If you can read this, either the style sheet didn't load or you have an older browser that doesn't support style sheets. Try clearing your browser cache and refreshing the page.

(BBC-US)   One man's quest to outrun the wind with a wind-powered car   (bbc.com) divider line 86
    More: Interesting, blackbirds, quests, new physics, cars, wind energy  
•       •       •

2837 clicks; posted to Geek » on 31 Jul 2012 at 11:53 PM (2 years ago)   |  Favorite    |   share:  Share on Twitter share via Email Share on Facebook   more»



86 Comments   (+0 »)
   
View Voting Results: Smartest and Funniest

Archived thread

First | « | 1 | 2 | » | Last | Show all
 
2012-07-31 08:43:19 PM
My dream is to someday invent a wind-powered boat. Could you imagine the possibilities that would open? The mind boggles sometimes. We truly live in the golden age.
 
2012-07-31 08:55:08 PM

brap: My dream is to someday invent a wind-powered boat. Could you imagine the possibilities that would open? The mind boggles sometimes. We truly live in the golden age.


Trim the jib matey, and when I say "coming about", pop it. We be jibin'!
 
2012-07-31 09:01:36 PM
I've been reading about this guy for years. The skeptics understandably claim that the wind can't push the vehicle once it hits wind speed, but what they are missing is that the transmission that connects the propeller to the wheels provides the power once it reaches current wind speed. It essentially becomes, after the inflection point, a fan aimed directly back at the wind, which is how he manages to multiply the wind power.

It's bizarre, I agree, but there's nothing to suggest it won't work. In his demonstration videos, you can watch streamers on the "fan" switch from blowing towards the front of the vehicle to blowing towards the back - that's the point where the wheels take over powering the fan and work against the wind blowing from behind.

It's actually pretty farking brilliant if you ask me. Work a similar system utilizing water drag to power a fan on the back of a boat, and you could increase speed performance of a sailboat.
 
2012-07-31 11:55:02 PM

Lsherm: I've been reading about this guy for years. The skeptics understandably claim that the wind can't push the vehicle once it hits wind speed, but what they are missing is that the transmission that connects the propeller to the wheels provides the power once it reaches current wind speed. It essentially becomes, after the inflection point, a fan aimed directly back at the wind, which is how he manages to multiply the wind power.

It's bizarre, I agree, but there's nothing to suggest it won't work. In his demonstration videos, you can watch streamers on the "fan" switch from blowing towards the front of the vehicle to blowing towards the back - that's the point where the wheels take over powering the fan and work against the wind blowing from behind.

It's actually pretty farking brilliant if you ask me. Work a similar system utilizing water drag to power a fan on the back of a boat, and you could increase speed performance of a sailboat.


So the faster it goes, the faster the wheels turn, the faster the propeller spins, the more thrust it produces, the faster it goes... So how is this not a perpetual motion machine?
 
2012-08-01 12:09:58 AM
Sounds a little dodgy. But supposedly iceboats do it:

"Modern designs of iceboats are very efficient, utilizing aerodynamic designs and low friction, and can achieve speeds as high as ten times the wind speed in good conditions."
 
2012-08-01 12:10:02 AM

CreamFilling: So how is this not a perpetual motion machine?


A stationary wind turbine extracts energy from the wind moving relative to the ground. This machine does the same thing, except it moves. If the wind stops the vehicle will also stop.
 
2012-08-01 12:11:24 AM

CreamFilling:

So the faster it goes, the faster the wheels turn, the faster the propeller spins, the more thrust it produces, the faster it goes... So how is this not a perpetual motion machine?


Because the system is receiving energy from an external source... the wind.
 
2012-08-01 12:13:45 AM

CreamFilling: Lsherm: I've been reading about this guy for years. The skeptics understandably claim that the wind can't push the vehicle once it hits wind speed, but what they are missing is that the transmission that connects the propeller to the wheels provides the power once it reaches current wind speed. It essentially becomes, after the inflection point, a fan aimed directly back at the wind, which is how he manages to multiply the wind power.

It's bizarre, I agree, but there's nothing to suggest it won't work. In his demonstration videos, you can watch streamers on the "fan" switch from blowing towards the front of the vehicle to blowing towards the back - that's the point where the wheels take over powering the fan and work against the wind blowing from behind.

It's actually pretty farking brilliant if you ask me. Work a similar system utilizing water drag to power a fan on the back of a boat, and you could increase speed performance of a sailboat.

So the faster it goes, the faster the wheels turn, the faster the propeller spins, the more thrust it produces, the faster it goes... So how is this not a perpetual motion machine?


There's a certain amount of energy in moving air. I know a wind turbine can't even theoretically extract 100% of the available wind energy, and all practical designs fall short of that limit too. Maybe he's found some way to get that extra percentage. There should be some limit of friction, drag, or whatever that would limit the speed to some number.

Got some reading to do, I've never heard of this before.
 
2012-08-01 12:31:29 AM
Yep, it's possible. If the sail has a good aerodynamic shape, is lightweight, and low drag, it's possible for the lift force applied by the wind to accelerate the object past the wind speed. The key is that the wind is blowing across the object (as with and airplane wing), rather than on it. The difference between an airplane wing and a sail is that 1. the wind is what blows air across a sail, whereas with airplanes the forward motion is what blows air across a wing, and 2. a wing produces lift, whereas a sail produces thrust.
 
2012-08-01 12:32:30 AM

robv83: CreamFilling:

So the faster it goes, the faster the wheels turn, the faster the propeller spins, the more thrust it produces, the faster it goes... So how is this not a perpetual motion machine?

Because the system is receiving energy from an external source... the wind.


Oh, sure, I guess that's the obvious answer. But I would think that once he he's going it would produce its own thrust regardless of whether here was wind or not. Does it need the wind to thrust against?
 
2012-08-01 12:36:51 AM

CreamFilling: robv83: CreamFilling:

So the faster it goes, the faster the wheels turn, the faster the propeller spins, the more thrust it produces, the faster it goes... So how is this not a perpetual motion machine?

Because the system is receiving energy from an external source... the wind.

Oh, sure, I guess that's the obvious answer. But I would think that once he he's going it would produce its own thrust regardless of whether here was wind or not. Does it need the wind to thrust against?


you leave pussy farts out of this
 
2012-08-01 01:06:54 AM
It's not perpetual motion, because at some point friction/drag, are balanced out by the extra thrust put out by the fan. The other limiter is that tip speed of your prop can't go supersonic. Thus your thrust will always be capped because after a certain speed, you can't get any more thrust from the propeller. In other words, it's a lot like a car, in that if you've got x amount of horsepower, y drag coefficient, and z transmission(speed limit on engine) you can only go so fast.

Now what's interesting is that if you had a machine designed correctly, you could probably 2-3x the wind speed in any direction, maybe even more if you had a more advanced machine (say 4x-5x if you had someone like Boeing making the car). The other thing, is that if you had the ability to pivot your propeller so that it was always facing directly into the wind, you could effectively go at high speed without regard to the wind direction.
 
2012-08-01 01:17:08 AM

CreamFilling: Lsherm: I've been reading about this guy for years. The skeptics understandably claim that the wind can't push the vehicle once it hits wind speed, but what they are missing is that the transmission that connects the propeller to the wheels provides the power once it reaches current wind speed. It essentially becomes, after the inflection point, a fan aimed directly back at the wind, which is how he manages to multiply the wind power.

It's bizarre, I agree, but there's nothing to suggest it won't work. In his demonstration videos, you can watch streamers on the "fan" switch from blowing towards the front of the vehicle to blowing towards the back - that's the point where the wheels take over powering the fan and work against the wind blowing from behind.

It's actually pretty farking brilliant if you ask me. Work a similar system utilizing water drag to power a fan on the back of a boat, and you could increase speed performance of a sailboat.

So the faster it goes, the faster the wheels turn, the faster the propeller spins, the more thrust it produces, the faster it goes... So how is this not a perpetual motion machine?


Because it stops accelerating at a certain point because of the wind - it actually follows a very clear waste energy equation after the inflection point. The mistake is reasoning that the wind is spinning the propeller - it isn't. The wind is pushing against the surface area of the propeller but it's spinning because of the friction provided by the wheels on the ground, transferred back to the propeller by the wheels.

The spinning propeller is extra energy provided by friction from the ground and the transmission. He just hit the magic design that keeps it going after wind speed. The whole design falls apart if the wheels can't drive the fan. But they do.
 
2012-08-01 01:29:52 AM

Dommo: It's not perpetual motion, because at some point friction/drag, are balanced out by the extra thrust put out by the fan. The other limiter is that tip speed of your prop can't go supersonic. Thus your thrust will always be capped because after a certain speed, you can't get any more thrust from the propeller. In other words, it's a lot like a car, in that if you've got x amount of horsepower, y drag coefficient, and z transmission(speed limit on engine) you can only go so fast.

Now what's interesting is that if you had a machine designed correctly, you could probably 2-3x the wind speed in any direction, maybe even more if you had a more advanced machine (say 4x-5x if you had someone like Boeing making the car). The other thing, is that if you had the ability to pivot your propeller so that it was always facing directly into the wind, you could effectively go at high speed without regard to the wind direction.


Altering course would be an interesting challenge.
 
2012-08-01 01:34:37 AM
I almost think you'd want to have two big props, rather than one, just because it'd be hard to deal with the torque of that spinning prop, when it's that big in comparison to the car. Set them to spin in opposite directions. Still, I'm not sure how you'd get the dang thing to turn very easily.
 
2012-08-01 01:54:15 AM
Weird, I was just reading about America's Cup yachts sailing faster than the wind earlier today, then this comes up.

Link
 
2012-08-01 03:41:21 AM
FTA: On the back of the car are two propeller blades. Although many people assume these are turned directly by the wind, they are not. Instead they are connected to the rear wheels by a long bicycle chain that runs through a transmission. As the wheels move, the propeller turns, creating thrust.

At the point when the vehicle's speed matches that of the wind, where does the power come from? It can't come from the wheels as they aren't providing power, only drag. Even the propeller is adding drag as it turns.

I could see momentarily exceeding the wind speed, but not maintaining a speed over the wind speed.
 
2012-08-01 05:09:40 AM
Though not exactly like this, in sailing it is called apparent wind
 
2012-08-01 07:38:56 AM
He is extracting energy, not velocity, from the wind. Would you say it is impossible for a fan powered wind-generated electricity to blow faster than the wind?
 
2012-08-01 08:21:21 AM

Lsherm: The spinning propeller is extra energy provided by friction from the ground and the transmission.


I don't know about "extra" it is energy taken from forawrd momentum.

Lsherm: He just hit the magic design that keeps it going after wind speed. The whole design falls apart if the wheels can't drive the fan. But they do


I am curious how long he can sustain it.

Sounds to me it is kind of like a helicopter auto rotation. The spining blades store up energy and he can change the pitch to drive it forward (or slow down in the case of autorotation), but that would anly allow for a temporary speed above wind speed and over a long distance a machine that didn't waste energy by storing it wuold go faster.

/first impression, so may not be right.


assjuice: He is extracting energy, not velocity, from the wind. Would you say it is impossible for a fan powered wind-generated electricity to blow faster than the wind?


No, but only if the wind is blown for a short time or if it is moving less air then it gets from the powering.

Which is why I would say he can't continually go faster than the wind.
 
2012-08-01 08:33:05 AM

AlwaysRightBoy: brap: My dream is to someday invent a wind-powered boat. Could you imagine the possibilities that would open? The mind boggles sometimes. We truly live in the golden age.

Trim the jib matey, and when I say "coming about", pop it. We be jibin'!


Aye, Jibbing downwind. Tacking upwind. Apparent angle of attack would allow ye to go faster than the breeze.

/LEEWARD!!
/Ever seen a brawl of fisticuffs between two crews during a J-24 race? You haven't sailed till you have.
/There's other good things about sailing too.
/Rum, Sodomy, the lash, Sodomy, and Scurvy!
 
2012-08-01 09:18:25 AM
No.
The stored energy is in the momentum of the vehicle itself. You may be able to use that to accelerate faster than the wind temporarily, but the car will eventually travel slower with this system engaged. You can't just look at a slice of this and say "he's moving faster than the wind'. I think that if you "closed the system' at the beginning and the end of movement and calculated everything, you would find no average speed above the wind. Which is sort of what they are inferring is possible. It isn't.
 
2012-08-01 09:35:22 AM

FC Exile: No.
The stored energy is in the momentum of the vehicle itself. You may be able to use that to accelerate faster than the wind temporarily, but the car will eventually travel slower with this system engaged. You can't just look at a slice of this and say "he's moving faster than the wind'. I think that if you "closed the system' at the beginning and the end of movement and calculated everything, you would find no average speed above the wind. Which is sort of what they are inferring is possible. It isn't.


Came here to say this.

I'll add: Travelling perpendicular to the wind can get you faster than the wind, depending on the angle of the sail and the design of the boat (as has been pointed out earlier in the thread).

Traveling straight downwind with an average speed faster than the wind is perpetual motion. It can't be done with a sailboat, a propeller car, or any other wind-based engine.

What this guy has done has been to basically store up some of the wind power to temporarily accelerate past wind speed. It's no more impressive than if he'd used the turbine to wind up a giant rubber band that briefly accelerated the car past the wind speed. Of course, if he had done that, everyone would see the trick right away.
 
2012-08-01 09:41:17 AM
E(in)=E(out)
 
2012-08-01 09:41:39 AM
Apparently the thing can go upwind faster than wind speed as well.
 
2012-08-01 09:45:57 AM

FC Exile: The stored energy is in the momentum of the vehicle itself. You may be able to use that to accelerate faster than the wind temporarily,


Except not, because under that assumption the faster you go the less energy you have to accelerate, which means you will simply approach wind speed asymptotically. The only way to exceed wind speed with this view is if the wind died down - which would be extremely obvious. These guys are getting up towards 3X tail wind speed.

Sailing faster than wind speed is common for boats, but they can't do it while sailing directly downwind because their sails are fixed. By essentially mounting the sails on an axle, we can overcome that.

Imagine a boat sailing perpendicular to the wind. The actual wind is trying to push the boat sideways, but the angle of the sails plus the forward speed of the boat results in a net forward speed that is much faster than true wind speed. The hull of the boat is pushed sideways against the water to counter the force of the true wind on the sails, so the boat necessarily drifts in that direction, but you don't want to go that way anyway.

Now imagine you stick that boat on the end of a pole, with the other end of the pole fixed to an axle. instead of sailing in a straight line perpendicular to the wind, it sails in a circle perpendicular to the wind. (If it helps, imagine the pole is as long as the radius of the planet and you're right back to sailing across the ocean as normal)

Remember that there is much more force pushing the boat forward than sideways, which is why it's moving so much faster than the true wind. We can harness this force as a torque on the axle, and use it to drive the wheels of our car. The car moving forward is analogous to the boat sliding sideways in the wind, except the driving force pushing the boat is the true wind alone and the driving force pushing the car forward is the true wind plus the torque harnessed from the apparent wind which is much greater.

As the car accelerates, the true wind will be effectively decreased - but the apparent wind on the "boat sails" of the turbine will still always be greater, and since that's what you're using to drive the car, you can go faster than the true wind.
=Smidge=
 
2012-08-01 09:46:36 AM
Define that. Cause they don't, and it would be the more impressive feat.
 
2012-08-01 09:49:44 AM
They claim this is traveling straight downwind.
 
2012-08-01 09:56:21 AM
This was debunked in Make magazine over a year ago.

His biggest accomplishment is building a full-scale model to demonstrate b.s. to gullible people.

If he added a motor and batteries, a rocket engine and fuel, a gas engine and fuel tank, etc. he could outrun the wind.
Having a car more powerful than it's energy source isn't in the cards.
 
2012-08-01 09:58:03 AM

Foundling: Make magazine


Got a link / reference for that?
=Smidge=
 
2012-08-01 09:59:05 AM

draypresct: FC Exile: No.
The stored energy is in the momentum of the vehicle itself. You may be able to use that to accelerate faster than the wind temporarily, but the car will eventually travel slower with this system engaged. You can't just look at a slice of this and say "he's moving faster than the wind'. I think that if you "closed the system' at the beginning and the end of movement and calculated everything, you would find no average speed above the wind. Which is sort of what they are inferring is possible. It isn't.

Came here to say this.

I'll add: Travelling perpendicular to the wind can get you faster than the wind, depending on the angle of the sail and the design of the boat (as has been pointed out earlier in the thread).

Traveling straight downwind with an average speed faster than the wind is perpetual motion. It can't be done with a sailboat, a propeller car, or any other wind-based engine.

What this guy has done has been to basically store up some of the wind power to temporarily accelerate past wind speed. It's no more impressive than if he'd used the turbine to wind up a giant rubber band that briefly accelerated the car past the wind speed. Of course, if he had done that, everyone would see the trick right away.


With a boat there are a few caveats, namely current.
You can also surf a forty foot boat down the front of the swell or jibe at an angle to get a bit more apparent wind.
Since ground speed and wind speed are two different things you can "rubber band" slightly past the apparent (causing the spinnaker to collapse and fun things like an accidental jibe (where the boom comes swinging back at you like a twenty foot long baseball bat because you forgot to tie it down).

I'm not sure what the physics are in this car, but I'm open to the concept of exceeding the speed of the wind. It may be a case where the drag is just a hair low enough for the gearing to work, not really perpetual motion.
 
2012-08-01 10:02:52 AM

JohnAnnArbor: Sounds a little dodgy. But supposedly iceboats do it:

"Modern designs of iceboats are very efficient, utilizing aerodynamic designs and low friction, and can achieve speeds as high as ten times the wind speed in good conditions."


Almost all modern sailboats can sail faster than the wind (with a low maximum due to drag through water), at an angle around 45 degrees of the wind. This is due to the wind blowing around the sails provides a life force. Its not the wind in the sails as much as the differential in air pressure infront and behind the sails.
 
2012-08-01 10:05:09 AM

MindStalker: JohnAnnArbor:


Lift force, not life force ! :)
 
2012-08-01 10:30:07 AM

way south: With a boat there are a few caveats, namely current.
You can also surf a forty foot boat down the front of the swell or jibe at an angle to get a bit more apparent wind.
Since ground speed and wind speed are two different things you can "rubber band" slightly past the apparent (causing the spinnaker to collapse and fun things like an accidental jibe (where the boom comes swinging back at you like a twenty foot long baseball bat because you forgot to tie it down).


True (and nice image!).
 
2012-08-01 10:51:06 AM
It goes a little something like this:
1:Wind moves faster than the ground. This gives you potential energy. Imagine a windmill that powers an electric cart. Obviously the cart can go faster than the wind depending on the windmill's size, the cart's friction and the efficiency of the setup.

2: Now we put the windmill on the cart. We use a direct mechanical coupling. It's the same energy, except used more efficiently. We start the cart by letting the wind blow over the blades. The blades deflect the air, which slows it down. The cart uses the rotational energy it gets from that by turning the wheels. Now, this is important: it looks like the wind is blowing against the cart's own inertia (as it would with a sailboat/car) But in fact, since the wheels are coupled to the ground, and the blades to the wheels, the wind is still blowing against the ground.

3: The TADAAA! moment. The cart is riding exactly as fast as the wind. (What does that tell you about the speed of the blades? Nothing! The speed of the blades is determined by the speed of the cart, and the ratio on the connection between the wheels and the blades. Anyway) It's still riding. Where is the energy coming from? Still from the difference in speed between the wind and the ground. To tap into this energy, the blades merely need to slow down the air- from the cart's point of view now, push the air backwards.
The blade is pushing against the air instead of the air against the blade. But it is only pushing because it's hooked up to the wheels that are now being rotated by the ground. Almost every newton the ground puts into slowing the cart down is instead put into slowing the wind down. Which makes the cart go faster.

That's what happens. Why they talk about the turbine being a propeller. And why it can go faster than the wind. The coupling between the ground and the wheels needs to be perfect for this to work. The whole cart needs to be super efficient. As do the propellers. And that total efficiency, weighed against inertia, times wind speed determines the speed of the cart.
 
2012-08-01 11:01:53 AM
This has been around for awhile. A longer, better article is on Wired's site. Also a paper on the concept dates back to 1969.
 
2012-08-01 11:16:53 AM

L2GX: It goes a little something like this:
*snip*


Actually, looking at the video, I believe it works a bit more like this:

Step 1: Basically what you said in steps 1 and 2. The rotor blades act like an actuator disk or sail to overcome the friction of the geartrain and start the vehicle moving. The rear wheels start rotating the blades.

Step 2: As the vehicle accelerates, the wind continues to provide thrust, but the driver has pitched the blades such that the tips of the blade begin to add additional thrust. Because of the blade twist, the portions of the blades near to the hub are stalled.

Step 3: As the vehicle approaches wind speed, it will begin to reach a constant speed; the rotor provides little lift. The key point here is that wind speed is not constant. What the driver is doing is waiting for a fluctuation in wind speed upward, then a fluctuation downward, leaving the vehicle momentarily going faster than the wind speed.

Step 4: With the vehicle moving faster than instantaneous wind speed, the driver pitches the blades to act as a turbine rather than a propeller. Key point here: a symmetric airfoil can do this without changing the direction of rotation by changing the angle of attack; the "top" surface of the blade functioning as a propeller becomes the "bottom" surface of the blade functioning as a turbine, and vice versa. You can see from the video that as the vehicle starts going faster than wind speed, the driver pitches the blades further from the plane of rotation, which would be necessary to function as a turbine. The mechanics are essentially the same as how a wind turbine starts, except the blades are already moving. The fact that the vehicle itself is moving is already accounted for by the change in apparent wind speed and direction, and whatever torque is placed on the geartrain from wind and mechanical resistances. Given a sufficient window of opportunity, the vehicle will accelerate past the point where it is affected by turbulence.

Step 5: As the blades spin faster, they must be pitched closer to the plane of rotation in order to create lift. This decreases the torque generated, and eventually the vehicle will stop accelerating.
 
2012-08-01 11:40:29 AM

FC Exile: No.
The stored energy is in the momentum of the vehicle itself. You may be able to use that to accelerate faster than the wind temporarily, but the car will eventually travel slower with this system engaged. You can't just look at a slice of this and say "he's moving faster than the wind'. I think that if you "closed the system' at the beginning and the end of movement and calculated everything, you would find no average speed above the wind. Which is sort of what they are inferring is possible. It isn't.


It is, but it takes a long time to think through it. It's fairly counterintuitive, but it works. Everyone who puzzles it out has the "aha!" moment differently. For me, it was thinking about a similar model in a canal. Water moves boat. Boat uses wheel to touch canal wall. Transmission transfers rotation to propeller... and you have a current powered boat going downstream faster than the current. But it may take you ten different conceptual models and a dozen YouTube videos to get there. Or even building a working model from a kit and doing the steady-state demo for yourself on a treadmill.
 
2012-08-01 11:52:08 AM

The Madd Mann: L2GX: It goes a little something like this:
*snip*

Actually, looking at the video, I believe it works a bit more like this:

Step 1: Basically what you said in steps 1 and 2. The rotor blades act like an actuator disk or sail to overcome the friction of the geartrain and start the vehicle moving. The rear wheels start rotating the blades.

Step 2: As the vehicle accelerates, the wind continues to provide thrust, but the driver has pitched the blades such that the tips of the blade begin to add additional thrust. Because of the blade twist, the portions of the blades near to the hub are stalled.

Step 3: As the vehicle approaches wind speed, it will begin to reach a constant speed; the rotor provides little lift. The key point here is that wind speed is not constant. What the driver is doing is waiting for a fluctuation in wind speed upward, then a fluctuation downward, leaving the vehicle momentarily going faster than the wind speed.

Step 4: With the vehicle moving faster than instantaneous wind speed, the driver pitches the blades to act as a turbine rather than a propeller. Key point here: a symmetric airfoil can do this without changing the direction of rotation by changing the angle of attack; the "top" surface of the blade functioning as a propeller becomes the "bottom" surface of the blade functioning as a turbine, and vice versa. You can see from the video that as the vehicle starts going faster than wind speed, the driver pitches the blades further from the plane of rotation, which would be necessary to function as a turbine. The mechanics are essentially the same as how a wind turbine starts, except the blades are already moving. The fact that the vehicle itself is moving is already accounted for by the change in apparent wind speed and direction, and whatever torque is placed on the geartrain from wind and mechanical resistances. Given a sufficient window of opportunity, the vehicle will accelerate past the point where it is affected by turbulence.

Step ...


Maybe I am misunderstanding what you are saying but I believe L2GX has the correct version. At first (speeds well below wind speed) the blades act as a turbine driving the wheels. At around wind speed and above the wheels drive the blades like a propeller pushing the car forward. I scanned through the 1969 paper and I believe this should work without adjusting the angle of attack of the turbine/propeller blades. By adjusting the angle of attack of the blades the Blackbird was able to maximize the effect and move at nearly 3 times the speed of the wind. I guess. I need to actually read the 1969 paper (which actually discusses the physics)
 
2012-08-01 12:52:18 PM

rwfan: Maybe I am misunderstanding what you are saying but I believe L2GX has the correct version. At first (speeds well below wind speed) the blades act as a turbine driving the wheels. At around wind speed and above the wheels drive the blades like a propeller pushing the car forward. I scanned through the 1969 paper and I believe this should work without adjusting the angle of attack of the turbine/propeller blades. By adjusting the angle of attack of the blades the Blackbird was able to maximize the effect and move at nearly 3 times the speed of the wind. I guess. I need to actually read the 1969 paper (which actually discusses the physics)


I'm going to have to run through the paper again as well, but consider this: the man in the article said that he had to put a turbine blade on in order to drive the vehicle upwind. If you look at the operation of the blades from a frame of reference moving at the same speed as the vehicle, shouldn't the principles of operation be the same for the vehicle moving upwind as it would be for the vehicle moving downwind faster than the wind speed? The only difference should be that the speed of the blades would be different for a given wind speed perceived by the blades, which would require modifications to the twist of the blades in order to optimize performance.

Of course, I could be completely wrong.
 
2012-08-01 01:30:41 PM

The Madd Mann: rwfan: Maybe I am misunderstanding what you are saying but I believe L2GX has the correct version. At first (speeds well below wind speed) the blades act as a turbine driving the wheels. At around wind speed and above the wheels drive the blades like a propeller pushing the car forward. I scanned through the 1969 paper and I believe this should work without adjusting the angle of attack of the turbine/propeller blades. By adjusting the angle of attack of the blades the Blackbird was able to maximize the effect and move at nearly 3 times the speed of the wind. I guess. I need to actually read the 1969 paper (which actually discusses the physics)

I'm going to have to run through the paper again as well, but consider this: the man in the article said that he had to put a turbine blade on in order to drive the vehicle upwind. If you look at the operation of the blades from a frame of reference moving at the same speed as the vehicle, shouldn't the principles of operation be the same for the vehicle moving upwind as it would be for the vehicle moving downwind faster than the wind speed? The only difference should be that the speed of the blades would be different for a given wind speed perceived by the blades, which would require modifications to the twist of the blades in order to optimize performance.

Of course, I could be completely wrong.


Maybe I am not sure what you are asking but it makes sense that you would use a turbine blade to run upwind and a propeller to run downwind. Upwind the apparent wind is always blowing front to back and the blades are always acting as a turbine. Downwind the apparent wind goes from back to front, then to zero and then to front to back (when the blades are acting as a propeller). I assume you could use either blade and get the effect regardless of upwind or downwind but the turbine blade works best up wind and the propeller works best downwind.

This kind of reminds me of the "Monty Hall" or "Let's Make a Deal" problem. At first it seems counterintuitive but once you understand the logic it seems obvious. Of course, not having read the paper, I am not positive I understand all that is going on.
 
2012-08-01 02:19:41 PM

rwfan: At first (speeds well below wind speed) the blades act as a turbine driving the wheels.


At low speeds, the whole thing (blades included) is just acting like a sail. In essence, the whole backside of the boat is just a slab to catch wind and push the entire thing forward. Ignore the rotating bits at this point.

In any event, no pitch changes to the prop or variations in wind speed are needed-- the system will operate above tailwind speed in steady state. There are plenty of youtube demos, but you can get a kit and test it out yourself. It took me months to convince myself it was workable. It helped me to think of an analogous arrangement in water, but everyone seems to have a different "aha" moment.
 
2012-08-01 03:02:01 PM

raygundan: rwfan: At first (speeds well below wind speed) the blades act as a turbine driving the wheels.

At low speeds, the whole thing (blades included) is just acting like a sail. In essence, the whole backside of the boat is just a slab to catch wind and push the entire thing forward. Ignore the rotating bits at this point.


Hmmm, now that you mention it, it seems like the propeller would need to rotate in the opposite direction the wind would initially try to push it in (assuming it was free to rotate in any direction). Right? I am trying to visualize this in my head so I might be wrong.

In any event, no pitch changes to the prop or variations in wind speed are needed-- the system will operate above tailwind speed in steady state. There are plenty of youtube demos, but you can get a kit and test it out yourself. It took me months to convince myself it was workable. It helped me to think of an analogous arrangement in water, but everyone seems to have a different "aha" moment.

Agreed.
 
2012-08-01 03:19:21 PM

rwfan: Hmmm, now that you mention it, it seems like the propeller would need to rotate in the opposite direction the wind would initially try to push it in (assuming it was free to rotate in any direction). Right? I am trying to visualize this in my head so I might be wrong.


I'm about 99% sure that's correct, but am having the same sort of niggling doubt that I've visualized the whole assembly properly. When it's in the above-windspeed steady-state, forward wheel motion is being translated into the prop blowing air out the back. Which I think means if you picked it up to let the wheels turn free, and blew air at the prop from the rear until it spun, the wheels would turn backwards.
 
2012-08-01 03:24:35 PM

raygundan: In any event, no pitch changes to the prop or variations in wind speed are needed-- the system will operate above tailwind speed in steady state.


I don't think that is possible.


raygundan: There are plenty of youtube demos, but you can get a kit and test it out yourself.


I am not sure if that is valid.

The link says, "For those no familiar with DDWFTTW, advancing on a treadmill in a still air room is *exactly* the same as moving above the wind speed while moving downwind." Which maybe right, but putting it on a moving treadmill isn't the same as having it in a room where the wind starts blowing behind it. The device they have on the ramp can store energy used to overcome inertia and use that to push the direction of the treadmill.

raygundan: It helped me to think of an analogous arrangement in water, but everyone seems to have a different "aha" moment


You can't do that in water with just tailwind, only if you are at an angle to the wind.

On all the articles about this none of them have said anything abotu how long it could sustain speed above the ambient wind.
 
2012-08-01 03:45:44 PM

liam76: raygundan: In any event, no pitch changes to the prop or variations in wind speed are needed-- the system will operate above tailwind speed in steady state.

I don't think that is possible.


I agree.

raygundan: There are plenty of youtube demos, but you can get a kit and test it out yourself.

I am not sure if that is valid.

The link says, "For those no familiar with DDWFTTW, advancing on a treadmill in a still air room is *exactly* the same as moving above the wind speed while moving downwind." Which maybe right, but putting it on a moving treadmill isn't the same as having it in a room where the wind starts blowing behind it. The device they have on the ramp can store energy used to overcome inertia and use that to push the direction of the treadmill.

raygundan: It helped me to think of an analogous arrangement in water, but everyone seems to have a different "aha" moment

You can't do that in water with just tailwind, only if you are at an angle to the wind.

On all the articles about this none of them have said anything abotu how long it could sustain speed above the ambient wind.


The demos I've seen show a short burst of speed 'upwind' (or towards the base of the treadmill in those examples), which we've covered upthread discussing the momentum issue. I haven't seen any sustained DDWFTTW videos.
 
2012-08-01 03:46:49 PM

liam76: You can't do that in water with just tailwind, only if you are at an angle to the wind.


I think the "broad reach" faster than wind example only helps in that it shows that it is possible to extract energy from the wind even if your speed is greater than the wind speed. This case is trickier but for me the key point (as someone else point out) is that while the apparent wind on the car has a speed of zero the propeller is being driven by the wheel and is therefore providing thrust. The next question is where is the energy coming from? Obviously it has to come from the wind but it's not obvious how. I am sure it would help to read the 1969 paper I linked to early (I hope to get to it tonight).

On all the articles about this none of them have said anything abotu how long it could sustain speed above the ambient wind.

Forever (well as long as the wind blows), just like the broad reach sailboat example.
 
2012-08-01 04:26:04 PM

liam76: raygundan: In any event, no pitch changes to the prop or variations in wind speed are needed-- the system will operate above tailwind speed in steady state.

I don't think that is possible.


Imagine a toy car with a long metal rod mounted horizontally on top of it. If you push the back end of this rod, the rod and the car will move along at the same speed as your finger.

Now put some gears on the car so that the rod extends out backwards as the car rolls forward (e.g. 1/2 inch extension per inch of forward travel). Again push on the back end of the rod. The back of the rod will still move at "finger speed" but due to the gearing the car has to go faster than this.

Now replace the solid rod and the finger with moving air.
 
2012-08-01 04:26:04 PM

liam76: I don't think that is possible.


It's been done.

I am not sure if that is valid.

The link says, "For those no familiar with DDWFTTW, advancing on a treadmill in a still air room is *exactly* the same as moving above the wind speed while moving downwind." Which maybe right, but putting it on a moving treadmill isn't the same as having it in a room where the wind starts blowing behind it. The device they have on the ramp can store energy used to overcome inertia and use that to push the direction of the treadmill.


There's plenty more videos by that group, each painstakingly addressing concerns like yours (as well as things like "you hid a spring" and "I see strings"). It's true that the ramp shows *only* the steady-state. They have separate videos showing it starting up from a stop and transitioning to the steady-state.

You can't do that in water with just tailwind, only if you are at an angle to the wind.

You're thinking of sailboats. I should have elaborated, but I meant a truly analogous setup. In my case, I like using a mental picture of a rowboat in a canal, with a wheel touching the side of the canal that is mechanically coupled to a propeller in the water.

Look-- I know it's a rough concept. I wrestled with it for months, drew all sorts of pictures and did more calculus than I have since college. But if you want to skip over the painful part, the simple truth is that they've done it.
 
2012-08-01 04:32:10 PM

Beer It's What's For Dinner: Weird, I was just reading about America's Cup yachts sailing faster than the wind earlier today, then this comes up.

Link


I remember reading an article years ago. The author was on a state-of-the-art catamaran doing 20 knots or so in the bay just using the masts.
 
Displayed 50 of 86 comments

First | « | 1 | 2 | » | Last | Show all

View Voting Results: Smartest and Funniest


This thread is archived, and closed to new comments.

Continue Farking
Submit a Link »






Report