Wind Power Vehicle Traveling Down Wind Faster Than The Wind

[chingel]

Do you drive a real car or motorcycle? Does the car accelerate faster in first gear than it does in top gear, and is the maximum speed you can go much lower in first gear than it is in top gear?

I thought that was due to inefficiency problems of the engine, i.e. the engine can only deliver a certain amount of force per explosion, because there is a certain amount of oxygen in the cylinder head and it can't run at very low speeds or it dies out. So that at 600 rpm it is not capable of outputting the same power as at 2000 rpm. Electric cars don't use transmissions because they don't have such problems as power depending on the rpm as much as the combustion engine does. But I am asking if the power is always the same, will the weight accelerate faster if it uses gears to multiply the force? Because I keep on reading that multiplying the force is the explanation, I don't understand it and I tend to think that multiplying the force shouldn't matter, but I am not a scientist.

I don't know any calculus so I can't do any calculations. I assume that you need calculus to calculate it out? Because as the engine applies force at a certain speed, the weight accelerates, offers less resistance, the engine has to move faster but reduce force since it only can do 500W etc and you have to use infinitely small steps because the weight is continuously accelerating. But, looking at the big picture, since no energy is lost in the transmission, all the power should go to the weight in my understanding.

Another idea I had is to put the propeller parallel to the wind and cover the bottom half with an aerodynamic shape, this way it is directly analogous to the ruler example, everything is exactly the same, but instead of the ruler pushing the top wheel the wind is pushing the propeller. Nevertheless I am still struggling with it, let's say the propeller is disconnected, then when I connect it, it uses energy to go slower than the wind and that energy does work to slow the cart down relative to the ground.

 

[rcgldr]

It is amazing that the wind powered car can go against (or should I say “against?”) the very wind that is propelling it

Upwind carts are possible, in this case the advance ratio or the effective gearing is greater than 1, and the propeller acts as a turbine instead, drving the wheels to move the cart against the wind. Even in this case the only limitation to how fast it can move upwind depends on the efficiency of the cart, and the limitation of dealing with higher aerodynamic drag than the downwind case.

I'll assume that you mean the cart outruns the wind that propels it. However what's propelling the cart is the combination of the wind and the ground moving at different speeds. Here's another link to that youtube video posted earlier about a cart propelled by a ruler and the ground moving at different speeds, geared so that the cart moves about twice as fast at the ruler (using the ground as a frame of reference) The gearing (advance ratio) is set so that the upper wheel surface speed is about 1/2 the surface speed of the lower spools:



Getting back to the wind + ground driven cart, although the cart does move faster than the wind, the air flow from the propeller does not. From a ground frame of reference, the cart can only work when the thrust from the propeller slows down the wind. Just aft of the propeller is a compressed (and moving) volume of air, that generates equal and opposing forces to the air and to the propeller, (the magnitude of this force is equal to the thrust force of the propeller). The force applied to the propeller drives the cart forwards,while the force applied to the air slows the air down (relative to a ground frame of reference).

Again this only works because the air speed is different than the ground speed, and in a downwind situation, because relative to the cart, the air speed is less than the ground speed. Similarly, a sailboat can't move faster than the wind unless the air speed is different than the water (or land or ice) speed.

Electric cars don't use transmissions because they don't have such problems as power depending on the rpm as much as the combustion engine does.

That's because they are willing to sacrifice acceleration and/or top speed. Some Tesla electric cars have two speed transmissions, but they are working on some issues:

http://en.wikipedia.org/wiki/Tesla_Roadster#Transmission

An ideal electric motor produces maximum power near the middle of it's rpm range. It produces peak torque at low rpm (unless overheat protection circuitry limits the current at very low rpms), and zero torque at it's peak rpm. Scroll down to section 3.2 to see a graph of this:

http://lancet.mit.edu/motors/motors3.html

Because as the engine applies force at a certain speed, the weight accelerates, offers less resistance.

The resistance to acceleration relative to weight (mass) is the same regardless of speed. Force = mass x acceleration, so acceleration = force / mass. Also power = force x speed, so in the case of constant power, as speed increases, force and acceleration decrease (assuming the weight (mass) isn't changing).

Another idea I had is to put the propeller parallel to the wind and cover the bottom half with an aerodynamic shape, this way it is directly analogous to the ruler example, everything is exactly the same, but instead of the ruler pushing the top wheel the wind is pushing the propeller.

That's essentially a water wheel. If using this concept works for you, note that a propeller is more efficient than a water wheel or squirrel cage type fan, so using a propeller will produce a faster cart. You could have a sail connected to a conveyor belt that moves upwind as the cart moves downwind, then have the sail collapse on the return trip and a second sail that would deploy as the first sail collapses. Again this two sail + conveyor belt setup would be less efficient than a propeller, but if it helps you understand the concept, ...

 

[A.T.]

After all, it also multiplies the force. I would think it does not make it accelerate faster,
...
F=ma,

You answered you own question there.

since nothing is lost in the transmission, all the 500W go to the weight's kinetic energy.

That is an impossible scenario that you describe here. The engine cannot deliver KE at 500W to a resting weight. If no losses are possible (perfectly rigid connection), the engine simply cannot work at 500W from rest. The power will start at 0W and then increase.

If you use a 500W laser to accelerate a perfectly reflective weight (non rigid connection), then at v=0 the reflected light has the same frequency (and thus power) that comes from the laser. So at v=0 the power that goes into the weights KE is zero.

Another reason why the increase in KE has nothing to do with "engine power": The first is frame dependent, while the later is frame independent.

But that would mean that the force would have to be infinite for any power, since the v is zero.

No, not for any power. For P = 0 the force can be a finite number at v = 0 and still satisfy P = F * v

 

[A.T.]

Another idea I had is to put the propeller parallel to the wind and cover the bottom half with an aerodynamic shape, this way it is directly analogous to the ruler example,

The spool example is even simpler:

https://www.youtube.com/watch?v=E7vcQcIaWSQ

The wind powered version would need an aerodynamic shape on top.

https://www.youtube.com/watch?v=Ufk6HVWdSzE

It could potentially work, but paddle-wheels are very inefficient. Here is a real model:

https://www.youtube.com/watch?v=TcSh0qqgYKc .

I think it would have to be much bigger to go faster than wind in "normal" winds. Propellers are more efficient and thus the easier way.

 

[chingel]

That is an impossible scenario that you describe here. The engine cannot deliver KE at 500W to a resting weight. If no losses are possible (perfectly rigid connection), the engine simply cannot work at 500W from rest. The power will start at 0W and then increase.

Ok, but what does this mean? Let's say that for the first Planck second the power is 0 and after that it is 500W. Or let's imagine that the weight is already moving at a speed of a Planck distance per decade. Cars can still start moving and engines can start running from a stop despite the fact that the engine can't produce any power when it is at rest.

All I want to know is that assuming a zero loss frictionless massless transmission and an engine producing constant power, if we use a different gear ratio to multiply force, would the weight be moving faster after 1 second? I can't see how, because if the engine does 500W of work, it cannot go anywhere else other than the kinetic energy of the weight.

Here is a pdf I found helpful:

http://www.greglondon.com/tumbleweed/tumbleweed.pdf

But I still feel somewhat confused due to the questions I am having.

 

[spork]

Here is a pdf I found helpful:

http://www.greglondon.com/tumbleweed/tumbleweed.pdf

But I still feel somewhat confused due to the questions I am having.

If you want to see some seriously flawed physics, Greg London is your man.

 

[A.T.]

Cars can still start moving and engines can start running from a stop despite the fact that the engine can't produce any power when it is at rest.

Acceleration is caused by forces, not power.

...engine producing constant power ... because if the engine does 500W of work, it cannot go anywhere else other than the kinetic energy of the weight.

Think about a moving observer. He will see the same engine but a different change of kinetic energy of the weight.

 

[Fun Value]

Here's as far as I've gotten with my point of view. I feel like I am on to something, but please stop me if I am headed in the wrong direction or on a dead end. Since I have taught 9th grade science for so long, I guess I am thinking at that level.

It makes sense that the vehicle could go almost the speed of the wind without the propeller. Now engage the propeller with the wheels to the ground. I think (not sure) one of three things would now happen:
1.) The propeller would cause an increase in speed if the propeller is of sufficient radius that most of the propeller is perpendicularly hitting the air at more than wind speed,
2.) The propeller would cause the vehicle to stay the same speed if the propeller radius is smaller,
3.) The propeller would cause the vehicle to slow if the propeller is even shorter.

When the propeller of sufficient radius is engaged, the vehicle can at least keep the same speed - I get that. I guess I can't prove it, but yes it does seem that the propeller would add to the speed of the vehicle. I do not see any reason it would slow it down. The perpendicular motion can only translate the velocity between wheels and ground into additional forward thrust (I think).

If the propeller is of small radius so that most of the propeller is slow, then I guess it seems like the oncoming wind from the front would tend to slow the propeller and the vehicle - wouldn't it?

Like I said, I am trying to see this from a 9th grade point of view.

 

[rcgldr]

The propeller ... radius.

The effective gearing of the propeller is related to it's pitch. The propellers "pitch" is how far it would tend to travel forward per revolution (similar to how far a screw would move in a solid per revolution).

what_is_propeller_pitch.html

The actual effect on the air depends on the propellers radius and pitch. The radius determines the amount of air affected, and the pitch and angular (rotating) speed determine by how much the affected air is accelerated. The actual thrust speed and force depend on radius, pitch, angular speed of the propeller, and the wind speed relative to the cart.

The "effective gearing" is the ratio of thrust speed divided by the ground speed, relative to the cart. If the effective gearing is less than zero (negative), the propeller acts as a turbine to drive the wheels at less than tailwind speed. If the effective gearing is zero, the propeller acts like a sail. If the effective gearing is greater than zero and less than one, the propeller tries to make the cart go downwind faster than the wind. If the effective gearing is one (or too close to one), it could tend to limit speed (via a braking effect on the wheels). If the effective gearing is greater than one, the propeller acts as a turbine, but now in reverse and tends to move the cart into the wind (upwind).

If the propeller radius is too small, then it doesn't generate sufficient force to affect the cart more than the affect of the wind on the cart itself. The larger the radius of the propeller, the higher the forces at the propeller and the tires. If the radius of the propeller is too large, then friction between the tires and ground could be an issue.

 

[A.T.]

The "effective gearing" is the ratio of thrust speed divided by the ground speed, relative to the cart. If the effective gearing is less than zero (negative), the propeller acts as a turbine to drive the wheels at less than tailwind speed. If the effective gearing is zero, the propeller acts like a sail. If the effective gearing is greater than zero and less than one, the propeller tries to make the cart go downwind faster than the wind. If the effective gearing is one (or too close to one), it could tend to limit speed (via a braking effect on the wheels). If the effective gearing is greater than one, the propeller acts as a turbine, but now in reverse and tends to move the cart into the wind (upwind).

See also that table:
http://i54.tinypic.com/2gv0kew.png

 

[Fun Value]

Thank you A.T. and rcgldr for all of your patient explanations and demos. I love this stuff and I am sure my students will also. For most of my students this really will have to be learned from hands on learning projects. Oh, and even my wife, a librarian is getting into this!

 

[spork]

Thank you A.T. and rcgldr for all of your patient explanations and demos. I love this stuff and I am sure my students will also. For most of my students this really will have to be learned from hands on learning projects. Oh, and even my wife, a librarian is getting into this!

Will you build a cart and demonstrate it in class?

 

[chingel]

Here is another question I have.

If I put brakes on a free spinning wheel, will the wheel stop faster when I put the brakes near the edge of the wheel than when I put the brakes near the center of the wheel, assuming constant force from the brakes? I would tend to think that when I apply them closer to the edge, it would slow down faster, and as long as the ratio of speed at contact point x force stays the same, it would stop in the same time. Essentially as long as the power is the same, J/s, that is how much kinetic energy the wheel will lose. Is this idea wrong or not with the spinning wheel and brakes example?

If I read about the work-energy principle, in my understanding it says that:

http://hyperphysics.phy-astr.gsu.edu/hbase/work.html

The change in an objects kinetic energy is equal to the work done on the object. So to make the cart move faster than the wind, ie to increase the kinetic energy of cart, work has to be done on it, it's not just force. Doesn't this also mean that as long as the engine is doing 500W of work, the objects kinetic energy is increasing at a rate of 500 J/s, no matter what the gearing is?

On the other hand these same questions can be asked about the Yo-Yo or the tumbleweed example, yet I can sort of understand why they move faster than the string is pulling or wind is pushing. What is the big picture?


Is there something wrong in the tumbleweed pdf or does the author generally have a bad reputation with applying physics the wrong way?

 

[spork]

If I put brakes on a free spinning wheel, will the wheel stop faster when I put the brakes near the edge of the wheel than when I put the brakes near the center of the wheel, assuming constant force from the brakes?

Yes, if we assume constant force from the brakes (no brake fading, etc.), the wheel will slow down much more rapidly if I apply the force at a greater radius. We can see that in two different ways. From a force point of view, we would be applying twice as much slowing torque if we apply the force at twice the initial radius.

From an energy point of view, we can see that we're doing much more work "against" the wheel because we're applying the same force against a faster moving point on the wheel (power = energy/time = force x velocity).

I would tend to think that when I apply them closer to the edge, it would slow down faster, and as long as the ratio of speed at contact point x force stays the same, it would stop in the same time.

You're correct.

If I read about the work-energy principle, in my understanding it says that:

http://hyperphysics.phy-astr.gsu.edu/hbase/work.html

The change in an objects kinetic energy is equal to the work done on the object. So to make the cart move faster than the wind, ie to increase the kinetic energy of cart, work has to be done on it, it's not just force.

Still correct.

Doesn't this also mean that as long as the engine is doing 500W of work, the objects kinetic energy is increasing at a rate of 500 J/s, no matter what the gearing is?

Yes. But we have to be careful - we may be losing 500 J/s of energy to aerodynamic drag, rolling resistance, etc. In that case the engine could do 500 J/s all day without the car ever accelerating.

Is there something wrong in the tumbleweed pdf or does the author generally have a bad reputation with applying physics the wrong way?

I read the original document. It was riddled with shamefully wrong statements. I know he's re-written it at least once. I don't care to grade every version of it for him. But I am happy to answer any questions you have.

 

[OmCheeto]

But I am happy to answer any questions you have.

Can you answer my question?

Where can I find Nikos's comments on the experiment?

 

[spork]

Can you answer my question?

I'll certainly try.

Where can I find Nikos's comments on the experiment?

Do you mean professor Nikos Mourtos?

 

[OmCheeto]

I'll certainly try.



Do you mean professor Nikos Mourtos?

Is there more than one Nikos that worked on the project?

 

[spork]

Is there more than one Nikos that worked on the project?

You're talking about someone that worked on the project?
(let's see how long we can answer a question with another question)


I don't know that you can read his opinions or comments about the project anywhere. I certainly don't recall seeing them. I'm quite certain he doesn't know how it works, and I guess it'd be a coin toss as to whether he believes it works.

Why do you ask about his comments in particular?

 

[OmCheeto]

You're talking about someone that worked on the project?
(let's see how long we can answer a question with another question)


I don't know that you can read his opinions or comments about the project anywhere. I certainly don't recall seeing them. I'm quite certain he doesn't know how it works, and I guess it'd be a coin toss as to whether he believes it works.

Why do you ask about his comments in particular?

To quote Leonard; "Fascinating"

10-26-2009, 04:43 PM
ThinAirDesigns ThinAirDesigns is offline
Senior Member

Join Date: Dec 2008
Rep: 40 Posts: 123
Location: USA
San Jose State University Aero Professor and Stanford Phd Dr. Nikos Mourtos along with a team of students and advisors have taken on a project to construct and document DDWFTTW in a more thorough way than ever before. Their goal is to achieve a documented 2x windspeed DDW.

Follow their blog at www.fasterthanthewind.org

http://www.boatdesign.net/forums/pr...ctly-downwind-faster-than-wind-25527-10.html"

and now, fasterthanthewind.org, has no mention of his existence?

sorry. You can PM the mentors/admin, and have them delete this post, as I have, on many occasions stated, that (I am of course paraphrasing, as I'm old, and have not a clue what I originally stated); "This is the most incredibly mindboggling physics homework problem in the world"

 

[spork]

and now, fasterthanthewind.org, has no mention of his existence?

We set out to take this project on with a group of SJSU aero students. Dr. Mourtos was their professor - and agreed to the project. And that was about the end of it. One of the students took it upon himself to start working with us relatively late in the project. The others were effectively no-shows. Neither they nor the professor contributed in any way to the project.

Because we had accepted money from sponsors to accomplish a stated goal, JB and I saw to it that the project was completed.

But if it eases your concerns in any way - I strongly suspect he still exists.

 

[OmCheeto]

We set out to take this project on with a group of SJSU aero students. Dr. Mourtos was their professor - and agreed to the project. And that was about the end of it. One of the students took it upon himself to start working with us relatively late in the project. The others were effectively no-shows. Neither they nor the professor contributed in any way to the project.

Because we had accepted money from sponsors to accomplish a stated goal, JB and I saw to it that the project was completed.

But if it eases your concerns in any way - I strongly suspect he still exists.

Why of course he does. I checked him out on facebook. I didn't try an befriend him. I figured he had way too many dd'er fans chasing him.

But I did just find a paper written by one of http://www.engr.sjsu.edu/nikos/projectsMSAE.htm" students:

2010 http://www.engr.sjsu.edu/nikos/MSAE/pdf/Shethal.F10.pdf"
Shethal Thomas Kodiyattu

Unfortunately, it's a 125 page pdf, and I was supposed to be in bed about 3 hours ago, so I don't have time at the moment to read much.

zzzzzzz:zzz:zzz:zzz:

 

[spork]

But I did just find a paper written by one of http://www.engr.sjsu.edu/nikos/projectsMSAE.htm" students:

Unfortunately, it's a 125 page pdf

That paper is incidental to the project. We take no credit or blame for it.

Incidentally, I think you'll find there are fewer than 30 pages to read. But I couldn't force myself to read much more than the abstract. The 17.5 foot propeller seems to have become a 16 inch propeller according to this paper.

 

[Fun Value]

Will you build a cart and demonstrate it in class?

Depends on the time and their motivation. If they are highly motivated I might be able to demonstrate it and expect them to build their own on their own time while we move on. I like to cover the entire text in order and usually the all important high tech stuff needed now days comes near the end. I do plan to have them watch AT's U.tube presentations of ATPFTTP and UTRFTTR and at least build those things.

 

[Subductionzon]

I don't know if spork mentioned it, but he does have a series of videos that tell how to build the small cart they used on the treadmill. Look under the name of spork33 on YouTube. I believe the parts should run under $40.00.

 

[A.T.]

Depends on the time and their motivation. If they are highly motivated I might be able to demonstrate it and expect them to build their own on their own time while we move on. I like to cover the entire text in order and usually the all important high tech stuff needed now days comes near the end. I do plan to have them watch AT's U.tube presentations of ATPFTTP and UTRFTTR and at least build those things.

The paper and ruler videos are not mine, but from someone who posts as "Michael C". My youtube channel contains some schematic animations:
http://www.youtube.com/user/eyytee#g/u

Personally I think it would be nice if you would do Michael's experiments live with the students, so they can see that it is not a trick. But you should show the nicely made ruler video first, and stop it after the puppets present their answers. Then let the students make their predictions.

Building your own propeller model and testing it on a treadmill is of course a more advance project. But useful to teach Galilean Relativity. Michael has a nice video on that too:

https://www.youtube.com/watch?v=9Yt4zxYuPzI

 

[chingel]

But then what is the correct way of thinking about the cart moving faster than the wind, and also the Yo-Yo and tumbleweed? What is the most intuitive and logical explanation?

 

[spork]

But then what is the correct way of thinking about the cart moving faster than the wind, and also the Yo-Yo and tumbleweed? What is the most intuitive and logical explanation?

We have never found that any given explanation is better than another overall. Each person that gets it seems to find one more intuitive while others claim that one makes no sense, and think another is more intuitive. Some are certain that none are intuitive, and that DDWFTTW is impossible.

 

[Fun Value]

Thank you Subductionzon and A.T. I am taking your advice. I am going to try to build my own this summer and demonstrate it to my students next fall. I have yet not looked at Spork's videos, but plan to asap. Right now it seems to me that maybe the most difficult part is the gear-box - one that changes direction from the plane of the wheels to a perpendicular plane so as to run the propeller. I thought I might find one at the local hobby shop, but they didn't have one. I guess I might have to make my own. Wonder if I can make it highly efficient.

 

[spork]

Right now it seems to me that maybe the most difficult part is the gear-box - one that changes direction from the plane of the wheels to a perpendicular plane so as to run the propeller. I thought I might find one at the local hobby shop, but they didn't have one. I guess I might have to make my own. Wonder if I can make it highly efficient.

For my first one, I purchased two bevel gears and made the gear box on a mill from Delrin. In the videos though, I use a tail-rotor gearbox from an R/C heli. I give the part numbers and source in the video as well.

 

[chingel]

We have never found that any given explanation is better than another overall. Each person that gets it seems to find one more intuitive while others claim that one makes no sense, and think another is more intuitive. Some are certain that none are intuitive, and that DDWFTTW is impossible.

But what are the explanations? One is that you can view the propeller as two sails moving at an angle and they also rotate. What are the others?