faster than wind with prop geared to drivetrain?

[sloper steve]

Is this possible or hoax?
http://youtube.com/watch?v=aJpdWHFqHm0
It makes sense if you are not going 100% downwind but they are.

[kbear]

Ok, here is what I noticed. The streamer is being blown forward and sometimes toward the cammera so the wind is blowing at the back of the vehicle, so yes the are traveling down wind. The problem is that if the wind was moving the prop, the prop would be turning in the opposite direction. Look at the pitch. It's set to spin the other way with the wind comming from behind. The other video has a vehicle traveling into the wind. I think this one is as well, but I cannot explain the streamer. I also don't heare any noise other than the wind and the wind vehicle. It appears that they are in a car or something and I don't hear an engine. considering how noisy the wind powered vehicle is, you would think you would hear something from the chase vehicle. Tire noise at least. I think I do here some engine noise toward the end from the chase vehicle but it's faint.

[sloper steve]

The wheels are geared to turn faster than the pitch speed.
When matching the speed of the wind-
If there is no air moving through the prop then there is no pitch speed.
So why would it speed up any more?
A gust was the best answer lol

If there is no airspeed over the prop then the wheels will be driving the prop.
since they are geared to spin faster they have little leverage working the other way around so it would slow down.

They do not claim perpetual motion but are claiming the sail/airfoil effect of the blades is giving more speed.

Their explanations ask you to picture boats etc... but not explanation that directly explains this effect with the prop.

Since a prop is in front of the ribbon it is silly- the ribbon should be up high away from the prop.

The subject seems difficult to talk about or figure out. very confusing.

I'm very interested in the mental fallacies behind this or my own mental fallacy for not understanding it!

[kbear]

Ok, let me see if I got this right. The wind is pushing the trike (let's call it). This causes the wheels to turn which makes the prop turn pushing the the vehicle faster. now the prop rotation makes since to me. I get it, but it doesn't seem possible does it. I'm willing to believe it, but now I want to build one just to see it work. Very interesting. does it give a web site for the creator? I can't seem to find a link.

[kbear]

Ok, I found the PDF. Sounds good to me.

[sloper steve]

I think they are saying the wind turns the prop.
The prop is geared to the wheels like a ten speed bicycle in hard gear.
So pedals would be on the prop shaft and the tire on the road for a bike analogy.
The wind turns the pedals which pushes it along faster than the wind.

Instead of pedals they use the prop to power the rotation.
A chain or shaft to translate it in a geared fashion to the wheels.

When it accelerates to the speed of the wind you need to remember there is no longer force on the prop. So it doesn't matter if it is geared to go faster than the pitch speed.

They are playing off a generic understanding of sailing diagonal to the wind and trying to mix us up with the prop pitch angles and gearing.

See this sailing page:
Downwind (diagram at left) is easy. If the wind is 10 kt, and the boat makes 6 kt in the same direction, then the crew feels a wind of 4 kt coming over the stern of the boat. The true wind vw equals the speed of the boat vb plus the relative wind vr.
The equation vw = vb + vr tells us the problem: as the boat speed approaches the wind speed, the relative wind drops towards zero and so there is no force on the sail. So you can't go faster than the wind. When the wind is at an angle, we have to add the arrows representing these velocities (vector addition). Upwind (right), exactly the same equation holds: vw = vb + vr.
(scroll down to how boats sail faster than the wind)
http://www.phys.unsw.edu.au/~jw/sailing.html

This person asks the question here and if you click the link below it there is an explanation:
http://www.madsci.org/posts/archives/jul99/932835143.Ph.q.html

Finally this explains why you can't sail faster than the wind and why sailing at an angle lets you go faster:
http://www.boatsafe.com/kids/bramp1099.htm

Note:
They have completely included the idea of airfoils in sails yet even with multiple sails state that you can not go faster downwind than the wind.

It is amazing that these silly sailors have not put windmill props on their boats which gear the underwater prop and make them go so fast! What are they thinking?

Kbear- you are the man! Thanks for trying it!

Holding it in the air with a fan blowing on it will make the wheels go faster than the airspeed of the fan!

Next turn off the fan to simulate when the trike matches the airspeed. How much do the wheels speed up?


Are we to think sailors are so against flight that they would not stick propellers on their boats instead of sails since it allows downwind travel faster than the wind?

[sloper steve]

Besides being wrong I've a 3rd explanation-
the air is not perfectly behind it
It is coming a bit diagonal.
The prop may do the sail tilt to get some lift from the side wind.
Since the wheels prevent it from sliding sideways with the wind that force is induced to forward speed.
The builders and testers may be honest but are not accounting for cross vector gusts.

[nmasters]

It's a brain teaser invented by an aeronautical engineer named Andy Bauer at Douglas Aircraft (before it became McDonnell-Douglas now a branch of Boeing). I think it has something to do with the lift to drag ratio of the blades. There's a picture of the full size windmill cart here:
http://www.solar2006.org/presentations/forums/f24-maccready.pdf

[kbear]

Here's the link to the creators explanation. He clearly states that the wheels drive the prop.

http://www.ayrs.org/DWFTTW_from_Catalyst_N23_Jan_2006.pdf

[nmasters]

So the air the fan is pushing is flowing against the natural wind and transferring the force of the fluid impact to the prop disk?

[allanflowers]

Years ago I build a much smaller trike powered by about a 10" balsa propeller, using a rubberband belt drive. It was very crude.
When placed in front of a fan, this little contraption would move steadily straight INTO the wind, which kind of amazed me. Nonetheless, it did it and there was no doubt about it.
A sailboat cannot sail directly into a wind but this thing could!
Therefore, I would have to be open to the idea that the thing in the video is legitimate. There is no question that a vehicle, in a broad reach, can easily travel faster than the wind - in the case of an iceboat, by a great deal.

[sloper steve]

but you have it reversed allan- this one is not going into the wind.
the wind is hitting it from the rear and as it speeds up the wind passes more slowly over it.
The iceboat can not travel directly into the wind nor can it travel directly downwind faster than the wind. See links above.

I believe your contraption worked and think that is very cool!
Here is a guess:
Because the wheels are connected to the ground and there is leverage from the prop over the wheels you can use the wind to drive it forward. The loss in forward speed makes up for the othwerwise loses on a 1:1 gear ratio due to wind resistance.
Because a sail boat does not have traction it can not do this.

We'd be able to talk about this reasonably if there was a scientific test for this being performed vs. a rural road with unknown everything.

[sloper steve]

Anyone have time to make a little boat?

A hybrid boat with a prop for a sail.
The prop is like a weather vane so it always faces into the wind.
The vane is connected to the keel to always steer the boat into the wind.
The air prop drives the water prop. The water prop has lots of torque but moves the boat very slowly forward- but with force.

[Brandano]

the wind is actually pushing a "sail" that moves "backward" on the contraption... it's a bit complex to explain it in full, but in this way it makes sense.
By the way, look up the terms "turbosail", there's a few interesting pages about this sort of things. and anyone remembers the movie "Waterworld"?

[Sparky Paul]

There was a thread on this subject a couple years back where the task was to have a propellor driven car advance into a head wind.
ISTR the task was accomplished with a prop geared to the wheels.

[Quacker]

If the machine is going upwind then it is not a hoax. I have personally seen another design do, this although it really looked impossible when I saw it!

If it is going down wind then it is a total hoax. If for no other reason than that the propeller would be spinning in the opposite direction than as shown.

Q

[Brandano]

No, it isn't a hoax. It's a clever bit of thinking. Consider this: disconnect the prop from the wheels and lock it still. The wind has enough force to push the cart this way, using the surface of the prop as a sail. Now gear the prop to the wheels. The cart moving forward pushed by the wind spins the prop in the dierction you see. This effectively means that the wind is pushing on a "sail" (the prop) that is being moved "backwards" (spun the opposite way the wind would spin it) by the wheels. You are effectively working around the speed of the wind limit by using the pressure rather than the speed.

[Sparky Paul]

without a transmission reversing the prop rotation relative to the wheels, the machine can't run downwind.

[JetPlaneFlyer]

Paul,
The prop is not driven round by the wind directly... It's driven round by a pulley from the wheels... i.e. the wheels drive the prop not the other way around.
If you study the start of the video carefully you will see that the prop is turning in the opposite direction to that predicted by your diagrams.

It's a real brain teaser for sure :)


Steve

[Sparky Paul]

The wheels don't appear to have sufficient diameter to provide enough torque that can be geared up to move the prop, which is moving "backwards" relative to the wind until the vehicle speeds up.
Looking from the front, the prop is pitched to rotate clockwise.. with wind from the rear, it would want to rotate in the opposite direction, and any force it develops would counter whatever the wheels are delivering to it.
I suspect there really is some other energy source there.

[Brandano]

Paul, the machine is following the wind, not going towards it. The whole purpose of the "trick" is to find a way to go faster than the wind in the same direction as the wind. a boat can usually go faster than the wind, but not when it travels in the same direction.

[Sparky Paul]

I see that.
The mechanics look like this...

[Quacker]

Guys,

I'm going to go with the author on this one. When he says "The key to understanding DWFTTW, is that the wheels are turning the propeller and that the propeller need only produce enough lift in still air to overcome the forces required to turn it."

Sure. As long as he can produce lift in still air, I guess it would work just fine ;>)

[Sparky Paul]

I'd like to see videos of that rig in these conditions.
1. Prop locked, disconnected from the wheels, facing upwind
2. Prop locked, disconnected from the wheels, facing downwind.
3. Prop unlocked, disconnected from the wheels, facing upwind.
4. Prop unlocked, disconnected from the wheels, facing downwind.
5. Prop unlocked, connected to the wheels, facing upwind.
6. Prop unlocked, connected to the wheels, facing downwind.

[ronmeister]

I'd like to see videos of that rig in these conditions.
1. Prop locked, disconnected from the wheels, facing upwind
2. Prop locked, disconnected from the wheels, facing downwind.
3. Prop unlocked, disconnected from the wheels, facing upwind.
4. Prop unlocked, disconnected from the wheels, facing downwind.
5. Prop unlocked, connected to the wheels, facing upwind.
6. Prop unlocked, connected to the wheels, facing downwind.
5 would be awesome to see.

[nmasters]

6. Prop unlocked, connected to the wheels, facing downwind.

In that situation wouldn't it ba a propeller driven vehicle going down wind?

[Sparky Paul]

In that situation wouldn't it ba a propeller driven vehicle going down wind?
.
Possibly... but the premise states the wheels drive the prop.
One data point (the video) proves nothing.

[Brandano]

Consider the prop in the current setup, but with no pitch in the blades, that is, with the blades perpendicular to the wind. The wind will start pushing the cart forward, and the prop will start spinning, pushed by the wheels. Then start angling the blades to "fight" the wind until the push on the prop will allow it, leaving enough extra energy to keep it spinning. That way you'll have the setup in the movie. Now, I know the blade's angle seems rather large, but you have to remember that the actual wind velocity relative to the contraption is the speed of the wind minus that of the cart. Where's the advantage in doing this? if you were to take just the section view of the cart you'd see that as it moves forward, the prop section view (the sail in this case) is moving backward a bit. That's a little more speed you can "steal" from the wind, meaning that by refining things further you could travel faster than it. It's a bit like an autogyro, reducing stall speed by having the wing move faster than the plane.

[rofujiyama]

Took awhile , for me to ponder over this.
Suppose i had a pinwheel, and enough wind to get it to spin, then i ran with it, with the wind, till i reached the speed of the wind,,, ok, suppose i surpass the speed of the wind, and create enough energy to act on the pinwheel, hahah,I think it would have to turn in the opposite direction, because the pitch angle, to me hasnt changed, but the ? wind pressure ? is now 180 degrees in the opposite direction ???,acting on the reverse side of the propblade.
Well something to think about ,hahah. :confused: :rolleyes: ;)
Roland

[Roj]

Paul, the machine is following the wind, not going towards it. The whole purpose of the "trick" is to find a way to go faster than the wind in the same direction as the wind. a boat can usually go faster than the wind, but not when it travels in the same direction.

Ok, initially i saw this as saying that you had a perpetual motion machine: one that basically has an airspeed above zero perpetually.

Now i see that the concept is perfectly ok. Of-course, when moving faster than the wind, the spinning prop is actually neither generating thrust nor drag, and in theory takes no energy to spin.

[vintage1]

Just consider what happens as this contraption accelerates. At a given point it is travelling at the same speed as the wind, and the prop therefore stops turning..

I think that should tell you all you need to know.

It is not possible to extract energy from the wind when your velocity downwind is greater than the windspeed.

Any resistance you put in its path will ADD energy TO the wind..and slow you down.

Analysing the energy transfer is the key.

[Roj]

Just consider what happens as this contraption accelerates. At a given point it is travelling at the same speed as the wind, and the prop therefore stops turning..

I think that should tell you all you need to know.

It is not possible to extract energy from the wind when your velocity downwind is greater than the windspeed.

Any resistance you put in its path will ADD energy TO the wind..and slow you down.

Analysing the energy transfer is the key.

Forget the contraption for a while. Correct me if i'm wrong but i think it applies to the "speed greater than wind speed" scenario:

Imagine a zero gravity setting, where a prop is set spinning. The prop is aerodynamically frictionless and is on perfect bearings.
Once equilibrium is reached, it will maintain steady state at some velocity relative to the air.

It appears as if we have an object that has an speed greater than windspeed perpetually, but there is no magic here - once in steady state, no work is being exchanged between the air and the device.

[MCarlton]

What we don't know is the gradient of the ground in the video......

[Brandano]

No, you are making a mistake, it's not a perpetual motion machine, just a neat trick. The speed of the wind isn't the energy of it, the machine is travelling faster than the wind the same way someone walking along a train travels faster than the train. And still getting energy from the wind to walk. The entire thing is arranged so that the surface the wind is pushing against is moving backward relative to the rig, the 2nd law of thermodynamic is still respected. The whole scope of the exercise is to be able to still get thrust from the wind as the speed of the sail approaches it, but with the way this thing is set up the sail still moves slower than the wind.

[Flight Engineer]

I'm begining to think the theory may be solid. By moving the local air rearwards the prop sail is converting the excess energy into velocity.

The key is to imagine the air mass around the prop as the sail. Its velocity relative to the cart is the speed above windspeed that the cart will travel.

[rofujiyama]

The thing that bothered me about this experiment was his telltale flag and its reaction, to the gust. However I realized that it's at rest position at dead stop is fluttering downwind,and its prevailing windspeed position is basically straight down and when the gust hit the vehicle the acceleration to the new speed ment the telltale flag had to lift from its prevailing windspeed position, until the vehicle could attain the new, gusting windspeed , and have the telltale settle down again. :eek: I think??? hahahah

[Roj]

Forget the contraption for a while. Correct me if i'm wrong but i think it applies to the "speed greater than wind speed" scenario:

Imagine a zero gravity setting, where a prop is set spinning. The prop is aerodynamically frictionless and is on perfect bearings.
Once equilibrium is reached, it will maintain steady state at some velocity relative to the air.

It appears as if we have an object that has an speed greater than windspeed perpetually, but there is no magic here - once in steady state, no work is being exchanged between the air and the device.

Now, placing this device on a wheeled cart with negligable drag and frictionless bearings on the wheels means it takes zero energy to keep this theoretical device moving at groudspeed greater than windspeed.

But when we include friction from wheels, bearings and prop the device would eventually want to slow.
Presumably the coupling from the wheels to the prop is meant to provide the energy needed to keep the prop rotating by drawing from energy from the ground moving below it.
This explanation seems flawed: we are trying to add a component of thrust from the prop by increasing drag on the wheels it itself is pushing?

[Brandano]

Ok, consider a glider. It travels faster than the speed it's falling. If it was actually on a thermal equal to the sink speed it would fly horizontally, much faster than the wind that is pushing it. It gets its energy from the wind and gravity, in this case, but still what keeps it up is the wind (the thermal) and the energy taken from gravity is zero, since there's no change in its energy potential. It's effectively converting that small wind speed to a speed some 40 times faster even when fighting against the drag. So the energy is there. The problem with this contraption is not the energy available, it's how to ensure there's an energy potential high enough for it to be used while still overcoming the drag of the wheels etc. If you were to examine the airflow against the prop you'd see that the leading edge is the one further back, and that the prop is "gliding" down on the wind, actually gliding backward relative to the contraption fast enough that it's "falling" slower than the wind. But here it's arranged as a prop, so that "moving away" is effectively running in circles. the energy the prop gains this way is transferred to the wheels. Another way to see it is this: consider a plane sectioning the prop vertically, and just see the section of the prop as the sail the wind is pushing against. As the prop turns the section moves backward. If it moves backward fast enough, the sail can catch a little extra energy to push the cart. It's no perpetual motion machine, doesn't really have any practical application, just a neat trick that requires some odd thinking.

[Roj]

No, you are making a mistake, it's not a perpetual motion machine, just a neat trick. The speed of the wind isn't the energy of it, the machine is travelling faster than the wind the same way someone walking along a train travels faster than the train. And still getting energy from the wind to walk. The entire thing is arranged so that the surface the wind is pushing against is moving backward relative to the rig, the 2nd law of thermodynamic is still respected. The whole scope of the exercise is to be able to still get thrust from the wind as the speed of the sail approaches it, but with the way this thing is set up the sail still moves slower than the wind.



While the device is moving faster than the wind the prop effectively sees a headwind. The prop has to spin with enough pitch speed to generate thrust. Are you saying the energy required to provide this comes from the wheels? The wheels themselves are only being pushed because of the prop.
There is a flaw here.

Also according to the explaination, the prop should temporarily slow if there was a wind gust and speed up in a lull. Again, correct me if i'm wrong.
Look carefully at the device in the video: isn't the prop speeding up when the tailwind increases?

[rofujiyama]

It appears that the designer has figured the amount of gear reduction to allow the prop to rotate in a tractor rotation,contrary to the force of the wind, however I think, he uses the telltale as a reason to show that its possible to go faster then the wind. But the only time it is trailing the machine is during the gust , and the telltale is not at its at rest position, but at the position, it is while already running with the wind, when it is hit by the gust.
So my point is that it would be trailing the vehicle from the start if it were going faster then the wind.

[Roj]

Ok, consider a glider. It travels faster than the speed it's falling. If it was actually on a thermal equal to the sink speed it would fly horizontally, much faster than the wind that is pushing it. It gets its energy from the wind and gravity, in this case, but still what keeps it up is the wind (the thermal) and the energy taken from gravity is zero, since there's no change in its energy potential. It's effectively converting that small wind speed to a speed some 40 times faster even when fighting against the drag. So the energy is there. The problem with this contraption is not the energy available, it's how to ensure there's an energy potential high enough for it to be used while still overcoming the drag of the wheels etc. If you were to examine the airflow against the prop you'd see that the leading edge is the one further back, and that the prop is "gliding" down on the wind, actually gliding backward relative to the contraption fast enough that it's "falling" slower than the wind. But here it's arranged as a prop, so that "moving away" is effectively running in circles. the energy the prop gains this way is transferred to the wheels. Another way to see it is this: consider a plane sectioning the prop vertically, and just see the section of the prop as the sail the wind is pushing against. As the prop turns the section moves backward. If it moves backward fast enough, the sail can catch a little extra energy to push the cart. It's no perpetual motion machine, doesn't really have any practical application, just a neat trick that requires some odd thinking.

I aknowledge your point.
But comparing apples to apples:
The glider flying horizontally at 40mph is indeed much faster than the upward current of wind in the thermal (lets say 3mph).
But it no matter how proficient the pilot or the aircraft, the glider can still not use this huge amount of kinetic energy to sustainably gain altitude faster than 3mph - the speed of the thermal itself.
In essence what the contraption is proposing that it is possible to out-thermal a thermal.

The reason is that the energy provided by the thermal to allow the glider to speed along at 40mph horizontally ceases to exist once the glider climbs at the same rate as the thermal itself.

[corsha99]

I think I have a grasp of how it works and have been working on an explanation. The key to it is keeping in mind that the wheels turn the rotating sail/prop. The mechanical advantage of the wheels over the prop don't let the prop rotate like we all think it should with a tail wind. Once the vehicle is in motion, its momentum will keep it traveling fwd and the prop spinning like it was powered. Some of the discussions so far do not necessarily consider this aspect of it.

Once the vehicle is traveling the same speed as the wind is where I have some questions which is why I want to build one. I believe that the rotating sail\prop is actually at an angle to the wind which allows it travel faster than the wind, just like real sailboats. Instead of having a path that is an angle to the wind, this device puts the sail at an angle to the wind.

[Brandano]

Roj: actually, you could outthermal a thermal, if it was strong enough, just not with a sailplane as it is. You'd need a rotor attached with a wire to the ground, a bit like the Brennan Torpedo. http://en.wikipedia.org/wiki/Brennan_Torpedo
Now, consider the brennan torpedo. It's moving forward because its prop is spun by a wire pulled from the shore. let's assume the water it's moving in is still, then it's moving at the speed of water plus its own speed. Now, let's fix the wire and have the water move in the same direction of the torpedo at the same speed we were pulling the wire. Nothing has changed from the point of view of the torpedo, the water around it is still at the same speed, the wire is moving away at the same speed. The torpedo must still move at the speed of water plus its speed. In that contraption the wire is replaced by the ground, but essentially it works the same way.

[Roj]

Roj: actually, you could outthermal a thermal, if it was strong enough, just not with a sailplane as it is. You'd need a rotor attached with a wire to the ground, a bit like the Brennan Torpedo. http://en.wikipedia.org/wiki/Brennan_Torpedo
Now, consider the brennan torpedo. It's moving forward because its prop is spun by a wire pulled from the shore. let's assume the water it's moving in is still, then it's moving at the speed of water plus its own speed. Now, let's fix the wire and have the water move in the same direction of the torpedo at the same speed we were pulling the wire. Nothing has changed from the point of view of the torpedo, the water around it is still at the same speed, the wire is moving away at the same speed. The torpedo must still move at the speed of water plus its speed. In that contraption the wire is replaced by the ground, but essentially it works the same way.

Damn good point.
I'll have to mull over it and get back to you...

[Roj]

Roj: actually, you could outthermal a thermal, if it was strong enough, just not with a sailplane as it is. You'd need a rotor attached with a wire to the ground, a bit like the Brennan Torpedo. http://en.wikipedia.org/wiki/Brennan_Torpedo
Now, consider the brennan torpedo. It's moving forward because its prop is spun by a wire pulled from the shore. let's assume the water it's moving in is still, then it's moving at the speed of water plus its own speed. Now, let's fix the wire and have the water move in the same direction of the torpedo at the same speed we were pulling the wire. Nothing has changed from the point of view of the torpedo, the water around it is still at the same speed, the wire is moving away at the same speed. The torpedo must still move at the speed of water plus its speed. In that contraption the wire is replaced by the ground, but essentially it works the same way.

edit: sorry double post.

[kcaldwel]

...

[kcaldwel]

...

[nmasters]

Their story of putting it on a flat treadmill and generating net thrust is a crock. It would be perpetual motion then.

Kevin Caldwell
Courtenay, BC

You noticed the steam engine running the torpedo but not the electric motor running the cart on the treadmill? :rolleyes: :p

[Roj]

This is not right. The cable is moving through the water in the opposite direction, and considerably faster than the torpedo is moving. It is a powered cable.

In your example, if the torpedo is fixed relative to the shore, and the water is moving, the cable would still have to be moving through the water considerably faster than the water is moving. That is how the power is suppled to the torpedo.

Don't laugh too loud guys, I'm afraid Brando's argument is valid.

eg:
Say the cable needs to pulled in by the steam engine at 20mph in order to propel the torpedo forwards at 1mph (choose any numbers you want) in calm water.

Then if we add 20mph velocity vector to our frame of reference:

The cable can be tied to the shore - it is stationary - now no steam engine needed.
The water is flowing North at 20mph.
The torpedo is moving North at 21mph - still 1mph faster than the water is flowing.


If the cable was fixed relative to the water, the torpedo would just drift backwards with the water.

No, if the cable was fixed the water would be in motion, and the torpedo also in motion but faster than the water.

This isn't perpetual motion. The flowing water relative to the fixed cable in this instance is the power source for this apparent paradox.

[kcaldwel]

,,,

[Roj]

I agree with your analysis, but that is not what he was proposing, or how the windmill cart is supposed to work.

You still have the cable moving relative to the water, in this case 20 mph. Changing the reference frame doesn't change the energy budget. If you isolate the system, there is energy coming from the cable.

This is the same as having the shore and water stationary, the cable wound in at 20 mph relative to the water and shore, and the torpedo moving forward at 1 mph. No mystery.

Agreed.
But reading his posts, this is how he proposed the device is meant to work.

Scenario above:
Stationary cable fixed to the shore.
Flowing water.
Torpedo able to flow in water downstream faster than the water itself.

Scenario of device:
Imagine the cable is the ground on which the wheels of the device are rolling.
Imagine the flowing water is a wind.
Device able to move downwind faster than the wind.
The physics is the same.


I admit i'm eating humble pie here because in earlier posts i was very skeptical about the device but now i can see the physics is sound: there is no free energy source, the energy is drawn from the relative motion of cable vs water, or in the case of the device, wind vs ground.

[rofujiyama]

I wonder if they ever tried pulling on the cable from the front of the torpedo, It would seem to me that, that is the direction of the force of the wind for the thing here :confused: :)

[Brandano]

rofujiyama, it would surely work, but then why would you want to torpedo yourself? Roj, that's exactly the scenario I was proposing. Neat trick, but requires thinking not sideways, but at right angles to reality :)

[markdrela]

Although the behavior of the cart in the video does seem surprising, there's nothing about it which violates physical laws. It's quite real. What makes it work is some fundamental thrust/power relations for a propeller.

Let's assume the simplest case where the cart is traveling at the speed of the wind. In this case, the propeller sees zero net airspeed, same as a helicopter rotor in hover. The shaft power needed to drive a hovering rotor (i.e. the propeller) is

P = sqrt[ T^3 / (2 rho pi) ] / (eta_h R)

T = thrust
rho = air density
pi = 3.14159
eta_h = hover figure of merit = hover efficiency ( = 0.8 typically)
R = rotor radius

If the cart is traveling at some ground speed V, there must be a "braking" force on the wheels in order for them to generate the power needed by the prop. This force is simply

F = P / V

and if the prop is big enough (R is sufficiently big), this braking force F can easily be far smaller than the thrust T produced by the prop, so the cart can accelerate.

Let's consider some reasonable numbers:

V = 5 m/s = 11 mph (looks like the speed of the cart)
T = 1 N (102 grams)
R = 0.4 m
eta_h = 0.5 (assume crummy prop to be conservative)
rho = 1.2 kg/m^3

This gives the following prop shaft power.

P = 1.8 W


The power-generating force on the wheels is

F = P/V = 1.8 W / 5 m/s = 0.36 N

So there's a net forward thrust on the cart equal to

T_net = T - F = 1.0N - 0.36N = 0.64 N = 65 gramf = 2.3 oz

This excess thrust can go into overcoming friction, or accelerating the cart.
In practice, the cart will continue to accelerate until the excess thrust T_net is balanced by air drag and rolling resistance.

[rofujiyama]

Very informative formula, however,I dont see the [N] acting on the axle between the wheels in this formula , that has made the vehicle reach the sustained speed noted. Seems to me, it would be effecting the value [T] as the vehicle would accelerate to the prevailing wind speed.
I am trying to visualize this in my mind, but so far, I am still skeptical. But I think this is the first formula i ever enjoyed going over :) roland


edit: Found it , thank you!

[kcaldwel]

Yep, I was wrong. It will actually go faster than the wind downwind, and generate thrust on a flat treadmill. Still seems wrong somehow, but it should work if the friction and drag of the windmill are resonably low.

[Roj]

Still seems wrong somehow...

Definitely.
Good thread - i have learnt something.

(Having said that, the video itself is poor quality and hardly scientific validation in itself. Could easily be on an incline, and the streamer most of the time is blowing forwards not backwards!)

[Sparky Paul]

As I mentioned, that's the last test I would do... I'd begin with a barebones no-prop carriage facing into the wind.. and go from there.

[rofujiyama]

I have a question, again, :confused: if the wire pulling the torpedo is going 10 mph , and the torpedo is going 10 mph, doesnt the wire go slack if the torpedo goes faster? Does this mean we are having a pulsing acceleration during slack wire? I have learned not dispute numbers or physics , but i am puzzled , again,still :confused: does this apply?

[Roj]

I have a question, again, :confused: if the wire pulling the torpedo is going 10 mph , and the torpedo is going 10 mph, doesnt the wire go slack if the torpedo goes faster? Does this mean we are having a pulsing acceleration during slack wire? I have learned not dispute numbers or physics , but i am puzzled , again,still :confused: does this apply?


The cable is being pulled backwards relative to the motion of the torpedo in the water. The torpedo is able to generate thrust that exceeds the backward pull of the cable itself, so is able to generate forward motion through the water.
You could of course pull the cable in the same direction of the torpedo - less force but higher speed (same work) of the cable is required - but this defeats the purpose of being able to power it (as well as guide it) from the shore.

I think I can see the source of your confusion - When comparing it to the windmill contraption, remember the cable represents the "ground", the water represents the "air".

[Flyingwingbat1]

The cable is being pulled backwards relative to the motion of the torpedo in the water. The torpedo is able to generate thrust that exceeds the backward pull of the cable itself, so is able to generate forward motion through the water.
You could of course pull the cable in the same direction of the torpedo - less force but higher speed (same work) of the cable is required - but this defeats the purpose of being able to power it (as well as guide it) from the shore.

I think I can see the source of your confusion - When comparing it to the windmill contraption, remember the cable represents the "ground", the water represents the "air".

This sounds like it relates to the thought experiment of pulling backwards on a bicycle pedal (at the bottom of its stroke) and causing the bike to go forward as a result. All this stuff STILL boggles my mind. :confused:

[Sparky Paul]

This sounds like it relates to the thought experiment of pulling backwards on a bicycle pedal (at the bottom of its stroke) and causing the bike to go forward as a result. All this stuff STILL boggles my mind. :confused:
.
That works!
If your shoe is clipped to the pedal and you can pull it up to the rear completing the normal pedaling motion.

[Brandano]

Well, replace the pedal with a spool and wrap some wire around it, then pull on the wire. Only that now the wire is the ground and the ground is the wind... :)