Sailing downwind faster than the wind

[spork]

Quite simple, but quite useless. You simply assume that there is a thrust from the prop.

Hmmm... a properly designed spinning prop should produce thrust. They have since 1903.

Well that takes power and since the vehicle is going faster than the wind which is supplying the power - Where in the heck is the power coming from???

From the difference in speed between the ground and the air. You have to try and follow the analysis. I gave you a free body diagram (in words). The forces on the cart are balanced. F pushing back on the wheels, and T pushing forward on the prop. F and T are equal. So in this frame the power comes from the ground turning the wheels.

Like I said all of the explanations seem nothing more than hand waving.

They may seem that way to you - but they are absolutely rigorously correct. If these seem like hand waving, there's no chance you'll be any happier with Drela's analysis.

Well that is really odd... A university is involved with this and you don't think they will write it up? Really?

Yes, really. I've been working with them for the past 6 months. If anyone writes it up it will be me. In fact I've already done so (along with another colleague on the project). We've been too lazy to produce the list of figures and format the equations so far.

Why would it not be closed if you build the vehicle and it is proven to go faster than the wind the except for a few deniers the case is closed.

Well we have built and tested that vehicle (in fact several). I guess it's all a matter of how many deniers. As far as I was concerned, the case was closed several years ago when I first did the simple vector analysis. But there are still people such as yourself that don't accept the physical evidence, the analogies, or the analyses. So I guess it's all a matter of what you mean by "the case is closed". Do you consider the case closed as to whether we've put men on the moon? Or whether there was a holocaust? I guess maybe we could hope for that level of "case closed".

 

[SubductionZone]

Origin, I see we cross posted a bit.

First off spork does not just assume that a spinning propeller will generate thrust. That is observed every day. He may already have linked a website that calculates the static thrust of a propeller when you input various values for the prop. Static thrust is the thrust measured when the air speed relative to the engine is zero, which is the same situation you have when the cart is going exactly downwind at the speed of the wind. If the cart is going twice the speed of the wind it will be spinning twice as fast, it is directly geared to the wheels, the wind speed will then be a headwind at the speed of the wind. Again, not to difficult of a problem to calculate the thrust for the cart if you know all of its parameters.

All that is required for the cart to be able to go faster than the wind is for its thrust to be greater than the back force at the wheels. You seem to be assuming that conservation of energy means conservation of force. But the simplest of levers will show you that that is not the case. You can have a much greater force on one end of a lever than the other. The cart is effectively "leveraging" the wind. Once again, after the cart passes by it leaves a trail of wind that has been slowed relative to the ground. You should ask yourself where did the energy and momentum of the passing air go to?

 

[ThinAirDesigns]

The "hand waving" is yours Origin.

You've been given the math, multiple explanations and analogies and been pointed to the documentation of the physical demonstrations by many independent parties. Still, the best 'hand waving' you can come up with is "where does the power come from when it goes faster than the wind".

That question has been and answered -- by math, by vector analysis, by explanation, by the America's Cup boats, by the vehicle itself --- the power comes from the relative motion between the ground and the wind, even when it's going faster than the wind.

Don't confuse "I don't get it" with "you haven't explained it". The mistake is yours (and that's OK). When you find the mistake in Drela's analysis (you won't) you can then claim 'hand waving'.

JB

 

[spork]

Just like some early aero engineers may have made the claim that it is impossible for a bumblebee to fly by the laws of aerodynamics, some people make the same claim of the various DDWFTTW vehicles that have been built.

There is one very key difference however... the aero engineers all knew that in real life bumblebees CAN fly. Whereas the deniers and skeptics clearly think DDWFTTW really is impossible.

 

[SubductionZone]

Just like some early aero engineers may have made the claim that it is impossible for a bumblebee to fly by the laws of aerodynamics, some people make the same claim of the various DDWFTTW vehicles that have been built.

There is one very key difference however... the aero engineers all knew that in real life bumblebees CAN fly. Whereas the deniers and skeptics clearly think DDWFTTW really is impossible.

Well to defend them on that point, almost everyone will see at least one bumblebee flying every year, usually you will see quite a few during the summer months. Too date DDWFTTW vehicles have been extremely rare. So the first reaction of many people when they first see one on YouTube, which already has a ton of faked videos, is that it is just another hoax.

But you have gone above and beyond the call of duty in being very open in every build project that you have done. Unlike builders of "magnetic motors" and other such nonsense you have not shown just a finished product that could be faked in several ways. You have gone out of your way to show how you have done it so that others could do the same thing. So the claim that you are an exploiter like various over unity kooks is busted.

To sum it up, I can see why many would have the gut reaction of it being a hoax. But there is no excuse for continued denial, once your explanations, which are at all levels of understanding, and videos, which show the cart working in more than one way, are shown.

 

[ThinAirDesigns]

Well that is really odd... A university is involved with this and you don't think they will write it up? Really?

The principle the vehicle uses to propel itself faster than the wind has been in use for centuries -- ever since the first sailboat managed to tack upwind. The spinning airfoils are sail and the wheels are the keel.

There's no scientific breakthrough here ... it only *looks* weird.

JB

 

[spork]

There's no scientific breakthrough here ... it only *looks* weird.

True, but there are some very interesting scientific implications.

 

[origin]

Well that is really odd... A university is involved with this and you don't think they will write it up? Really?

The principle the vehicle uses to propel itself faster than the wind has been in use for centuries -- ever since the first sailboat managed to tack upwind. The spinning airfoils are sail and the wheels are the keel.

There's no scientific breakthrough here ... it only *looks* weird.

JB

No, I disagree this is completely different than a ship tacking. When a sail boat is tacking there is a wind that is felt and is filling the sails because the boat is not going directly down wind. There ALWAYS is a wind that is driving the boat, no sail boat ever goes faster than the wind in the direction of the wind - never. Your vehicle does that - it would be a break through.

 

[ThinAirDesigns]

No, I disagree this is completely different than a ship tacking. When a sail boat is tacking there is a wind that is felt and is filling the sails because the boat is not going directly down wind.

And just as the sails on the sailboat are not going directly downwind, neither are the sails on our vehicle. The ONLY difference between the path of the sailboat sail and the path of the vehicle sail is the diameter of the helix they BOTH carve.

The mistake is yours -- it's a very simple mistake, but an easy one to make for those who cling to their initial intuitive answer.

JB

 

[SubductionZone]

Well that is really odd... A university is involved with this and you don't think they will write it up? Really?

The principle the vehicle uses to propel itself faster than the wind has been in use for centuries -- ever since the first sailboat managed to tack upwind. The spinning airfoils are sail and the wheels are the keel.

There's no scientific breakthrough here ... it only *looks* weird.

JB

No, I disagree this is completely different than a ship tacking. When a sail boat is tacking there is a wind that is felt and is filling the sails because the boat is not going directly down wind. There ALWAYS is a wind that is driving the boat, no sail boat ever goes faster than the wind in the direction of the wind - never. Your vehicle does that - it would be a break through.

Are you claiming that an iceboat cannot average four or even five times the speed of the wind by tacking downwind? In other words if an iceboat were to race a neutral buoyancy balloon what would you expect to happen?

 

[origin]

Origin, I see we cross posted a bit.

First off spork does not just assume that a spinning propeller will generate thrust. That is observed every day. He may already have linked a website that calculates the static thrust of a propeller when you input various values for the prop. Static thrust is the thrust measured when the air speed relative to the engine is zero, which is the same situation you have when the cart is going exactly downwind at the speed of the wind. If the cart is going twice the speed of the wind it will be spinning twice as fast, it is directly geared to the wheels, the wind speed will then be a headwind at the speed of the wind. Again, not to difficult of a problem to calculate the thrust for the cart if you know all of its parameters.

All that is required for the cart to be able to go faster than the wind is for its thrust to be greater than the back force at the wheels. You seem to be assuming that conservation of energy means conservation of force. But the simplest of levers will show you that that is not the case. You can have a much greater force on one end of a lever than the other. The cart is effectively "leveraging" the wind. Once again, after the cart passes by it leaves a trail of wind that has been slowed relative to the ground. You should ask yourself where did the energy and momentum of the passing air go to?

You guys are talking in circles, you talk of thrust from the prop and this and that but you cannot explain where the power is coming from except for invoking the incorrect comparison of a tacking sailboat. You realize that if you can go faster than the wind, then essentially there will be a head wind that the vehicle is experiencing (this is shown in your video by the streamers going backwards). So if the vehicle really can still maintain or increase it's speed in that case, then there is no reason that you can't just push the vehicle in still air and have it keep going and accelerating. This certainly would be a scientific break through!

Here is my humble analysis. I think the vehicle picks up speed and if the wind slows you can go faster than the wind for a short period of time because that big prop is acting like a fly wheel and transfers it's momentum to wheels. Good for a you tube video but that is about it.

But like I said that is the way it seems to me now - let me look at the math of the MIT guy and that should answer it.

 

[SubductionZone]

Origin, you have been told over and over again that the power comes from the wind. Forget how the cart does it for right now, let us just analyze the results. Question one, will a spinning prop change the speed of the air it is interacting with? The answer is an obvious yes. Question two, is the air that the prop interacts with moving slower relative to the ground after it interacts with it than before? Again the answer is yes, it might take a little more analysis, but again this is a fairly simple answer. Question three, where did the energy and momentum of that air go to? I will let you answer that one for yourself.

These are fairly simple questions and answers. Do you disagree with any of them?

 

[origin]

Are you claiming that an iceboat cannot average four or even five times the speed of the wind by tacking downwind? In other words if an iceboat were to race a neutral buoyancy balloon what would you expect to happen?

Nope. Listen closely (or read carefully I guess). The ice boat cannot continue to increase speed or maintain a constant speed once the ice boat is moving in the direction of the wind at the speed of the wind. This is simple basic stuff, once it is going in the direction of the wind at the wind speed there is no force on the sail!!

Example if the wind is blowing at 20 mph to the north and the ice boat is moving at 40 mph to the north east (45 deg angle) it cannot go any faster. Now if it heads in a more easterly direction it can go faster than 40. If it heads in a more northerly direction it will go slower than 40 mph. If it heads due north it will max out close to 20 mph.

 

[origin]

Origin, you have been told over and over again that the power comes from the wind.

If you are going the speed of the wind then there is no wind relative to the vehicle...

Forget how the cart does it for right now, let us just analyze the results. Question one, will a spinning prop change the speed of the air it is interacting with? The answer is an obvious yes. Question two, is the air that the prop interacts with moving slower relative to the ground after it interacts with it than before? Again the answer is yes, it might take a little more analysis, but again this is a fairly simple answer. Question three, where did the energy and momentum of that air go to? I will let you answer that one for yourself.

These are fairly simple questions and answers. Do you disagree with any of them?

Fairly simply. And if there was a motor attached to the prop I would not have a problem with any of this.

 

[SubductionZone]

And I missed the second part of your last post. The cart runs off of the difference between the velocity of the wind and the velocity of the ground. Regardless of the cart's velocity that quantity is the same. It lowers that difference in those two velocities. The difference in those velocities is a source of energy. Now lets see what happens if there is no wind, i.e. it is a clam day. On this day the velocity of the air and the velocity of the ground would always be equal regardless of your velocity, therefore the difference between them would always be zero, meaning that the cart could no get any power from those differences and therefore would not run.

The temporary faster than the wind claim has been made before. There are several problems with it. Number one, Jack Goodman's cart can be observed for over a minute with its streamers flowing backwards. So how long do you think it can coast faster than the wind? On the Vimeo videos of the man sized cart you can see that the prop is spinning fastest well after the streamers indicate it is going faster than the wind. In other words he continued to accelerate after he hit wind speed, something you claim is impossible.

 

[SubductionZone]

Are you claiming that an iceboat cannot average four or even five times the speed of the wind by tacking downwind? In other words if an iceboat were to race a neutral buoyancy balloon what would you expect to happen?

Nope. Listen closely (or read carefully I guess). The ice boat cannot continue to increase speed or maintain a constant speed once the ice boat is moving in the direction of the wind at the speed of the wind. This is simple basic stuff, once it is going in the direction of the wind at the wind speed there is no force on the sail!!

Example if the wind is blowing at 20 mph to the north and the ice boat is moving at 40 mph to the north east (45 deg angle) it cannot go any faster. Now if it heads in a more easterly direction it can go faster than 40. If it heads in a more northerly direction it will go slower than 40 mph. If it heads due north it will max out close to 20 mph.

You could not be more wrong. A typical DN racing course goes around two points, one directly downwind, or upwind, from the other. Going with the wind they have no problem tacking to an average speed of four times the wind speed on a VMG course. I could supply some links if I searched a little, though I am sure that JB or spork already have them bookmarked.

So once again are you claiming that an iceboat could not beat a neutral buoyancy balloon in a race downwind? If you don't think it can don't worry too much, I used to believe that as well.

 

[SubductionZone]

Origin, you have been told over and over again that the power comes from the wind.

If you are going the speed of the wind then there is no wind relative to the vehicle...

Forget how the cart does it for right now, let us just analyze the results. Question one, will a spinning prop change the speed of the air it is interacting with? The answer is an obvious yes. Question two, is the air that the prop interacts with moving slower relative to the ground after it interacts with it than before? Again the answer is yes, it might take a little more analysis, but again this is a fairly simple answer. Question three, where did the energy and momentum of that air go to? I will let you answer that one for yourself.

These are fairly simple questions and answers. Do you disagree with any of them?

Fairly simply. And if there was a motor attached to the prop I would not have a problem with any of this.

There is no wind relative to the body of the cart, but the prop is moving in a circle, and any one point does feel a wind. That wind increases the further from the hub that you get.

And my questions did not involve a motor on the prop. Please answer the question as asked, if I don't know exactly where you disagree it is hard to argue properly with you.

 

[origin]

And I missed the second part of your last post. The cart runs off of the difference between the velocity of the wind and the velocity of the ground. Regardless of the cart's velocity that quantity is the same. It lowers that difference in those two velocities. The difference in those velocities is a source of energy. Now lets see what happens if there is no wind, i.e. it is a clam day. On this day the velocity of the air and the velocity of the ground would always be equal regardless of your velocity, therefore the difference between them would always be zero, meaning that the cart could no get any power from those differences and therefore would not run.

That makes no sense at all.

Lets look at two cases:

1. We have a 5 mph tail wind. The craft accelerates and begins traveling at 10 mph, therefore there is a 5 mph head wind. It makes no difference how the airfoil is arranged it is experiencing a 5 mph head wind.

2. There is no wind. Push the vehicle up to a speed of 5 mph. It will experience a 5 mph head wind. The forces on an airfoil in these 2 cases would be identical.

If the vehicle in the first case can maintain the velocity then the craft the in the second case should also be able to maintain it's velocity.

 

[origin]

So once again are you claiming that an iceboat could not beat a neutral buoyancy balloon in a race downwind? If you don't think it can don't worry too much, I used to believe that as well.

Yes, that is correct. I have no worries (about this).

 

[SubductionZone]

So then if someone could show you a credible link that showed otherwise, that is of an iceboat going several times the speed of the wind by tacking with the wind, you might reconsider your other claims?

 

[ThinAirDesigns]

So once again are you claiming that an iceboat could not beat a neutral buoyancy balloon in a race downwind? If you don't think it can don't worry too much, I used to believe that as well.

Yes, that is correct. I have no worries (about this).

Holy Cow Origin -- this is even done to the tune of *more* than 2x in watercraft and more than 4x in ice-boats and land-yachts.

I already provide you a link to the America's Cup boats to prove this ... do you think that the America's Cup committee were lying and the course wasn't as long as they said it was?

JB

 

[SubductionZone]

I found a link for you with GPS information for a typical DN race:http://www.nalsa.org/Articles/Cetus/Iceboat Sailing Performance-Cetus.pdf

If you scroll down the page you will see where with a ten mph wind . The iceboat easily goes faster than the wind on a VMG course. So by tacking an iceboat can outsail the wind. The propeller on the cart is on an effective tack, are you still saying that it cannot outsail the wind?

 

[origin]

I found a link for you with GPS information for a typical DN race:http://www.nalsa.org/Articles/Cetus/Iceboat Sailing Performance-Cetus.pdf

If you scroll down the page you will see where with a ten mph wind . The iceboat easily goes faster than the wind on a VMG course. So by tacking an iceboat can outsail the wind. The propeller on the cart is on an effective tack, are you still saying that it cannot outsail the wind?

I assumed you were talking about directly down wind. An ice boat can beat the wind because it carries it's high speed into the tack and at each tack it moves much faster than the wind.

It still does not relate to your situation because a sailing craft on a steady path cannot go faster than the wind directly in the direction of the wind. Because the sail would feel no wind so there would be no force.

The same is true with your vehicle once it exceeds the speed of the wind there will be a head wind. If you can accelerate in a head wind then you can sail in dead air.

edited to add- I have enjoyed discussing this but I need to do something productive now. (honey do list...)

 

[SubductionZone]

I thought we were always fairly clear about the iceboats tacking with the wind. Now look what one point is doing on the propeller. It always has a constant angle to the wind and is moving in a helical path. By opening up that helix we see that the prop at that point is effectively on a downwind tack with the wind. As you move up and down the propeller you will see that that is always the case. So if the propeller is effectively putting the cart on a tack why can't if provide thrust like a sail on a downwind tack does?

 

[eyytee]

It still does not relate to your situation because a sailing craft on a steady path cannot go faster than the wind directly in the direction of the wind. Because the sail would feel no wind so there would be no force.

Unless the sail is moving relative to the craft, like the propeller blades do. You said, that you already understand this basic kinematic fact, and are "OK" with it:

But it makes a difference how the airfoils are moving relative to the cart. The total velocity vector of the airfoils is the sum of the carts velocity (parallel to wind direction) and the tangential velocity due to rotation (perpendicular to wind direction). This total velocity vector is NOT parallel to the wind direction.

OK

See it is you who is going in circles, falling behind things you have already accepted