Down wind faster than the wind

[spork]

"Have you considered the possibility that your interpretation could be wrong? "

Yes, but ask Spork.

I concur. Your interpretation is wrong.

 

[Michael C]

Is there anything incorrect with these statements?

To travel at the speed of the wind the thrust of the propeller against still air must equal the rolling resistance of the wheels.

To travel at greater than the speed of the wind the thrust of the propeller against a headwind must be greater than the rolling resistance of the wheels.

Yes there is something incorrect. Correct would be:

To travel at any constant speed (be it less, the same or greater than that of the wind), the thrust produced by the propeller must be equal to the total loss of energy from the friction in the wheels and drag of the air on the cart.

 

[humber]

Yes there is something incorrect. Correct would be:

To travel at any constant speed (be it less, the same or greater than that of the wind), the thrust produced by the propeller must be equal to the total loss of energy from the friction in the wheels and drag of the air on the cart.

So how do you explain that there is no appropriate thrust on the treadmill?

 

[humber]

I concur. Your interpretation is wrong.

But not that the conclusion is trivial?

 

[John Freestone]

What a fascinating thread. I've just found it, haven't read much, it seems to be increasing in length faster than the speed of threads. I'm going to dive in with my penny's worth.

BIG EDIT: I've just realised I'm a dipstick. I'll leave the following just to prove it. I'm sure now it's wrong, and the claim is a hoax. I'm forgetting that down-wind, you're losing your source of power as you accelerate. Across or up wind that doesn't apply.

The original proposition that something can be propelled downwind faster than the windspeed only sounds ridiculous because it has the effect of a trick question on the mind. We intuitively imagine a fixed sail pushing a kind of land yacht along. Clearly, it's not going to go faster than the wind. But it's not like that.

In one of the first posts there was a link to a discussion of an early version of the vehicle (link below), where a vital clue is given to understanding this: if the gears are reversed, it was said, the machine will even propel itself up wind. If you're having trouble getting your head round something being wind-powered faster than the wind downwind, just start imagining the most difficult opposite version - driving itself up wind!...

But what's strange about that? Try this idea on for size: you get a propeller on a frame that can move along another piece of framework, like a little trolley along a horizontal ladder, and you take a gear chain from it to a winch, or just a gear on the ladder. Now, add wind blowing on the prop, such that it turns the gear train, winding itself along the ladder in the opposite direction to the wind. If you're having difficulty thinking of the gears, just imagine the prop winding up a big rubber band, like in a toy plane. As the wind turns the prop, it will wind up the band, which will twist, get shorter, and pull the trolley closer to where it's attached - up wind.

ETA: Unfortunately, there's a problem with the band version, which is that energy is stored and will try to reverse the prop. The gears, however, could turn with only their friction being a slight hindrance. A screw thread would do it. You could fit a prop to a nut and make it screw itself along a bolt up wind no problem.

Now, what's so different between that and said vehicle driving itself up wind on wheels? Only that instead of a fixed, unslippable gear system wrenching it along the ladder, or a twisty bit of elastic pulling it along, it is driving its wheels, and there is potentially some slipping, but with rubber wheels on tarmac, not that much. It will depend on the weight, so that they are held in contact with the tarmac, and the efficiency and gear ratios and wind speeds would be important, but I imagine such a vehicle could even go directly against the wind as reported. DWFTTW is a doddle.

Another way to 'prove' it to yourself in imagination (the downwind version is easiest) is to remember that the propeller is gathering the energy of the wind. If you say that it can't work with one propeller, just imagine harnessing a much greater cross section of the wind round the vehicle - assuming you had infinitely strong and light materials you could have a square acre of props all being driven by the passing wind, all that power being delivered to that little frame at the bottom. Now imagine a rather light breeze. Our machine only needs to beat 1/4 mph. Still, with a vast area of props, you're going to be catching massive amounts of energy, and it's all going to end up at those wheels, and you'll be burning rubber.

ETA: that link again - it was actually via the mythbusters website, is http://www.ayrs.org/DWFTTW_from_Catalyst_N23_Jan_2006.pdf - see last few paragraphs.

 

[H'ethetheth]

That is the basis of the treadmill. It is said to be true, that dV is zero at windspeed. That is also the idea behind the 'van' tests.
The belt is moving at (-windspeed) leaving dV= 0 around the cart, so the cart is said to be moving at a (slightly) greater velocity than windspeed.
ETA:
I just noticed that text moved when actually displayed, so there is an X at (0,0) and at (windspeed,0)




It might, if the wind and belt were truly equivalent.
It could then be determined what actually happens at windspeed, rather than first assuming the conclusion. It could be moving at >windspeed, or a few cm/sec relative to the ground.

My question is if that if former assumption is valid, then there must be intermediate values between the other case of dV=0, that of standstill.

Hm, this makes no sense to me. In that graph you're asking me to fill in the dots between moving treadmill at standstill, and moving at wind speed on the road; There are no dots between them; they are the same X; the windspeed X.

If you compare standstill on the street with motion at windspeed on the street, then there is no X at (0,0), but at (0,windspeed).

If you compare standstill on a motionless treadmill with standstill on the street, you're comparing non-equivalent situations and can expect to see different behaviour.

And lastly this: The equivalent to putting the cart on the street in a breeze is not putting the cart vertically down on a rolling treadmill. It's putting the cart on the treadmill while standing still on the rolling treadmill (don't try that at home), in which case the X at the beginning of the run is again (0,windspeed).

 

[ThinAirDesigns]

What a fascinating thread. I've just found it, haven't read much, it seems to be increasing in length faster than the speed of threads. I'm going to dive in with my penny's worth.

BIG EDIT: I've just realised I'm a dipstick. I'll leave the following just to prove it. I'm sure now it's wrong, and the claim is a hoax. I'm forgetting that down-wind, you're losing your source of power as you accelerate. Across or up wind that doesn't apply.

Perhaps it's just me, but I had trouble deriving a conclusion from your posts.

Do you believe that DDWFTTW is impossible and the demonstrations are a hoax?

Thanks.

JB

 

[H'ethetheth]

What a fascinating thread. I've just found it, haven't read much, it seems to be increasing in length faster than the speed of threads. I'm going to dive in with my penny's worth.

BIG EDIT: I've just realised I'm a dipstick. I'll leave the following just to prove it. I'm sure now it's wrong, and the claim is a hoax. I'm forgetting that down-wind, you're losing your source of power as you accelerate. Across or up wind that doesn't apply.

Read more of the thread John. You're assuming, like I did, that the propeller functions as a wind turbine, which it doesn't.

 

[John Freestone]

Perhaps it's just me, but I had trouble deriving a conclusion from your posts.

No, I'm not surprised you had trouble. I'm sorry. I posted in haste, thinking that I had a sudden insight as to how this proposed machine could be wind-powered and run downwind faster than the wind powering it. Those were my bits that I put in quotes in a later edit, when I stopped and put the kettle on. I then realised I'd been had, or at least that I had made a fundamental mistake or two.

Do you believe that DDWFTTW is impossible and the demonstrations are a hoax?

I should be careful, having just made such a silly mistake, but at the moment I can't think of any other explanation. My guess is that it must be a fake.

I'm actually surprised there is so much complicated maths and physics supposedly being posted here. Maybe some of it is wishful thinking. If you reduce the problem to its simplest principles (as I'm sure I'll find some have already posted, and then gone on to more interesting things!) - if there is no source of power other than wind passing over a turbine or prop, which drive the wheels directly down wind, then as the machine increases speed towards that of the wind, its source of power reduces. It could never reach wind speed, in fact, due to all the inefficiencies of the system, which are considerable.

When I first started reading the thread I got into all sorts of silly ideas - as some were posted - that once it gets going, the wheels are driving the prop, via the gears, which is pushing the air 'backwards' against the wind, making it go even faster. But that is clearly nonsense - there is nothing driving the wheels other than the wind turbine itself (supposedly!). They can't suddenly become a source of power for the turbine.

As I suggested in my correction, if mounted a wind turbine (I'm meaning directly driving wheels, not an electrical turbine) on a cart at right angles from the ones shown, you could drive the machine across the wind. I'm not clear how fast it could go, but I'll guess it could be faster than wind speed - now my earlier argument about just collecting larger amounts of energy from bigger props would work - so theoretically it could go very fast. In the directly downwind version, it doesn't matter if I collect a much bigger area. Assuming that it's all going the same speed, when my vehicle reaches that speed there is no further source of power. The upwind scenario makes my head hurt.

I also would be eager to dispell a myth I keep seeing here that a sail-boat can tack at an angle to the wind faster than the wind speed. Surely that can't be right. There is even less reason to imagine a sailboat could go downwind faster than the wind, and across it at an angle it is just taking a reduced proportion of the velocity - it will be slower than it would racing downwind. I think.

Anyway, yes, I reckon it's a fake. On the vid linked to in the OP, the prop's vanes seem to be rotating such that they would be the driving force - like a prop on an airplane - they appear to be pushing it along, or at least helping it move along, and I wouldn't be surprised to find that there's a battery somewhere!

The treadmill experiments are another thing entirely - there there is clearly a big hefty source of power to the wheels in the form of an electric treadmill, and the wheels are then driving the prop, I imagine, in the video I watched. Indeed, they would only do this if there were some resistance. The demonstrator made a big thing about the treadmill being angled, with the model driving itself up the slope, but if it weren't trying to roll down the slope with gravity, it would just travel along with the belt and fall off the end. He even pushes against it. Of course, this push helps it to stay in one place, while the wheels are turned. Once it's got going, it may drive itself faster than the treadmill via the prop.

Electric power to the wheels doesn't prove anything. If it can move faster than the treadmill, that's a different problem. You might be able to get it to go much faster than the treadmill, I imagine. When the proper machine is moving along the road, the motion of the wheels aren't a source of power - no-one's got the road plugged into the mains.

 

[John Freestone]

Read more of the thread John. You're assuming, like I did, that the propeller functions as a wind turbine, which it doesn't.

What's it function as?

 

[Modified]

I also would be eager to dispell a myth I keep seeing here that a sail-boat can tack at an angle to the wind faster than the wind speed.

That such is possible is common knowledge. Even humber wouldn't dispute that.

 

[John Freestone]

That such is possible is common knowledge. Even humber wouldn't dispute that.

I think common knowledge is in this case mistaken.

 

[Modified]

I think common knowledge is in this case mistaken.

It is simple from a physics perspective and more than obvious from experience. Do a bit of googling, or just take a trip to the beach and watch a windsurfer.

 

[spork]

I should be careful, having just made such a silly mistake, but at the moment I can't think of any other explanation. My guess is that it must be a fake.

Several of us have now demonstrated this and have posted videos of same. You are showing yourself to be a fool by jumping in with both feet (in your mouth) without even having read the thread.

That you choose to call our credibility into question because of your own lack of understanding is shameful. Fortunately, no one here puts any weight behind your words as you've admitted to jumping in with a conclusion without understanding what's being claimed.

Well, I guess it's off to another thread for you - so you can offer your opinion on other matters you don't understand - and then move on.

I also would be eager to dispell a myth I keep seeing here that a sail-boat can tack at an angle to the wind faster than the wind speed. Surely that can't be right. There is even less reason to imagine a sailboat could go downwind faster than the wind, and across it at an angle it is just taking a reduced proportion of the velocity - it will be slower than it would racing downwind.

Did you study intuition or physics?

The treadmill experiments are another thing entirely - there there is clearly a big hefty source of power to the wheels in the form of an electric treadmill...

What if we'd tested it in a very long wind tunnel? Would you insist we unplug the fan? What if we'd tested it outdoors? Would you insist we do it when the wind is not blowing - despite this being a wind powered vehicle!?

and the wheels are then driving the prop, I imagine, in the video I watched. Indeed, they would only do this if there were some resistance. The demonstrator made a big thing about the treadmill being angled, with the model driving itself up the slope, but if it weren't trying to roll down the slope with gravity, it would just travel along with the belt and fall off the end.

And yet it DOES NOT do that when we do it on a flat belt. Instead it advances on the treadmill. So in addition to not reading the thread, you must have watched the video while shaving.

 

[Dan O.]

Welcome to the discussion John.


Before you read the rest of this thread, how about entering into a bit of a wager? Since JREF probably frowns upon using it's site for gambling, this is what I propose we do...

Let the believers and non-believers put up equal sums of money to back their belief. We'll then enlist the services of the JREF staff to test the device to see if it performs as claimed. The losers pay their wager as a donation to JREF.

[As a bonus, if JREF can be convinced beforehand that the device must be violating the laws of physics in order to perform as claimed and therefore qualifies for the MDC, All parties will split the $1M prize money.]

 

[humber]

Hm, this makes no sense to me. In that graph you're asking me to fill in the dots between moving treadmill at standstill, and moving at wind speed on the road; There are no dots between them; they are the same X; the windspeed X.

If you compare standstill on the street with motion at windspeed on the street, then there is no X at (0,0), but at (0,windspeed).

If you compare standstill on a motionless treadmill with standstill on the street, you're comparing non-equivalent situations and can expect to see different behavior.

And lastly this: The equivalent to putting the cart on the street in a breeze is not putting the cart vertically down on a rolling treadmill. It's putting the cart on the treadmill while standing still on the rolling treadmill (don't try that at home), in which case the X at the beginning of the run is again (0,windspeed).

Sorry if I did not make it clear. This is for the real environment. (0,0) is the trivial case of a motionless cart in still wind. The other
(0, Windspeed) downwind, is the assumed state when tested on the treadmill.
I say that the latter is conjecture. If not, then there must be states between, that have a non-zero dV. What is sought is a plot of that w.r.t. velocity.

I do understand what the treadmill represents. I considered that you could start from stasis with the cart held against the wind, rather than still wind, but there would still be a need to explain that latter case. If you would to start with dV= windspeed, then please do.

Of course a simple monotonic line would do, but where is the support ?

The (0,0) to (0, windspeed) case will then (presumably) have a maxima.
As you say, the treadmill does start from the other position. The fact that it can get instantaneously to windspeed, is one of its curiosities.
This not trivial, because it is of significant consequence regarding the power transfer.

 

[Michael C]

So how do you explain that there is no appropriate thrust on the treadmill?

The cart is certainly developing thrust on the treadmill. Consider a theoretical "perfect" rolling vehicle with no friction loss in the wheels and no resistance to the air. If you place it on the treadmill, it will stay there indefinitely with its wheels turning. A real cart, with friction losses in the wheels, will lose speed on the treadmill and move towards the back end. If the cart keeps at a constant speed on the treadmill, it means that it is developing thrust that compensates for the losses due to friction.

 

[humber]

Welcome to the discussion John.


Before you read the rest of this thread, how about entering into a bit of a wager? Since JREF probably frowns upon using it's site for gambling, this is what I propose we do...

Let the believers and non-believers put up equal sums of money to back their belief. We'll then enlist the services of the JREF staff to test the device to see if it performs as claimed. The losers pay their wager as a donation to JREF.

[As a bonus, if JREF can be convinced beforehand that the device must be violating the laws of physics in order to perform as claimed and therefore qualifies for the MDC, All parties will split the $1M prize money.]

Interesting, but the problem is always verification. Similar proposals (not wagers) have been put forward for Homeopathy.
Involving unbiased 3rd parties is difficult and expensive. A wind tunnel would be needed, I expect.
There is a claim in one of the videos along those. Not breaking the laws, but not explained by those laws. Something like that.

ETA:
John Freestone,
Also welcome. I generally agree that there is a lot of unnecessary "physics", but that is often the way.
The faster than downwind sailing devices are not relevant, if they exist or not. Some change their shape along the way; sliding components and adjustable sails, and there is also the matter of the information that the pilot adds.

This is a pilotless, propeller driven device, so I do not see the connection.
Fake is probably the wrong word. The treadmill is much as you have described, and so irrelevant.
The wind test could be anything. Without speed monitoring, it makes no claim in my view.

 

[Michael C]

Hey Spork, I think this is the treadmill you need for more serious testing:

180 mph enough for you? It's built by Windshear, who put it in an enormous wind tunnel so that pretty much any road situation can be simulated.

Of course, if Humber's right they must be operating a gigantic scam...

 

[humber]

The cart is certainly developing thrust on the treadmill. Consider a theoretical "perfect" rolling vehicle with no friction loss in the wheels and no resistance to the air. If you place it on the treadmill, it will stay there indefinitely with its wheels turning. A real cart, with friction losses in the wheels, will lose speed on the treadmill and move towards the back end. If the cart keeps at a constant speed on the treadmill, it means that it is developing thrust that compensates for the losses due to friction.

No. It is clearly easily moved. It can be pushed, back and side to side, because the friction is very low.
Also, the torque cancels itself out, so there won't be any.
If you don't think so, then try to see if you can provide torque, while it remains on the belt. There is no opposing load, so it would run off the belt.
No motion = forces in balance. That is also true if the cart is not accelerating from windspeed, but then there would be a significant 'static' thrust.
There is none, for the above reason, and the lack of friction shows that it could not be supported if that were the case.

ETA
Of course, if Humber's right they must be operating a gigantic scam...
Michael_C,

Which would you prefer, mistaken or fraudster?
Note that this does have real wind, which makes them really stupid if you are right, so there would be no point in listening to them, would there? Take your pick.