thought the prop was acting as a turbine and driving the wheels. Once you get past this and ask, "how does it work", then you begin to grasp what's happening. It's not immediately obvious and not being a "blow boater" ( ) the sailing analogies don't help me as much as they might others.

So what's next ? Build a vehicle with a variable pitch "prop" so you could reverse the energy flow and "sail" directly upwind (as well as downwind and presumably all points between) ? :ugeek:

 

[eyytee]

But to put it simply, if I have to turn a crank to turn the prop, I get more thrust from the prop (for a given input torque) when the wind is blowing "downwind" vs when there's no wind or it's blowing "upwind". This "additional thrust" is what pushes the balance in favor of the prop vs the retarding forces.

Yes, the true wind (air movement relative to ground) creates "better conditions" for the prop to operate in. So the prop can produce more thrust with the same shaft power (torque * angular velocity), than without that true wind.

It's not immediately obvious and not being a "blow boater" ( ) the sailing analogies don't help me as much as they might others.

Sailing with a downwind VMG greater than windspeed is itself counter-intuitive and seems impossible to some. The analogy to a squeezed wedge is helpful and works also for the prop-cart:
[youtube]http://www.youtube.com/watch?v=H_OKNr120t4[/youtube]
[youtube]http://www.youtube.com/watch?v=zPFzHoubQzg[/youtube]

 

[ThinAirDesigns]

So what's next ? Build a vehicle with a variable pitch "prop" so you could reverse the energy flow and "sail" directly upwind (as well as downwind and presumably all points between) ? :ugeek:

You are correct that with some modifications a vehicle like this can sail all points. Interestingly enough, it doesn't even take a variable pitch prop to enable it to sail straight up wind - merely change the gear ratio between the wheels and the propeller (or even just put smaller drive wheels on it which effectively also changes this ratio) and it will go backwards into the wind rather than downwind.

The craft you see in the video are optimized however for only DDWFTTW. While a propeller can act as a turbine, it's a rather inefficient one because the camber is on the wrong side of the airflow. Same thing with a designed turbine blade acting as a propeller -- it can, but not particularly well. If one wants to sail all points, compromises would need to be made.

JB

 

[Mee_n_Mac]

So what's next ? Build a vehicle with a variable pitch "prop" so you could reverse the energy flow and "sail" directly upwind (as well as downwind and presumably all points between) ? :ugeek:

You are correct that with some modifications a vehicle like this can sail all points. Interestingly enough, it doesn't even take a variable pitch prop to enable it to sail straight up wind - merely change the gear ratio between the wheels and the propeller (or even just put smaller drive wheels on it which effectively also changes this ratio) and it will go backwards into the wind rather than downwind.

The craft you see in the video are optimized however for only DDWFTTW. While a propeller can act as a turbine, it's a rather inefficient one because the camber is on the wrong side of the airflow. Same thing with a designed turbine blade acting as a propeller -- it can, but not particularly well. If one wants to sail all points, compromises would need to be made.

JB

It would make an interesting design exercise; whether it's better to have a compromised "prop-bine" or carry one of each, optimized for their respective tasks and eat the drag of the inoperative one (presumably feathered). But now I've got the cart so far in front of the horse the latter has stopped "pulling" and started grazing. Let me get back to understanding the details of how the existing machine works.

 

[Shpaget]

Ok, I've done some thinking and I've come from completely dismissing the theory to being quite intrigued.
Some of my thought experiments seem to work (in my mind) so I'm definitely going to try to build something when I find time...
I'm currently too busy, but hopefully in next couple of weeks I'll have some spare time.

 

[Couerl]

Hi, the reason any given craft (say the Flying Cloud, fastest clipper ship ever made circa 1800's) can move faster than the wind behind (or in opposition for that matter) is simply because of the aerodynamic properties of wind resistance/drag over an airfoil (sail, wing, propeller) combined with the angular momentum of the force (wind, treadmill, motor, whatever..) driving it the same way as lift! The wind speed itself (like the treadmill speed in the videos) is not representative of the entire net-force of energy available in the system to begin with and so of-course anything can go faster than either of them. I don't see anything in these videos or the mumbo-jumbo accompanying it that violates any fundamental laws in physics known since at least the days of Newton. Was there something that was supposed to? Are we insinuating that more kinetic energy can be derived from this system than actually exists to begin with or merely pointing out that the existing potential energy in the system can be leveraged? A wheel with a frictionless bearing on a level treadmill will simply spin in place as the treadmill surface moves beneath it (action=opposite reaction) and if you add a prop to that system and the resistance is lower in front than it is in back it will move forward and it will even move uphill. Now try the exact same experiment on a tilted treadmill with a 5-ton tricycle and see which direction it goes, up or through the floor and in to your basement? :lol:

It's all about available net energy and resistance and not some kind of hocus-pocus..

 

[eyytee]

I don't see anything in these videos or the mumbo-jumbo accompanying it that violates any fundamental laws in physics known since at least the days of Newton.

Good for you. But many people (including physics professors) disagree and claim what you see there violates these laws.

 

[ThinAirDesigns]

Ok, I've done some thinking and I've come from completely dismissing the theory to being quite intrigued.
Some of my thought experiments seem to work (in my mind) so I'm definitely going to try to build something when I find time...
I'm currently too busy, but hopefully in next couple of weeks I'll have some spare time.

Very cool Shpaget -- always fun to interact with folks who go out and test stuff. Keep us posted and feel free to ask questions if you feel the need. I've built more than my share of these craft and can help with what's critical and what's not.

Best wishes.

JB

 

[ThinAirDesigns]

I don't see anything in these videos or the mumbo-jumbo accompanying it that violates any fundamental laws in physics known since at least the days of Newton. Was there something that was supposed to?

No laws were violated in the making of these vehicles or videos.

It's the brainteaser aspect of DDWFTTW that makes it interesting for most -- for every person that recognizes that it's merely a different *looking* application of principles in use for centuries, there are scores who scream "IT VIOLATES EVERY LAW OF PHYSICS!".

JB

 

[Couerl]

Well those guys are tards. :lol: :ugeek:

 

[spork]

Ok, I've done some thinking and I've come from completely dismissing the theory to being quite intrigued.
Some of my thought experiments seem to work (in my mind) so I'm definitely going to try to build something when I find time...
I'm currently too busy, but hopefully in next couple of weeks I'll have some spare time.

You should definitely try it. I posted a set of detailed build videos for exactly that reason:

http://www.youtube.com/watch?v=T-ArigMKhi4

http://www.youtube.com/watch?v=p0rhgop5wEM

http://www.youtube.com/watch?v=gSHNqrF93MU

 

[origin]

I have thought about this for a while and I think it is BS. There has been quite a bit of talk about sail boats and how that relates to this situation and I just don't think it actually does relate.

Lets look at a sailboat and how they work. They are quite simply a wind powered craft, that is they get there power from the wind - so if there is no wind there is no power. In the example of a boat going down wind, the wind fills the sail which imparts a force due to the delta pressure across the sail. The foce is a pressure so it is in lbs/in2, and that times the surface area of the sail gives the total force.

Lets assume a 20 knt wind and going down wind. The boat begins to accelerate and this results in 2 things. One is that the friction on the hull begins to produce a force in the opposite direction of travel. The other thing is as the boat increases in speed the relative wind speed and therefore the force decreases. At 10 knts/hr the hull drag has increased significately and the force on the sail has decreased due to the smaller relative wind speed. At 13 or 14 knts/hr the force on the sail equals the hull drag and you go at a steady velocity. You can never exceed or even approach the speed of the wind because you have to overcome hull drag to maintain your velocity.

Now lets assume the same 20 knts wind blowing to the north and the sailboat moving to the north east. The force on the sail increases the speed of the boat by transfering the force to the hull and keel. The keel forces the boat to move to the north east. So if the boat is traveling at 10 knts/hr, it is only moving 5 knts/hr to the north so the relative wind speed to the sail is still 15 knts. So, assume that at 22 knts/hr the force on the sail equals the hull drag and you have reached a constant speed. It is kind of cool to go faster than the wind speed but the point is you are NOT going faster than the wind in the direction of travel, in fact the relative wind speed on the sail would still be 9 knts/hr.

This pretty obvious - a wind powered vehicle gets its power from the wind, if there is no wind, or no relative wind there is no power.

So I have a very simple question:

On the wind powered propeller contraption, once it exceeds the speed of the wind, where is the power coming from to cause it to accelerate?

 

[ThinAirDesigns]

I have thought about this for a while and I think it is BS. There has been quite a bit of talk about sail boats and how that relates to this situation and I just don't think it actually does relate.

You'll need to keep thinking to come to the correct answer, but OK.

Lets assume a 20 knt wind and going down wind. The boat begins to accelerate and this results in 2 things. One is that the friction on the hull begins to produce a force in the opposite direction of travel. The other thing is as the boat increases in speed the relative wind speed and therefore the force decreases.

That's fine for a spinnaker, but you'll notice that BMW Oracle didn't use a spinnaker on their downwind legs of the America's Cup. (there's a good reason ... it's slower that way)

At 10 knts/hr the hull drag has increased significately and the force on the sail has decreased due to the smaller relative wind speed. At 13 or 14 knts/hr the force on the sail equals the hull drag and you go at a steady velocity. You can never exceed or even approach the speed of the wind because you have to overcome hull drag to maintain your velocity.

Should we just believe you and your paragraph, or should we believe the readily available data from the latest America's Cup which show BWMO more than doubling the speed of the wind (VMG) while on the downwind leg?

You need to exit your little bubble and learn what's happening in the real world Origin -- it's enlightening.

JB

 

[ThinAirDesigns]

This pretty obvious - a wind powered vehicle gets its power from the wind, if there is no wind, or no relative wind there is no power.

You are 100% correct in the above statement -- stick to it (but there is a twist so be careful)

So I have a very simple question: In the wind powered propeller contraption, once it exceeds the speed of the wind, where is the power coming from to cause it to accelerate?

Here's the simple answer you seek: from the wind.

Now here's where you're going wrong in your understanding -- you're continuing to insist that there be wind relative to the *chassis* rather than think about the wind relative to the airfoil. Just like BMWOracle continues to have wind over it's airfoil when it gets above windspeed, so does the airfoil of the cart. Yes, there is no wind where the driver is sitting when it reaches windspeed, but there no power produces from were the driver sits so who cares? The fact that you, the driver, feel no wind at windspeed doesn't mean that the wind has stopped blowing.

It's as simple an answer as you will get (though it may not help you understand). The mistake is yours and it's a very common one. I don't mean any insult with that last statement ... it's just a fact that perhaps someday you'll come to realize.

Origin, I'll tell you what's going to happen soon if you keep up with http://www.fasterthanthewind.org:

That site is the project blog for a group from a reputable USA University (see Bios at site). This group (of which I'm a part) set out late last year to answer this question in a scientifically documented way. The money for this project came from Joby Energy (a wind energy research company) and Google.

NALSA (NALSA.org) is the ratifiying organization for wind powered land speed records -- they ratified the recent Greenbird record of 126mph (not ddw). Sometime in the next two or three months (we're entering our windy season) we will run this vehicle down a NALSA specified course and under their scrutiny and instrumentation we will set a record with this vehicle traveling something near 3 times the speed of the wind while it's travelling directly downwind. We know we can do this because we have already done this with them watching (to prove to them we could).

Perhaps when an outside organization whose business and expertise is in measuring and ratifying speed records for wind powered vehicle measures and verifies our claim, you will rething your "it's BS" position. If not -- well the loss is yours.

Best wishes.

JB

 

[origin]

Here's the simple answer you seek: from the wind.

Now here's where you're going wrong in your understanding -- you're continuing to insist that there be wind relative to the *chassis* rather than think about the wind relative to the airfoil.

This statement makes no sense to me. There is no wind at the chasis but there is wind at the airfoil (propeller), huh? How is that possible if the propeller and the chassis are moving at the same speed!

Just like BMWOracle continues to have wind over it's airfoil when it gets above windspeed, so does the airfoil of the cart.

No, it is not the same situation. When the boat is going faster than the wind speed, it is only because they are not directly down wind. There is in fact a relative wind, it is felt by the people on the boat and the sails - it is a real wind. If they were going down wind no matter how they trimed their sails or how much sail they had they could never even go as fast as the wind.

Yes, there is no wind where the driver is sitting when it reaches windspeed, but there no power produces from were the driver sits so who cares? The fact that you, the driver, feel no wind at windspeed doesn't mean that the wind has stopped blowing.

But it has stopped blowing relative to that frame of reference!

It's as simple an answer as you will get (though it may not help you understand). The mistake is yours and it's a very common one. I don't mean any insult with that last statement ... it's just a fact that perhaps someday you'll come to realize.

I am not insulted at all. But neither am I satisfied with the response. Saying it is just a fact is not very convincing. The simple question was where does the power come from after you have achieved a velocity equal to the wind, and what I got from your reply was: if there is a 20 mph wind and your contraption is going 20 mph then the contraption will feel a 0 mph wind speed except for the propeller which will continue to have a tail wind.

That sounds like utter nonsense to me.

Origin, I'll tell you what's going to happen soon if you keep up with http://www.fasterthanthewind.org:

That site is the project blog for a group from a reputable USA University (see Bios at site). This group (of which I'm a part) set out late last year to answer this question in a scientifically documented way. The money for this project came from Joby Energy (a wind energy research company) and Google.

NALSA (NALSA.org) is the ratifiying organization for wind powered land speed records -- they ratified the recent Greenbird record of 126mph (not ddw). Sometime in the next two or three months (we're entering our windy season) we will run this vehicle down a NALSA specified course and under their scrutiny and instrumentation we will set a record with this vehicle traveling something near 3 times the speed of the wind while it's travelling directly downwind. We know we can do this because we have already done this with them watching (to prove to them we could).

Perhaps when an outside organization whose business and expertise is in measuring and ratifying speed records for wind powered vehicle measures and verifies our claim, you will rething your "it's BS" position. If not -- well the loss is yours.

Best wishes.

JB

Well good luck on the run. I cannot fathom how this thing could possibly exceed the speed of the wind, but I could be wrong - wouldn't be the first or last time.

 

[Mee_n_Mac]

Here's the simple answer you seek: from the wind.

Now here's where you're going wrong in your understanding -- you're continuing to insist that there be wind relative to the *chassis* rather than think about the wind relative to the airfoil.

This statement makes no sense to me. There is no wind at the chasis but there is wind at the airfoil (propeller), huh? How is that possible if the propeller and the chassis are moving at the same speed!

Just like BMWOracle continues to have wind over it's airfoil when it gets above windspeed, so does the airfoil of the cart.

No, it is not the same situation. When the boat is going faster than the wind speed, it is only because they are not directly down wind. There is in fact a relative wind, it is felt by the people on the boat and the sails - it is a real wind. If they were going down wind no matter how they trimed their sails or how much sail they had they could never even go as fast as the wind.

Let me interject and thus see if I correctly understand the situation. When the vehicle is at "windspeed" there's no wind relative to the chassis but there's still wind relative to the propellor, it's rotating. Every point on the prop, except for the center, is both rotating and moving "downwind". So the question is how does the wind, relative to the prop, get harnessed ? I note that if I were a fly stuck somewhere on the prop, I'm not travelling directly downwind.

 

[eyytee]

This statement makes no sense to me. There is no wind at the chasis but there is wind at the airfoil (propeller), huh? How is that possible if the propeller and the chassis are moving at the same speed!

airfoil = propeller blade

When the boat is going faster than the wind speed, it is only because they are not directly down wind.

And neither are the propeller blades.

Did you miss this:

[youtube]http://www.youtube.com/watch?v=zPFzHoubQzg[/youtube]

 

[ThinAirDesigns]

Here's the simple answer you seek: from the wind.

Now here's where you're going wrong in your understanding -- you're continuing to insist that there be wind relative to the *chassis* rather than think about the wind relative to the airfoil.

This statement makes no sense to me. There is no wind at the chasis but there is wind at the airfoil (propeller), huh? How is that possible if the propeller and the chassis are moving at the same speed!

The propeller is NOT taking the same path through the air as the chassis -- it's taking a longer spiral path. Two different paths, two different speeds.

Let me interject and thus see if I correctly understand the situation. When the vehicle is at "windspeed" there's no wind relative to the chassis but there's still wind relative to the propellor, it's rotating. Every point on the prop, except for the center, is both rotating and moving "downwind".

Yes, that's a perfect understanding so far.

So the question is how does the wind, relative to the prop, get harnessed ? I note that if I were a fly stuck somewhere on the prop, I'm not travelling directly downwind.

You've nailed it -- the airfoils are not travelling directly downwind, even though the chassis of the vehicle is. The airfoils are on the same broad reach that the sailboat takes it's airfoil on -- the difference is that unlike the sailboat, where the chassis (hull) and airfoil (sail) must take the same path, we've separated the two and allowed them to take separate paths.

Here's a simple version -- take two identical BMWO boats. Send them out on mirrored broad reaches headed for their top speed (which we know can be higher VMG than WS). They reach out, gybe, reach back until they almost touch each other at the center line of the course, gybe and repeat ... out and back. Now put a long telescoping pole between them and place a pilots chair right smack dab in the middle of this pole. This person is now traveling *directly downwind* while the boats tack back and forth on either side. Once the two boats reach *exactly* WS (VMG) the person in this chair will feel *no wind*, but if you're on either BMWO, you're like the fly on the prop -- there is still wind where you are. Once the two boats exceed the speed of the wind (VMG), the person in this chair will feel a headwind and will be going DDWFTTW.

Sitting in the drivers seat of the vehicle in question is like sitting in the center of that telescoping pole -- the prop blades are happily running at an angle to the wind while the pilot sits in the middle going DDWFTTW.

JB

 

[Mee_n_Mac]

Your quotes are misplaced.

 

[origin]

So the question is how does the wind, relative to the prop, get harnessed ? I note that if I were a fly stuck somewhere on the prop, I'm not travelling directly downwind.

You've nailed it -- the airfoils are not travelling directly downwind, even though the chassis of the vehicle is. The airfoils are on the same broad reach that the sailboat takes it's airfoil on -- the difference is that unlike the sailboat, where the chassis (hull) and airfoil (sail) must take the same path, we've separated the two and allowed them to take separate paths.

Am I missing something here? How can you say the airfoils are not traveling directly down wind, because they are taking a spiral path? If the wind is blowing a 20 mph and the contraption is moving at 20 mph it makes no difference how the airfoils are positioned, since they are traveling down wind with the vehicle, at the speed of the wind, they are experiencing stagnant air.

 

[ThinAirDesigns]

Your quotes are misplaced.

Thanks -- I think I got them right this time.

JB

 

[Mee_n_Mac]

So the question is how does the wind, relative to the prop, get harnessed ? I note that if I were a fly stuck somewhere on the prop, I'm not travelling directly downwind.

You've nailed it -- the airfoils are not travelling directly downwind, even though the chassis of the vehicle is. The airfoils are on the same broad reach that the sailboat takes it's airfoil on -- the difference is that unlike the sailboat, where the chassis (hull) and airfoil (sail) must take the same path, we've separated the two and allowed them to take separate paths.

Am I missing something here? How can you say the airfoils are not traveling directly down wind, because they are taking a spiral path? If the wind is blowing a 20 mph and the contraption is moving at 20 mph it makes no difference how the airfoils are positioned, since they are traveling down wind with the vehicle, at the speed of the wind, they are experiencing stagnant air.

Can we agree that a wing travelling through the air experiences drag in the direction of the airflow and lift in an orthogonal direction ? That this is true whether the wing is moving or the air is moving ? So a wing moving in a helical path is moving relative to any wind moving in a linear (downwind) path. And in this case producing lift (as a result of the relative motion) in the direction of the wind.

 

[Couerl]

The simple question was where does the power come from after you have achieved a velocity equal to the wind, and what I got from your reply was: if there is a 20 mph wind and your contraption is going 20 mph then the contraption will feel a 0 mph wind speed except for the propeller which will continue to have a tail wind.

That sounds like utter nonsense to me.

Well that's because it is. :lol:
The simplest answer is that windspeed is non-representative of the total amount of energy in the wind itself and therefore, windspeed does not present a physical limit to any given craft. Think of a surfer: When a surfer goes diagonally on the face of a wave he is travelling much faster than the wave speed because he is facing less resistance and leveraging further energy from gravity and angular momentum. Does the speed of the wave represent any kind of limit on the surfer? Does speed represent the total amount of energy in that wave or simply the amount of energy that the water molecules absorb/transmit? Indeed the amount of energy that drives a wave or wind is far greater than the speed of either and so going faster downwind or downwave is not only possible, but simple.