Wind powered vehicle, or perpetual motion machine?

[crashtestdummy]

From this article:
http://www.wired.com/magazine/2011/02/ff_fasterthanwind/

Can a wind-driven vehicle go faster than the wind, going downwind? The video would appear to validate the result, and outside of a very elaborate hoax I have to buy it, but I can't wrap my head around it.

The claim is that once the device gains speed, the wheels push the turbine, which then generates forward lift (like an airplane propeller). This lift is enough that the car can accelerate to more than twice the wind speed and sustain it. What I suppose I don't understand is the energetics. I can see that the propeller would provide forward force, but I don't see how that can possibly provide a net kinetic energy gain for the machine. Wouldn't that mean that the car somehow builds energy for itself out of nothing?

 

[Matt1970]

Not even perpetual motion machine, has to be a hoax. As soon as you were to start going faster than the wind you were in, the wind will want to push the propeller in the opposite direction.

 

[crashtestdummy]

Not even perpetual motion machine, has to be a hoax. As soon as you were to start going faster than the wind you were in, the wind will want to push the propeller in the opposite direction.

The claim is that the wind doesn't drive the rotation of the propeller, that the wheels do. At that point, though, the wind doesn't actually provide any power at all except drag. In which case, the car is gaining kinetic energy from its own kinetic energy. I must be missing something.

 

[Evadman]

The wind is not directly pushing the propeller, the propeller is going though the wind at an angle, just like a tacking sailboat. Therefore, the wind is providing a pushing power, and creating lift (to the front of the vehicle instead of up on a plane). The 2 combined make for enough power to go faster than the direct downwind speed.

Just like a boat tacking into the wind, it is possible to move faster than the wind itself. That is also in a high friction environment (water). Use the same energy on a low friction environment (wheels on land) and you can go much faster. It makes perfect sense to me.

<edit>
Heh, check the vid. The right rear wheel isn't even on the ground from the torque from the propeller.

 

[Matt1970]

I just looked at it again. It has to be a hoax. They are claiming the wind starts driving the propeller from behind but it is spinning in the wrong direction to have wind driving it from behind.

 

[crashtestdummy]

The wind is not directly pushing the propeller, the propeller is going though the wind at an angle, just like a tacking sailboat. Therefore, the wind is providing a pushing power, and creating lift (to the front of the vehicle instead of up on a plane). The 2 combined make for enough power to go faster than the direct downwind speed.

Just like a boat tacking into the wind, it is possible to move faster than the wind itself. That is also in a high friction environment (water). Use the same energy on a low friction environment (wheels on land) and you can go much faster. It makes perfect sense to me.

<edit>
Heh, check the vid. The right rear wheel isn't even on the ground from the torque from the propeller.

As stated above me, the propeller spins the wrong way for it to be used the way a sail would. Furthermore, a sailboat can move faster than the wind, but not in the direction of the wind. Once the windward component of the boat/car reaches the speed of the wind, the sails would fall flat.

 

[Evadman]

As stated above me, the propeller spins the wrong way for it to be used the way a sail would.

No, it is the correct way. The sail is acting as an airfoil, not like a parachute stuck into the wind.

Furthermore, a sailboat can move faster than the wind, but not in the direction of the wind.

That's what tacking is; when a boat wants to go directly against or with the wind. The boat doesn't go directly into the wind, but the net effect is that the boat can go faster than the wind. A plain old regular boat did that int he America's cup IIRC so net downwind faster than the wind, certainly possible. Directly downwind is the issue, because you need to turn the sail into the wind, without changing the direction of travel of the vehicle.

Changing the sail into something resembling a propeller puts the sail at a different angle than the wind. Thus, lift pulls the sail (propeller) forward in addition to just pushing. Honestly, I think the problem is that the sail is not a propeller. The sail only looks like a propeller, and the article calls it one. It is a sail, or airfoil.

 

[CycloWizard]

Impeller design is everything. The point of the impeller is to collect the wind's kinetic energy and transform it into rotation, thereby driving the wheels. Thus, there is no reason for the vehicle's speed to be limited by the wind speed, regardless of the vehicle's direction relative to the wind. Creating an ideal impeller which does this efficiently without creating mechanical instabilities in such a lightweight setup, on the other hand, is hardly trivial.

 

[Matt1970]

No, as stated in the video the propeller begins spinning by wind from the rear. That propeller is spinning the wrong direction if the wind is from the rear. Grab a small fan if you can and blow on the blades and see which way it spinns.

Also, they are going pretty much in a straight path with the wind. To go faster than the wind you need to be going at an angle to the wind to create a wing from a large sail.

 

[CycloWizard]

No, as stated in the video the propeller begins spinning by wind from the rear. That propeller is spinning the wrong direction if the wind is from the rear. Grab a small fan if you can and blow on the blades and see which way it spinns.

Also, they are going pretty much in a straight path with the wind. To go faster than the wind you need to be going at an angle to the wind to create a wing from a large sail.

Impellers are not sails. Sails convert kinetic energy from the wind directly to kinetic energy of the vehicle. Impellers convert the wind's kinetic energy into mechanical energy via a drive train, which then converts the mechanical energy into the vehicle's kinetic energy. The net result is the same (kinetic energy to kinetic energy), but the thermodynamic path taken is substantially different.

 

[Matt1970]

Ok, never mind the fact that the propeller is spinning the wrong direction. They are going directly downwind. Even if they were able to reach the exact speed of the wind. Once the vehicle goes faster than the wind, the you will lose all force on the propeller as the wind will now be flowing in the opposite direction as far as the propeller is concerned.

 

[Matt1970]

The car will accelerate as a direct result of the difference between the true wind and the Apparent wind. Once apparent wind becomes zero, there is no longer a wind to drive the propeller.

 

[CycloWizard]

Ok, never mind the fact that the propeller is spinning the wrong direction. They are going directly downwind. Even if they were able to reach the exact speed of the wind. Once the vehicle goes faster than the wind, the you will lose all force on the propeller as the wind will now be flowing in the opposite direction as far as the propeller is concerned.

Not necessarily. Impellers can also be driven by pressure differences. The "front" of the impeller will have a higher pressure than the back. The shape of the impeller again dictates the utility of this driving force.

 

[Skyhanger]

The world land speed record for a wind powered vehicle was broken on 26 March 2009 by Richard Jenkins in his yacht Greenbird with a speed of 126.1 mph (202.9 km/h).[5] Wind speeds were fluctuating between 30–50 mph (48–80 km/h) at that time.[6][7]

From wikipedia.

 

[crashtestdummy]

No, it is the correct way. The sail is acting as an airfoil, not like a parachute stuck into the wind.



That's what tacking is; when a boat wants to go directly against or with the wind. The boat doesn't go directly into the wind, but the net effect is that the boat can go faster than the wind. A plain old regular boat did that int he America's cup IIRC so net downwind faster than the wind, certainly possible. Directly downwind is the issue, because you need to turn the sail into the wind, without changing the direction of travel of the vehicle.

Changing the sail into something resembling a propeller puts the sail at a different angle than the wind. Thus, lift pulls the sail (propeller) forward in addition to just pushing. Honestly, I think the problem is that the sail is not a propeller. The sail only looks like a propeller, and the article calls it one. It is a sail, or airfoil.

I'd like to see an explanation of the America's Cup downwind speed if you have a reference. My google-fu has failed me and I haven't found anything showing sailboats having windward speeds greater than the wind speed. It might help me understand a bit better.

I understand on a force diagram level what's going on, but I lose it on conservation of momentum. The wind, outside of local effects, has no relative velocity to the car at the zero point. How, then, does the car accelerate? There is no overall wind momentum to transfer to the vehicle.

The world land speed record for a wind powered vehicle was broken on 26 March 2009 by Richard Jenkins in his yacht Greenbird with a speed of 126.1 mph (202.9 km/h).[5] Wind speeds were fluctuating between 30&#8211;50 mph (48&#8211;80 km/h) at that time.[6][7]

From wikipedia.

Jenkins was traveling at an angle from the wind, so the windward component of his velocity was likely less than the wind speed.

 

[Skyhanger]

I'd like to see an explanation of the America's Cup downwind speed if you have a reference. My google-fu has failed me and I haven't found anything showing sailboats having windward speeds greater than the wind speed. It might help me understand a bit better.

I understand on a force diagram level what's going on, but I lose it on conservation of momentum. The wind, outside of local effects, has no relative velocity to the car at the zero point. How, then, does the car accelerate? There is no overall wind momentum to transfer to the vehicle.



Jenkins was traveling at an angle from the wind, so the windward component of his velocity was likely less than the wind speed.

I don't believe its possible to faster than wind speed downwind, however its very easy to go many times the wind speed crosswind. All you need is a sail shaped like a airfoil.

 

[Evadman]

I'd like to see an explanation of the America's Cup downwind speed if you have a reference.

I suggest starting with a book titled High Performance Sailing. At 10 degrees off axis, a vehicle can go ~5.7 times faster than the wind. (Ice boats can go faster with only 7 degrees off axis, and DDFTTW should be able to go even faster with an alpha of zero.). Some geometry can give you a result with the ground speed of between 5 and 6 times faster than the wind at 10 degrees alpha. Just switch (starboard to port) after traveling some distance. Multihull boats are expected to sail upwind at 1.2 times the speed of the wind, and downwind at 1.6 times the speed of the wind in the America's Cup.

It may also help to check out the difference between actual wind and apparent wind. It doesn't matter what the apparent wind of the vehicle is, it matters what the apparent wind of the sail/impeller/airfoil is.

I don't believe its possible to faster than wind speed downwind, however its very easy to go many times the wind speed crosswind. All you need is a sail shaped like a airfoil.

You just proved out your argument. The 'propeller' on the back of the vehicle is moving perpendicular to the wind. The sail on a boat is fixed, and thus would need to be at an angle to the wind.

 

[crashtestdummy]

I suggest starting with a book titled High Performance Sailing. At 10 degrees off axis, a vehicle can go ~5.7 times faster than the wind. (Ice boats can go faster with only 7 degrees off axis, and DDFTTW should be able to go even faster with an alpha of zero.). Some geometry can give you a result with the ground speed of between 5 and 6 times faster than the wind at 10 degrees alpha. Just switch (starboard to port) after traveling some distance. Multihull boats are expected to sail upwind at 1.2 times the speed of the wind, and downwind at 1.6 times the speed of the wind in the America's Cup.

It may also help to check out the difference between actual wind and apparent wind. It doesn't matter what the apparent wind of the vehicle is, it matters what the apparent wind of the sail/impeller/airfoil is.



You just proved out your argument. The 'propeller' on the back of the vehicle is moving perpendicular to the wind. The sail on a boat is fixed, and thus would need to be at an angle to the wind.

Thanks! That'll start me, though I may have to track down that book, as the information on the pdfs don't provide a detailed explanation. I'll dig around the rest of that site as well. Question, though: When they talk of going 1.6xTWS, do they mean that as the overall speed of the boat, or the velocity component of the boat in the direction off the wind. Since they talk of both upwind and downwind speeds, I'm inclined to think the former. That's the equivalent to saying that the propeller spins with a outer edge tangential speed greater than that of the wind, which I have no problem believing in, but is quite different than the problem at hand.

 

[KillerCharlie]

People who are comparing this to a sailboat are making a crucial mistake. Sailboats are powered by wind. They get energy from the moving air. The air is moving because of atmospheric conditions - the sailboat is NOT creating the wind by it's motion.

This guy's magic car is being powered by wind that he's creating (by moving in a forward direction). He's taking energy from the wind he's creating.

I'm a professional aerodynamicist so I'm probably wrong though.

 

[Matt1970]

People who are comparing this to a sailboat are making a crucial mistake. Sailboats are powered by wind. They get energy from the moving air. The air is moving because of atmospheric conditions - the sailboat is NOT creating the wind by it's motion.

This guy's magic car is being powered by wind that he's creating (by moving in a forward direction). He's taking energy from the wind he's creating.

I'm a professional aerodynamicist so I'm probably wrong though.

Are you being sarcastic?

 

[Murloc]

looks like a hoax to me, as long as the wind is faster or equal, then it's okay.
But if it goes faster and the fan/propeller changes direction and pushes the thing using power from the wheels, then it's like an airplane I guess.
That means it will lose energy to drag while not acquiring any and it's a hoax.

 

[bwanaaa]

my understanding is that the 'propeller' is coupled to the axle. as the car moves forward, the wheels drive a belt that spins the propeller generating force. It is the wind from the rear that starts pushing the vehicle because the large surface area of the 'propeller' is being blown downwind. As the vehicle picks up speed, it reaches wind velocity and exceeds it because the there are two forces on it- the wind from behind and the power from the propeller. Of course, this really depends on there being very few frictional losses and the airfoil having the proper shape.

 

[reksio]

I'd like to see an explanation of the America's Cup downwind speed if you have a reference.

Start here:

http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind#Speed_made_good

Example vector diagram:

Animated diagram:

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

The wind, outside of local effects, has no relative velocity to the car at the zero point.

The air has always a relative velocity to the rotating airfoils,

Jenkins was traveling at an angle from the wind, so the windward component of his velocity was likely less than the wind speed.

Nope:

during the 2009 land speed record, the yacht Greenbird was proceeding at about 3 times the speed of the wind on a course about 120 degrees off the true wind.[17] Thus, its speed made good downwind was about 1.5 times the speed of the wind.

But I'm sure he could do even more downwind VMG, if he was going for it and not just for absolute speed.

 

[reksio]

This guy's magic car is being powered by wind that he's creating (by moving in a forward direction). He's taking energy from the wind he's creating.

No, it is taking energy from the true wind, which is slowed down by the vehicle.

 

[reksio]

I can see that the propeller would provide forward force,

That means a backwards force on the air, which slows the air down relative to the ground.

Wouldn't that mean that the car somehow builds energy for itself out of nothing?

Not out of nothing. Out of the true wind.