Downwind faster than the wind. Poll

[snowleopard]

That would work, if there were an additional surface to place the wheels against and that is stationary relative to the cart.

If you imagine the treadmill running at the bottom driving wheels attached to the top of the cart, the wheels on top of the cart are moving. Now place a board against the top wheels like a ceiling, so we have a stationary ceiling with wheels geared to wheels running on the moving floor.

That works and here is the film of it in action. (skip the 1st 1min 40 secs). That shows a mechanism that uses the relative movement of two surfaces to produce motion faster than the original.

Now replace the ruler and wheel with air and prop. The coupling is a lot less efficient but it does the same job.

 

[lw395]

I hope we would all agree that, when the vessel (vehicle, whatever) is moving DDW at exactly the wind speed, it see NO apparent wind....
. So - can we agree on this? The vessel, and all of its components, see NO apparent wind.

I'm at work and can't access the video etc, but I think the flaw in your argument is that you are thinking in one dimension.
Your second statement does not follow from the first, because parts of the machine move relative to each other, they can have an apparent wind. Each prop blade will have an apparent wind at 90 degrees to the true wind. Of course the prop would have to be moving to start with, but then things like aeroplanes are not so good from a standing start either!

Because using the 'two or more BMW oracles' model, a machine can be imagined, does not mean that somebody will make it work. There is a huge gulf between 'possible' and 'practical'.

A sailing machine can 'possibly' go at 100mph, in only 25kts of breeze, but 'practically' this is only achieved by ice yachts.

It's the difference between fundamental limits and technology limits. For many years foiling boats were a big disappointment, only working in freak conditions, now 600FF's and moths are available retail, and can race reliably around the whole circuit.

Failing proves nothing. Leonardo would have failed with helicopters, due to the materials of the day.


Proper research is not done like this nowadays, if you were serious, you would simulate a design then build it knowing it would work.

 

[xxyyzz]

I met a person once who knew a person who had seen a demonstration of a car that ran on water as its fuel. He really saw it: they poured water into the petrol tank, started the engine and drove it. Absolutely true, real, for sure.

“Witchcraft to the ignorant, …. Simple science to the learned.”

Although your mate's mate's car sounds like a con.

 

[Ubergeekian]

Quite. But a prop is a pretty inefficient way of imparting thrust. Much more efficient to have those wheels (which are going ten to the dozen) driving some other wheels.

You're so nearly there!

Yes, wheels are more efficient than a prop, but the wheels are pushing against something fixed and the prop is pushing against something moving in the right direction.

The question is simply: does the boost you get from pushing against a moving thing make up for the lower efficiency of the pushing?

 

[bbg]

The question is simply: does the boost you get from pushing against a moving thing make up for the lower efficiency of the pushing?

You've forgotten one element of the equation. Does the boost you get from pushing against a moving thing make up for the lower efficiency of the pushing AND the drag created in order to extract the energy in order to do the pushing?

Note to self. Must stop feeding the trolls.

 

[snowleopard]

A question purely for those who said yes...

How did you come to that conclusion?

For myself, my first reaction was no way - no apparent wind. Then after watching the videos I started to think 'could this work'?

After studying the mechanism I caught on to the fact that the energy was coming from the ground/air difference and from there it became clear how it worked.

 

[nmeyrick]

How did you come to that conclusion?

For myself, my first reaction was no way - no apparent wind. Then after watching the videos I started to think 'could this work'?

After studying the mechanism I caught on to the fact that the energy was coming from the ground/air difference and from there it became clear how it worked.

One question please Snowleopard, and I am not trying to be funny here. I don't believe that this can work but, as you said am genuinely trying to understand how it could.

You talk about the difference between the speed of the ground and the speed of the air as something different to and separate from the wind, and explaining how the cart can get energy above the wind speed. But isn't "wind" just the name we give to a difference in speed between the air and ground/water/boat?

 

[snowleopard]

One question please Snowleopard, and I am not trying to be funny here. I don't believe that this can work but, as you said am genuinely trying to understand how it could.

You talk about the difference between the speed of the ground and the speed of the air as something different to and separate from the wind, and explaining how the cart can get energy above the wind speed. But isn't "wind" just the name we give to a difference in speed between the air and ground/water/boat?

Yes, I am talking about the difference in velocity between ground and air. Most of the antis recognise only the difference between air and vessel velocity. That's where the problem arises as in the latter case it falls to zero as the velocity of the vessel approaches air velocity while the former remains the same regardless of the vessel's motion.

 

[Ubergeekian]

You've forgotten one element of the equation. Does the boost you get from pushing against a moving thing make up for the lower efficiency of the pushing AND the drag created in order to extract the energy in order to do the pushing?

Efficiency comes into it, of course, but it's a second-order effect.

 

[Porthandbuoy]

I'm still waiting for someone to tell me what happens in a gust. Does this wondrous machine accelerate or decelerate? Does its apparent wind increase, decrease or stay the same?
If you give it a push start on a calm day will it keep on rolling?

We may have solved the longitude problem, but perpetual motion still eludes us.

 

[snowleopard]

I'm still waiting for someone to tell me what happens in a gust. Does this wondrous machine accelerate or decelerate? Does its apparent wind increase, decrease or stay the same?
If you give it a push start on a calm day will it keep on rolling?

We may have solved the longitude problem, but perpetual motion still eludes us.

It operates on the difference in speed between the ground and the wind. If that increases the speed increases. On a calm day there is a zero difference so there is no power to make it go.

No need for silly quips about perpetual motion. I went through the maths in a previous post and showed there is no need to imagine more input coming out than going in, it uses 20% or less of the available energy.

 

[Porthandbuoy]

It operates on the difference in speed between the ground and the wind. If that increases the speed increases. On a calm day there is a zero difference so there is no power to make it go.

No need for silly quips about perpetual motion. I went through the maths in a previous post and showed there is no need to imagine more input coming out than going in, it uses 20% or less of the available energy.

I didn't mean to offend. I've read and re-read your post and I just can't get my head around your explanation. At some point this machine has to pass from a tail wind, through a zero relative wind, to a relative headwind. I believe this to be physically impossible.

No reply needed. I'm already convinced.

 

[Ubergeekian]

I didn't mean to offend. I've read and re-read your post and I just can't get my head around your explanation. At some point this machine has to pass from a tail wind, through a zero relative wind, to a relative headwind. I believe this to be physically impossible.

Why?

The propeller is always pushing backwards against the tailwind.

 

[mikemonty]

How did you come to that conclusion?

For myself, my first reaction was no way - no apparent wind. Then after watching the videos I started to think 'could this work'?

After studying the mechanism I caught on to the fact that the energy was coming from the ground/air difference and from there it became clear how it worked.

We should keep this to the original thread but...
My take on it is that the energy is coming from the air - the ground has no energy to impart - it is not moving.
The grounds role is to form the fulcrum (the point of contact of the wheel with the ground) of the lever (the wheel) that is turning the force of the air into work.

 

[nmeyrick]

Is it possible to sail directly downwind slower than the wind?

Thanks for clearing that up Snowleopard.

The problem I have remains the same; lets take the original situation where the vehicle is outside on the road, moving downwind at windspeed. It sees no aparent wind, but a difference in speed between the road and the vehicle. Can it use that differential to generate further forward motion?

The anti's will tell you no, the differential you are looking at is its own forward motion, and you can't use your own forward motion to generate further forward motion. The pros will tell you that the difference in speed between the ground and the wind is able to provide you with extra power.

I think the fundamental difference here is whether you believe that the vehicle can be made to travel slower than the wind, or whether you believe it will always travel at least at windspeed inn which case anything extra is a bonus and will make you go faster.

Ok, lets try one of these thought experiments everyone is so fond of. Lets use the original vehicle and put a clutch in the drive shaft so that we can engage and disengage the prop. Lets assume the clutch is perfectly efficient so that it introduces no losses, it is simply a device to let us turn the drive to the prop on or off. We'll let the cart set off dead downwind with the prop disengaged and free-wheeling. It will accelarate under its windage until it reaches windspeed and is now travelling directly downwind at the speed of the wind.

This is the really crucial bit - we now engage the clutch to drive the prop, what happens to the cart? Lets assume that there is no energy lost in engaging the clutch, or bringing the prop up to speed as that will confuse matters. What is happening is that the wheels of the cart are now performing work, ie turning the prop to generate thrust. This creates resistance to turning the wheels and will cause the cart to slow to less than wind speed. This will in part be counteracted by the thrust provided by the prop, but due to frictional losses in the system this will be less than the power input at the wheels and the net effect will be to slow the vehicle.

The whole thing works only if you believe that the work done by the wheels does not slow the cart to less than windspeed; that the cart is somehow "fastened" to the wind and cannot go slower than it.

Or look at it another way. We have two identical carts just like the original, both running downwind at windspeed. On one cart we have an extra wheel, which turns a dynamo which powers an electric lightbulb. Most of us will have done this experiment at school; if you flick a switch to break the circuit between the dynamo and the bulb it is easy to turn the dynamo. When you close the circuit the dynamo gets harder to turn because it is doing work to power the bulb, which creates resistance.

Question is, will the work done to light the bulb cause the cart to slow down to less than windspeed? If not why not? What if the bulb drew 10x more power? 100x more? And will it travel at the same speed as the cart with no bulb (assuming weight, windage etc are equal) or at some other speed slower than the other cart but still faster than the wind?

 

[DJE]

What is happening is that the wheels of the cart are now performing work, ie turning the prop to generate thrust. This creates resistance to turning the wheels and will cause the cart to slow to less than wind speed. This will in part be counteracted by the thrust provided by the prop, but due to frictional losses in the system this will be less than the power input at the wheels and the net effect will be to slow the vehicle.

The San Jose team have been testing their vehicle in still air on a rolling road. This is equivalent to the situation where the cart is moving at exactly wind speed downwind. They measure the thrust generated by the prop and the force applied to the wheels by the rolling road. They've been finding that the thrust from the prop exceeds the driving force from the rolling road indicating that there is thrust available to drive the cart above wind speed.

 

[RAI]

The problem I have remains the same; lets take the original situation where the vehicle is outside on the road, moving downwind at windspeed. It sees no aparent wind, but a difference in speed between the road and the vehicle. Can it use that differential to generate further forward motion?

The structure of the cart may see no apparent wind, but what do the propeller blades see?

They are in an analogue situation to the wing on BMW Oracle, or an ice yacht. Just as the yacht gets resistance from it's keel/dagger-board/ice skates to balance against the pressure of the apparent wind on its wing, the propeller on this contraption gets resistance from the wheels (not wanting to go yet faster) and the pressure of the apparent wind on its blades. In both case they can go faster if they can get an positive net balance of forces in their direction of motion.

When one sees BMW Oracle tacking downwind at three times the wind speed in the down-wind direction, 18 knots down-wind VMG in 6-8 knots of wind, it seems counter intuitive. It is clearly out-running the wind by a large margin. If a wing on a boat (or ice yacht) can out-run the wind, why can't a propeller blade do the same in three dimensions that the yacht does in two D?

 

[Pierrome]

Thanks for clearing that up Snowleopard.

The problem I have remains the same; lets take the original situation where the vehicle is outside on the road, moving downwind at windspeed. It sees no aparent wind, but a difference in speed between the road and the vehicle. Can it use that differential to generate further forward motion?

Etc. Etc.......

That's a good post and very well argued.


However, if the cart is moving downwind at windspeed, then the air is stationary with respect to the cart.
In other words you can say the cart is stationary, with respect to the airmass.
The road is moving underneath with respect to the cart. No-one can disagree with that I'm sure.

The cart works be taking power from one medium, the road, to propel itself through the other medium, the air.

If you say it cannot do that, then you are also saying that an upwind windmill boat cannot sail directly into the wind.
Because in the case of the upwind windmill boat, the windmill is taking power from the flow of air coming towards it, and pushing the boat through the water to sail against the same flow of air coming towards it. Some have had difficulty in accepting that, saying that something cannot move against the force that powers it, but it is a fact that this can be done.

That also, in effect is what the doubters say about downwind faster than the wind. They say that it is impossible for the cart to move through the air (remember its part of the airmass as explained above) in the opposite direction to the force that powers it - the road.
But we know it IS possible to obtain movement directly against the powering force by the example of the upwind windmill boat. So there is no reason why this cannot happen. And indeed this is what the experimental videos demonstrate and hopefully will also be demonstrated by the manned test soon.



Sailing DDWFTTW with the propeller cart is exactly that same thing, but the other way round.

 

[Pierrome]

The structure of the cart may see no apparent wind, but what do the propeller blades see?

They are in an analogue situation to the wing on BMW Oracle, or an ice yacht.

You're wrong here and a little confused.
The propeller blades are not an analogue of the wing on BMW Oracle, just because propeller blades look a little like wings you have fallen into an obvious trap.
The wing on BMW Oracle is in fact an analogue of the road wheels on the downwind cart, the wing is the device taking the power from the wind and pushing the boat along via its keel or lateral resistance. This lateral resistance is the true analogue of the downwind cart's propeller.

 

[Ubergeekian]

This is the really crucial bit - we now engage the clutch to drive the prop, what happens to the cart? Lets assume that there is no energy lost in engaging the clutch, or bringing the prop up to speed as that will confuse matters. What is happening is that the wheels of the cart are now performing work, ie turning the prop to generate thrust. This creates resistance to turning the wheels and will cause the cart to slow to less than wind speed. This will in part be counteracted by the thrust provided by the prop, but due to frictional losses in the system this will be less than the power input at the wheels and the net effect will be to slow the vehicle.

You've missed the crucial bit. The propeller is pushing against the wind, which is moving faster than the road. A forward force of 1N at the propeller will do more work than a backward force of 1N at the wheels.