"One Man’s Quest to Outrace Wind"

Sailing directly downwind faster than the wind

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"One Man’s Quest to Outrace Wind"

#1  Postby Davian » Mar 03, 2011 11:55 pm

I checked, and it's not the April edition, so I don't think this is a hoax, although it took a bit for the concept to sink into my brain:

... a fast-talking inventor named Rick Cavallaro, says that his craft—Blackbird—can, while sailing directly downwind, go faster than the wind that is propelling it.

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[youtube]http://www.youtube.com/watch?v=UGRFb8yNtBo[/youtube]
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Re: "One Man’s Quest to Outrace Wind"

#2  Postby Davian » Mar 04, 2011 6:03 am

An earlier article from Aug 2010:

'A friend following our progress online commented, “Someday one of these will hang from the rafters of the Air and Space Museum with a plaque which will read, ‘In the early part of the century, this device caused physics and aero professors everywhere to storm out of their classrooms in absolute frustration.’” When we had only equations to present, we were nearly universally ridiculed. When we built and demonstrated a manned cart beating the wind, people complained we had no theory to back it up. To this end another friend offered, “Sure it works in practice – but can you prove it works in theory?”'
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Re: "One Man’s Quest to Outrace Wind"

#3  Postby The_Metatron » Mar 04, 2011 6:50 am

I'd need one question answered:

Where does the energy come from to reverse the relative windspeed between the wind and the vehicle?

Once you match wind speed and direction, the relative windspeed is zero. You're traveling exactly along with the air molecules. This is easy to replicate in still air. So, this vehicle should just be able to tear ass around in still air.

Bullshit.

Edit: My original judgment on this phenomenon was mistaken. My retraction here. Rick's achievement is fascinating.
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Re: "One Man’s Quest to Outrace Wind"

#4  Postby CdesignProponentsist » Mar 04, 2011 7:16 am

The_Metatron wrote:I'd need one question answered:

Where does the energy come from to reverse the relative windspeed between the wind and the vehicle?

Once you match wind speed and direction, the relative windspeed is zero. You're traveling exactly along with the air molecules. This is easy to replicate in still air. So, this vehicle should just be able to tear ass around in still air.

Bullshit.


Yeah, but once you bridge that gap between positive relative wind speed to negative relative wind speed you can travel backwards through time.

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Re: "One Man’s Quest to Outrace Wind"

#5  Postby Aca » Mar 04, 2011 7:21 am

not this again:D

There are more than 20 threads on talkrational about this and if i recall there was a thread on RDF too
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Re: "One Man’s Quest to Outrace Wind"

#6  Postby zulumoose » Mar 04, 2011 7:22 am

Metatron you have not read the article, basically what he is doing is setting a 'sail' to tack angle and then shifting it backwards and forwards rapidly so that it is tacking to both sides equally. He achieves this by having two rigid and opposing sails rotate, ie a propellor.

It is known fact that a sail can outperform the wind speed, if tacking at an offset angle, so if you do this to both sides with great efficiency you can easily go faster downwind than actual wind speed. All he has achieved is to 'trick' the setup with a propellor design such that he can tack while keeping the hull pointed steadily downwind.
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Re: "One Man’s Quest to Outrace Wind"

#7  Postby Aca » Mar 04, 2011 8:22 am

Aca wrote:...if i recall there was a thread on RDF too


there was

http://forum.richarddawkins.net/viewtop ... 23&start=0
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Re: "One Man’s Quest to Outrace Wind"

#8  Postby CdesignProponentsist » Mar 04, 2011 8:59 am

Blows my mind. I guess it is possible. I'll have to read up more on this.
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Re: "One Man’s Quest to Outrace Wind"

#9  Postby Darkchilde » Mar 04, 2011 11:19 am

We had a thread back at RDF which was about exactly that same subject, and was very interesting because of the physics and technology that people employed in their tests.
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Re: "One Man’s Quest to Outrace Wind"

#10  Postby Made of Stars » Mar 04, 2011 11:31 am

Cue post from Recursive Prophet in 3... 2...
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Re: "One Man’s Quest to Outrace Wind"

#11  Postby The_Metatron » Mar 04, 2011 5:24 pm

zulumoose wrote:Metatron you have not read the article, basically what he is doing is setting a 'sail' to tack angle and then shifting it backwards and forwards rapidly so that it is tacking to both sides equally. He achieves this by having two rigid and opposing sails rotate, ie a propellor.

It is known fact that a sail can outperform the wind speed, if tacking at an offset angle, so if you do this to both sides with great efficiency you can easily go faster downwind than actual wind speed. All he has achieved is to 'trick' the setup with a propellor design such that he can tack while keeping the hull pointed steadily downwind.

It peeves me to no end to be told what I have or haven't read.

While sailing at an angle to the wind direction can, and does, achieve vehicle speeds in its direction of travel that are greater than the wind speed in its direction, in no case does the vector component of the vehicle's velocity in the same direction as the wind equal the wind velocity, let alone exceed it.

There's no such thing as a free lunch.
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Re: "One Man’s Quest to Outrace Wind"

#12  Postby Macdoc » Mar 04, 2011 5:34 pm

Yup - short tacks. TANSTAAFL

I asked someone about how the wheeled version would perform on ice.

No answer.

•••

Met
in no case does the vector component of the vehicle's velocity in the same direction as the wind equal the wind velocity,


that seems to say that you think the velocity of the vehicle downwind cannot exceed windspeed with the same vector?
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Re: "One Man’s Quest to Outrace Wind"

#13  Postby reksio » Mar 04, 2011 10:29 pm

The_Metatron wrote:
in no case does the vector component of the vehicle's velocity in the same direction as the wind equal the wind velocity, let alone exceed it.

Wrong. The downwind VMG (component of the vehicle's velocity in the same direction as the wind) can be greater than 2 x true wind speed for water sail craft, and more than 3 x for ice boats and land yachts.

For references check out:
http://en.wikipedia.org/wiki/Sailing_fa ... _made_good
It sailed 20 nautical miles (37 km) downwind in 1 hour 3 minutes, so its velocity made good downwind was about 2.5 times windspeed, consistent with being able to sail about 14 degrees off the apparent wind when sailing downwind.


For ice boats check out page 4 of this:
http://www.nalsa.org/Articles/Cetus/Ice ... -Cetus.pdf

Here is how it works:

Image

[youtube]http://www.youtube.com/watch?v=63hvQABLOaE[/youtube]

The_Metatron wrote:There's no such thing as a free lunch.

Of course not. Just like every wind powered vehicle, this cart slows down the true wind to extract wind energy. The air looses kinetic energy, which is used by the vehicle.

[youtube]http://www.youtube.com/watch?v=FqJOVHHf6mQ[/youtube]
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Re: "One Man’s Quest to Outrace Wind"

#14  Postby The_Metatron » Mar 05, 2011 7:40 am

Yes, yes, reksio. What you've done is confuse speed with velocity. True wind is a force, it has both magnitude and direction. If the wind is blowing due south at 10 meters per second, there is no trick of physics you can do to make that wind push anything due south at faster than 10 meters per second. Period. In fact, you're never going to make that wind push anything due south at exactly 10 meters per second, because the laws of conservation of energy are going to get in the way of that. You will have energy loss in the transfer of kinetic energy of the air molecules to whatever they're pushing, you'll have drag, friction. Like I said before, there's no such thing as a free lunch.

While you can use that wind blowing due south to push something southeast at a greater speed than the prevailing southerly wind speed, the vector component of that pushed vehicle's travel due south is never, ever, going to be greater than 10 meters per second due south.

Wind is a force, just like gravity is a force. For our purposes on earth, gravity is a downward force. Are you telling us there is a way to increase that force simply by changing the direction of falling from straight down to falling at some other angle? This would, of course, instantly negate the study of ballistics. I bring this up as an example to illustrate that you cannot break up vector components of a force in such a way that their vector totals exceed the original force. Ever.
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Re: "One Man’s Quest to Outrace Wind"

#15  Postby reksio » Mar 05, 2011 8:19 am

The_Metatron wrote:
True wind is a force,

Wrong. True wind is the velocity difference between air and surface. Velocity and force are different physical quantities.

The_Metatron wrote:it has both magnitude and direction.

Yes, true wind is a vector. See the vector diagram:

Image

As you see the vector component of the vehicle's velocity in the same direction as the true wind(downwind VMG) is 1.5 time greater in magnitude than the true wind vector. And still the sail can produce a force that has a positive vector component in the direction of travel. The boat can accelerate or hold it's speed.

The_Metatron wrote:
If the wind is blowing due south at 10 meters per second. There is no trick of physics you can do to make that wind push anything due south at faster than 10 meters per second. Period.

Reality disagrees. Period.

http://en.wikipedia.org/wiki/Sailing_fa ... n_the_wind

Indeed iceboats can make good both upwind and downwind at speeds far greater than the wind.[35][36] And so can sand yachts:
...
the yacht Greenbird was proceeding at about 3 times the speed of the wind on a course about 120 degrees off the true wind.[17] Thus, its speed made good downwind was about 1.5 times the speed of the wind.
...
It sailed 20 nautical miles (37 km) downwind in 1 hour 3 minutes, so its velocity made good downwind was about 2.5 times windspeed,
...
The polar chart in Figure PS1 of the cited book High Performance Sailing[5] shows that boats that were sailing in 1996 were able to make good downwind at about 1.5 times the speed of the wind.


The_Metatron wrote:
Wind is a force, just like gravity is a force.

Wrong, see above.

The_Metatron wrote:you cannot break up vector components of a force in such a way that their vector totals exceed the original force.

I have posted a vector diagram that shows how downwind VMG greater than true wind is possible. So far, you have failed to point out any errors in the diagram.
Last edited by reksio on Mar 05, 2011 10:12 am, edited 1 time in total.
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Re: "One Man’s Quest to Outrace Wind"

#16  Postby Onyx8 » Mar 05, 2011 9:13 am

I don't get this either.

In the video quite a deal is made when the ribbons fluttering from the mast start being blown to the rear of the craft, this not only shows that the craft is now moving faster than the wind, but it also would seem that it would mean that the wind is now acting to slow the craft down. Or has the pitch on the propellor changed to now provide forward force to the cart even though the wind is now from the front? If that is the case why wasn't it mentioned?

Variable pitch propellors are non-trivial things, you notice if you happen to build one.

Just a thought and a bookmark, this is counter-intuitive to me and I'm skeptical. :grin: :grin:
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Re: "One Man’s Quest to Outrace Wind"

#17  Postby reksio » Mar 05, 2011 10:04 am

Onyx8 wrote:
In the video quite a deal is made when the ribbons fluttering from the mast start being blown to the rear of the craft,

You mean this at about 3:00 ?
[youtube]http://www.youtube.com/watch?v=EEuAqq8FINw[/youtube]

Onyx8 wrote:it also would seem that it would mean that the wind is now acting to slow the craft down.

The apparent wind at the chassis is creating drag that acts to slow the craft. The apparent wind at the rotating airfoils is creating lift that acts to accelerate the craft.

Onyx8 wrote:
Or has the pitch on the propellor changed to now provide forward force to the cart even though the wind is now from the front?

No, on this early prototype the blade pitch was fixed. You don't need to change the pitch, to accelerate past windspeed. Just like you don't need to change the sail setting on an ice boat, to accelerate past downwind VMG = windspeed on a downwind tack.

Onyx8 wrote:
Variable pitch propellors are non-trivial things, you notice if you happen to build one.

They later implemented variable pitch to optimize acceleration. But it works without it, just very slowly at the start as you see in the video.
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Re: "One Man’s Quest to Outrace Wind"

#18  Postby The_Metatron » Mar 05, 2011 1:49 pm

Sure, reksio. Maybe you can explain to us how to extract kinetic energy from moving air molecules that haven't yet reached your craft, then.
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Re: "One Man’s Quest to Outrace Wind"

#19  Postby reksio » Mar 05, 2011 2:12 pm

The_Metatron wrote:Maybe you can explain to us how to extract kinetic energy from moving air molecules that haven't yet reached your craft, then.

You don't. You extract kinetic energy (in the ground frame) from molecules currently passing at the airfoil.
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Re: "One Man’s Quest to Outrace Wind"

#20  Postby Xaihe » Mar 05, 2011 2:43 pm

This whole technology is easier to understand when you understand that a cart with a propellor (connected to the wheels) can start moving directly upwind. I'm sure it's mentioned somewhere in the article (the fact that the prop is connected to the wheels), but it would be helpful if people would be more straightforward in explaining how this works. My guess is that it's so obvious to people who know it, that they fail to realize that it's not obvious to people who don't know it.
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