Sailing downwind faster than the wind

[spork]

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 could do that - but it can also beat the balloon to a downwind finish line without ever tacking. In other words, we make a long start line and a long finsh line, such that the finish line is directly downwind from the start line (both are perpendicular to the wind), we can start the ice-boat and balloon at the same time and place on the start line. The ice boat will reach the finish line LONG before the balloon. The difference is that the balloon will end up directly downwind, while the ice-boat will have gone crosswind as well (if we don't allow it to tack).

The same is true with your vehicle once it exceeds the speed of the wind there will be a head wind.

Yes - there will. I've felt that 30 mph headwind on my face while driving the vehicle directly downwind.

If you can accelerate in a head wind then you can sail in dead air.

Sadly - no. We extract energy from the motion of the air moving over the ground. Take away our power source and we're dead in the water so to speak.

 

[origin]

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

The problem is it is irrelevent. If you are going faster than the wind the airfoils will not develop any thrust they will slow the vehicle.

 

[origin]

The same is true with your vehicle once it exceeds the speed of the wind there will be a head wind.

Yes - there will. I've felt that 30 mph headwind on my face while driving the vehicle directly downwind.

If you can accelerate in a head wind then you can sail in dead air.

Sadly - no. We extract energy from the motion of the air moving over the ground. Take away our power source and we're dead in the water so to speak.

That is proposterous. You mean to tell me if there is a tail wind of 20 mph when you are traveling at 50 mph, such that there is a relative head wind of 30 mph, that the energy to maintain your speed is coming from the 20 mph wind? Oh yeah, that is clearly violating physics as I understand it.

This is getting very reduntant, like I have said, I will look at the MIT guys analysis and then approach this problem from that angle (so to speak),

 

[SubductionZone]

I am sorry to tell you this origin, but if you don't understand the relatively simple explanations given to you here then it is highly doubtful that you will understand Drela's paper.

Even when spork was going 45 mph directly downwind with a wind of 15 mph, net result a 30 mph headwind, he can still extract energy from the wind. You are thinking of the wind as an object that only hits a sail from one side. You have to think of it as a fluid that the propeller is interacting with. Once again for right now forget the how, think of the what is happening. Is it or isn't it fairly obvious that the cart leaves behind a trail of locally slowed down air relative to the ground in its wake? Where did you think that energy of the wind went?

 

[ThinAirDesigns]

Origin, here is the very best thing you could do to begin to understand -- forget this vehicle and spend an hour doing research on the capabilities of typical land-yachts and ice-boat and also the best sailboats in the world. No need to use our links, you can find plenty of your own. For example, I typed in "Land-yacht polar" into Google and first return was a link to a .pdf from San Diego State where they did wind tunnel testing showing steady state downwind VMGs for land-yachts to be multiples of windspeed. Again, we're talking *steady state* -- without a single tack or gybe, just a constant angle to thw wind.

Without the understanding that this violates no laws of physics, it's pretty hard to imagine you climbing out of your rut of wrongness here. After all -- it's not really productive for your own knowledge base to be arguing that something is impossible when it's being done world over day after day.

JB

 

[MeteorWayne]

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?

Yes, a credible link...all the Utubevideos have no measurents of the ambient wind, tne relative "to whatever" velocity, or any real data that would be considered real scientific data...it's all "we have YouTube Videos" which are worth exactly nothing.

 

[origin]

I am sorry to tell you this origin, but if you don't understand the relatively simple explanations given to you here then it is highly doubtful that you will understand Drela's paper.

Even when spork was going 45 mph directly downwind with a wind of 15 mph, net result a 30 mph headwind, he can still extract energy from the wind. You are thinking of the wind as an object that only hits a sail from one side. You have to think of it as a fluid that the propeller is interacting with. Once again for right now forget the how, think of the what is happening. Is it or isn't it fairly obvious that the cart leaves behind a trail of locally slowed down air relative to the ground in its wake? Where did you think that energy of the wind went?

Well the energy of a 30 mph head wind would be slowing you down!

What you fail to understand is that if I am in a car traveling 45 mph with a 15 mph tail wind and I put a propeller out the window it will react exactly the same as if I held a propeller and was stationary in a 30 mph wind.

I just ain't buying, plus you guys must be knuckle heads, if you got this wonderous machine why are you wasting your time arguing on internet forums and making you tube videos - all you have to do is demonstrate this thing to the aeronautical engineering departments at major university and they would fall all over themselves to help you out.

 

[SubductionZone]

Wayne, did you not see the link I gave of iceboat races? Or weren't you following the discussion at that point?

And origin I see you are still thinking about the wind "blowing" on something rather than thinking about that objects reaction to being in a moving fluid. This cart does not break any aerodynamic laws, it is just an interesting application of them. It is those who don't understand these laws who think that the cart is revolutionary. It is not even a particularly practical vehicle on areas where it can sail. It can only go downwind, and the further off of true downwind that you get the slower it goes.

There are several treadmill videos of these carts, done by different people, many of them were doubters before they made their own carts. I hope you guys do understand what the treadmill tests represent.

 

[SubductionZone]

I just watched the BUFC blowup video again and I have some questions for the doubters. Did you notice that JB, who was driving the pickup truck that was pushing the cart backed off as soon as the streamers indicated that the cart was going faster than the wind? Did you notice that the prop sped up after that? In other words the prop that is directly geared to the wheels indicated that the cart continued to accelerate after the cart got to the speed of the wind. How fast does a cloud of dust that is blown by the wind go as a fraction of wind speed? I would estimate on the order of 99% of the speed of the wind. Did you notice how fast the cart went through the cloud of dust raised by the pickup truck?

I know that this is highly counterintuitive, but that is exactly what makes this a very entertaining topic. The cart itself will have very limited usage.

And as an aside, spork and JB are seriously considering going after the Greenbird's land speed record. There first actual trial run was extremely successful, and going directly downwind means that they do have a bit of an advantage of vehicles that run cross wind, they can have a smaller effective headwind than the Greenbird had.

 

[spork]

If you are going faster than the wind the airfoils will not develop any thrust they will slow the vehicle.

Wrong. When an airplane is taxiing at 5 mph on a no-wind day it's going downwind faster than the wind, and still seems to develop plenty of thrust.

That is proposterous.

No it's not.

You mean to tell me if there is a tail wind of 20 mph when you are traveling at 50 mph, such that there is a relative head wind of 30 mph, that the energy to maintain your speed is coming from the 20 mph wind?

Yes. Tricky - ain't it?

Oh yeah, that is clearly violating physics as I understand it.

That we can agree on.

I will look at the MIT guys analysis and then approach this problem from that angle (so to speak),

The "MIT guy" describes exactly what I've told you - only with lots of symbols that make it more obscure for a non aerodynamicist.

the Utubevideos have no measurents of the ambient wind, tne relative "to whatever" velocity, or any real data that would be considered real scientific data...it's all "we have YouTube Videos" which are worth exactly nothing.

The utube videos tell you everything you need to know unless you think they're intentional hoaxes. Streamers blow with the relative wind. Done. The cart on a treadmill goes against the belt - even when it has to climb a hill to do so. It doesn't matter exactly how fast it's going. It's going faster than the belt. And yes, that is absolute genuine, hard core scientific data. Just not the kind you prefer.

Well the energy of a 30 mph head wind would be slowing you down!

Not if you're clever.

What you fail to understand...

Keep in mind - we're describing something that happens in the real world. Something we've DONE. You're describing why it is that it CAN'T happen in the real world. So I think it's fair to say that we DO understand.

you guys must be knuckle heads

Perhaps. But we're knuckle heads that understand how this works.

all you have to do is demonstrate this thing to the aeronautical engineering departments at major university and they would fall all over themselves to help you out.

First off - we don't need help. It's working just fine.

 

[spork]

Did you notice that JB, who was driving the pickup truck that was pushing the cart backed off as soon as the streamers indicated that the cart was going faster than the wind?

That's what it looks like, but JB never backed off. He simply stopped accelerating and maintained constant speed once he saw the streamers go limp. He did this intentionally to provide that unique perspective. Unfortunately, Ken continued to zoom the camera as I pulled away. You can see a few seconds later that JB speeds up to 60 mph to catch up with me, and Ken zooms back out at that point.

 

[origin]

Holy crap! Just took a quick look and you guys have been crashing forums for years with this stuff! I would think with years of arguments you would come up with something better than this strawman

If you are going faster than the wind the airfoils will not develop any thrust they will slow the vehicle.

Wrong. When an airplane is taxiing at 5 mph on a no-wind day it's going downwind faster than the wind, and still seems to develop plenty of thrust.

It is silly to argue anyway - you have the vehicle, you even have a pilot that has gone 45 mph in a 15 mph breeze. So what is there to argue about? I am looking forward to seeing this in the papers or TV or a peer reviewed journal. I mean what could possibly be holding you back?

 

[SubductionZone]

So what gives origin, are you a sore loser? Yes, this is spork's pet project and it has been on many websites. He did not start it here, as he has done on others. But the same old mistaken ideas came up. It is one of the more enjoyable debating topics on the web. Sadly, that will probably end shortly. Their story is being picked up by various television stations. Their first run of the BUFC was just too successful :roll:

 

[spork]

I would think with years of arguments you would come up with something better than this strawman

"Strawman" = "explanation I can't understand" ?

you guys have been crashing forums for years with this stuff!

"crashing forums" = "showing up to offer information about an ongoing topic" ?

It is silly to argue anyway - you have the vehicle, you even have a pilot that has gone 45 mph in a 15 mph breeze.

Actually, I am the pilot. And we're not arguing any more than you are - it's just that we're right.

So what is there to argue about? I am looking forward to seeing this in the papers or TV or a peer reviewed journal. I mean what could possibly be holding you back?

Nothing's holding us back. You can see it on the "Daily Planet" on Discovery next month, and soon you'll see the NALSA ratified direct downwind record. Papers? What's that? Peer reviewed journal? Maybe - but why?

 

[nimbus]

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?

Yes, a credible link...all the Utubevideos have no measurents of the ambient wind, tne relative "to whatever" velocity, or any real data that would be considered real scientific data...it's all "we have YouTube Videos" which are worth exactly nothing.

I don't have time to read the whole thread (unfortunately), but the first thing I thought when I saw this youtube vid documenting this contraption was "why didn't they have a conventional sail of equal thrust alongside for comparison?".

 

[SubductionZone]

nimbus, first off people would not believe what a sailed vehicle would indicate. spork in one of his BUFC videos does just that. Second, the source I was talking about there was a source that showed an iceboat could beat a neutrally buoyant balloon in a downwind race. It was a related discussion to this debate. Meteor Wayne missed that and now you have.


I would suggest that you go back to the start of this thread, after all it is only 6 pages long. There is one thread that I know of that is over 1,500 pages long and there are still deniers on this subject. And this is after spork and JB built a man sized version of the cart and ran it clearly DDWFTTW.

 

[SubductionZone]

And nimbus I have to add your statement "why didn't they have a conventional sail of equal thrust alongside for comparison?". does not make much sense. I assume that you are referring to a spinnaker type of sail, the force on that sort of sail will drop as the vehicle that it is attached to approaches the speed of the wind. Jack Goodman did have streamers, that were mounted well outside of the path of the wind from the prop to indicate relative wind speed. Once again there have been quite a few videos made of these type of vehicles running faster than the wind. Both outside and inside. Now what force would keep the streamers blowing backwards in a tailwind for an extended period of time if the cart was not going faster than the wind?

 

[eyytee]

but the first thing I thought when I saw this youtube vid documenting this contraption was "why didn't they have a conventional sail of equal thrust alongside for comparison?".

It is difficult to arrange a race from dead stop, because of the initially low acceleration of the prop-cart. But they did it in the test run, where the prop-cart was pushed to windspeed and then released, in line with a sail cart going directy downwind:

Here the sail-cart just a moment before the release of the prop-cart:
http://www.youtube.com/watch?v=c5VlX-xEk00#t=0m50s

Here later the sail-cart in the background, far behind the prop-cart:
http://www.youtube.com/watch?v=c5VlX-xEk00#t=2m8s

But as SZ noted, this is not real proof of anything. The sail cart is not going at windspeed. However, the dust hanging in the air behind the truck can be assumed to travel at winspeed. Look how the prop-cart overtakes it:
http://www.youtube.com/watch?v=c5VlX-xEk00#t=2m15s

 

[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

The problem is it is irrelevent.

It is relevant, because it kills your argument above, about why the "situation does not relate" to a sail-craft tacking with a downwind VMG > winspeed.

If you are going faster than the wind the airfoils will not develop any thrust they will slow the vehicle.

Obviously wrong. Rotating propellers can create forward thrust in air that moves backwards relative to them.

 

[origin]

If you are going faster than the wind the airfoils will not develop any thrust they will slow the vehicle.

Obviously wrong. Rotating propellers can create forward thrust in air that moves backwards relative to them.

Rotating propellers will create a forward thrust
It takes power to rotate the propeller
The craft is powered by the wind
If the craft is going in the direction of the wind faster than the wind then there will be no relative wind for power
The craft will slow

Quite simple really.

 

[eyytee]

If you are going faster than the wind the airfoils will not develop any thrust they will slow the vehicle.

Obviously wrong. Rotating propellers can create forward thrust in air that moves backwards relative to them.

Rotating propellers will create a forward thrust
It takes power to rotate the propeller

Good so far

The craft is powered by the wind

Yes, by the "true wind" : air movement relative to the ground. By using inaccurate language and ambiguous terms like "wind" you confuse yourself, and fail to see the inconsistency in your argument:

If the craft is going in the direction of the wind faster than the wind then there will be no relative wind for power

Wrong. The cart is not driven by the relative wind on the chassis.

 

[spork]

Rotating propellers will create a forward thrust
It takes power to rotate the propeller
The craft is powered by the wind
If the craft is going in the direction of the wind faster than the wind then there will be no relative wind for power

You claim we're hand waving and using circular arguments, but look at your argument... you say the thing can't go faster than the wind, because the prop can't produce thrust when the cart is going faster than the wind. When we explain that a rotating prop CAN produce thrust when going faster than the wind, you say, "only if that prop is powered, and yours can't be powered, because your cart can't go faster than the wind - therefore your cart can't go faster than the wind".

The craft will slow

Does it truly not occur to you that there must be a slight kink in your analysis when it concludes the exact opposite of what happens in the real world? Are you prepared to prove to us that bumble bees can't fly.

Quite simple really.

Except for the part about your analysis and the real world being in direct opposition.

 

[centsworth_II]

Excuse me if I missed this, but have any tests been done with a model on a flat surface facing into a controlled wind?

The open air results are in question because the vehicle is not really always going directly into the wind, which, because it is natural, shifts around. I am not comfortable with the treadmill results because I imagine the possibility of energy transfer by friction between the powered tread and the vehicle.

It would be easy to test a model on a flat surface facing into a fan. Has this been done?

 

[eyytee]

I am not comfortable with the treadmill results because I imagine the possibility of energy transfer by friction between the powered tread and the vehicle.

This not a "possibility" but an obvious fact, in both cases: treadmill and outside in natural wind. In the reference frame of the cart the moving surface provides the power input to drive the propeller, in both cases.

 

[spork]

Excuse me if I missed this, but have any tests been done with a model on a flat surface facing into a controlled wind?

The open air results are in question because the vehicle is not really always going directly into the wind, which, because it is natural, shifts around. I am not comfortable with the treadmill results because I imagine the possibility of energy transfer by friction between the powered tread and the vehicle.

It would be easy to test a model on a flat surface facing into a fan. Has this been done?

The problem with the demonstrations and tests we've performed are that we've already performed them. I can give you 10 reasons to deny any test with any instrumentation. Incidentally, "facing into a fan" has a far more basic issue - this thing isn't designed to go upwind.