Sailing downwind faster than the wind

[ThinAirDesigns]

Am I missing something here?

Yes you are. It's a brainteaser ... you can't trust your intuition.

How can you say the airfoils are not traveling directly down wind, because they are taking a spiral path?

I can say it because the wind doesn't take a spiral path and the airfoils do -- thus the airfoils can't be traveling DDW. Was that some sort of trick question?

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.

A: tell that to the fly on the tip of the propeller as it feels an apparent wind of ~100mph

B: tell that to the sailors on the twin BMWO boats I described in my response to mac above.

JB

 

[ThinAirDesigns]

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.

Nicely done Couerl. It's funny how people will make up their own wrong theory and keep saying "IT'S WRONG!!" -- well, yes it is.

JB

 

[Mee_n_Mac]

[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?

Not to dispute your general point but yes, the speed of the wave, in the direction normal to it's face, does limit the surfer. Except for a short run (where potential energy is exchanged for kinetic) the surfer can't go faster than the wave in the direction of it's travel. Diagonally ... yes, "downwave" ... no.

 

[ThinAirDesigns]

A quick repeat note to Origin regarding spiral paths of airfoils vs directly downwind:

A good thing to remember is that BOTH the sails of BMWO and the propeller on the "contraption" are taking the same spiral path -- the only difference is the diameter of the spiral. The BMWO sail is on a spiral the diameter of the earth and ours are taking on just a bit smaller. Imagine a propeller the diameter of the earth and it's tips would happily run right alongside that of the sailboats.

JB

 

[ThinAirDesigns]

[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?

Not to dispute your general point but yes, the speed of the wave, in the direction normal to it's face, does limit the surfer. Except for a short run (where potential energy is exchanged for kinetic) the surfer can't go faster than the wave in the direction of it's travel. Diagonally ... yes, "downwave" ... no.

Agreed that his point was an excellent one, and agreed that it can't be applied directly to this case. It is however easy to see the difference between the wind and a wave and then make the connection -- a wave has a defined length (or width, depending on definitions) while the wind or current is infinite as applied in a pratical sense to this problem.

With wind or current, one need not 'jump' a barren wasteland of no energy to get to the next wave -- the energy is available throughout.

JB

 

[origin]

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.

Yes

 

[origin]

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.

Well based on this I should be able to put a stationary wing on my bicycle and when a breeze comes along I can accelerate continually.

 

[ThinAirDesigns]

Well based on this I should be able to put a stationary wing on my bicycle and when a breeze comes along I can accelerate continually.

Nope.

Let me ask you this Origin -- are the videos I posted faked?

Has the Aeronautical Engineering Department at San Jose State University started committing fraud?

Has Adam Cort (Senior Editor at "Sail Magazine") started pulling hoaxes?

http://sailmagazine.com/racing/running_ ... _the_wind/

Was Bob Dill, a former wind powered land speed world record holder, technical guru and BOD member of NALSA (the ratification organization for wind powered land speed records) playing a trick on Editor Adam Cort when after personally witnessing the tests he wrote to him and said: "Having seen their runs, and having looked over the craft for large, tightly wound rubber bands, small but powerful motors running off small but powerful batteries, JATO bottles, etc (and finding none), I can say empirically (and definitively) that it worked well. As you may know, there are many people who believe what they are doing requires perpetual motion or is not possible due to other physical laws. I am happy to report that no violations of the laws of thermodynamics were required."

Has the Discovery Channel suddenly started promoting perpetual motion? (They filmed us last week for one of their science shows).

Has Mark Drela (Google him ... MIT Professor and one of the foremost aerodynamicsts in the worl) gone nuts when he says that DDWFTTW with a propeller based vehicle is not only possible, but rather easy.

All that (and more), or could it just be that Origin just hasn't wrapped his brain around how it works just yet?

Hint: it's actually happening - with or without your understanding.

JB

 

[origin]

All that (and more), or could it just be that Origin just hasn't wrapped his brain around how it works just yet?

Don't know why you are getting hostile. Like I have said I don't see how this could work. I have asked questions that should be able to be answered. You have presented evidence that sail boats can go faster than the wind, which is true because they go at an angle to the wind so there is still wind that is applying a force, so it is not the same thing.

I cannot see how your wind powered vehicle can still get power when it is going faster than the wind. It would seem that this would be something that can be answered. Saying that the airfoil does see wind does not make any sense to me.

If I am in a car that is driving at 90 mph and there is a tail wind of 90 mph then the air is moving at the same speed as the car. I could in theory place a ballon that had enough He to give it neutral boyancy outside the car window and it would move at the same speed as the car. So where is the force that is making your vehicle accelerate?

 

[ThinAirDesigns]

Don't know why you are getting hostile.

There's nothing but fair questions there ... no hostility intended.

You openly claim that what we're doing is "BS" -- some would interpret that as hostile.

JB

 

[eyytee]

You've nailed it -- the airfoils are not travelling directly downwind, even though the chassis of the vehicle is.

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,

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 (prependicular to wind direction). This total velocity vector is NOT parallel to the wind direction.

since they are traveling down wind with the vehicle, at the speed of the wind, they are experiencing stagnant air.

So you think that the blade of a rotating propeller experiences stagnant air, if the air doesn't move relative to the propeller's hub. What is the point of rotating propellers then?

If I am in a car that is driving at 90 mph and there is a tail wind of 90 mph then the air is moving at the same speed as the car. I could in theory place a ballon that had enough He to give it neutral boyancy outside the car window and it would move at the same speed as the car. So where is the force that is making your vehicle accelerate?

Take a long stick with a sail on it. Place one end on the ground, the sail in the middle the hold the other end. It will push you forward, beyond windspeed:

[youtube]http://www.youtube.com/watch?v=g8bxXRQtcMY[/youtube]

 

[spork]

So you think that the blade of a rotating propeller experiences stagnant air, if the air doesn't move relative to the propeller's hub. What is the point of rotating propellers then?

And in fact how could a plane taxi and takeoff on a no-wind day? No wind over the propeller - right?

When sitting in the run-up area on a no-wind day, standing on the breaks, and giving the engine throttle, there's no question that I generate loads of thrust. The prop is not moving through the air. There's no "wind" over it. But the blades are moving through the air like mad. They feel plenty of apparent wind.

But some will say "sure, but the plane has an engine". True - but that's getting ahead of ourselves. Question 1 is this: do the blades of the prop feel an apparent wind when the prop is spinning and the vehicle is going downwind at windspeed? And of course the answer is yes.

 

[MeteorWayne]

That's not really the relevant question. If the plane is doing 90 mph relative to the ground with a 90 mph tailwind, guess what? No lift over the wings since the speed of the air over it is zero. No takin' off.

 

[origin]

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 (prependicular to wind direction). This total velocity vector is NOT parallel to the wind direction.

OK

So you think that the blade of a rotating propeller experiences stagnant air, if the air doesn't move relative to the propeller's hub. What is the point of rotating propellers then?

I think you know that an engine is turning the propellers to propel the craft. But you are talking about an wind powered vehicle, so, no wind no power.

If I am in a car that is driving at 90 mph and there is a tail wind of 90 mph then the air is moving at the same speed as the car. I could in theory place a ballon that had enough He to give it neutral boyancy outside the car window and it would move at the same speed as the car. So where is the force that is making your vehicle accelerate?

Take a long stick with a sail on it. Place one end on the ground, the sail in the middle the hold the other end. It will push you forward, beyond windspeed:

Huh?

[youtube]http://www.youtube.com/watch?v=g8bxXRQtcMY[/youtube]

I am sorry I watched a couple of the videos and they seemed to be trying to sell me on the craft without adequately explaining the physics - I don't feel impelled to sit through more youtubes.

 

[eyytee]

That's not really the relevant question.

You mean your question below?

If the plane is doing 90 mph relative to the ground with a 90 mph tailwind, guess what? No lift over the wings since the speed of the air over it is zero. No takin' off.

The wings don't move relative to the plane, the propeller blades do. Therefore the propeller blades experience a different apparent wind than the wings and the fuselage.

 

[origin]

Don't know why you are getting hostile.

There's nothing but fair questions there ... no hostility intended.

You openly claim that what we're doing is "BS" -- some would interpret that as hostile.

JB

Oh, I didn't know this was your project - I thought you were just commenting on the videos. I don't mean to insult your project.

The point is I have a sailboat, I have flown an airplane, I even did some studying to fly sail planes but I never actually flew one (yet), but the point is I know a bit about this stuff and what you are claiming does not compute. I have also learned a bit of physics since I am an engineer so I am not exactly unteachable. It seems that someone should be able to explain this vehicle in a way that makes some physical sense.

 

[ThinAirDesigns]

I don't mean to insult your project.

None taken. Moving on.

The point is I have a sailboat, I have flown an airplane, I even did some studying to fly sail planes but I never actually flew one (yet), but the point is I know a bit about this stuff and what you are claiming does not compute.

You're in very good company -- it doesn't compute with many, many folks.

It seems that someone should be able to explain this vehicle in a way that makes some physical sense.

Everyone responds differently to different explanations. Some response more visually (videos), some more to math and some to descriptions. I don't know what your preferred learning methods are, but let's try the math next.

I apologize that I can't attach .pdf files directly in this forum, but here is a link to a zip file that contains a full math treatment. I promise that the .zip file contains nothing more than the text and accompanying images and won't screw up your computer.

http://www.mediafire.com/?gnznnd3dwct

Here are two math treaments by Mark Drela (aforementioned MIT Professor). These are primarily for a water based version of the craft (where his interest lie), but he also directly applies it to the wheeled version as well:

http://www.mediafire.com/?xdmjm0von5q

http://www.mediafire.com/?xbzbcyjy0b1

Let me know if the math treatments help at all.

JB

 

[eyytee]

So you think that the blade of a rotating propeller experiences stagnant air, if the air doesn't move relative to the propeller's hub. What is the point of rotating propellers then?

I think you know that an engine is turning the propellers to propel the craft. But you are talking about an wind powered vehicle, so, no wind no power.

Before you start to talk about dynamics, you schould first understand the kinematics and that the blades experience a different apparent wind, than the chassis.

And when you say "wind" you should specify what wind you mean: true wind (relative to ground) or apparent wind (relative to airfoil).

If I am in a car that is driving at 90 mph and there is a tail wind of 90 mph then the air is moving at the same speed as the car. I could in theory place a ballon that had enough He to give it neutral boyancy outside the car window and it would move at the same speed as the car. So where is the force that is making your vehicle accelerate?

Take a long stick with a sail on it. Place one end on the ground, the sail in the middle and hold the other end. It will push you forward, beyond windspeed

Huh?

Yes, it is called "a lever".

[I am sorry I watched a couple of the videos and they seemed to be trying to sell me on the craft without adequately explaining the physics

I've posted links to an analysis in post #4.

 

[spork]

Are we all agreed then... the cart can go directly downwind faster than the wind, powered only by the wind - without violating all sorts of physical laws?

 

[MeteorWayne]

No, not at all.

 

[eyytee]

Are we all agreed then... the cart can go directly downwind faster than the wind, powered only by the wind - without violating all sorts of physical laws?

No, not at all.

So which physical laws were violated in Ivanpah, a month ago? Can we at least agree, that nobody here can show exactly how physical laws are being violated by this vehicle?

 

[origin]

Are we all agreed then... the cart can go directly downwind faster than the wind, powered only by the wind - without violating all sorts of physical laws?

I certainly do not agree with this statement. The explanations have not been nearly good enough to convince me. A youtube video does not convince me either.

Thinair was kind enough to supply links to an MIT professor that apparently has an analysis on this subject. Once I have slugged through that analysis I will see if my opinion changes. It is going to take a while for me to understand the analysis though because I don't have a lot of time to spare on this and there are many terms he uses that I am not farmiliar with so I will have to first look up the terms.

I don't understand why it is so important that we all believe it is possible. If the process really works and you are working with a university once the final results are written up, it will be in journals and the case will be closed.

 

[spork]

Are we all agreed then... the cart can go directly downwind faster than the wind, powered only by the wind - without violating all sorts of physical laws?

I certainly do not agree with this statement. The explanations have not been nearly good enough to convince me. A youtube video does not convince me either.

Fair enough. I'm curious - do you think we've somehow hoaxed the videos? Or do you think they simply don't show what we claim they show?

Thinair was kind enough to supply links to an MIT professor that apparently has an analysis on this subject. Once I have slugged through that analysis I will see if my opinion changes. It is going to take a while for me to understand the analysis though because I don't have a lot of time to spare on this and there are many terms he uses that I am not farmiliar with so I will have to first look up the terms.

I can give you a simple analysis that won't take much slugging at all. Here it is...

If we consider the vehicle at some steady speed above wind speed (we don't care how we get it there for now), we have some retarding force on the wheels, and we have some thrust generated by the prop. If we ignore the aerodynamic drag of the chassis for the moment, those two forces have to be the same in order for the cart to maintain that speed (neither accelerating no decelerating).

So, let's imagine we're going twice the wind speed, 20 mph in a 10 mph wind. This means the retarding force on the wheels (F) is giving us F x 20 horsepower. And the thrust (T) of the prop is spending T x 10 horsepower (as the relative headwind over the cart is only 10 mph). Since we agreed that T and F have to be equal if there were no aerodynamic drag on the chassis, that means we're harnessing twice as much power as we're spending. That's good since we will lose power through the transmission, prop, and rolling resistance. In fact, the 2X power that we get is enough to cover all those losses AND the aero drag that we neglected for now.

There are other simple analogies; but I think this is the simplest analysis without lots of greek letters, subscripts and fancy aero terms.

I don't understand why it is so important that we all believe it is possible.

It's not at all important. It's just a really fun problem. I assumed you must have found it interesting as well since you've posted a good bit about it here.

If the process really works and you are working with a university once the final results are written up, it will be in journals and the case will be closed.

Perhaps we'll write it up and submit it to a journal, but quite likely not. In any event, I assure you it will never be "case closed" for many people.

 

[origin]

I can give you a simple analysis that won't take much slugging at all. Here it is...

If we consider the vehicle at some steady speed above wind speed (we don't care how we get it there for now), we have some retarding force on the wheels, and we have some thrust generated by the prop. If we ignore the aerodynamic drag of the chassis for the moment, those two forces have to be the same in order for the cart to maintain that speed (neither accelerating no decelerating).

So, let's imagine we're going twice the wind speed, 20 mph in a 10 mph wind. This means the retarding force on the wheels (F) is giving us F x 20 horsepower. And the thrust (T) of the prop is spending T x 10 horsepower (as the relative headwind over the cart is only 10 mph). Since we agreed that T and F have to be equal if there were no aerodynamic drag on the chassis, that means we're harnessing twice as much power as we're spending. That's good since we will lose power through the transmission, prop, and rolling resistance. In fact, the 2X power that we get is enough to cover all those losses AND the aero drag that we neglected for now.

Quite simple, but quite useless. You simply assume that there is a thrust from the prop. 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???
Like I said all of the explanations seem nothing more than hand waving. I will look at the MIT fellows work and see what I think.

Perhaps we'll write it up and submit it to a journal, but quite likely not. In any event, I assure you it will never be "case closed" for many people.

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

 

[SubductionZone]

Origin, as a former skeptic, to put it mildly, I can feel where you are coming from. You keep asking "Where does the energy for the DDWFTTW cart come from?" And the answer, as already provided is "the wind". All you have to do is to analyze the speed of the wind relative to the ground before and after it passes through the prop. This is exactly the same thing that a sail does when a boat is on a downwind reach with a velocity made good (for non sailors out there that is you average velocity in a direction when you are tacking either with the wind or against the wind) greater than the speed of the wind. In either case if you look at the wind as "blowing" on a sail or a propeller blade you will probably get the wrong answer, that it is impossible. Ice boaters were probably the first to achieve a VMG greater than wind speed. The surface of the ice offers very little resistance to the crafts advancing.

I don't see spork or JB writing this up for a scientific journal. Though an interesting approach to sailing it is not revolutionary nor does it mean there has to be any changes at all in the current theories of aerodynamics. spork came up with this independently, as have others before him. He is not the first to make a man sized vehicle either. He and JB will be the first to get well documented evidence that it can be done. I know that you don't like "YouTube videos", but if there is trickery it is usually fairly obvious. And the Vimeo links provided are not exactly YouTube quality, they are quite a bit higher resolution than you will find on most YouTube videos. And not only do they have their outside full scale tests. Long before then they made their own versions of the Jack Goodman cart of which they made several YouTube videos. The one that convinced me was their first one of the cart running on a treadmill.

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. But bumblebees do fly, and they don't break any laws of aerodynamics in doing so. And the cart does run faster than the wind without achieving over unity or any other such nonsense. Your natural skepticism is keeping you from analyzing what is going on.