Wind powered vehicle, or perpetual motion machine?

[Matt1970]

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.

If you are traveling in the same direction as the wind from behind it no longer can be a source of propulsion once you have exceeded its speed. That would be perpetual motion. The power from the propeller is also being driven from the wind from behind and would have enormous resistance from the now reversed direction of the wind on it.

 

[reksio]

If you are traveling in the same direction as the wind from behind it no longer can be a source of propulsion once you have exceeded its speed. That would be perpetual motion.

Nope. Perpetual motion would be an isolated system, that keeps going. This is not isolated at all. It takes energy from the wind (air motion relative to ground) which is slowed down.

 

[reksio]

That propeller is spinning the wrong direction if the wind is from the rear.

Propellers spin like that. Turbines spin the other way:
http://www.youtube.com/watch?v=nudBjrOF3LE

To go faster than the wind you need to be going at an angle to the wind

The propeller blades are going at an angle to the wind:
http://www.youtube.com/watch?v=UGRFb8yNtBo

 

[Matt1970]

A blade, whether on a turbine, fan, propeller or heat pump will always have air flow move from the are where it encounters the air first to the trailing edge whether the wind drives it or it drives the wind. The propeller and vehicle are traveling in the same direction as the wind as stated in the video. If you seriously think this thing works, then I have some land I want to sell you.

 

[kevinsbane]

Wait, I think I've gotten it. Let me get this straight;

It seems to me there are 3 regimes of forces working here;

1. The wind blows on the cart from behind, and pushes the cart forward. This holds true up to the point at which the cart reaches windspeed.

2. A traditional "propellor" driven regime, dominant just after reaching wind speed. In this regime, the propellor cannot rotate due to air movement alone; as far as it's concerned, there is no apparent wind. Since the wheels are driving it around, it will turn; since there is no apparent windspeed, it will push the air backwards, providing forward thrust. Therefore, it will increase in speed.

3. Once enough headwind is built up, it transitions to an airfoil type regime; here, the wheels are still turning the propellor, but now the propellor type turning motion can no longer "push back" against the wind due to purely brute force. Now, the turning motion is simply sufficient to maintain the correct "tacking" angle for a given headwind. The spinning, in and of itself, no longer provides any thrust directly, similar to how the engines of a plane do not provide thrust directly, but drive the airplane wing (airfoil) through the air, which provides the lift. The wind itself is now providing "lift" along the spin axis, because the vector addition of the apparent headwind and rotation speed results in the correct airfoil shape to produce what is essentially a spinning airplane wing.

I realise that in both 2 and 3, the air is still technically "pushing" against each other; just in 2, the propellor pushes on the air causing the air to push back against it, whereas in 3, the high pressure air below the airfoil pushes against the prop, causing the prop to push against the air.

Is that right?

 

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[Matt1970]

What a thread full of fail. Matt1970, do you have any knowledge of what you are talking about? Or are you just venting smelly gas?

Smelly gas would have a better chance of going faster that the wind than that car does.

 

[kevinsbane]

The spinning, in and of itself, no longer provides any thrust directly, similar to how the engines of a plane do not provide thrust directly, but drive the airplane wing (airfoil) through the air, which provides the lift.

Oops, that should be lift.

 

[reksio]

The propeller and vehicle are traveling in the same direction

A point on the propeller blade moves in a different direction than the vehicle . It is simple vector addition:

blade_velocity = cart_velocity + tangential_blade_velocity

And since tangential_blade_velocity is perpendicular to the cart_velocity & wind, the blade_velocity is not parallel to the wind. The blade is going across the wind.

 

[Matt1970]

3. Once enough headwind is built up, it transitions to an airfoil type regime; here, the wheels are still turning the propellor, but now the propellor type turning motion can no longer "push back" against the wind due to purely brute force. Now, the turning motion is simply sufficient to maintain the correct "tacking" angle for a given headwind. The spinning, in and of itself, no longer provides any thrust directly, similar to how the engines of a plane do not provide thrust directly, but drive the airplane wing (airfoil) through the air, which provides the lift. The wind itself is now providing "lift" along the spin axis, because the vector addition of the apparent headwind and rotation speed results in the correct airfoil shape to produce what is essentially a spinning airplane wing.

I realise that in both 2 and 3, the air is still technically "pushing" against each other; just in 2, the propellor pushes on the air causing the air to push back against it, whereas in 3, the high pressure air below the airfoil pushes against the prop, causing the prop to push against the air.

Is that right?

Lift from an airplane wing design is very precise. Any disruption in the flow over the wing will catastrophically disrupt lift. Any lift created by the propeller blades acting like a wing would surely be disrupted by the apparent wind flow now coming from the front.

 

[Matt1970]

A point on the propeller blade moves in a different direction than the vehicle . It is simple vector addition:

blade_velocity = cart_velocity + tangential_blade_velocity

And since tangential_blade_velocity is perpendicular to the cart_velocity & wind, the blade_velocity is not parallel to the wind. The blade is going across the wind.

You are exactly correct. My bad. I meant there isn't an angle of the blades to the wind like how a sailboat would use.

 

[reksio]

Wait, I think I've gotten it. Let me get this straight;

It seems to me there are 3 regimes of forces working here;

1. The wind blows on the cart from behind, and pushes the cart forward. This holds true up to the point at which the cart reaches windspeed.

2. A traditional "propellor" driven regime, dominant just after reaching wind speed. In this regime, the propellor cannot rotate due to air movement alone; as far as it's concerned, there is no apparent wind. Since the wheels are driving it around, it will turn; since there is no apparent windspeed, it will push the air backwards, providing forward thrust. Therefore, it will increase in speed.

3. Once enough headwind is built up, it transitions to an airfoil type regime; here, the wheels are still turning the propellor, but now the propellor type turning motion can no longer "push back" against the wind due to purely brute force. Now, the turning motion is simply sufficient to maintain the correct "tacking" angle for a given headwind. The spinning, in and of itself, no longer provides any thrust directly, similar to how the engines of a plane do not provide thrust directly, but drive the airplane wing (airfoil) through the air, which provides the lift. The wind itself is now providing "lift" along the spin axis, because the vector addition of the apparent headwind and rotation speed results in the correct airfoil shape to produce what is essentially a spinning airplane wing.

I realise that in both 2 and 3, the air is still technically "pushing" against each other; just in 2, the propellor pushes on the air causing the air to push back against it, whereas in 3, the high pressure air below the airfoil pushes against the prop, causing the prop to push against the air.

Is that right?

It's not wrong. But the transition between the cases 1-3 is smoother than you describe it. The airfoils are completely stalled at first, and then with increasing speed they gradually become unstalled (from the tips inwards) and start to work more efficiently.

Some of the differences you try to make between 1,2 and 3 do not exist:

In this regime, the propellor cannot rotate due to air movement alone

The propeller is never turned directly by the relative air movement. A propeller always rotates against the aerodynamic torque. That is the difference to a turbine.

but now the propellor type turning motion can no longer "push back" against the wind due to purely brute force.

The propeller always applies a backwards force to the air, even when unstalled.

I realise that in both 2 and 3, the air is still technically "pushing" against each other; just in 2, the propellor pushes on the air causing the air to push back against it, whereas in 3, the high pressure air below the airfoil pushes against the prop, causing the prop to push against the air.

There is always a pressure difference between the two sides of the airfoil. There is no difference between "air pushing" and "pressure pushing".

 

[kevinsbane]

Lift from an airplane wing design is very precise. Any disruption in the flow over the wing will catastrophically disrupt lift. Any lift created by the propeller blades acting like a wing would surely be disrupted by the apparent wind flow now coming from the front.

True. It's just that the "lift" in this case is not that critical; it's not like you're going to fall out of the sky just because your lift efficiency drops 50%. The "catastrophic" failure of lift in an airplane is only catastrophic because, well, if it falls below a certain threshold, the plane falls out of the sky. "Catastrophic" failure of the "lift/thrust" generation on a cart like this means your thrust output drops; which just means the headwind slows you down. This also occurs at the upper speed ranges, that is, faster than the wind.

If I were to make something like this, I'd make a "broad spectrum" airfoil, one shaped to have a relatively large (although perhaps inefficient) operating range; so that for a given range of headwinds, I know that the craft would work in that range.

Theoretically, when I think about the energy extraction situation, there's no reason why it shouldn't work. For a square sailed car moving directly downwind, your efficiency of energy extraction is directly related to your speed relative to windspeed. Once you get to windspeed, you cannot *extract* any more energy from the wind using a square sail; you in a "no wind" situation. A square sail just won't work at that point. But what if you could continue to extract energy at the same efficiency as when you stand still, from the wind while on a moving platform?

 

[reksio]

I meant there isn't an angle of the blades to the wind like how a sailboat would use.

It is exactly the angles a sailcraft would use on a broad reach with downwind VMG > windspeed. Look at the diagram below and replace:

boat velocity -> blade velocity
downwind VMG -> vehicle velocity

As you see vehicle velocity(downwind VMG) is 1.5 x true wind, and the blade is still pulled forward.

Any lift created by the propeller blades acting like a wing would surely be disrupted by the apparent wind flow now coming from the front.

No, because the apparent wind at the airfoil is not coming directly from the front, but at an angle. So you just have the adjust the propeller pitch to have an opitmal angle of attack at the airfoil.

 

[kevinsbane]

It's not wrong. But the transition between the cases 1-3 is smoother than you describe it. The airfoils are completely stalled at first, and then with increasing speed they gradually become unstalled (from the tips inwards) and start to work more efficiently.

Some of the differences you try to make between 1,2 and 3 do not exist:


The propeller is never turned directly by the relative air movement. A propeller always rotates against the aerodynamic torque. That is the difference to a turbine.

The propeller always applies a backwards force to the air, even when unstalled.

Ah, ok. I didn't do a holistic analysis, I just did the "time shots" in my head at each of the three potential situations I was thinking of; I figured they overlapped, I just didn't know how much!

There is always a pressure difference between the two sides of the airfoil. There is no difference between "air pushing" and "pressure pushing".

I think this is just a different way of saying what I was trying to get across, heh. I suppose what I was trying to get at was the difference in what was providing the forward thrust: between an airplane wing (on the high speed end) and a boat propellor (on the low speed end). Is that a fair way of putting it?

 

[Matt1970]

But what if you could continue to extract energy at the same efficiency as when you stand still, from the wind while on a moving platform?

That would basicly be a wind turbine if I undrstand you correctly. You are just transfering the energy to counteracting the motion of the moving platform.

 

[reksio]

I suppose what I was trying to get at was the difference in what was providing the forward thrust: between an airplane wing (on the high speed end) and a boat propellor (on the low speed end). Is that a fair way of putting it?

Yeah, I guess you mean the difference between attached flow at the blades and separated flow. But keep in mind that even with propellers the aim is to have attached flow, like on a wing.

 

[kevinsbane]

Yeah, I guess you mean the difference between attached flow at the blades and separated flow. But keep in mind that even with propellers the aim is to have attached flow, like on a wing.

Wait...

Do airplane propellors utilize same principle as airplane wings? Ie, Bernoulli's Principle? Or do they act more like boat propellors, directly forcing a fluid backwards?

 

[Matt1970]

By definition they do create lift and are shaped much like the wing of a plane.

 

[reksio]

Wait...
Do airplane propellors utilize same principle as airplane wings? Ie, Bernoulli's Principle? Or do they act more like boat propellors, directly forcing a fluid backwards?

There are no different principles utilized by different foils. Just different laws (based on models of different abstraction level), which are satisfied by all foils. Every foil accelerates the fluid opposite to the force it creates (Newtons 3rd). Every foil obeys Bernoulli's Laws.

But if by "Bernoulli's Principle" you mean the explanation of lift, based on equal transit time of air particles on opposite sides of the airfoil, then forget it. It's nonsense. Two separated air particles don't have to meet again behind the airfoil:
http://www.youtube.com/watch?v=6UlsArvbTeo