Naked Science Forum

On the Lighter Side => Science Experiments => Topic started by: hamdani yusuf on 14/06/2021 04:24:34

Title: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 14/06/2021 04:24:34
Veritasium made a video showing that we can build a wind powered car that goes faster than the wind itself.
Alexander Kusenko, a UCLA Physics Prof. disagree.

https://twitter.com/veritasium/status/1403130178197278720
Quote
Big News! UCLA Physics Prof, @alexkusenko bet me $10,000 that I'm wrong about going downwind faster than the wind. Our wager was witnessed by
@neiltyson
@BillNye
@seanmcarroll
If I win, to what charitable cause should I donate the funds?
(https://pbs.twimg.com/media/E3jpZa4VUAABl48?format=jpg&name=small)
(https://pbs.twimg.com/media/E3jq0fQVUAAgtUL?format=jpg&name=360x360)
(https://pbs.twimg.com/media/E3jq0fQVkAI7pKP?format=jpg&name=360x360)



Here is the response from Alexander Kusenko trying to debunk the claim in Veritasium's video.
Quote
Here is a set of 10 slides that (1) explain what is seen in the video (2) point out the errors theoretical arguments, and (3) provide a complete solution to the problem.
https://docs.google.com/presentation/d/1xuN-9C1Gs6MAVTJkhh1vMJw0atmSsTsKDkhEfN898K4/edit#slide=id.gdc0eb9892c_0_223

Quote
Replying to
@thephysicsgirl
 @alexkusenko
 and 3 others
Come on! If someone emails me saying I’m wrong, first I try to convince them that I’m right. Then if we continue to disagree I suggest we make it interesting. The key is to get to the truth. The best, scientifically accurate explanation should win.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 14/06/2021 04:35:04
Let me make the first move. I'm on Veritasium's side. I'll explain my reasoning later.
Will someone give it a try to show their prowess in physics?
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 14/06/2021 10:39:51
Steve Mould already made a response video.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 14/06/2021 10:47:36
And here is one slide found in Google Document of Alexander Kusenko (UCLA and Kavli IPMU) titled
Debunking the claim of propeller-assisted straight-downwind land sailing faster than the wind

Erroneous theoretical arguments
Quote
Some literature on the subject
Wikipedia page: https://en.wikipedia.org/wiki/Blackbird_(land_yacht)
[1]   Md. Sadak Ali Khan, et al, “Analysis of Down-Wind Propeller Vehicle”,
International Journal of Scientific and Research Publications, 3, 4. (2013)  http://www.ijsrp.org/research-paper-0413/ijsrp-p16135.pdf
[2]    M. Drela, "Dead-Downwind Faster Than The Wind (DFTTW) Analysis", unpublished
https://www.blueplanettimes.com/wp-content/uploads/2010/08/Drela-DDWFTTW-Analysis.pdf
[3]    Presentation by Rick Cavallaro (from 54:47 until 59:00)  These references contain the same errors.  Let us discuss them.


And here is the conclusion slide
Quote
  • The experiment in the Veritasium video is consistent with the car’s behavior expected in variable wind.  The experiment does not prove the possibility of a wind-powered sustained (v=const, a=0) straight-downwind land sailing faster than the  wind.
  • Incorrect theoretical arguments are found in the literature.
  • The correct description confirms the intuitive result that, in a steady state (v=const), the speed of the car is always smaller than the speed of the wind.
    If the car moves faster than the wind, the acceleration is always negative (v≠const, a<0), so that the car decelerates until the speed drops below the speed of the wind.

Title: Re: Can we go downwind faster than the wind?
Post by: alancalverd on 15/06/2021 21:35:04
For the windmill to turn, there must be a net wind passing through the propellor disc..

If the wind vector over the ground is v and the car is moving downwind at v, there is no net wind passing through the disc.
Title: Re: Can we go downwind faster than the wind?
Post by: Halc on 15/06/2021 22:00:18
I cannot read the legal document, the resolution of which is insufficient.
I did not read any of the links.


I presume the document disallows such cheats, because the task would be pretty easy in all 3 cases.
If the vehicle is moving straight downwind, then in the inertial frame of the vehicle, the wind motion is directly from the front as is the ground motion, and it is hard to initially imagine generating positive net force from that.

Second thoughs:  Imagine an axle with 30 cm tires on a road with a trench between the wheels.  In the middle of the axle is a 50 cm spool of string wound up with the free end below the road facing the rear.  You take the end of the string and pull backwards, and the axle assembly moves forward away from the direction of pull.
It could work that way.  The car has like almost no mass, so the pressure from the front turns the propeller, and that energy is applied at the wheels to add more positive thrust at the wheels than the negative thrust against the propeller.

So despite my initial skepticism, I'd consider it.

Third thought:  In the optimal scenario, there would be no wind at all.  In the reference frame of the vehicle, it's the same air movement across the propeller, but maximum forward movement of the road helping the vehicle. That makes it a perpetual motion machine.  I vote no again.
Title: Re: Can we go downwind faster than the wind?
Post by: Bored chemist on 15/06/2021 22:28:55
Imagine you are standing on the boat when it reaches the same speed as the wind.
You and the wind are both doing, for example, 20 miles per hour (WRT the ground).
So your speed with respect to the air around you is zero.
What is driving the windmill?

I hate to say it, but Alan is right.

Now, what I'm less sure about is whether you can sail into the wind faster than the wind.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 15/06/2021 23:14:59
Here is the theory part of Wikipedia article.
Quote
Gaunaa, et al. describe the physics of rotor-powered vehicles. They describe two cases, one from the vantage point of the earth and the other from the vantage point of the air stream and come to the same conclusions from both frames of reference. They conclude that (apart from forces that resist forward motion):[6]

There is no theoretical upper limit to how fast a rotor-driven craft can go directly upwind.
Likewise, there is no theoretical upper limit to how fast a rotor-driven craft can go directly downwind.
These conclusions hold both for land and water craft.

Required for wind-powered vehicle (or water craft) motion are:[6]

Two masses moving with respect to each other, e.g. the air (as wind) and the earth (land or water).
The ability to change the velocity of either mass with a propeller or a wheel.

In the case of a rotor-powered vehicle, there is a drive linkage between the rotor and the wheels. Depending on one's frame of reference—the earth's surface or moving with the air mass—the description of how available kinetic energy powers the vehicle differs:[6]

As seen from the vantage point of the earth (e.g. by a spectator), the rotor (acting like a wind turbine) decelerates the air and drives the wheels against the earth, which it accelerates imperceptibly.
As seen from the vantage point of the air stream (e.g. by a balloonist), the wheels impede the vehicle—decelerating the earth imperceptibly—and drive the rotor (acting like a propeller), which accelerates the air and propels the vehicle.
The connection between the wheels and the rotor causes the rotor to rotate faster with increasing vehicle speed, thereby allowing the rotor blades to continue to obtain forward lift from the wind (as seen from the ground) or to propel the vehicle (as seen from the air stream).[6]
And the achievement
Quote
On July 2, 2010, Blackbird set the world's first certified record for going directly downwind, faster than the wind, using only power from the available wind during its run on El Mirage Dry Lake. The yacht achieved a dead downwind speed of about 2.8 times the speed of the wind.[1][16]

On June 16, 2012, Blackbird set the world's first certified record for going directly upwind, without tacking, using only power from the wind. The yacht achieved a dead upwind speed of about 2.1 times the speed of the wind.[1]
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 15/06/2021 23:17:55
So your speed with respect to the air around you is zero.
What is driving the windmill?
As mentioned in the article I quoted above, it's the earth. So if the vehicle is running on a treadmill that has the same velocity as the wind, it can't accelerate.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 15/06/2021 23:23:49
Now, what I'm less sure about is whether you can sail into the wind faster than the wind.
Yes, it can. The record is 2.1 times the speed of the wind.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 15/06/2021 23:32:03
For the windmill to turn, there must be a net wind passing through the propellor disc..

If the wind vector over the ground is v and the car is moving downwind at v, there is no net wind passing through the disc.

What's needed is difference of velocity between the wind and the earth. Without the difference, it can't harvest the kinetic energy.
It should be more convincing if the video shows the case of running against the wind. Unfortunately it doesn't.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 15/06/2021 23:48:39
IMO, people who are sceptic about it think that it would enable perpetual motion and overunity energy generators. But that's not the case.
Someone who is not familiar with buck converter may think that an electronic device can't produce higher dc voltage than the battery that powers it. It's a  similar case here.
Title: Re: Can we go downwind faster than the wind?
Post by: alancalverd on 16/06/2021 07:36:57
No. The buck converter, electromechanical inverters, etc., generate less power than they consume, but at a different voltage. 

Sailboats and land yachts are interesting. On a beam reach you can certainly sail faster than the wind, but essentially perpendicular to the wind vector. Running downwind there is no net flow of wind over the sail so you can't exceed the wind speed.

A "crosswind" rotor, like a sail on beam reach, can generate as much power as you want by just making it bigger, so physics says your speed perpendicular to the wind depends only on the ratio of drag to rotor size, and the practical limit is set by engineering considerations. But a "downwind" rotor still requires an actual mass flow from one side of the disc to the other, and that will be zero if the disc is moving at the same speed as the wind. 
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 16/06/2021 08:35:06
No. The buck converter, electromechanical inverters, etc., generate less power than they consume, but at a different voltage.
I'm pretty sure that the Blackbird also produces less power than they consume, even when it's running faster than the wind. What makes you think otherwise?

think that an electronic device can't produce higher dc voltage than the battery that powers it.
Replacing the word for word,
think that a land yacht can't achieve higher speed than the wind that powers it.
Title: Re: Can we go downwind faster than the wind?
Post by: Bored chemist on 16/06/2021 08:38:56
As mentioned in the article I quoted above, it's the earth.
The Earth isn't touching the windmill.
The only thing in contact with the windmill is the air.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 16/06/2021 09:12:33
As mentioned in the article I quoted above, it's the earth.
The Earth isn't touching the windmill.

Not directly. They are connected by the wheels and through a gear mechanism.
As seen from the vantage point of the air stream (e.g. by a balloonist), the wheels impede the vehicle—decelerating the earth imperceptibly—and drive the rotor (acting like a propeller), which accelerates the air and propels the vehicle.
The connection between the wheels and the rotor causes the rotor to rotate faster with increasing vehicle speed, thereby allowing the rotor blades to continue to obtain forward lift from the wind (as seen from the ground) or to propel the vehicle (as seen from the air stream).[6]
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 16/06/2021 09:21:54
Another analogy I can think of. This time is using thermoelectric effect.
Two reservoirs are given. The first is cool, it's 0°C. The second is hot, it's 100°C. They are placed in a large fully hermetic room that can't exchange energy to the outside world.

Is it possible to make a device that produce temperature higher than 100°C?
Is it possible to make a device that produce temperature lower than 0°C?

It's easy to produce temperature between 0°C and 100°C, by simply connecting those reservoirs with a thermal conductor. There would be temperature gradient in the conductor, spanning from 0°C near the cool reservoir to 100°C near the hot reservoir. At the center of the conductor, it's around 50°C.

Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 16/06/2021 09:33:38
Is it possible to make a device that produce temperature higher than 100°C?
Is it possible to make a device that produce temperature lower than 0°C?
https://en.wikipedia.org/wiki/Thermoelectric_effect
A device combining Seebeck effect and Peltier effect can produce temperature higher than the hot reservoir, as well as lower than the cool reservoir.
(https://upload.wikimedia.org/wikipedia/commons/thumb/c/c1/Thermoelectric_effect.svg/220px-Thermoelectric_effect.svg.png)
(https://upload.wikimedia.org/wikipedia/commons/thumb/3/3b/Thermoelectric_Cooler_Diagram.svg/220px-Thermoelectric_Cooler_Diagram.svg.png)

The device can't work if both reservoirs have the same temperature. Similar to land yacht that can't work when the wind has the same velocity as the earth surface. I hope this analogy makes sense to you, Halc.

Third thought:  In the optimal scenario, there would be no wind at all.  In the reference frame of the vehicle, it's the same air movement across the propeller, but maximum forward movement of the road helping the vehicle. That makes it a perpetual motion machine.  I vote no again.
Title: Re: Can we go downwind faster than the wind?
Post by: Bored chemist on 16/06/2021 09:51:17
Is it possible to make a device that produce temperature higher than 100°C?
Is it possible to make a device that produce temperature lower than 0°C?
Yes*.
So what?

* for example, you could run a "steam" engine from the temperature difference using alcohol as the working fluid.
That engine could turn a generator and run an electric fridge or furnace.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 16/06/2021 11:15:16
Is it possible to make a device that produce temperature higher than 100°C?
Is it possible to make a device that produce temperature lower than 0°C?
Yes*.
So what?

* for example, you could run a "steam" engine from the temperature difference using alcohol as the working fluid.
That engine could turn a generator and run an electric fridge or furnace.
So, we can build a machine that runs faster than the wind without violating known physical laws.
From the perspective of a space observer, the earth surface moves at v1, while the wind moves at v2. Energy can be harvested as long as v1≠v2.
Title: Re: Can we go downwind faster than the wind?
Post by: Halc on 16/06/2021 12:17:11
Concerning these Gaunaa lines quoted from wiki:
As seen from the vantage point of the earth (e.g. by a spectator), the rotor (acting like a wind turbine) decelerates the air and drives the wheels against the earth, which it accelerates imperceptibly.
Gaunaa is wrong here, and the wiki doesn't have a secion pointing out the flaws in his argument. In the frame of Earth, the rotor accelerates the air since it is pushing from behind. Since both air and car accelerate, that violates energy conservation.

Quote
As seen from the vantage point of the air stream (e.g. by a balloonist), the wheels impede the vehicle—decelerating the earth imperceptibly—and drive the rotor (acting like a propeller), which accelerates the air and propels the vehicle.
Again wrong. Relative to air, the earth and car are moving in opposite directions, so the thrust on the Earth again serves to accelerate both Earth and car, same violation.
Apparently there is no peer review section in the wiki article, which just reports Gaunaa's faulty physics.

IMO, people who are sceptic about it think that it would enable perpetual motion and overunity energy generators. But that's not the case.
That's my argument, yes. Show me where It's not the case. I'm referring to the case of going with the wind.

I'm pretty sure that the Blackbird also produces less power than they consume
Exactly. That means it must be consuming stored energy, which is what a normal car does when going faster than the wind. It's cheating. There's a battery in it somewhere.

Is it really supposedly a mechanical linkage between prop and wheels? I'd have gone for electric connection like they use with railroad engines. I've been in one diesel train in my life with an actual mechanical linkage and it was hilariously awkward.
Title: Re: Can we go downwind faster than the wind?
Post by: charles1948 on 16/06/2021 20:18:45
Can't sailing-ships go in an opposite direction to the wind.  They can sail westwards against an easterly wind.

That might seem a physical impossibility.  A sailing-ship is blown along by the wind in its sails, so how can the ship go in the opposite direction from the wind?
Title: Re: Can we go downwind faster than the wind?
Post by: Bored chemist on 16/06/2021 21:24:57
Is it really supposedly a mechanical linkage between prop and wheels? I'd have gone for electric connection like they use with railroad engines. I've been in one diesel train in my life with an actual mechanical linkage and it was hilariously awkward.
I was rather hoping they would train a small army of squirrels to carry small clockwork motors around the system, winding them up using the power of the  prop, and then taking them down and connecting them to the wheels as drive.



Can't sailing-ships go in an opposite direction to the wind.  They can sail westwards against an easterly wind.

That might seem a physical impossibility.  A sailing-ship is blown along by the wind in its sails, so how can the ship go in the opposite direction from the wind?
You would know this if you had bothered to read the stuff cited but...
https://en.wikipedia.org/wiki/Tacking_(sailing)
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 16/06/2021 22:44:19
Gaunaa is wrong here, and the wiki doesn't have a secion pointing out the flaws in his argument. In the frame of Earth, the rotor accelerates the air since it is pushing from behind. Since both air and car accelerate, that violates energy conservation.
No. The energy harvesting will make the velocity of the air closer to the velocity of earth surface. That's still true even when the vehicle runs faster than the wind.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 16/06/2021 23:13:38
Again wrong. Relative to air, the earth and car are moving in opposite directions, so the thrust on the Earth again serves to accelerate both Earth and car, same violation.
They accelerate to the opposite direction. Your reasoning implies that the air doesn't accelerate due to the interaction. That's not the case in the experiment.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 16/06/2021 23:56:13
That's my argument, yes. Show me where It's not the case. I'm referring to the case of going with the wind.
The vehicle collects energy by reducing relative velocity between earth surface and the air. It would be easier if the analysis is done in a reference frame where the total momentum of interacting system is 0. Conservation of momentum guarantees that observers staying in this frame doesn't change their velocity due to the interaction.

The fact that the vehicle only interacts with a small part of the wind adds the complexity of the problem. So, to simplify the system, we can replace the salt lake bed with a conveyor on the floor moving to the left. The wind is replaced by a conveyor hung on the ceiling, moving to the right. The vehicle starts at the same velocity as bottom conveyor.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 17/06/2021 01:40:56
Exactly. That means it must be consuming stored energy, which is what a normal car does when going faster than the wind. It's cheating. There's a battery in it somewhere.
The energy was stored in the wind that was moving relative to the earth surface.

Quote
British science fiction writer Arthur C. Clarke formulated three adages that are known as Clarke's three laws, of which the third law is the best known and most widely cited. They are part of his ideas in his extensive writings about the future.[1] These so-called laws are:

When a distinguished but elderly scientist states that something is possible, he is almost certainly right. When he states that something is impossible, he is very probably wrong.
The only way of discovering the limits of the possible is to venture a little way past them into the impossible.
Any sufficiently advanced technology is indistinguishable from magic.
https://en.m.wikipedia.org/wiki/Clarke%27s_three_laws

Before accusing someone of cheating, you should be pretty sure that it really is physically impossible.
Extraordinary claims requires extraordinary evidence. In the bet, Derek's claim is that a wind powered vehicle can go faster than the wind without additional power sources. The evidence is that he has driven the vehicle himself. He accepted the explanation using sail boat analogy for the turbine.
On the other hand, Alex' claim is that it cannot be done sustainably. It occurs due to wind gusts. The evidence is mostly theoretical. It would be better if he replicates the experiment and show that it doesn't work as claimed.

Your claim that there's a hidden battery is even more extraordinary than theirs. Before putting his money on the bet, Derek must have checked the possibility of cheating, which he didn't find any. Otherwise the bet won't happen. Your only evidence is your application of established theory of physics, which is not more extraordinary than theirs.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 17/06/2021 03:22:33
You would know this if you had bothered to read the stuff cited but...
https://en.wikipedia.org/wiki/Tacking_(sailing)
I realized that not everyone is willing to follow a link, so I guess I'll just quote it here.
Quote
Tacking is a sailing maneuver by which a sailing vessel, whose desired course is into the wind, turns its bow toward and through the wind so that the direction from which the wind blows changes from one side of the boat to the other, allowing progress in the desired direction.[1] The opposite maneuver to tacking is called jibing, or wearing on square-rigged ships, that is, turning the stern through the wind. No sailing vessel can move directly upwind, though that may be the desired direction, making this an essential maneuver of a sailing ship. A series of tacking moves, in a zig-zag fashion, is called beating, and allows sailing in the desired direction.
(https://upload.wikimedia.org/wikipedia/commons/thumb/2/22/Beating_to_windward.svg/600px-Beating_to_windward.svg.png)


Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 17/06/2021 10:38:21
The vehicle collects energy by reducing relative velocity between earth surface and the air. It would be easier if the analysis is done in a reference frame where the total momentum of interacting system is 0. Conservation of momentum guarantees that observers staying in this frame doesn't change their velocity due to the interaction.

The fact that the vehicle only interacts with a small part of the wind adds the complexity of the problem. So, to simplify the system, we can replace the salt lake bed with a conveyor on the floor moving to the left. The wind is replaced by a conveyor hung on the ceiling, moving to the right.
Let's start the analysis using this simplified version of the experiment. First, using Rand's razor, any relevant assumptions should be made explicit.
- The mass of the vehicle (m) is much smaller than bottom or top conveyor (M, they are identical).
- At the start of the experiment, the vehicle's velocity is the same as bottom conveyor's which is -v, while the top conveyor's is +v.
- Loss of mechanical energy due to internal friction in the vehicle is negligible.

The question is, is it possible to make the vehicle move at velocity more than +v?

At the beginning of the experiment, the kinetic energy of the vehicle is ½ m v².
At the end of the experiment, the kinetic energy of the vehicle is > ½ m v².
This additional energy must come from the reduction of kinetic energy of the conveyors. But since M is much bigger than m, their change of velocities would be insignificant.
Title: Re: Can we go downwind faster than the wind?
Post by: Bored chemist on 17/06/2021 10:52:46
So, we can build a machine that ...
As far as I can tell, that's a non sequitur.
Just because we can use one form of energy supply to do one job, does not mean we can use a different form to do some other job.
Title: Re: Can we go downwind faster than the wind?
Post by: Bored chemist on 17/06/2021 11:07:37
I think I might have changed my mind about this.

Where does this go wrong.
Imagine I'm in the basket of a helium  balloon and there's a steady wind blowing.
From the point of view of someone on the ground I'm already travelling at the same speed as the wind. Let's say I'm heading North (from their point of view)
I see the ground rushing past me to the South.
I put a generator on the end of a stick and fit a wheel and tyre to the shaft of the generator.

I can lean out of the basket and put the wheel in contact with the ground so it spins the generator and creates electricity.
I can use that electricity to run a motor and have that drive a propeller and I can use that to move my balloon through the air in any direction I choose.

If I choose, I can point the propeller to blow air Southward and thus move myself North WRT the air.

Now consider what my ground based  observer sees.
He saw me heading North at the same speed as the wind. (and not moving WRT the air- because I'm a drifting balloon)
Then he saw me turn on a propeller and start to move North WRT the air and thus he sees me moving North faster than the wind.

This is true whether or not he knows that I am getting my motive power from a battery or from the generator on a stick.

So he sees my "craft" going faster than the wind.
It isn't powered by anything apart from the wind.

If I dangle a chassis with low friction castor wheels from my balloon so it just touches the ground, I can call the balloon a "car".
And if I use a paddle wheel on a stick (and floats rather than wheels) I can use the same principle to travel across water. If I do that I can call my balloon a boat.
So, as far as I can see, that's a way to travel faster than the wind powered by the wind, and call it "sailing"
Title: Re: Can we go downwind faster than the wind?
Post by: Halc on 17/06/2021 12:34:57
I read the line in the article: "the rotor (acting like a wind turbine)" to mean that the air drove the propeller, the energy of which was sent to the wheels, which violates energy conservation per my post above. But reading closer, the opposite is what is going on. The rotor acts as a propeller, not a turbine. The wheels apply a braking action and the propeller applies thrust. Thus energy is taken from the air movement and no violation of energy conservation takes place.

The wheels generate the energy.  Work is force*distance and so say F=1, then work is proportional to distance. The road is moving past the vehicle faster than is the air when the vehicle is moving with the wind, so more energy is taken from the road than is given to the air.  I.E. less work is done by the propeller because distance is lower. Hence the thing works. Wet noodle to me for reading it wrong, and to the website for incorrectly label the prop as a turbine.
Title: Re: Can we go downwind faster than the wind?
Post by: Origin on 17/06/2021 12:59:44
I think this contraption can go faster than the wind for a few moments but that would be it.  When the wind causes the vehicle to begin moving the wheels will cause the fan blade to to spin and it can certainly get it to move a bit faster than the wind, but it will immediately begin to decelerate as it exceeds the wind speed due to friction (both from the ground and air).  So while the vehicle could exceed the speed of the wind for short periods it will on average move slower than the wind.  If there was a perfectly steady wind over a level course that is say a kilometer in length the vehicle will certainly average a speed less than the wind, even though there could be times in the run where the speed of the vehicle is faster than the wind.
If the average speed was faster than the wind that would violate conservation laws, clearly if there is no force from the wind (because you are moving faster than the wind) why would you continue to maintain speed?  It is nonsensical.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 17/06/2021 14:53:06
So, we can build a machine that ...
As far as I can tell, that's a non sequitur.
Just because we can use one form of energy supply to do one job, does not mean we can use a different form to do some other job.
I used the analogies merely to break the mental barrier that keeps our minds inside the box, which says that energy users can't get to higher energy density than the sources.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 17/06/2021 15:04:03
I think this contraption can go faster than the wind for a few moments but that would be it.  When the wind causes the vehicle to begin moving the wheels will cause the fan blade to to spin and it can certainly get it to move a bit faster than the wind, but it will immediately begin to decelerate as it exceeds the wind speed due to friction (both from the ground and air).  So while the vehicle could exceed the speed of the wind for short periods it will on average move slower than the wind. 
How long is short period?

How do you think it can go against the wind?
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 17/06/2021 15:11:04
If the average speed was faster than the wind that would violate conservation laws, clearly if there is no force from the wind (because you are moving faster than the wind) why would you continue to maintain speed?  It is nonsensical.
Did you watch the videos?
Did you read the Wikipedia article?
Did you even read previous posts by me and other members?

Your comment indicates that the answers are no for all of the questions above.
Title: Re: Can we go downwind faster than the wind?
Post by: Origin on 17/06/2021 15:14:15
How long is short period?
87 seconds or less.
Title: Re: Can we go downwind faster than the wind?
Post by: Origin on 17/06/2021 15:15:24
Did you watch the videos?
Did you read the Wikipedia article?
Did you read previous posts by me and other members?
Your comment indicates that the answers are no for all of the questions above
Then why ask?
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 17/06/2021 15:20:15
How long is short period?
87 seconds or less.
Where does that number come from?
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 17/06/2021 15:30:31
Did you watch the videos?
Did you read the Wikipedia article?
Did you read previous posts by me and other members?
Your comment indicates that the answers are no for all of the questions above
Then why ask?
To give you a chance to defend yourself from accusations. The discussion is meant to exchange ideas to find the best explanation to the problem. If your mind has already been made up before even considering competing ideas, why bother joining a discussion?
Title: Re: Can we go downwind faster than the wind?
Post by: Bored chemist on 17/06/2021 18:24:24
If the average speed was faster than the wind that would violate conservation laws,
Nope.
Title: Re: Can we go downwind faster than the wind?
Post by: Bored chemist on 17/06/2021 18:25:40
If there was a perfectly steady wind over a level course that is say a kilometer in length the vehicle will certainly...
Your "certainty" seems unfounded.
Could you please explain in detail where my idea fails?
Title: Re: Can we go downwind faster than the wind?
Post by: Origin on 17/06/2021 19:22:53
Nope
I could certainly be wrong.  My simple minded way of looking at this is; if the vehicle is powered by air movement and on average the relative air movement is 0 fps or negative fps, where is the force coming from to maintain the movement?
Title: Re: Can we go downwind faster than the wind?
Post by: Bored chemist on 17/06/2021 19:38:31
Nope
I could certainly be wrong.  My simple minded way of looking at this is; if the vehicle is powered by air movement and on average the relative air movement is 0 fps or negative fps, where is the force coming from to maintain the movement?
That's an understandable mistake to make.
Here's where I made it.


https://www.thenakedscientists.com/forum/index.php?topic=82462.msg643362#msg643362

And here is where I changed my mind (and why)

https://www.thenakedscientists.com/forum/index.php?topic=82462.msg643539#msg643539
Title: Re: Can we go downwind faster than the wind?
Post by: Origin on 18/06/2021 00:39:20
I can lean out of the basket and put the wheel in contact with the ground so it spins the generator and creates electricity.
I can use that electricity to run a motor and have that drive a propeller and I can use that to move my balloon through the air in any direction I choose.
That wheel on the ground will be a big drag and slow you down considerably.  Since the generator and the motor are not 100% efficient (the propeller is not 100% efficient either) I do not think that you will be able to even make it back up to your speed before you lowered the wheel.
Title: Re: Can we go downwind faster than the wind?
Post by: Halc on 18/06/2021 02:34:15
That wheel on the ground will be a big drag and slow you down considerably.
No, it's a relatively small drag.  Compute the work done by the wheels to the work done by the prop. In equilibrium, force is the same in both cases, so it's a matter of difference in distance.  It might be a 3-1 ratio at slower speeds, and efficiency lost between prop and wheels isn't that bad.
Title: Re: Can we go downwind faster than the wind?
Post by: Bored chemist on 18/06/2021 08:38:07
That wheel on the ground will be a big drag and slow you down considerably. 

The whole windward surface of the balloon is being pushed by the wind, that's doing nearly all the work of moving the balloon WRT the ground.
The load from the wheel on a stick is small by comparison.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 18/06/2021 09:09:31
Update on the wager.
https://twitter.com/veritasium/status/1405699594013470723
(https://pbs.twimg.com/media/E4IMU9MVkAMKGAQ?format=jpg&name=small)

He even shows a clear close-up image of the exposed gear assembly. It shows that the whole system is purely mechanical.
(https://pbs.twimg.com/media/E4JEkoPVoAwlum-?format=jpg&name=360x360)
(https://pbs.twimg.com/media/E4JEkoRVkAAjQvR?format=jpg&name=360x360)

His transparency brings credibility.
The ratio of the gears seems to be 1:1
So the speed multiplier must come from wheels radii vs blades radii, and also blades angle.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 18/06/2021 10:45:21
That's an understandable mistake to make.
Here's where I made it.


https://www.thenakedscientists.com/forum/index.php?topic=82462.msg643362#msg643362

And here is where I changed my mind (and why)

https://www.thenakedscientists.com/forum/index.php?topic=82462.msg643539#msg643539

Wise Quotes. When an honest man discovers he is mistaken, he will either cease being mistaken, or he will cease being honest.

I'd like to express my respect to your scientific integrity. We all make mistakes sometimes. How we response when we discover one will show who we are.

Albert Einstein Quotes ... A person who never made a mistake never tried anything new.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 18/06/2021 10:56:23
The vehicle collects energy by reducing relative velocity between earth surface and the air. It would be easier if the analysis is done in a reference frame where the total momentum of interacting system is 0. Conservation of momentum guarantees that observers staying in this frame doesn't change their velocity due to the interaction.

The fact that the vehicle only interacts with a small part of the wind adds the complexity of the problem. So, to simplify the system, we can replace the salt lake bed with a conveyor on the floor moving to the left. The wind is replaced by a conveyor hung on the ceiling, moving to the right. The vehicle starts at the same velocity as bottom conveyor.

They say that a picture speaks a thousand word. So here we are.
(https://www.thenakedscientists.com/forum/index.php?action=dlattach;topic=82462.0;attach=32034)
- belt velocity is 1 m/s.
- no slip between belt and wheels.
- front (right side) wheels turn independently, and installed for balance only.
- rear wheels are coupled using a V-belt.
Since everything is balanced, the speed of the car is 0.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 18/06/2021 11:03:14
Now replace the pulley of the top rear wheel with a bigger one, e.g. three times bigger.
(https://www.thenakedscientists.com/forum/index.php?action=dlattach;topic=82462.0;attach=32036)
It would make the rear top wheel rotates slower than the others. So we need to replace the wheel with a slippery type.
This will enable the car to get a push from the top conveyor, hence accelerates to the right. Acceleration stops when the rear top wheel has the same tip speed as the top conveyor.
Title: Re: Can we go downwind faster than the wind?
Post by: Origin on 18/06/2021 12:55:03
If the average speed was faster than the wind that would violate conservation laws, clearly if there is no force from the wind (because you are moving faster than the wind) why would you continue to maintain speed?  It is nonsensical
I looked at wiki and some other sources and no where did is see that the average speed of the vehicle over the run was faster than the speed of the wind.  I only saw that the speed of the vehicle could exceed the speed of the wind at some point. 
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 18/06/2021 14:22:46
If the top wheel has the same tip speed as the top conveyor, it wouldn't rotate at all, but then the bottom wheel shouldn't.
Why not? Remember that the bright grey wheel is slipping. It doesn't have a problem to spin at different tip speed than top conveyor.

Let's start with the car stationary to the frame of the conveyors. Dark wheels all spin at 1 m/s since they don't slip. The pulleys and V-belt will force the bright wheel to spin at lower tip speed, i. e. 1/3 m/s. It's possible since it is slipping. But as long as it's not perfectly slippery, there would be some friction forcing the bright wheel to increase its tip speed. This force is transmitted through pulleys and V-belt to bottom rear wheel, increasing its tip speed too. This will make the car move to the right, since the front wheels can spin independently.
Title: Re: Can we go downwind faster than the wind?
Post by: Halc on 18/06/2021 14:41:51
Why not?
OK, I read you wrong. You said this:
Acceleration stops when the rear top wheel has the same tip speed as the top conveyor.
You're saying that acceleration stops when the wheel stops slipping, but I took it as when the wheel moves at the conveyor speed.
I agree. My bit about getting the car to move faster than either conveyor still stands. Like the real car, it is powered by the different speeds of the medium against which the forces are applied.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 18/06/2021 22:44:11
Let's start with the car stationary to the frame of the conveyors. Dark wheels all spin at 1 m/s since they don't slip. The pulleys and V-belt will force the bright wheel to spin at lower tip speed, i. e. 1/3 m/s. It's possible since it is slipping. But as long as it's not perfectly slippery, there would be some friction forcing the bright wheel to increase its tip speed. This force is transmitted through pulleys and V-belt to bottom rear wheel, increasing its tip speed too. This will make the car move to the right, since the front wheels can spin independently.
In real life experiment, the starting condition is not symmetrical. The bottom wheels don't spin and the car moves along with bottom conveyor. The bright wheel doesn't spin since it's locked by the V-belt. Top dark wheel spins at 2m/s, which is the speed difference between top and bottom conveyors. Once the brake is released, the bright wheel starts to spin due to friction. It turns the bottom wheel to turn 3x as fast. It makes the car to accelerate to the right until the tip spin of the bright wheel equals top conveyor. When the force of internal friction in the car mechanism is non-zero, the bright wheel will spin at lower speed, hence generate balancing force from friction with top conveyor.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 18/06/2021 23:14:23
I looked at wiki and some other sources and no where did is see that the average speed of the vehicle over the run was faster than the speed of the wind.  I only saw that the speed of the vehicle could exceed the speed of the wind at some point.
They don't care about average speed, since it would take initial speed into account, which is 0.
They care about terminal speed, where total acceleration is 0. It occurs when the total force pushing the car forward is balanced by friction in car mechanism.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 18/06/2021 23:46:10
In real life experiment, the starting condition is not symmetrical.
This asymmetry skews our perspectives in analyzing the problem. We have been accustomed to see various phenomena from earth perspective. In many cases it's fine, but in some other cases it gives us false assumptions which lead to unexpected results.   
If we see the motions of planets from earth perspective, they don't seem to make sense. Similar thing might have happened in this case.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 19/06/2021 10:40:20
My bit about getting the car to move faster than either conveyor still stands.
What does it mean? Do you still believe that to go faster than top conveyor, the car needs additional power source?
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 19/06/2021 13:09:34
Ernest Rutherford Quote: “All of physics is either impossible or trivial. It is impossible until you understand it, and then it becomes trivial.”
Once you understand the problem, it becomes trivial. In retrospect, we might think that it's too trivial to worth $10k.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 19/06/2021 13:26:54
So how do you get it to go faster than the top conveyor?  Simple: Loop the belt in figure-8 around the unequal size pulleys.  The closer in size the pulleys are, the faster it goes.
There is no figure-8 in my picture. When the size of the pulleys are equal, we get back to my previous picture. Tip speed of bottom wheels are 1 m/s. The car would be stationary wrt observer, or 1 m/s relative to bottom conveyor.
With 2:1 ratio, the car would move 2 m/s relative to bottom conveyor, or 0 m/s relative to top conveyor.
With 3:1 ratio, the car would move 3 m/s relative to bottom conveyor, or 1 m/s relative to top conveyor.
When the ratio is negative, like in 8 figure V-belt, the car would move backward.
Note: don't worry about the right dark wheels. Their solely function is to prevent the car from tumbling.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 19/06/2021 13:30:57
You need to learn to read. I never suggested the need for an additional power source for the car between the conveyors.
You haven't retracted your claim that the land yacht needs a battery to go faster than the wind.
Title: Re: Can we go downwind faster than the wind?
Post by: Halc on 19/06/2021 15:04:45
You need to learn to read. I never suggested the need for an additional power source for the car between the conveyors.
You haven't retracted your claim that the land yacht needs a battery to go faster than the wind.
You need to learn to read.  See post 31.

When the size of the pulleys are equal, we get back to my previous picture. Tip speed of bottom wheels are 1 m/s. The car would be stationary wrt observer, or 1 m/s relative to bottom conveyor.
With 2:1 ratio, the car would move 2 m/s relative to bottom conveyor, or 0 m/s relative to top conveyor.
If it's moving at 0 m/sec relative to the top conveyor, the wheel would not be turning at all, which contradicts a 2:1 ratio between the turn rates of the two wheels. Your algebra is off.
Quote
With 3:1 ratio, the car would move 3 m/s relative to bottom conveyor, or 1 m/s relative to top conveyor.
It will not.  It will move at 1.5 m/sec right relative to the bottom conveyor and 0.5 left relative to the bottom. Only then is the ratio 3:1 with the wheels turning in the same direction as you have it pictured. If you figure-8 the belt, the wheels will turn with a ratio of 3:1, but in the opposite direction, which gets the car going at 3 m/s as you indicate. The 2:1 ratio makes it go even faster.
Quote
When the ratio is negative, like in 8 figure V-belt, the car would move backward.
Your assertion contradicts the algebra. If you want the assembly to move left, the lower pulley needs to be the larger one, regardless of regular or figure-8 looping of the belt.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 19/06/2021 22:02:04
You need to learn to read.  See post 31.
Do you mean this one?
I read the line in the article: "the rotor (acting like a wind turbine)" to mean that the air drove the propeller, the energy of which was sent to the wheels, which violates energy conservation per my post above. But reading closer, the opposite is what is going on. The rotor acts as a propeller, not a turbine. The wheels apply a braking action and the propeller applies thrust. Thus energy is taken from the air movement and no violation of energy conservation takes place.
I'm sorry for missing this one. I guess I didn't count it since it says nothing about battery.
My analogy shows that bottom wheel always act to push the conveyor belt to the left. It never acts as a brake. It can have tip speed higher than the wind because of the speed multiplication effect provided by the pulley ratio. In the case of land yacht, it's also determined by angle of the blades in the turbine, as explained in Steve Mould's video.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 20/06/2021 07:21:42
If it's moving at 0 m/sec relative to the top conveyor, the wheel would not be turning at all, which contradicts a 2:1 ratio between the turn rates of the two wheels. Your algebra is off.
Remember that the bright wheel is slippery, hence it can spin in any speed without much problem.  On the other hand, dark wheels always spin according to speed difference between car and conveyor. So, if the car speed is +1 m/s, bottom wheels must spin at 2 m/s tip speed. Bright wheel is then spin at 1m/s tip speed. But that means that its top surface is moving at 2 m/s, which is faster than the top conveyor.

You made me realize that before achieving 1 m/s, the car must accelerate from 0 and gradually increase the speed. At some point, it can't absorb kinetic energy from the top conveyor anymore.
But the experiment shows that the car can move faster than the wind, so there must be something else at play.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 21/06/2021 07:34:10
Let's analyze the process more thoroughly, for both straight and inverting pulleys. I used spreadsheet to generate the trend.
(https://www.thenakedscientists.com/forum/index.php?action=dlattach;topic=82462.0;attach=32042;image)
The left image shows straight pulleys with positive ratio, while right image shows inverting pulleys with negative ratio.
vc : speed of the car
vbc : tip speed of bottom wheels relative to the car
vtc : tip speed of bright wheel relative to the car
vt : tip speed of bright wheel touching top conveyor
dvt : speed difference between highest point of bright wheel and top conveyor

dvt determines the force acted to the car. Negative values means the car is pushed to the right, while positive value means it is pushed to the left.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 21/06/2021 08:22:50
vc : speed of the car
vbc : tip speed of bottom wheels relative to the car
vtc : tip speed of bright wheel relative to the car
vt : tip speed of bright wheel touching top conveyor
dvt : speed difference between highest point of bright wheel and top conveyor
For those who want to reproduce the table, just use following formula.
vbc = vc - v1
vtc = vbc / ratio
vt = vtc + vc
dvt = vt - v2

Here is an interesting thing. With ratio=-1, dvt is constant. No matter how fast the car is running, it will always feel a push to the right by top conveyor. It seems like the only force countering it is friction. Traction between bottom wheels and conveyor can be a limiting factor. So does the slipperiness of the bright wheel.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 21/06/2021 08:28:46
Your assertion contradicts the algebra. If you want the assembly to move left, the lower pulley needs to be the larger one, regardless of regular or figure-8 looping of the belt.
Thanks for pointing this out. I guess my intuition is not a good guidance for problems I'm not familiar with.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 21/06/2021 10:20:21
Here is an interesting thing. With ratio=-1, dvt is constant. No matter how fast the car is running, it will always feel a push to the right by top conveyor. It seems like the only force countering it is friction. Traction between bottom wheels and conveyor can be a limiting factor. So does the slipperiness of the bright wheel.
With ratio of -0.3, the car gets even bigger force as it gets faster.
 (https://www.thenakedscientists.com/forum/index.php?action=dlattach;topic=82462.0;attach=32056;image)
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 21/06/2021 10:28:23
The twisted belt can be replaced by gears to achieve the same effects.
(https://www.thenakedscientists.com/forum/index.php?action=dlattach;topic=82462.0;attach=32058;image)
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 21/06/2021 10:41:07
Unless there's some error in my formula, it should be clear beyond a reasonable doubt that the answer to the OP question is yes. My analogy confirms the conclusion of Blackbird team that there is no inherent theoretical limit for going faster than the wind.
Title: Re: Can we go downwind faster than the wind?
Post by: Halc on 21/06/2021 13:13:13
The twisted belt can be replaced by gears to achieve the same effects.
(https://www.thenakedscientists.com/forum/index.php?action=dlattach;topic=82462.0;attach=32058;image)
This is equivalent to the twisted belt with identical size pulleys top and bottom.
In such an arrangement, there is no speed at which the one wheel will not forever slip at dvt=2.
The size of the yellow idler gears has no effect on the relative rate of turn of the wheels. The only size that matters is the red gears, and you drew those the same.

You also drew the arrow for the large yellow gear larger than the smaller one. The smaller one will turn faster, but indeed, the actual wheels with the red gears will turn the fastest as you have drawn them.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 21/06/2021 13:45:36
This is equivalent to the twisted belt with identical size pulleys top and bottom.
In such an arrangement, there is no speed at which the one wheel will not forever slip at dvt=2.
The size of the yellow idler gears has no effect on the relative rate of turn of the wheels. The only size that matters is the red gears, and you drew those the same.

You also drew the arrow for the large yellow gear larger than the smaller one. The smaller one will turn faster, but indeed, the actual wheels with the red gears will turn the fastest as you have drawn them.
Thanks for pointing out those facts. Looks like we can solve problems better by collaboration.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 21/06/2021 22:24:47
The left image shows straight pulleys with positive ratio, while right image shows inverting pulleys with negative ratio.
It seems counter-intuitive that both straight and negative ratio push the car to the right. So, it's understandable that many of us got it wrong. Only after thorough analysis the whole process becomes clear.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 22/06/2021 02:35:11
Someone from twitter shared an old video demonstrating this experiment in simplified version.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 22/06/2021 10:58:04
Here is a diagram showing the setup from the video. No slippery wheel is used. Instead, to simulate wind, the top conveyor can be made slippery. All wheels are non-slippery type. The ratio is inverting.
(https://www.thenakedscientists.com/forum/index.php?action=dlattach;topic=82462.0;attach=32066;image)
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 24/06/2021 10:31:50
The result of the wager.
Quote
Professor @alexkusenko has graciously conceded the wager.
Changing your mind in light of new evidence is the hallmark of a great scientist.
I want to thank Alex for the many good points he raised, which have sharpened my thinking about this problem. https://t.co/UEzYHsYjd9
https://twitter.com/veritasium/status/1407566432699781126?s=19
Title: Re: Can we go downwind faster than the wind?
Post by: MarkV on 24/06/2021 15:57:47
Here is a diagram showing the setup from the video. No slippery wheel is used. Instead, to simulate wind, the top conveyor can be made slippery. All wheels are non-slippery type. The ratio is inverting.
(https://www.thenakedscientists.com/forum/index.php?action=dlattach;topic=82462.0;attach=32066;image)

Not quite. Note the two different radii of the spools used in the video. The are key to have it move faster than the ruler. I'm attaching the diagrams for various examples, including the formula to compute their speed.

(https://www.thenakedscientists.com/forum/index.php?action=dlattach;topic=82462.0;attach=32080)




Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 24/06/2021 22:30:53
Not quite. Note the two different radii of the spools used in the video. The are key to have it move faster than the ruler. I'm attaching the diagrams for various examples, including the formula to compute their speed.
What would happen if R=r?
Title: Re: Can we go downwind faster than the wind?
Post by: MarkV on 25/06/2021 03:09:12
Not quite. Note the two different radii of the spools used in the video. The are key to have it move faster than the ruler. I'm attaching the diagrams for various examples, including the formula to compute their speed.
What would happen if R=r?
Then Coco from the video would be right: It can't work at all. It would slide or lock or break.
Title: Re: Can we go downwind faster than the wind?
Post by: MarkV on 25/06/2021 07:30:55
In the frame of Earth, the rotor accelerates the air since it is pushing from behind.
When going downwind faster than the wind, the rotor speeds up the air relative to the cart, which implies slowing down the air relative to the ground. It extracts wind energy by reducing the velocity difference between air & ground.
Title: Re: Can we go downwind faster than the wind?
Post by: MarkV on 25/06/2021 07:40:49
Sailboats and land yachts are interesting. On a beam reach you can certainly sail faster than the wind, but essentially perpendicular to the wind vector.

The downwind velocity component can also be greater than windspeed:


* downwind_vectors_en_3.png (68.85 kB . 600x542 - viewed 2525 times)

Here the vectors for a section of a propeller blade on the cart that goes directly downwind faster than the wind:


* downwind_propeller_vectors.png (75.75 kB . 644x542 - viewed 2527 times)
Title: Re: Can we go downwind faster than the wind?
Post by: MarkV on 25/06/2021 07:52:47
I think this contraption can go faster than the wind for a few moments but that would be it.
No, it can go faster than the wind in steady sate, because it cannot use stored energy for propulsion and is able to accelerate while being above windspeed.

Derek Mueller posted the data from the official NALSA record runs here:
https://twitter.com/veritasium/status/1407103269961273355
More details here:
https://www.nalsa.org/DownWind.html

Video analysis of Derek's own run also shows acceleration while being above windspeed:
https://twitter.com/veritasium/status/1407473631521771522



Title: Re: Can we go downwind faster than the wind?
Post by: MarkV on 25/06/2021 08:00:32
Now, what I'm less sure about is whether you can sail into the wind faster than the wind.
Yes you can, and for upwind travel the rotor acts as a windmill. But in the downwind faster than wind case it's a propeller.This vehicle did both:
- It went 2.8x windspeed directly downwind
- it went 2.1x windpseed directly upwind
Title: Re: Can we go downwind faster than the wind?
Post by: MarkV on 25/06/2021 08:20:27
He even shows a clear close-up image of the exposed gear assembly. It shows that the whole system is purely mechanical.

His transparency brings credibility.
The ratio of the gears seems to be 1:1
So the speed multiplier must come from wheels radii vs blades radii, and also blades angle.
The radius of the blades doesn't matter for the speed multiplier. Just the wheel radius, transmission ratio and the pitch of the propeller.
Title: Re: Can we go downwind faster than the wind?
Post by: Bored chemist on 25/06/2021 09:54:45
Now, what I'm less sure about is whether you can sail into the wind faster than the wind.
Yes you can, and for upwind travel the rotor acts as a windmill. But in the downwind faster than wind case it's a propeller.This vehicle did both:
- It went 2.8x windspeed directly downwind
- it went 2.1x windpseed directly upwind

Well, I'm glad that's sorted.


Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 25/06/2021 10:24:39
Then Coco from the video would be right: It can't work at all. It would slide or lock or break.
In my diagram, one of the conveyors is slippery. Nothing has to break.
What does it take to move in reverse direction?
Title: Re: Can we go downwind faster than the wind?
Post by: MarkV on 25/06/2021 11:23:30
Then Coco from the video would be right: It can't work at all. It would slide or lock or break.
In my diagram, one of the conveyors is slippery. Nothing has to break.

Then it will slide.


What does it take to move in reverse direction?

Just turn it upside down.

But if that's not allowed then look at my diagram. The formula for this case is:

V/W = 1 / (1 - r/R)

To make V/W negative, you must have:

r > R

But note that r still touches the upper wheel, while R still touches the ground/lower belt. To make this work requires a double rail / split belt on which the now smaller R parts roll. So turning it upside down is really easier.


Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 25/06/2021 13:57:12
What does it take to make it move slower than the wind?
Title: Re: Can we go downwind faster than the wind?
Post by: MarkV on 25/06/2021 17:35:32
What does it take to make it move slower than the wind?

For this the advance ratio A must be negative, so you have to get rid of the blue wheel or apply the ruler at the bottom of the blue wheel.
Title: Re: Can we go downwind faster than the wind?
Post by: MarkV on 25/06/2021 20:03:09
I can lean out of the basket and put the wheel in contact with the ground so it spins the generator and creates electricity.
I can use that electricity to run a motor and have that drive a propeller and I can use that to move my balloon through the air in any direction I choose.
That wheel on the ground will be a big drag and slow you down considerably.  Since the generator and the motor are not 100% efficient (the propeller is not 100% efficient either) I do not think that you will be able to even make it back up to your speed before you lowered the wheel.

The thrust at the propeller can be higher than the drag at the wheels because the relative velocity of the ground is higher than the relative velocity of the air.

Power = Force x Velocity
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 25/06/2021 22:19:36
What does it take to make it move slower than the wind?

For this the advance ratio A must be negative, so you have to get rid of the blue wheel or apply the ruler at the bottom of the blue wheel.
My analysis shows that to go faster than the wind, the bottom wheels must spin in reverse direction than the top. Halc pointed this out earlier. Even your own formula implies that. It just turns out that spinning in reverse direction  requires less components.
Title: Re: Can we go downwind faster than the wind?
Post by: MarkV on 26/06/2021 04:01:42
What does it take to make it move slower than the wind?

For this the advance ratio A must be negative, so you have to get rid of the blue wheel or apply the ruler at the bottom of the blue wheel.
My analysis shows that to go faster than the wind,...

Just to clarify: Above I was answering the question about making it move slower than the wind.

... the bottom wheels must spin in reverse direction than the top.

Well, it depends on which side of the wheel the ruler/belt is interacting with it (see the single spool). But if you stick to:
- top-side for the top wheel
- bottom-side for the bottom wheel
then yes.

The advance ratio A is the ratio of relative velocities, that your gearbox is trying to achieve:

A = air_relative_velocity / ground_relative_velocity

When moving with the air but faster, or against the air, the air_relative_velocity and ground_relative_velocity have the same sign so A > 0. When moving with the air but slower, they have opposite signs so A < 0.

I'm attaching a plot of advance ratio vs. maximal speed. For a gear box with no slippage you can assume the ideal velocity (blue line). With slippage and drag (like the propeller cart) you get something like the red velocity line.To make sense of the diagrams in the table below, you should watch the video on the bottom.


* rotor_carts_01.png (209.85 kB . 812x1000 - viewed 2731 times)


Title: Re: Can we go downwind faster than the wind?
Post by: MarkV on 01/07/2021 08:45:30
Follow up video:

Title: Re: Can we go downwind faster than the wind?
Post by: Just thinking on 04/07/2021 11:11:47
Yes. By capturing the wind in a large area and venturing it that increasing the pressure = higher velocity.
Title: Re: Can we go downwind faster than the wind?
Post by: Eternal Student on 08/07/2021 13:04:41
Hi all.

This has been well covered in YouTube Videos,  many of which are cited in this thread.
I think there are now videos and web pages showing people how to print their own working model using a 3D printer.

   Further improvements might include adding a gear box or continuously variable transmission.  Rotate the propeller slower at low road speeds to get the thing started and increase the gear ratio between road wheels and propeller to reach higher speeds.   I wonder if you could get more control and better performance by replacing the mechanical linkage between road wheels and propeller with an electrical linkage instead.   Have dynamos on the road wheels and an electric motor to drive the propeller  etc.
    Also, put more propellers on the vehicle (stacked one above the other on a longer mast but also stacked side-by-side) and more wheels on the road to drive those propellers if you wish.  I suspect the next set of developments will be more about practical and conventional engineering rather than conceptual physics.  It's been interesting watching how the conventional wisdom has been changed and this is one of those times when when something has been discovered that didn't require a million dollars and a particle accelerator.  It's a great example of classical invention and discovery that could almost have been done in someone's garden shed.
Title: Re: Can we go downwind faster than the wind?
Post by: hamdani yusuf on 08/07/2021 15:36:02
In engineering, simplicity is a virtue. Complexities should not be added unnecessarily. "No part is the best part", Elon Musk once said.
Title: Re: Can we go downwind faster than the wind?
Post by: MarkV on 18/07/2021 11:42:27
Further improvements ...

Improvements with what goal? I don't think this will ever be used for transport. And if you want to beat their record, you have to prove that you cannot use any stored energy for propulsion. Adding variable or electric transmission will just make it more difficult to get the vehicle qualified.