Pierrome
Well-Known Member
Yes, you could, but it would have to have a large vertical dimension to get sufficient differential.
Downwind faster than the wind. Poll
- Thread starter snowleopard
- Start date
Yes, you could, but it would have to have a large vertical dimension to get sufficient differential.
RAI
Well-Known Member
Maybe Keen Ed was right when he said:
"Sadly, those who can understand the proof don't need it, and those who need it can't understand it. "
If Mark Drela's analysis is wrong, please point us to his error.
H
halfway
Guest
RAI: "If Mark Drela's analysis is wrong, please point us to his error. "
There are two document on the web I have found:
1. DDWFTTW Power Analysis 2 jan 09
2. Dead-Downwind Faster Than The Wind (DDWFTTW) Analysis 1 jan 09
The first is useless as a proof. The second does have the key element in it V/(V-W) so hence why I said he would be good to use as a performance analysis person on a known working system.
However he glosses over the main issue that sceptics first have. The air prop is impossible!
Which it is, in certain circumstances, but nothing to do with frictional losses (assuming your explanation of a perfect aerofoil is down to flows and pressures and not friction)
Interestingly he does have a symbol in the diagram for the rotational speed of the prop and the radius. However the only reference to these in the text is to dismiss them:
"rather than the more conventional prop tip speed"
He then follow it with
"although these details can be worked out later and do not need to be considered at this stage"
Which is where I fall off my seat laughing. To go into so much detail on friction and losses and then not even show that the overall water to rotational gear ratio must be 1:1. Any significant increase does not provide any enhancement, but increases losses, and any decrease ruins the whole concept. It is like saying you can sail to windward at any angle. Actually there is a key set of angles that are appropriate and deviations from this ruin the concept. But it is a "detail" to be sorted out later. Absolutely hilarious. Try telling a student to sail to wind ward and not give them a clue as to how sailing at 10 degrees to the wind just will not work.
All the way through he is so concentrated on friction and performance he has missed HOW it works.
So the people who read these proofs just assume you need a NORMAL prop. Anyone will do it seem. As long as it works like his equations. They put very little effort into the selection and end up with something that looks like the front of a rubber band model plane or a old Sopwith camel with the blade ends sawn off. The guy probably spent hours sanding that thing. What a waste.
However, if the proof showed how the prop worked and its limitations they would put more effort and design into the prop, and air assembly, and not work on the weight of the vehicle so much. It is like polishing the bottom of a day boat expecting it to become as efficient as a racing boat.
So at best this is an analysis of the frictional losses in the system. It fall massively short on being a proof of a concept.
One final example, I could run a water generator and power a computer while sailing down wind faster than the wind. The energy I steal from the system does not invalidate the concept of going faster than the wind. So the concept of faster than wind can not be proven by any relation to friction, which is just the real world stealing energy.
I rest my case.
There are two document on the web I have found:
1. DDWFTTW Power Analysis 2 jan 09
2. Dead-Downwind Faster Than The Wind (DDWFTTW) Analysis 1 jan 09
The first is useless as a proof. The second does have the key element in it V/(V-W) so hence why I said he would be good to use as a performance analysis person on a known working system.
However he glosses over the main issue that sceptics first have. The air prop is impossible!
Which it is, in certain circumstances, but nothing to do with frictional losses (assuming your explanation of a perfect aerofoil is down to flows and pressures and not friction)
Interestingly he does have a symbol in the diagram for the rotational speed of the prop and the radius. However the only reference to these in the text is to dismiss them:
"rather than the more conventional prop tip speed"
He then follow it with
"although these details can be worked out later and do not need to be considered at this stage"
Which is where I fall off my seat laughing. To go into so much detail on friction and losses and then not even show that the overall water to rotational gear ratio must be 1:1. Any significant increase does not provide any enhancement, but increases losses, and any decrease ruins the whole concept. It is like saying you can sail to windward at any angle. Actually there is a key set of angles that are appropriate and deviations from this ruin the concept. But it is a "detail" to be sorted out later. Absolutely hilarious. Try telling a student to sail to wind ward and not give them a clue as to how sailing at 10 degrees to the wind just will not work.
All the way through he is so concentrated on friction and performance he has missed HOW it works.
So the people who read these proofs just assume you need a NORMAL prop. Anyone will do it seem. As long as it works like his equations. They put very little effort into the selection and end up with something that looks like the front of a rubber band model plane or a old Sopwith camel with the blade ends sawn off. The guy probably spent hours sanding that thing. What a waste.
However, if the proof showed how the prop worked and its limitations they would put more effort and design into the prop, and air assembly, and not work on the weight of the vehicle so much. It is like polishing the bottom of a day boat expecting it to become as efficient as a racing boat.
So at best this is an analysis of the frictional losses in the system. It fall massively short on being a proof of a concept.
One final example, I could run a water generator and power a computer while sailing down wind faster than the wind. The energy I steal from the system does not invalidate the concept of going faster than the wind. So the concept of faster than wind can not be proven by any relation to friction, which is just the real world stealing energy.
I rest my case.
RAI
Well-Known Member
But would you accept any theoretical proof of this concept? You reject his analysis for both the water and wheeled vehicles and offer only generalisations and accusations in return.
The guys building that new machine have probably used a similar analysis to scale it and clearly think it is worth making one to test. Just because they are mostly students, doesn't make them stupid.
A more scientific approach would be to provide a theoretical proof why the concept will not work. After all, it is usually easier to disprove a theory. Mark's analysis might be a good basis to start, you only have to prove that the coefficients that he identified all have values that prevent V>W.
As for your criticisms of his simplifying assumptions, I think you need to show that these are not conservative assumptions before claiming that they are inadequate in detail and then therefore deduce that the whole thing will not work. See
http://www.mh-aerotools.de/airfoils/propuls3.htm
For example
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H
halfway
Guest
RAI "therefore deduce that the whole thing will not work"
I AM A BELIEVER IN THE CONCEPT. It is the proof that is the major problem to its acceptance. How can you not see that I was arguing FOR the concept by statements like "ruins the whole concept".
Can we get it straight: There are three camps here at present.
1. Those FOR the concept and holding tight to the friction, efficiency, energy transfer proof.
2. Those AGAINST the concept as they can not see a proof worth having.
3. Me. That is FOR the concept, but keeps falling of my chair in laughter at the proofs given by these self proclaimed experts.
This is so like the forums banned subject. Everyone knows the world is being destroyed. Someone comes up with a so called proof and all the masses cling to the theory as a testament of the downfall.
Then, using the incorrect theory, they endeavour to solve the issue and concentrate on only those bits that occur in the theory.
The outcome is the same. A total waste of time, or worse, they make the problem worse or fail to demonstrate the concept.
But in the end it is just a group of people clinging to a security blanket.
Concepts can be incorrectly proven by incorrect theories, as much as, incorrect theories seem to disprove perfectly correct concepts. Opponents and proponents are equally susceptible to being duped.
This concept is easily shown with a simple apparent wind diagram on a short blade, but large diameter, turbine fan type prop. Using a 1:1 rotation ratio from the ground velocity to the blade velocity. The proof shows the limitation, for a given foil, to the speed multiplying factor. You do not need any equations for a foil, just one example foil characterization data from the real world. Just a simple example point off a real attack angle, lift angle graph. The whole proof does not talk about energy or efficiency it just shows that at a certain point (faster or equal to the wind speed) that there is a net forward force. What you do with this force is up to you. You can make electricity and boil a kettle to make tea and warm the environment like friction does, or you can use the force to accelerate faster. I would choose the later, being a speed merchant, and not liking tea.
All you have to prove, is a net forward force. Just like if you tried to teach someone how to sail to windward. No one goes into how much friction is on the hull, they just show that the apparent wind can be used to make forward motion and the position and direction limitations of the sail and boat keel. That is all that is necessary for the proof. It has no equations other than simple vector angles.
I AM A BELIEVER IN THE CONCEPT. It is the proof that is the major problem to its acceptance. How can you not see that I was arguing FOR the concept by statements like "ruins the whole concept".
Can we get it straight: There are three camps here at present.
1. Those FOR the concept and holding tight to the friction, efficiency, energy transfer proof.
2. Those AGAINST the concept as they can not see a proof worth having.
3. Me. That is FOR the concept, but keeps falling of my chair in laughter at the proofs given by these self proclaimed experts.
This is so like the forums banned subject. Everyone knows the world is being destroyed. Someone comes up with a so called proof and all the masses cling to the theory as a testament of the downfall.
Then, using the incorrect theory, they endeavour to solve the issue and concentrate on only those bits that occur in the theory.
The outcome is the same. A total waste of time, or worse, they make the problem worse or fail to demonstrate the concept.
But in the end it is just a group of people clinging to a security blanket.
Concepts can be incorrectly proven by incorrect theories, as much as, incorrect theories seem to disprove perfectly correct concepts. Opponents and proponents are equally susceptible to being duped.
This concept is easily shown with a simple apparent wind diagram on a short blade, but large diameter, turbine fan type prop. Using a 1:1 rotation ratio from the ground velocity to the blade velocity. The proof shows the limitation, for a given foil, to the speed multiplying factor. You do not need any equations for a foil, just one example foil characterization data from the real world. Just a simple example point off a real attack angle, lift angle graph. The whole proof does not talk about energy or efficiency it just shows that at a certain point (faster or equal to the wind speed) that there is a net forward force. What you do with this force is up to you. You can make electricity and boil a kettle to make tea and warm the environment like friction does, or you can use the force to accelerate faster. I would choose the later, being a speed merchant, and not liking tea.
All you have to prove, is a net forward force. Just like if you tried to teach someone how to sail to windward. No one goes into how much friction is on the hull, they just show that the apparent wind can be used to make forward motion and the position and direction limitations of the sail and boat keel. That is all that is necessary for the proof. It has no equations other than simple vector angles.
mikemonty
Well-Known Member
You held your corner a lot longer than I held mine, and for what it's worth, I CAN actually see the theory - in fact I think I can even give an analogy which works well.
Take three or four Oracles, cut off the keels along the keelbolts and the sails at the deck.
Put the sail rigs radially on one wheel and the keels on another - each at the same attitude to the wind/water that they would have when going downwind at greater than windspeed.
Now join the two wheels with a shaft arranged so that the sails stay out the water and the keels stay in. Set the whole thing running.
The sails would sail as if on a whole boat and the keels would er, keel as if they were on the boat, and the resultant "across the wind" velocity of a normal boat is eliminated.
It's as if each individual sail/keel is gybing continuously
The whole lot SHOULD (not taking anything else into account), work, going straight downwind in line with the driveshaft... maybe.
That's OK as far as it goes but I doubt that the real world will allow one to get the whole thing going faster than the wind because of the tiny margins on which the original boat is sailing will be insufficient to power the machine.
Or else we would all be running about in one.
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PaulJS
Well-Known Member
Conservation of energy
It seems to me to be impossible to move directly down wind faster than the wind behind you.
In the case of the cart on the treadmill extra energy is being imparted to the system by the treadmill motor - the only way this could work without a treadmill would be if you were on a downwind facing decline so that gravity could impart energy to the carts wheels.
In the case of a totally efficient boat sailing downwind in a 5 knot wind against a 5 knot tide, I don't believe it can even if it tacks because although the relative difference between the waters velocity and the wind's is 10 knots and the totally efficient boat will be able to easily exceed 10 knots at an angle to the wind and tide it's net upstream velocity will be less than 5 knots, there is not sufficient energy being imparted to the boat to allow it to move upstream.
Sorry to all the believers, but you cannot make energy out of nothing, nor can a systems efficiency ever, even in the theoretical world exceed 100%.
Good brain teaser though!
It seems to me to be impossible to move directly down wind faster than the wind behind you.
In the case of the cart on the treadmill extra energy is being imparted to the system by the treadmill motor - the only way this could work without a treadmill would be if you were on a downwind facing decline so that gravity could impart energy to the carts wheels.
In the case of a totally efficient boat sailing downwind in a 5 knot wind against a 5 knot tide, I don't believe it can even if it tacks because although the relative difference between the waters velocity and the wind's is 10 knots and the totally efficient boat will be able to easily exceed 10 knots at an angle to the wind and tide it's net upstream velocity will be less than 5 knots, there is not sufficient energy being imparted to the boat to allow it to move upstream.
Sorry to all the believers, but you cannot make energy out of nothing, nor can a systems efficiency ever, even in the theoretical world exceed 100%.
Good brain teaser though!
RAI
Well-Known Member
Then you need to explain how BMW Oracle in the America's cup race covered 20 nm downwind in a little over an hour with only 6-8 knots of wind behind it. 18 knots down wind in 6 knots of wind, is that possible?
PaulJS
Well-Known Member
Sorry, bad wording and worse thinking in my last post, of course a boat can sail upstream in a 5 knot tide with a 5 knot wind. It's simply a case of having more resistance to the wind offered by the sails than resistance to water flow by the hull. A bit of a "Doh" moment.
However as to the above, I just cannot believe a boat can sail directly downwind at faster than the speed of the wind behind it. Maybe the report was from a wind speed indicator onboard the Oracle, in which case true wind speed would have been 24 knots; I could easily believe that a racing machine can maintain 2/3 or more of the speed of a following wind...
RAI
Well-Known Member
BMW Oracle and Alinghi tack down wind, close hauled the whole way. They don't pop up a spinnaker and run dead down wind. Were they do do so, you would be right, no faster than the wind. But they don't run before the wind.
The commentators call it "bending the wind". Have a look at the videos on BMW Oracles' web site. Average speed on the down wind leg is 18 knots VMG down wind and a boat speed of around 25 knots. All done in 6-8 knots of wind.
It's a bit mind boggling but they, and other fast sailing multi-hulls, do it. If they had released smoke or balloons at the windward mark, the balloons/smoke would take three hours to reach the finish line. Those yachts did it in just over one hour.
mono
Well-Known Member
I don't see this is relevant - they were not sailing directly downwind. A reach is always faster than a dead run and, depending on boat type and conditions, it's perfectly possible to sail faster - a lot faster - than the wind. The issue here is whether a cart/boat can be made to run downwind at a greater speed than the wind itself.
This concept is not logical because as soon as a cart/boat/whatever - when running directly before the wind - exceeds the speed of the wind, it is no longer moving downwind - it is moving upwind.
I think the reply by HRChivers was correct. The cart was placed on the treadmill and held there until it was running at the same speed as the treadmill. The wheels turned the prop at a high enough speed to propel the cart once it was released.
That was nothing to do with running faster than the wind and everything to do with the gear that connected the prop to the wheels. The cart only had to move about one foot before it was stopped by the operator - depending on its gear ratio there could have been plenty of surplus energy in the prop to carry it this distance.
RAI
Well-Known Member
Hey Ho! Here we go again. The major bit of aerodramics on this cart is its propeller. Just as on BMW Oracle it is the wing main sail. The propeller is "close hauled", so is BMW Oracle's wing, when "tacking" down wind. The yacht makes 3 times wind speed with its down wind VMG. How much might the propeller do?
mono
Well-Known Member
False analogy. The boat is moving across the wind...the wind applies force to the sail. The maximum speed of the boat is determined by more factors than the mere speed of the wind - sail area and hull design for example.
However, when going directly downwind, the sole factor in assessing the maximum speed that the cart can travel at is the speed of the wind. The aerodynamics of the prop are not relevant - though they might be if the cart were moving across the wind.
RAI
Well-Known Member
On the contrary, the propeller blades are moving across the wind when the cart is travelling dead down wind.
mono
Well-Known Member
On the contrary, if the wind is blowing at 10k and the boat is travelling at 10k there is NO wind.
Pierrome
Well-Known Member
RAI I'm on your side but I would like to point out your slight error. I have pointed this out before.
The propeller on the downwind cart is not analagous with the wing on BMW Oracle because on BMW Oracle the wing is the COLLECTOR of energy from the wind and the boat is "driven" by it's keel, and on the cart the propeller is the driving force through the air and the wheels are the COLLECTORS or energy from the road.
Pierrome
Well-Known Member
It's doesn't look here as if you have fully understood the point of the experiment. Of course the treadmill motor is imparting energy, that's what it's for. You maybe do not understand that in the experiment the air was not moving - there is no wind, and if the treadmill was not imparting energy nothing would happen - there would be no experiment!
Pierrome
Well-Known Member
This argument has been advanced before - in this video they show that this is not the case. http://www.youtube.com/watch?v=1pSYALWQ-nI
wingsailor
New Member
There will always be disbelievers?
Hi
Many posters have claimed their qualifications to prove whether or not they know better than anyone else whether downwind faster than the wind is possible.
I'm open to correction, but as far as I know I am probably the only person on the forum who has built and tested an experimental a direct-drive windmill powered boat that can sail directly against the wind (you can see it here http://www.youtube.com/watch?v=NNbNNSDljGI ) which is a similar thing but the "other way round" - If you look at the youtube video comments you can see there are still a lot of people who do not believe it. However, it is easily done.
From my experience and knowledge of these matters I can categorically state that sailing directly downwind faster than the wind is possible and by all accounts it will be proven further in the next few weeks (although it is proven already really!)
Hi
Many posters have claimed their qualifications to prove whether or not they know better than anyone else whether downwind faster than the wind is possible.
I'm open to correction, but as far as I know I am probably the only person on the forum who has built and tested an experimental a direct-drive windmill powered boat that can sail directly against the wind (you can see it here http://www.youtube.com/watch?v=NNbNNSDljGI ) which is a similar thing but the "other way round" - If you look at the youtube video comments you can see there are still a lot of people who do not believe it. However, it is easily done.
From my experience and knowledge of these matters I can categorically state that sailing directly downwind faster than the wind is possible and by all accounts it will be proven further in the next few weeks (although it is proven already really!)
mono
Well-Known Member
I've watched the youtube video and I don't see that it proves the point at all. The cart is being prevented from running out of 'steam' by the operator. The demonstration of how long the prop will carry on turning when lifted off the treadmill is yet another red herring because the wheels are no longer in contact with the treadmill, thereby losing their driving force - which in turn means that the prop also loses its driving force.
All the time the cart is on the treadmill and - more importantly, being held there - it is going to carry on developing power through the wheels to drive the propellor at sufficient speed to make the cart travel backwards.
