Downwind Faster than the Wind - Successful Run by manned cart

[mikemonty]

MM.

Your wrong here the boat IS being driven by water, have you forgotten Newton's third law? "Every action has a reaction equal in magnitude and opposite in direction". The boat is being driven by an interaction between two mediums - it depends on your point of view which one is doing the driving.

Oh dear - you see, the problem is that if I just ignore this it looks like I agree...

An unbalanced force produces a motion, if the force on the hull from the sails was BALANCED nothing would happen.
In fact the sideways component of the force of the sails does exactly what you say - it makes the keel push sideways against the water, which pushes back and stops (reduces) leeway.
BUT
The forwards component is not resisted significantly by the water and so motion results.
At no point is the water driving the boat along, the lift from the air over the sails is driving the boat along.

On this there is no doubt, and if your arguments are based on any other premise then they are wrong.

You can shift frames of reference all you like but it will resolve to this.

 

[Bosun Higgs]

You said that the pitch of the prop on the cart was in "neutral" when the cart was in static air. (I'm still not sure why you think this).

For once I agree with Pierrome. When the cart is going at the speed of the wind, the prop is still rotating quite fast. If the thrust from that prop is more than the rolling resistance of the cart then the cart will continue to accelerate. Only when prop thrust, transmitted to the wheels via the gearing, matches air resistance will the max speed be reached.

I was wrong to say in the first place that it was impossible, but there is nothing other than the videos to prove that it is possible in practise as opposed to theory.

 

[A.T.]

I have given a reasonable and hopefully understandable explanation of my theory and it hinges on a plausible chain of cause and effect.

Using linear cause and effect reasoning to analyse a feedback system like this, is tricky. You end up in a loop. But it is not an isolated loop, like in perpetual motion machines. One of the effects (forward force on the cart) has 2 causes:
- Air movement relative to ground (external power input)
- Propeller rotation driven by the wheels (feedback loop)

A simple example (ruler = air, blue wheel = propeller)

The purpose of the carts propellor is to drive the wheel (aerodynamic force to torque at the prop spindle to the wheel)

No, this would be a turbine (or windmill). You cannot go DDWFTTW in turbine mode, because it would violate energy conservation: Above windspeed the turbine would push the air forward, so it would increase the air speed relative to the ground. It would create wind out of nothing.

To comply with energy conservation the cart must always reduce the air speed relative to the ground (slow down the true wind). Therefore the rotor is a propeller that converts the torque from the wheels into a backward force on the air.

I posted links to a detailed analysis here

 

[A.T.]

I was wrong to say in the first place that it was impossible, but there is nothing other than the videos to prove that it is possible in practise as opposed to theory.

- Theory says it is possible
- Experiments show it is possible

What else is missing?

 

[mikemonty]

Using linear cause and effect reasoning to analyse a feedback system like this, is tricky. You end up in a loop. But it is not an isolated loop, like in perpetual motion machines. One of the effects (forward force on the cart) has 2 causes:
- Air movement relative to ground (external power input)
- Propeller rotation driven by the wheels (feedback loop)

A simple example (ruler = air, blue wheel = propeller)






No, this would be a turbine (or windmill). You cannot go DDWFTTW in turbine mode, because it would violate energy conservation: Above windspeed the turbine would push the air forward, so it would increase the air speed relative to the ground. It would create wind out of nothing.

To comply with energy conservation the cart must always reduce the air speed relative to the ground (slow down the true wind). Therefore the rotor is a propeller that converts the torque from the wheels into a backward force on the air.

I posted links to a detailed analysis here

I'm sure you read my boring and protracted post carefully but I'd encourage you - if you can be bothered - to read it again carefully-er.
At no point do I get into a feedback loop or violate energy conservation (in fact energy does not come into it) and the prop drives the wheel quite happily DWFTTW.
The geared ruler thing is not necessary or even, as far as I can see, particularly illustrative.

 

[mikemonty]

For once I agree with Pierrome. When the cart is going at the speed of the wind, the prop is still rotating quite fast. If the thrust from that prop is more than the rolling resistance of the cart then the cart will continue to accelerate. Only when prop thrust, transmitted to the wheels via the gearing, matches air resistance will the max speed be reached.

I was wrong to say in the first place that it was impossible, but there is nothing other than the videos to prove that it is possible in practise as opposed to theory.

This is the problem with taking P's interpretation of what I said.
I never said...what he said I said, as far as I know!

 

[mikemonty]

- Theory says it is possible
- Experiments show it is possible

What else is missing?

A healthy dose of scepticism and the substitution of the word "suggests" for "says" and
"shows".

 

[Bosun Higgs]

(in fact energy does not come into it) .

Thats wrong Mike. The one law of physics that definitely applies to the cart is that of conservation of energy. So if the cart is doing work, which it is in overcoming rolling resistance and in accelerating, it has to get energy from somewhere to do that work. The somewhere is the wind - the prop is changing the direction and speed of the wind that hits it and in so doing is taking energy.

 

[A.T.]

At no point do I get into a feedback loop

But this is one

or violate energy conservation (in fact energy does not come into it)

Sorry, you cannot ignore energy conservation. And your explanation does violate it:

The purpose of the carts propellor is to drive the wheel (aerodynamic force to torque at the prop spindle to the wheel)

A turbine cart (as you describe here) going DWFTTW would accelerate the true wind (air relative to ground speed would increase) creating more wind energy there was before, out of nothing. That is not how wind powered vehicles work. They always slow donwn the true wind, to extract energy from it.

prop drives the wheel quite happily DWFTTW.

Depends what you mean by "drive" here. You could correctly say:

"The prop drives the wheels indirectly and non-exclusively by helping the wind to push the cart forward, so the wheels spin."

What is wrong, is the idea that the propeller drives the wheels directly by acting as a turbine to create a torque that drives the wheels forward.

The geared ruler thing is not necessary or even, as far as I can see, particularly illustrative.

It is completely analogous:

"The blue wheel drives the cotton reels indirectly and non-exclusively by helping the ruler to push the cart forward, so the cotton reels spin."

 

[snowleopard]

At no point is the water driving the boat along, the lift from the air over the sails is driving the boat along.

And that, in a nutshell, is your fundamental error and that of everyone who is hung up on the 'apparent wind seen by the cart'.

It is the combination of wind and water moving at different speeds that is essential to make the boat go. If the wind and water are moving in the same direction at the same speed the boat can make no way through the water and will simply drift.

If you take the water out of the equation and mount the mast on a hovercraft it will drift helplessly with the wind.

 

[RAI]

I the San Jose team are going to make some interesting improvements to their cart. It looks like they might achieve a startling DDWFTTW speed.

http://www.fasterthanthewind.org/

 

[mikemonty]

But this is one


Sorry, you cannot ignore energy conservation. And your explanation does violate it:

A turbine cart (as you describe here) going DWFTTW would accelerate the true wind (air relative to ground speed would increase) creating more wind energy there was before, out of nothing. That is not how wind powered vehicles work. They always slow donwn the true wind, to extract energy from it.




Depends what you mean by "drive" here. You could correctly say:

"The prop drives the wheels indirectly and non-exclusively by helping the wind to push the cart forward, so the wheels spin."

What is wrong, is the idea that the propeller drives the wheels directly by acting as a turbine to create a torque that drives the wheels forward.



It is completely analogous:

"The blue wheel drives the cotton reels indirectly and non-exclusively by helping the ruler to push the cart forward, so the cotton reels spin."

Sorry, you've completely misread my recent posts and I'm afraid I can't be bothered re-phrasing it or show you where you have misinterpreted.

Not your fault, I have written screeds here after the point where I said I couldn't be bothered - I'm not trying again.

 

[Ubergeekian]

Pierrome,
Your reply contains the same elements of trollerie that Ubers do.

You might get a little further in your understanding if you didn't dismiss anything beyond your comprehension as "trolling"

First - the difficult frame of reference - an insistence on using the air as a frame of reference and taking energy from the road - Dont do that! Why would you do that?

Why not? There is nothing wrong about using moving axis systems to analyse dynamics problems - on the contrary, choosing an axis system to use one of the fundamental and initial stages in any analysis. It doesn't affect the answer, but it can make finding it a lot easier.

Next the misrepresentation - I didn't suggest "drag" and "transfer of energy" were mutually exclusive terms.

That does NOT equate to the downwind scenario working.

Doesn't the existence of a working, human carrying, faster-than-the-wind-downwind cart worry you at all?

The wheel is no more being driven by the ground than the boat is by the water. No energy is being transferred from water to boat or ground to wheel, other than by frictional losses.

Anything that pushes against the ground moves it, a bit. Anything that pushes and moves does work.

As it passes windspeed the apparent wind starts to take over and starts to DRIVE the wheel(s).

Would that work on a calm day? I really think you need to do some thinking about conservation of energy...

 

[mikemonty]

And that, in a nutshell, is your fundamental error and that of everyone who is hung up on the 'apparent wind seen by the cart'.

It is the combination of wind and water moving at different speeds that is essential to make the boat go. If the wind and water are moving in the same direction at the same speed the boat can make no way through the water and will simply drift.

If you take the water out of the equation and mount the mast on a hovercraft it will drift helplessly with the wind.

Make life easy on yourself and those around you - keep the water as a stationary frame of reference. There is no need to have it move, it just confuses the issue.

 

[snowleopard]

Make life easy on yourself and those around you - keep the water as a stationary frame of reference. There is no need to have it move, it just confuses the issue.

Pick whatever frame of reference you like or feel comfortable with. Just so long as you have air and water or air and road moving relative to each other.

 

[mikemonty]

You might get a little further in your understanding if you didn't dismiss anything beyond your comprehension as "trolling"

I have not dismissed anything beyond my comprehension - I have dismissed some stuff that is incomprehensible.

Why not? There is nothing wrong about using moving axis systems to analyse dynamics problems - on the contrary, choosing an axis system to use one of the fundamental and initial stages in any analysis. It doesn't affect the answer, but it can make finding it a lot easier.

Back at Ya, Why?
The ground or water is the ideal reference.
It's wrong to use anything else when it confuses the issue. The fundamental reference here is the fundament (geddit? see what I did there?)

Doesn't the existence of a working, human carrying, faster-than-the-wind-downwind cart worry you at all?

No, should it? are they going to put scythes on the wheels?

Anything that pushes against the ground moves it, a bit. Anything that pushes and moves does work.

Ah yes, "work done equals force times distance", so the force pushing back against the wheel is equal and opposite, how far does the Earth move?
I admit that work is done in moving the boat forward BY THE AIR. Any work done by the water is resisting that motion.

Regarding your final try:
I wrote:
"As it passes windspeed the apparent wind starts to take over and starts to DRIVE the wheel(s). "
You wrote:

Would that work on a calm day? I really think you need to do some thinking about conservation of energy...

Previously I had written:
"The other conceptual problem is in how the boat/cart "knows" that there is a true component to the wind - why can't it simply keep on sailing/carting if the true wind dies?
Try it - do the vector diagram - as you decrease the true wind, the apparent moves to the point where it is "on the nose". In irons for the boat and neutral or reverse pitching the prop on the cart - no more power and the vehicle stops"

Now apart from addressing the issue of windspeed reducing to zero here, the situation where you are trying to START a wind powered vehicle on a calm day sounds a bit ludicrous to me - presumably not to you.

 

[PaulJS]

At first I was a sceptic, but after reading SL's explanation in post #275 I'm convinced.

At first I couldn't see how DTWFTTW could work (In my defence I was unable to view the original videos!), then I saw the videos of the desert runs and was converted...

Then I saw SnowLeopards explanation of how it was staged...


And fell for it...

So now I'm a converted April Fool.

 

[snowleopard]

Then I saw SnowLeopards explanation of how it was staged...


And fell for it...

So now I'm a converted April Fool.

Handsome admission sir. Take my hat off to you.

 

[[32511]]

OK, chaps, one more explanation; no angles or apparent speeds, so it might be easier to grasp.It also covers the vital bit when the cart is at wind speed.
Firstly, we have the cart stationary in the wind. The propellor is loose on the shaft, and not connected to the wheels, and will speed up and freewheel in the wind. The rotational speed stabilises at a certain no. of revs, when the blades are probably at an angle of 5 or 10 degrees to the wind, and the freewheeling prop is producing a goodly amount of thrust or lift. The energy comes from the blades slowing the wind down a bit and changing it's direction. If that doesn't work, every gyroplane, with it's freewheeling rotor, will fall out of the sky! OK so far?
Now we replace the prop with one having no pitch whatsoever, but geared to the wheels, and pull the cart at wind speed. At wind speed the forces we are overcoming are rolling resistance, the friction in the mechanism, and a bit of drag from the rotation of the flat propellor spinning. No prop thrust at all, as it is flat and travelling at the speed of the wind, and no aerodynamic drag from the machine either, so we don't need a lot of power to move the machine.
Now the rotating propellor has been geared to the same revs it was making when stationary and windmilling, and if we instantly replace the flat prop with a pitched prop, with a pitch of 5 or 10 degrees, we will get exactly the same thrust as the freewheeling prop.
Same revs; same angle to the wind, and the wheels were only spinning the prop, not providing the thrust. Some of this thrust energy WILL be taken out to pull the cart, and keep the prop spinning, but there will be plenty left to continue accelerating the cart beyond wind speed.
As the column of air in front of the prop speeds up, and the cart speeds up further, we obviously need to increase the blade angle, with a variable pitch control, just as we need to sheet in as a yacht speeds up, and like the yacht or ice yacht, will eventually reach a steady state, faster than the wind.
The fixed pitch prop at present used is angled at about the eventual VP prop angle, and will be very inefficient at lower speeds, producing poor acceleration compared to the variable pitch version that the team are about to use.
Anybody get it now?
Edited at 13.58 to clarify one minor point.

 

[A.T.]

The ground or water is the ideal reference. It's wrong to use anything else when it confuses the issue.

It is never "wrong" to use a different reference frame. And in this specific case, clinging to the idea that ground or water define an ideal or preferred reference frame is very counter productive for the understanding. It is exactly what gives the deniers the wrong idea, that you cannot extract energy from the true wind, when you are at rest relative to the air. Of course no one sees a problem in extracting energy from the true wind, while at rest relative to the ground. But since there is no preferred reference frame in physics, the two cases are in fact not fundamentally different:

If you can extract energy from the speed difference of air & ground (true wind) while at rest to the ground, then you can also do this while at rest to the air.