Down wind faster than the wind

[spork]

I right the first time.

Actually you weren't right any time. I just posted instructions for you to build your own. So you can go do it yourself or you can keep telling us it's B.S.

But keep this in mind - when there are only two people on an elevator, and one farts, everyone knows the score.

 

[technoextreme]

Thabiguy:
Someone needs to break it to techno that Thabi has presented a perfectly good definition of "drag" and it is he/she not Thabi who is confused in this regard. Ok, I'll do it.
JB

I thank you for not reading. Drag forces are opposite of thrust. You could have bothered to read his posts because he was talking about thrust. The drag would be the force of the propeller opposite of that to the direction of rotation.

 

[Thabiguy]

Thats not drag. Every single time you've said drag you mean thrust. Every single time.

No, I do not mean thrust. I mean drag.

I was mainly speaking about the blower model. The force of the air retarding the movement of the blades is drag, not thrust. You can say that the blades provide forward thrust to the vehicle (which in turn experiences drag as it moves through the air), but this is not what I was talking about.

You can also say that the inertia of the vehicle exerts force on the blades as it pushes them against the wind; this would be thrust. Again, this is not the force I was talking about. I was talking about drag, a force in the opposite direction - force exerted by air on the blades as it retards their movement through the air.

 

[my_wan]

I thank you for not reading. Drag forces are opposite of thrust. You could have bothered to read his posts because he was talking about thrust. The drag would be the force of the propeller opposite of that to the direction of rotation.

You say "Drag forces are opposite of thrust", true. Thabiguy says drag, you claim he is talking about thrust therefore he must be wrong. So in order for you to be right you have to claim he said the opposite of what he said... Hmmm.

 

[spork]

Does it accelerate to be faster than the wind or start there? It would seem to lose power as it approached wind speed, and be limited to under the speed of the wind.

It will accelerate from a dead stop until it is going faster than the wind, and it will continue along faster than the wind until the wind dies. Build one and try it.

It's always going to stop because it's going in the wrong direction.

Apparently it doesn't know this.

Airplane on a treadmill! Airplane on a treadmill! Airplane on a treadmill! Im going to scream if I hear one more person say that this has nothing to do with airplane on a treadmill I will scream.

Tantrums aside, this has nothing to do with the airplane on a treadmill. Not everything involving a treadmill does.

That's a very good reason why the device in the first video can't go faster downwind than wind-speed...

Well there must be an even better reason why it can - because it does.

I've a question for everyone, just to make sure I'm not jumping to false conclusions.

When a windmill/propeller is facing directly into the wind, can the force from the wind turning the blades ever exceed the force from the wind pushing the blades backward?

I'm not sure I understand your question, but I think I do. The answer would be no - the blades will never pull themselves forward into the wind steady-state.

I'm not a native English speaker. The irony is, even though you imagine I "have no clue what I'm talking about", it's you who weren't right once in this entire topic.

Well your command of the language is better than most natives it seems. And I applaud your choice to use punctuation, spelling, sentences, etc. That's better than most of us Americans for sure.

I told you how it fails. DRAG DRAG DRAG DRAG DRAG DRAG DRAG.

Man you almost had me. Probably just one more "DRAG" and you would have convinced me.

Look up the definition of DRAG. Read it. Embrace it. Hug it. Kiss it. Because you make the assumption that it doesn't stop the propeller.

They told me all about it when I got my M.S. in aero. They had a pretty different idea about it than you do.

You forgot about DRAG. DRAG DRAG DRAG DRAG DRAG DRAG DRAG DRAG. And more drag.

Shoot!!! You're right. Thanks for the reminder.

Now go build the darn thing. I hope you don't implode in a cloud of improbability when it does as advertised. Be sure and use a spotter.

 

[technoextreme]

Place it on your treadmill at 10 mph. If it doesn't go straight ahead (and it won't) adjust the steering by bending the soft aluminum tube very slightly. Make sure this doesn't mess up the alignment and cause any drag.

Take a video and post it on youtube.

Don your flamesuit.

Of course I'm going to try your experiment from which the physics has nothing to do with the problem at hand. I've said it. Someone else has said it. The treadmill will work but it has nothing to do with the actual issue of your cart going faster than the wind.

 

[my_wan]

Uggg... I right the first time. It's the problem with the drive ratio. You need to flip direction sometimes and you can't do that with a single belt drive.

Wow, I finally got an answer to the question I asked after his first post, even though he wasn't actually trying to answer any question here.

No, you never want the flip the props directions no matter which way the wind is blowing relative to the craft. You always want the prop to blow air out behind the direction of road travel no matter which direction the wind is blowing. If you ever switch directions then you are putting the breaks on the craft.

 

[TjW]

If kinetic energy is being extracted from the wind, it is being extracted by the wind turbine.

The formula for kinetic energy in general is:
1/2 mV²
where m is the mass and V is the velocity through the turbine.
The mass of a volume of air through the wind turbine in one second would be:
(air density) (swept area of turbine)(velocity in linear units per second)

Substituting that into the 1/2mV²
1/2 [(air density)(swept area)(velocity)]velocity²
That reduces to:
1/2 (air density)(swept area)velocity³
Since the volume of the mass I calculated is the volume that goes through the swept area in one second, that's kinetic energy per second, meaning the result has units of power.

(This is actually a way overoptimistic estimate. The power is extracted by slowing the air down. If you don't slow it, you don't extract the energy. If you extract it all, the air is stopped,no longer moving through the turbine, and you can't extract any more energy. So there's a limit to the extractable energy, the Betz limit, and it's about 59% of the kinetic energy in the mass of air moving through the turbine.)

When any of the terms in the equation go to zero, the result goes to zero.
As the wind turbine accelerates downwind, the velocity of the air through it decreases. The power available drops off very rapidly because of the cubed term. As it reaches wind speed, the velocity of air through the turbine goes to zero. Therefore the power the turbine can extract from the air goes to zero.
It doesn't matter much what it's attached to.

 

[my_wan]

Of course I'm going to try your experiment from which the physics has nothing to do with the problem at hand. I've said it. Someone else has said it. The treadmill will work but it has nothing to do with the actual issue of your cart going faster than the wind.

So build the same craft and put it on a tennis court on a good windy day and take good measurements.

 

[spork]

Therefore the power the turbine can extract from the air goes to zero.

Those are some lovely equations you've got there. Were you looking for me to point out where you went wrong, or are you just telling me my cart can't do what I can see it doing?

 

[JWideman]

spork, nobody (in this thread at least) disputes what is shown in the treadmill video. In that, the treadmill is serving as the motor, and thrust from the prop pushes the vehicle forward. This is in keeping with what we know of physics.
But that's not what's supposedly happening in the first video. In that, the tailwind is turning the prop, which turns the wheels. But once it reaches wind speed, what turns the prop?

 

[ThinAirDesigns]

I thank you for not reading. Drag forces are opposite of thrust. You could have bothered to read his posts because he was talking about thrust. The drag would be the force of the propeller opposite of that to the direction of rotation.

I've read them -- he correctly means drag.

JB

 

[Thabiguy]

As the wind turbine accelerates downwind, the velocity of the air through it decreases. The power available drops off very rapidly because of the cubed term. As it reaches wind speed, the velocity of air through the turbine goes to zero. Therefore the power the turbine can extract from the air goes to zero.

The thing is that when the device reaches wind speed, the velocity of air going through the propeller is not zero. (ETA: Actually, that wasn't the best way to say it. It's true, in essence, but it probably won't help anyone understand what's going on.)

I feel almost ashamed to promote it again, but if you have a time, check out the blower model that I proposed in post #41. It is conceptually equivalent, but much simpler (and it can also be interpreted as a turbine - a Pelton wheel). I really think people should start with that one, because it's easier to grasp.

If you look at it, you will see that when the device reaches wind speed, the air will still move at non-zero speed with respect to the "turbine", and will still push it forward.

With the propeller, the end effect is the same, but it's less visual.

 

[ThinAirDesigns]

spork, nobody (in this thread at least) disputes what is shown in the treadmill video. In that, the treadmill is serving as the motor, and thrust from the prop pushes the vehicle forward. This is in keeping with what we know of physics.
But that's not what's supposedly happening in the first video. In that, the tailwind is turning the prop, which turns the wheels. But once it reaches wind speed, what turns the prop?

JWideman, what is happening in both videos is *exactly* the same from a physics perspective.

It's been established from the days of Galileo and Newton that motion is relative. There is no physics experiment, no matter how sensitive the instrument, which can determine if the road is still and the air is moving or if the road is moving and the air is still. If you can devise an experiment to do the above -- even if it just involves a silly DDWFTTW cart, and you will be in instant Nobel territory.

I know it seems wrong, but it's the truth.

JW, if you still have issues with the above, I'm happy to help anyway I can. I've got some simple thought demonstrations that might help you.

JB

 

[my_wan]

If kinetic energy is being extracted from the wind, it is being extracted by the wind turbine.

The formula for kinetic energy in general is:
1/2 mV²
where m is the mass and V is the velocity through the turbine.
The mass of a volume of air through the wind turbine in one second would be:
(air density) (swept area of turbine)(velocity in linear units per second)

Substituting that into the 1/2mV²
1/2 [(air density)(swept area)(velocity)]velocity²
That reduces to:
1/2 (air density)(swept area)velocity³
Since the volume of the mass I calculated is the volume that goes through the swept area in one second, that's kinetic energy per second, meaning the result has units of power.

(This is actually a way overoptimistic estimate. The power is extracted by slowing the air down. If you don't slow it, you don't extract the energy. If you extract it all, the air is stopped,no longer moving through the turbine, and you can't extract any more energy. So there's a limit to the extractable energy, the Betz limit, and it's about 59% of the kinetic energy in the mass of air moving through the turbine.)

When any of the terms in the equation go to zero, the result goes to zero.
As the wind turbine accelerates downwind, the velocity of the air through it decreases. The power available drops off very rapidly because of the cubed term. As it reaches wind speed, the velocity of air through the turbine goes to zero. Therefore the power the turbine can extract from the air goes to zero.
It doesn't matter much what it's attached to.

Yes, this is exactly like my starting point. As it turns out the propeller is acting more like a propeller than a turbine such that efficiency is not limited Betz' limit. Although Betz' law limits absolutely perfect efficiency to 59% airplane propellers often have a practical efficiency of over 90%. The difference is the direction of torque on the prop shaft. When the prop torques the prop shaft Betz' law applies. When the shaft torques the prop it doesn't. The later applies to this craft for the reasons given below.

Wind driven case (no treadmill): The pressure difference front to back of the craft places a forward vector force on the frame of the craft. This forces the craft forward which torques the wheels. This force is then feed through the wheels to increase the props angular speed to maintain (not increase) the pressure difference across the frame of the craft. Although at high speed the wind is much faster front to back of the craft the available power is not increased. The power available to the craft itself is limited to the difference between the air and road, regardless of the apparent air speed relative to the craft. This is accomplished by having the wheel directly tied to the prop so this difference is maintained and all speeds. When the drag from going faster than the wind matches the power available through this air to road difference the craft no longer accelerates, as you indicated. Top speed is reached.

There remains much more slippage in the air to craft interface than the road to wheel interface. However, even here efficiency improves at high craft to air relative speed because the effective density of air is higher relative to the craft. The prop act more like an air screw than a windmill.

 

[spork]

I feel almost ashamed to promote it again, but if you have a time, check out the blower model that I proposed in post #41

I like it. It's very much like a cart I proposed (only by way of explanation)

 

[Thabiguy]

When the drag from going faster than the wind matches the power available through this air to road difference the craft no longer accelerates, as you indicated. Top speed is reached.

I feel I should clarify this somewhat. The drag certainly does lower the top speed, but even if there was no unwanted drag at all, the device still wouldn't be able to accelerate indefinitely. The propelling assembly provides forward force only up to a certain speed (v_wind / f). At this speed, the propeller is pushing back the air at the same speed as the air is already flowing back (with respect to the device), so the air will no longer exert any force on the propeller. The drag makes the actual limit lower than that, but this is the fundamental limit of the concept.

 

[CaveDave]

Because of limitations of my OS, machine, and connection, I don't attempt you-tube or any other video downloads: it takes an hour for 5 minutes of unviewable images and chopped audio.

Would someone post some good still shots and also a plan-view (overhead view) diagram showing the road, vehicle, direction of travel, wind direction, and propeller axis orientation with blade angle/rotation direction all included?

Cheers

Dave

 

[spork]

Because of limitations of my OS, machine, and connection, I don't attempt you-tube or any other video downloads

Would someone post some good still shots and also a plan-view (overhead view) diagram showing the road, vehicle, direction of travel, wind direction, and propeller axis orientation with blade angle/rotation direction all included?

Hey, I don't want you to risk a youtube download. I'll be happy to get all that documentation together for you. But that doesn't seem like enough. I could bring the cart by your house, and we'll do the experiment together. When is good for you?

 

[humber]

thought[/I]) experiment when all spork has to do is set the craft on the treadmill backwards and no thought is required. Perhaps you should add that skepticism back into the equation, self skepticism.

Well, it is undeniable that physics is empirical, but that is not the same as simple trial and error, or dare I say it, speculation. You misunderstand me, I think. There is no trial or battles of wills for me. In this case, intuition and Newton's laws are enough to describe what is happening. Counter-intuitive examples, may also be sophisms
How it got beyond that point, is interesting.

Before the Laws of Physics can be declared broken, it is at least necessary to first give them a beating.
If I am wrong, you will be able to tell me so. I ask you to please tell me how the following is flawed.

The real cart shown in the video, works this way.

As I have mentioned, this cart will only run faster than the wind, if the wind is variable. It will tend to run at the peak wind speed. The builder mentions that the wind is variable, and it is obviously so.

Imagine that the cart is running along at walking pace. A gust of wind pushes the cart, so that its velocity is increased by say, 10%. The propeller is directly connected to the wheels, so it too will increase by 10%. Agreed?

The momentum of the body of the cart is therefore increased by 10%, (mv) but that of the propeller also increased by the same amount. If the mass of the propeller is high (as it appears to be), then much more energy is stored than by a cart alone. Note that the kinetic energy is increased by the square of that increase in velocity (1/2mv^2). The cart has now raised its kinetic energy by much more than is needed to maintain its current velocity.
It absorbs more momentum from the wind, because it presents a higher load than a simple cart, but that energy is not dissipated due to simple resistance, but isstored.

If the wind falls again, the overall momentum of the cart will mean that it tends to remain the previous velocity. Do you agree?

The next gust does the same, so the velocity "rathchets up" in this manner until the cart runs closer to the speed of the gusts, than any true average. That is all that this device does. It's a wind-driven flywheel.

I can point to clues and indicators shown in the video, that support this contention, but can tell me why that explanation does not work ?

Spork's model is similar. Its momentum keeps it running in a very similar manner. The belt is the wind. If the cart slows, momentum drives it until some small disturbance allows it to pick up more. That's it.

Newton's law of opposite and equal reaction, means that the cart will certainly not follow the belt backwards, because of the opposing 'thrust' of the propeller.
The cart 'mirrors' its input. Momentum does the rest. If it slows, then energy comes from the stored momentum to maintain that speed.
I agree it appears circular in description. The wheels drive the propeller, which drives the wheels, which drives..... Yes, but it is only a balance of forces.
Find something to upset that balance, and it will move in the appropriate direction. However, there is a bias, because if it slows, the momentum of the flywheel, a reservoir of energy, can be called upon to drive it forward again. It takes what energy it it needs; directly from the belt, or from stored momentum, so as to maintain that balance.
It is visibly so. It slips and slides about due to the low rolling resistance, and its precarious balance. So little work (force x distance) is being done.