44 replies to this topic

[scolbourne]

http://www.popsci.co...the-wind,378020

 

Can some of the knowledgeable engineers or aerodynamicists on this forum tell me whether this is real or not. It appears as though it is possible to make a wind powered vehicle travel downwind faster than the wind ie approx 30km/h in 10km/h wind. Apparently this is sustained and is not using the flywheel effect.

 

The wheels drive the propeller which then propels the vehicle.

 

https://www.youtube....DzWh9J1dk4#t=87

 

 

A slightly modified vehicle was also able to travel upwind at about two times the speed of the wind. This is more believable to me.

 

Also of interest is Greenbird achieving 126mph

 

 

 

A 1969 paper on the subject

 

http://projects.m-qp...t-Interface.pdf

Edited by scolbourne, 25 December 2014 - 06:56.

[kikiturbo2]

I do not know about downwind, but windsurfers regularly go faster than the wind...

[Greg Locock]

Windsurfers don't move downwind faster than the wind, sustainably. Ice boats do.The general challenge is to beat a drifting balloon downwind.

 

The geared propeller to an axle machine is for real. http://www.lhup.edu/...eum/ddwfttw.htm and many other attempts to explain it without maths.

 

Upwind seems conceptually easier to me, it is 'obvious' that you could mount a windturbine on a crawler and use the windturbine to power the crawler's motor, and that the upwind speed would involve some tradeoff between the thrust needed to generate a certain power, and the rolling resistance etc of the combination.

Edited by Greg Locock, 10 December 2014 - 18:26.

[gruntguru]

Either should be possible. Think of the wind and ground as two geared racks translating relative to each other. It is straightforward to devise a system of gearing to use this relative motion to propel a vehicle at any multiple of "windspeed" you want. If you have got your head around that (whether upwind or downwind) all you have to do is reverse the names of the two racks - replace the "wind" with the "ground" and vice versa.

 

"Real wind" falls short of this ideal, because there is slippage and aero drag etc but aprt from that a propellor is just a "gear" for connecting to the air.

[scolbourne]

Downwind car racing may be an interesting  sport for the designers. They have an upwind race in holland but I feel it would be more of a challenge to build downwind racers.

 

http://www.windenergyevents.com/

 

 

I expect a variable pitch propeller with a variable ratio/direction transmission system would be required.

 

While going less than wind speed the propeller-.windmill would drive the wheels, and then above wind speed, the wheels drive the propeller.

The geared racks you mentioned do give a good explanation of how this works, although it still seems a bit suspicious to me.

[imaginesix]

Either should be possible. Think of the wind and ground as two geared racks translating relative to each other. It is straightforward to devise a system of gearing to use this relative motion to propel a vehicle at any multiple of "windspeed" you want. If you have got your head around that (whether upwind or downwind) all you have to do is reverse the names of the two racks - replace the "wind" with the "ground" and vice versa.
 
"Real wind" falls short of this ideal, because there is slippage and aero drag etc but aprt from that a propellor is just a "gear" for connecting to the air.

This sounds like an excellent analogy. So can someone draw for me one of these gears that drives forward against two gear racks that translate relative to each other?

[Greg Locock]

I think the yoyo one is  most easily understood

[saudoso]

I'm having a very hard time grasping this, even the geared racks proposition.

 

Really would like to see an example of the moving racks thingy.

 

I don't agree the yoyo is a good example, because even if you do get some spinning induced the COG is right there with the wind.

[Greg Locock]

um, wrong yoyo

[Allan Lupton]

I'll make a few of fundamental points and then withdraw, as I had enough of this in another place.

Downwind at less than wind speed, that's o.k.
Downwind at wind speed a windmill will do nothing and a fixed sail works better.
If downwind at more than wind speed with the pitch reversed gave power equal to, or greater than that required to move it at that speed that's the basis of a perpetual motion engine.

 

There was some cod-maths used to show how it worked and it was done by introducing a clever fallacy which was the idea that the thrust from a propellor is not dependent on its airspeed.
The power absorbed by a propellor is thrust times speed and not (as they stated) thrust times airspeed.

It followed that for propelling power to exceed resistance power the product of the efficiencies has to be greater than 1.
 

[saudoso]

And last, but not least, why isn't anything like that commercially available yet? Oil cartel lobbying?

[imaginesix]

And last, but not least, why isn't anything like that commercially available yet? Oil cartel lobbying?

Wind cartel holding back production.

[BRG]

If downwind at more than wind speed with the pitch reversed gave power equal to, or greater than that required to move it at that speed that's the basis of a perpetual motion engine.

 

There's always some mug who thinks you can get something for nothing.

[imaginesix]

There's always some mug who thinks you can get something for nothing.

He's not describing a perpetual motion machine. That's just what he thinks he's describing based on his internalized model of the laws of physics that he built up over the years.

[gruntguru]

Two coaxial gears and two racks. Same as yo yo (thanks Greg).

Edited by gruntguru, 12 December 2014 - 00:24.

[scolbourne]

The above picture also shows how a simple device can move faster downwind than the wind.

 

Assume the above was a wheel (e.g.2 LP records spaced 1ft apart by some pivoting blades, pivoting approx on the top gear track in picture).

 

The wind will push the device faster than the wind if the blades can swing to reduce drag when neccessary.

[rubberduck]

This sounds like an excellent analogy. So can someone draw for me one of these gears that drives forward against two gear racks that translate relative to each other?

 

The simplest one has already been posted by gruntgury:

 

 

Here another variant showing both directions:

 

[rubberduck]

If downwind at more than wind speed with the pitch reversed gave power equal to, or greater than that required to move it at that speed that's the basis of a perpetual motion engine.

 

It's not perpetual motion because it stops, when the true wind stops. The energy comes from reducing the true wind (like for every wind powered vehicle).

 

 

It followed that for propelling power to exceed resistance power the product of the efficiencies has to be greater than 1.

 

The math shows the opposite: That it works for efficiencies less than 1. See for exmaple:

 

Theory and Design of Flow Driven Vehicles Using Rotors for Energy Conversion

Mac Gaunaa, Stig Øye, Robert Mikkelsen

http://orbit.dtu.dk/...3748519/content

 

Physics Olympiad 2013 Semifinal (page 11-12)
American Association of Physics Teachers 

http://www.aapt.org/...-solutions.pdf 

 

Dead-Downwind Faster Than The Wind (DFTTW) Analysis

Mark Drela 

http://www.boatdesig...n-wind-ddw2.pdf

[saudoso]

The simplest one has already been posted by gruntgury:

 

 

Here another variant showing both directions:

 

Ok, now I got it. 

[rubberduck]

if W<V the term

V     
(V −W)  can never be positive 

 

On the contrary: If 0<W<V the term V/(V−W) is not only positive, but also > 1.

 

 

Oh and whilst you can change the velocity of the wind locally if you are moving faster than that wind, you cannot harvest energy by so doing.

 

Why not?

Edited by rubberduck, 12 December 2014 - 12:32.

[Kelpiecross]

Feliks was right after all.

Edited by Kelpiecross, 13 December 2014 - 06:22.

[gruntguru]

[ray b]

no sailors here ?

 

apparent wind shifts forward as the vehicle gains speed 

even down wind at a slight angle off dead down wind

so lift is moving the airfoil not just drag do to the change in apparent wind angle

[gruntguru]

Sure Ray

 

but a boat with a conventional sail or foil

cannot sail faster than the wind

either directly downwind

or directly upwind.

Edited by gruntguru, 14 December 2014 - 23:42.

[Greg Locock]

Here's the polar for a rather undistinguished yacht, http://www.boatdesig...rboats-p440.gif

 

As you can see downwind it struggles to hit 50% of the wind speed.

 

[Kelpiecross]

My explanation of how it all works: Wind moving relative to a stationary point on the ground has energy that can be harvested by a propeller (or turbine) that is fixed relative to the ground. No matter how fast or in what direction the wheeled cart moves, the point of contact of the wheel on the ground remains stationary - thus the wind speed relative to this stationary point always remains the same - and can be used to supply energy. (This is assuming that the propeller can swivel to always point directly into the wind). The wheel and the propeller must be mechanically linked to maintain this relativity between a fixed point on the ground and the wind speed etc.
If the same propeller was not linked mechanically to the wheels and was towed directly downwind on the cart at the same speed as the wind - the propeller would not turn at all.

Still - I wouldn't have guessed that it all would actually work as it does.

[rubberduck]

no sailors here ?

 

Greg already mentioned that ice boats easily achive downwind VMGs of multiple true wind speeds. Some very efficient boats can do it too. The trick of DDWFTTW: Instead of tacking with the entrie vehicle, only the airfoils move across the wind:

 

[scolbourne]

I thought ice boats achieved their downwind speed by going across wind to pick up speed and then simply coasted down wind using their momentum. They can do the same upwind as well.

 

As I lived in England when I decided to build one , I also found that when it is cold enough for thick ice , we  very rarely have any wind.

It did save a skaters life though when my brother was able to lay his sail across the thin  ice to allow a skater to climb out after he had fallen through.

[rubberduck]

I thought ice boats achieved their downwind speed by going across wind to pick up speed and then simply coasted down wind using their momentum.

 

They can also achieve a sustained downwind VMG > true wind, in steady state without loosing momentum. Of course they don't move directly downwind, but on broad reach like shown in the video above. But their directly downwind velocity component is greater than true wind. So they can release a ballon and catch it again by doing two tacks.

 

See diagram on page 4:

http://www.nalsa.org...mance-Cetus.pdf

Edited by rubberduck, 15 December 2014 - 13:29.

[rubberduck]

Wind moving relative to a stationary point on the ground has energy that can be harvested by a propeller (or turbine) that is fixed relative to the ground. No matter how fast or in what direction the wheeled cart moves, the point of contact of the wheel on the ground remains stationary - thus the wind speed relative to this stationary point always remains the same - and can be used to supply energy. (This is assuming that the propeller can swivel to always point directly into the wind). The wheel and the propeller must be mechanically linked to maintain this relativity between a fixed point on the ground and the wind speed etc.

 

That is a correct explanation for this specific case, but in more general terms you don't need that fixed point. For example a DDWFTTW boat would have an underwater turbine, instead of the wheels, so it would have slippage at both media. This makes it more difficult to work, but is still within our engineering possibilities according to Drela's analysis:

http://www.boatdesig...n-wind-ddw2.pdf

Edited by rubberduck, 16 December 2014 - 07:35.

[scolbourne]

I have added a poll to see whether people believe Blackbird achieved a sustained speed faster than the wind dead downwind (without cheating ie no flywheels , motors etc.)

[rubberduck]

I have added a poll to see whether people believe Blackbird achieved a sustained speed faster than the wind dead downwind (without cheating ie no flywheels , motors etc.)

 

I wasn't there, but the guys from the North American Land Sailing Association checked the vehicle, and ratified the record. Also, since the Blackbird performance matches quite well what the math predicts, I see little reason to suspect cheating.

[Kelpiecross]

I have added a poll to see whether people believe Blackbird achieved a sustained speed faster than the wind dead downwind (without cheating ie no flywheels , motors etc.)

Do you believe it?

I would be inclined to believe that the effect certainly does happen but as to the accuracy and honesty of the measurement I don't really know.

[gruntguru]

Though the measurement would have to be wildly inaccurate (more than 180% exaggerated) for it to not have outrun the wind.

[scolbourne]

How come after several years we do not see more of these craft. I thought races for these would have become popular as there are quite a few ways to improve the performance in construction and whilst driving (eg. varying prop pitch or changing gear ratio and working as a windmill for initial acceleration).

 

I noticed that the video has a cut at 1:22 just as the craft is starting  which could mean nothing or it could be hiding a push start allowing the prop to be spun up.

[rubberduck]

How come after several years we do not see more of these craft.

Several years? Bauer build and described his cart decades ago:

Andrew B. Bauer
Faster than the Wind (1969)
http://projects.m-qp...t-Interface.pdf


 

I thought races for these would have become popular

Given that the only benefit is being able to go on certain courses directly instead of tacking, I guess most land sailors prefer the simplicity of a conventional sail cart, over the added complexity, weight and failure possibilities of a rotor craft.
 

as there are quite a few ways to improve the performance in construction and whilst driving (eg. varying prop pitch or changing gear ratio and working as a windmill for initial acceleration).

The final version of the Blackbird had variable positve pitch. Bauer's rotor could even go negative, and work as a turbine for initial start up.
 

I noticed that the video has a cut at 1:22 just as the craft is starting which could mean nothing or it could be hiding a push start allowing the prop to be spun up.

Of course you can push it to save time and space. It's not like starting to roll downwind is the big challenge here. But there are several videos showing it self start downwind:

[Kelpiecross]

Bloody thing refuses to accept my vote - I voted for the second option.

[scolbourne]

To race these you could have a long straight (Nullabor Plain maybe) and maybe have the cars race both upwind and down wind legs.

 

By allowing the "windmill" to  turn to face the wind a normal race course could be used.

 

I guess these are not very safe in close proximity to each other but I hope the Dutch organisers of the Aeolus (upwind) race can include a down wind leg as well

 

http://www.windenergyevents.com/

[gruntguru]

I guess these are not very safe in close proximity to each other but I hope the Dutch organisers of the Aeolus (upwind) race can include a down wind leg as well

http://www.windenergyevents.com/

Those vehicles have smaller diameter turbines, which seems to answer the question of proximity.

[Tenmantaylor]

Someone get that guy a dead ferret to put over the mic.

[imaginesix]

This is the one time when wind noise in a video is beneficial!

Start -> Wind noise (from wind blowing past stationary car)

Accelerate -> No wind noise (from moving at the speed and direction of the wind)

Accelerate more -> Wind noise (from going faster than the wind)

[Charles E Taylor]

Wind Driven Speed Records

 

 

505 mph in a 65 mph wind.....

 

 

 

No engine - No Wheels, remarkable all the same.

 

 

Dynamic Soaring. 

 

https://www.youtube....notation_508371

 

505 mph DS world record 22nd Nov 2014 Weldon CA.

 

 

 

 

 

 

Charlie

[rubberduck]

 

505 mph in a 65 mph wind.....

 

 

It's the same principle: Using a small velocity difference of two big masses to achieve a much higher speed of a small mass. Albatrosses use this to travel continuously upwind and downwind faster than the wind.

[Kelpiecross]

 I am not sure it is the same principle.  

[rubberduck]

 I am not sure it is the same principle.  

 

In a general sense it is, with some differences:

 

Sailcraft are powered by the velocity difference between surface and airmass. Dynamic soaring gliders are powered by the velocity difference between two airmasses.

 

Sailcraft exchange momentum with surface and airmass simultaneously. Dynamic soaring gliders exchange momentum with the two airmasses alternately.