Ubergeekian
Well-Known Member
Relative to the cart, the road has plenty of kinetic energy.
Downwind faster than the wind. Poll
- Thread starter snowleopard
- Start date
Relative to the cart, the road has plenty of kinetic energy.
Ubergeekian
Well-Known Member
There is no fundamental reason why not, as long as you thing of a suitable mechanism. For example, you might use a fan to blow back against the wind. The net result, after the boat has bassed, is that some of the air has been blown back and slowed down, thereby losing kinetc energy (relative to the ground).
Yes you can, if there is a tide to move the boat and fill the sails. Once again, it's the difference in wind and water velocities which does the trick, not the absolute value of either. As you yourself have said:
Ubergeekian
Well-Known Member
We don't ignore it. We take account of it, very simply and easily. Let's go over it one more time, shall we?
- At the wheels, reaction work (force x velocity) is turned into shaft work - less a bit from frictional losses.
- At the fan, shaft work is turned into reaction work - less a bit from frictional losses.
- If the air was stationary relative to the ground the absolute best you could do would be to produce the same thrust forward at the prop as you had drag backwards at the wheel.
- You can achieve this - quite apart from friction, no turbine can run at 100% efficiency for well established reasons. If you want the gory detail, you need to learn Kelvin-Froude actuator disk theory - I wouldn;t recommend this unless you already have a fairly solid backkground in theoretical fluid dynamics
- So if there is no relative movement between air and road, everybody agrees that any possible arrangement of gearing, drive and fan will act only as a brake.
- However, if the wind is travelling faster than the road in the direction of cart travel things change. First and foremost, you can now get more reaction work from the fan than you put in as shaft work. That doesn't violate conservation of energy: it shows that the wind still does work on a fan blowing back into it, even if the fan itself is moving with the bulk windspeed.
- Now you have two competing effects: losses in the drive train versus extra power from the wind blowing a turning fan. The $64,000 question is whether the latter can cancel out the former.
So trying to implement this is an engineering problem, not a physics one. Whether it can be done will depend on many factors: wind speed, payload, scale and so on. However it is fundamentally just another way of slowing down some of the wind and using the energy released to overcome the frictional losses involved in moving something.
Pierrome
Well-Known Member
It maybe a question for this argument. But in the outside world it is proven.
1. It has been done many times.
2. It is being done all the time.
3. As much evidence as anyone would need is available for observing it is being done.
4. It's easy to do.
5. A simple demonstration can be done.
6. Anyone can build a simple demonstrator and instructions are free online.
7. Another bigger demonstration will be held shortly.
After all this, it's hardly surprising that non-believers are regarded as flat-earthers.
To be fair, I guess most unbelievers are new to this. They need to think deeper and study thoroughly before making their comments!
Danny Jo
Well-Known Member
Judging by my "Have you changed your mind" poll, these well-argued posts have failed to influence opinion, which is evidently firmly held on both sides. But the argument has convinced me that it is possible.
What cracked it for me were the analogies, on the one hand with attitude of a sail on a boat tacking downwind and the movement of the propellor blade relative to the wind, and on the other hand with the resistance of the keel against the tendency to be blown directly downwind and the resistance provided by the wheels on the cart facing directly downwind.
I haven't read the whole thread, but has anyone considered kitesurfers? A simple conceptual model for sailing directly downwind without a propellor is provided by a light boat whose sail consists of a kite, with the kite tacking repeatedly to and fro, downwind, but the boat being steered so as to provide a direct downwind course. The kite could even be flown just above the surface, with keel in the water (one that looks like BMW Oracle will do).
What cracked it for me were the analogies, on the one hand with attitude of a sail on a boat tacking downwind and the movement of the propellor blade relative to the wind, and on the other hand with the resistance of the keel against the tendency to be blown directly downwind and the resistance provided by the wheels on the cart facing directly downwind.
I haven't read the whole thread, but has anyone considered kitesurfers? A simple conceptual model for sailing directly downwind without a propellor is provided by a light boat whose sail consists of a kite, with the kite tacking repeatedly to and fro, downwind, but the boat being steered so as to provide a direct downwind course. The kite could even be flown just above the surface, with keel in the water (one that looks like BMW Oracle will do).
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snowleopard
Well-Known Member
Quite so. I am amazed at the logic that accepts that a reciprocating motion (sail boat/land yacht tacking) can produce DDWFTTW motion but considers a rotary mechanism breaks the 'laws of physics'.
snowleopard
Well-Known Member
Well, call me a fanatic or whatever but I'll have one more go at explaining how it can work.
In the diagram the black curve is the prop blade rotating at velocity R.
V is the speed of the vehicle and W the wind. That means the vehicle sees an apparent headwind of V-W. The blade, turned by the wheels, is travelling across the direction of motion so resolving the headwind V-W and the motion of the blade R, the apparent wind hitting the blade is A which passes over the blade producing the resultant forward force F.
So that's where the propulsion comes from to drive the vehicle into the apparent headwind.
In the diagram the black curve is the prop blade rotating at velocity R.
V is the speed of the vehicle and W the wind. That means the vehicle sees an apparent headwind of V-W. The blade, turned by the wheels, is travelling across the direction of motion so resolving the headwind V-W and the motion of the blade R, the apparent wind hitting the blade is A which passes over the blade producing the resultant forward force F.
So that's where the propulsion comes from to drive the vehicle into the apparent headwind.
DJE
Well-Known Member
nmeyrick
Well-Known Member
Before I can add any more to the discussion I would like to make a public apology. I did get rather enthusiastic about this whole thing - and from reading widely on the subject on the web I'm certainly not the only one. I made a post which read humorously in my head, but on re-reading now comes across as smug and is not one I am proud of.
I also cast aspertions on the motives of Snowleopard and Ubergeekian, which I believe that their subsequent responses have shown to be totally unfounded. I did say that I meant no offence to anybody, but in re-reading my message and the subsequent replies it is clear it was seen as offensive. For that I can only apologise unreservedly - I'm sorry guys.
I think in hindsight the reason I got so excited was because of the treadmill test more than the theory in itself. My concern with this is not because of anything to do with inertial frames of reference, but because of flaws in the design of the experiment.
In short the video could show that the vehicle is operating as claimed, but I can think of at least two other ways you could get the same result in absolute good faith.
In particular the vehicle could be sapping energy from the belt, speeding up at the cost of slowing down the belt:
1. Belt is moving from right to left at 10mph. Vehicle is moving from left to right at 10mph over the belt.* Relative to the belt the vehicle has a velocity of 10+10 = 20mph. Relative to the frame of the treadmill the velocity of the cart is 10-10 = 0 mph, and to an outside observer it is stationary in the middle of the belt moving neither to left nor right.
2. Vehicle draws energy from the belt, slowing down the movement of the belt relative to the frame. The belt is now moving from right to left at only 8mph, and the vehicle has increased in speed to 11mph.* The actual numbers aren't too important only the direction of change, and I'm only trying to address the way a vehicle moves on a treadmill so it works just as well with a battery powered car if that makes things easier.
Looking at the numbers again, the velocity of the vehicle relative to the belt is now only 8+11 = 19mph, in other words it is moving more slowly relative to its road. However relative to the frame it is moving at 11-8 = 3mph, and to an outside observer it moves from left to right relative to the frame, appearing to outrun the belt even thought its velocity over the belt has actually slowed down.
The problem with the experiment is that as it stands, you couldn't tell whether it is behaving as I describe, or is indeed doing what it should and so it provides no evidence that it is doing either one or the other.
* I know, you can't really describe the velocity of the vehicle without reference to the belt. For the sake of argument I mean the speed at which it would be travelling if you suddenly stopped the belt. If it helps imagine that the vehicle is battery powered and the controls are set so that the rpm of the wheels is fixed. If it were put on a stationary belt this is the speed it would move at.
I wonder whether this changes anyone's mind? For myself I'm now firmly in the camp of maybe not impossible but certainly not yet proven, so I can only look forward to the results of the full size tests.
I also cast aspertions on the motives of Snowleopard and Ubergeekian, which I believe that their subsequent responses have shown to be totally unfounded. I did say that I meant no offence to anybody, but in re-reading my message and the subsequent replies it is clear it was seen as offensive. For that I can only apologise unreservedly - I'm sorry guys.
I think in hindsight the reason I got so excited was because of the treadmill test more than the theory in itself. My concern with this is not because of anything to do with inertial frames of reference, but because of flaws in the design of the experiment.
In short the video could show that the vehicle is operating as claimed, but I can think of at least two other ways you could get the same result in absolute good faith.
In particular the vehicle could be sapping energy from the belt, speeding up at the cost of slowing down the belt:
1. Belt is moving from right to left at 10mph. Vehicle is moving from left to right at 10mph over the belt.* Relative to the belt the vehicle has a velocity of 10+10 = 20mph. Relative to the frame of the treadmill the velocity of the cart is 10-10 = 0 mph, and to an outside observer it is stationary in the middle of the belt moving neither to left nor right.
2. Vehicle draws energy from the belt, slowing down the movement of the belt relative to the frame. The belt is now moving from right to left at only 8mph, and the vehicle has increased in speed to 11mph.* The actual numbers aren't too important only the direction of change, and I'm only trying to address the way a vehicle moves on a treadmill so it works just as well with a battery powered car if that makes things easier.
Looking at the numbers again, the velocity of the vehicle relative to the belt is now only 8+11 = 19mph, in other words it is moving more slowly relative to its road. However relative to the frame it is moving at 11-8 = 3mph, and to an outside observer it moves from left to right relative to the frame, appearing to outrun the belt even thought its velocity over the belt has actually slowed down.
The problem with the experiment is that as it stands, you couldn't tell whether it is behaving as I describe, or is indeed doing what it should and so it provides no evidence that it is doing either one or the other.
* I know, you can't really describe the velocity of the vehicle without reference to the belt. For the sake of argument I mean the speed at which it would be travelling if you suddenly stopped the belt. If it helps imagine that the vehicle is battery powered and the controls are set so that the rpm of the wheels is fixed. If it were put on a stationary belt this is the speed it would move at.
I wonder whether this changes anyone's mind? For myself I'm now firmly in the camp of maybe not impossible but certainly not yet proven, so I can only look forward to the results of the full size tests.
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Ubergeekian
Well-Known Member
Handsomely spoken, and thank you very much. But please don't worry about it - I'm all in favour of passionate arguments about science, and I am very grateful to you for making me think hard about this.
I look forward to some full scale tests too. I can't say I hold out great hope for the American attempt - it looks awful rickety / Heath Robinson, and I have a gut feeling (how unscientific!) that the energy balance will be so close at that scale that it will take really beautiful engineering for a result.
mono
Well-Known Member
Anyone who believes that 'downwind faster than the wind' is impossible might be interested in this thread on the Advanced Physics forum - http://www.advancedphysics.org/forum/showthread.php?t=10550&page=1 - this was enough to convince me that it is possible - in theory at least.
What I was failing to grasp was that the cart can be powered by two forces simultaneously - the force of the wind and the force supplied by the motion of the cart relative to the ground. Once this concept is understood, it all begins to make sense.
I am no longer a dyspeptic sceptic - though I would like to see a properly and independently monitored practical demonstration.
What I was failing to grasp was that the cart can be powered by two forces simultaneously - the force of the wind and the force supplied by the motion of the cart relative to the ground. Once this concept is understood, it all begins to make sense.
I am no longer a dyspeptic sceptic - though I would like to see a properly and independently monitored practical demonstration.
fireball
Well-Known Member
Eh? I think you're over thinking this ...
The belt is moving from Right to Left at 10mph. The vehicle appears to be stationary in the middle of the belt neither moving to the left or the right. So therefore the vehicle is moving at 10mph relative to the belt - can't be more or less.
Blimey - I only did A-Level physics and I know this!!
H
halfway
Guest
It seems, in my absence, that the explanation in sailing terms and forces is getting the upper hand, over energy and power equations. Quite rightly, since this is a sailing forum. The diagram above is wonderful Snowleapod.
However, for those that still think that it has been demonstrated; try these observations about the applicability of the treadmill videos:
----
For this we assume the treadmill is going 10kts, and it is meant to represent the wind and the cart going forward at the same velocity on a stationary road.
A: Assume the "Blow" off the back of the cart is 20kts in the prop column.
So re-reference this to a stationary ground. That means the wind behind the cart is going the completely opposite direction to the wind it was using to make the motion.
So the wind has not only be slowed to nothing it has been completely reversed by using its own energy! OK, better than perpetual energy. Like a pelton wheel rotating faster than the water jet speed.
IMPLICATION: If you sail up behind Oracle, then you need to start tacking instead of gybing, and sail INTO the wind to catch up to her.
Whoops!
B: Assume the "Blow" of the back of the cart is 10kts in the prop column.
This is the case where you have removed ALL the energy from the supplied wind. However in this case on the treadmill the wind would have to circulate back to the front of the cart. Since we are in a room, it has to circulate, or the guy in front (poking the cart) is going to die of a lack of air. The circulation will mean air will travel forward around the prop.
If this is re-referenced to a stationary road this means the air to either side of the cart is going OVER 10kts! (And the Air behind is going 0 kts)
IMPLICATION: If you have a cart going faster than the wind, then the cart overtaking it will experience faster wind from behind! Which will mean overtaking would be easy! But then they would both be helping one another! They will therefore need brakes, to stop them hitting MACH 1.
Whoops!
C: So the wind must be slower off the prop than the treadmill speed.
So look now at the wind in front of the prop. That is being effected some distance ahead of the cart. It must be, since, it is just a toy plane prop with a treadmill motor driving it. Especially when the cart is held. It will create a "suck" effect.
Assume say a 5kt velocity backwards in front of the prop on the treadmill.
If this is re-referenced to a stationary road it is a 5kt forward wind. But yet the wind that is going forwards is POSITIONED at least a cart length in front of the cart!
Are you saying that a boat in front of Oracle, going faster than the wind, will have their wind altered? As well as the boat behind! Since the flow in the tread mill example must go through the prop and have approximately equal speed both sides.
This would mean that once past Oracle you would see a wind hole and stay at equal positions. So both in front and behind you would not experience the same wind as Oracle and in both cases be starved of wind. However, you would also starve Oracle of their wind in both cases. So the whole thing would grind to a halt.
Whoops!
I do not know if all these "whoops" are totally valid (Some are implications of others) but it does shine a little light of doubt on the assumption that the treadmill matches reality. It is also fun, and something to get your head around. As for more points on my record because of late night posting, can I admit guilt now, and save court time.
Off back to the real world.
However, for those that still think that it has been demonstrated; try these observations about the applicability of the treadmill videos:
----
For this we assume the treadmill is going 10kts, and it is meant to represent the wind and the cart going forward at the same velocity on a stationary road.
A: Assume the "Blow" off the back of the cart is 20kts in the prop column.
So re-reference this to a stationary ground. That means the wind behind the cart is going the completely opposite direction to the wind it was using to make the motion.
So the wind has not only be slowed to nothing it has been completely reversed by using its own energy! OK, better than perpetual energy. Like a pelton wheel rotating faster than the water jet speed.
IMPLICATION: If you sail up behind Oracle, then you need to start tacking instead of gybing, and sail INTO the wind to catch up to her.
Whoops!
B: Assume the "Blow" of the back of the cart is 10kts in the prop column.
This is the case where you have removed ALL the energy from the supplied wind. However in this case on the treadmill the wind would have to circulate back to the front of the cart. Since we are in a room, it has to circulate, or the guy in front (poking the cart) is going to die of a lack of air. The circulation will mean air will travel forward around the prop.
If this is re-referenced to a stationary road this means the air to either side of the cart is going OVER 10kts! (And the Air behind is going 0 kts)
IMPLICATION: If you have a cart going faster than the wind, then the cart overtaking it will experience faster wind from behind! Which will mean overtaking would be easy! But then they would both be helping one another! They will therefore need brakes, to stop them hitting MACH 1.
Whoops!
C: So the wind must be slower off the prop than the treadmill speed.
So look now at the wind in front of the prop. That is being effected some distance ahead of the cart. It must be, since, it is just a toy plane prop with a treadmill motor driving it. Especially when the cart is held. It will create a "suck" effect.
Assume say a 5kt velocity backwards in front of the prop on the treadmill.
If this is re-referenced to a stationary road it is a 5kt forward wind. But yet the wind that is going forwards is POSITIONED at least a cart length in front of the cart!
Are you saying that a boat in front of Oracle, going faster than the wind, will have their wind altered? As well as the boat behind! Since the flow in the tread mill example must go through the prop and have approximately equal speed both sides.
This would mean that once past Oracle you would see a wind hole and stay at equal positions. So both in front and behind you would not experience the same wind as Oracle and in both cases be starved of wind. However, you would also starve Oracle of their wind in both cases. So the whole thing would grind to a halt.
Whoops!
I do not know if all these "whoops" are totally valid (Some are implications of others) but it does shine a little light of doubt on the assumption that the treadmill matches reality. It is also fun, and something to get your head around. As for more points on my record because of late night posting, can I admit guilt now, and save court time.
Off back to the real world.
Ubergeekian
Well-Known Member
This is why you need Kelvin-Froude actuator disk theory. As you have demonstrated, simplistic models don't cut it ... and can't be used to prove or disprove anything.
RAI
Well-Known Member
Not very surprising, since BMW Oracle is close hauled herself. Maybe that explains why they call it tacking down wind instead of gybing down wind.
fireball
Well-Known Member
Have you never sailed close to another sailing boat?
I suggest you go racing ..
nmeyrick
Well-Known Member
Fair point well made - I think its the head twisting involved in looking at everything from different perspedctives that makes this so infuriatingly interesting. I'll have a think and see if I can restate my maths more correctly, unless someone else wants to have a go.
I still think my point is valid; the car will stay in the middle of the treadmill however fast the treadmill is moving, as long as it is travelling at the same speed.
Therefore the car moving from left to right could be caused by the car speeding up, the belt slowing down, or a combination of the two, and as you can't tell which the experiment as it stands doesn't prove anything.
As Ubergeekian says the man carrying prototype does look rather Heath Robinson so may be a struggle to get working - and of course if it doesn't work that won't prove that it can't be done, only that they haven't succeeded.
In that case you could improve the treadmill test to make it more reliable, simply by accurately measuring the speed of the belt so you can demonstrate that it doesn't drop therefore the car moving forward must mean it has speeded up relative to the belt. Of course you would also need to accurately measure the input power of the motor to ensure that it doesn't start pulling more juice to keep up the speed, as this wouldn't model the real world.
Perhaps if the US team don't get the goods we should club together and get a YBM cart from Mr Spork - or maybe I should just get out more!
Thanks for accepting my apology in good grace. I agree, I'm all for passionate arguments and have really had to think about this. It has also been a sharp reminder to me that when passions raise the internet loses a lot of nuances that would be there face to face and makes it much easier to cause offence without meaning to.
fireball
Well-Known Member
Your welcome to have a go ... but the if the vehicle is stationary to fixed objects in the room then it is going the same speed as the treadmill belt ... FACT ... how it gets that speed is a completely different matter
RAI
Well-Known Member
Have you looked at all of spork's videos on youtube? There is one where they reduced the treadmill speed to get the cart to stay fairly stationary on the belt, another where the had the belt at max speed for the treadmill, and the cart was determined to travel faster forward than the belt was carrying it back. Yet another has the treadmill inclined, so that the cart has to climb as well as keep up. It succeeds.
My reason for being uncertain whether the treadmill test is an accurate representation is that the propeller is working in the static or hover mode, which is a special case in propeller thrust analysis - See Mark Drela's paper on DDWFTTW. However, it's a case applicable to the free running cart when it reaches wind speed, so it has to be passed through to exceed the wind speed down wind.
boomerangben
Well-Known Member
I voted that it doesn't prove what it claims:
DDWFTTW is misleading.
What this cart shows is that a wheel can extract enough energy from the tyre/ground interface to provide a propeller mechanism with enough power to generate thrust equal to or greater than the friction on the tyres.
For those who think energy:
Treadmill example:
Chemical energy in coal > heat energy in steam > kinetic energy in turbine rotor > kinetic energy in alternator > electical energy to 13 amp plug > kinetic energy in treadmill motor > kinetic energy in belt > kinetic energy into cart's wheel > kinetic energy in propeller > kinetic energy in air moved by propeller > blows butterfly off flower on other side of the world.
Road example:
Kinetic energy of machine required to get cart up to speed > kinetic energy in cart > kinetic energy in cart's wheel etc
In terms of forces and moments:
When cart travels at exactly the same speed as the wind, ie no relative wind on the cart forces acting on the cart are:
Friction at the tyre ground interface
Thrust imparted on cart by propeller
Gravity.
Edit Reaction of ground against gravity (to stop the cart falling through the treadmill/road
Resolving horizontally:
friction from tyres = horizontal component of rotor thrust
Resolving vertically
gravity and vertical component of thrust = reaction at tyre contact
Friction from tyres causes a couple (monent or torque) in the axle.
That torque is transmitted through the linkages and gears to propeller.
Now this is where the wind plays it's only roll. The thrust generated by a given static propeller ie one that is rotating in still air is proportional to the angle of attack. Angle of attack is the angle between the cord of the aerofoil section (cross section of propeller blade) and the relative airflow. A static propeller induces air through the propeller (that's what we feel from a desk top fan). The air that a blade feels as it rotates is not in the plane of the rotor disc, it is angled slightly. That is the relative airflow. What is demonstrated here is that the propeller is rotating such that the relative airflow generates sufficient thrust to overcome the friction at the tyre/ground interface which would otherwise cause the cart to slow or fall off the back of the treadmill.
To go faster than the wind, more air will flow through the propeller, causing the angle of attack to reduce and conversely if the cart moves slower than the wind, the angle of attack will increase. Now change the propeller for one with a greater overall pitch angle and the propeller will generate more thrust from the same speed and will tend to accelerate the cart in the direction it is travelling. But there will be a narrow band of speeds that a given propeller would be able to demonstrate this since movement of the cart relative to the air changes the relative airflow and therefore thrust from the propeller.
DDWFTTW is misleading.
What this cart shows is that a wheel can extract enough energy from the tyre/ground interface to provide a propeller mechanism with enough power to generate thrust equal to or greater than the friction on the tyres.
For those who think energy:
Treadmill example:
Chemical energy in coal > heat energy in steam > kinetic energy in turbine rotor > kinetic energy in alternator > electical energy to 13 amp plug > kinetic energy in treadmill motor > kinetic energy in belt > kinetic energy into cart's wheel > kinetic energy in propeller > kinetic energy in air moved by propeller > blows butterfly off flower on other side of the world.
Road example:
Kinetic energy of machine required to get cart up to speed > kinetic energy in cart > kinetic energy in cart's wheel etc
In terms of forces and moments:
When cart travels at exactly the same speed as the wind, ie no relative wind on the cart forces acting on the cart are:
Friction at the tyre ground interface
Thrust imparted on cart by propeller
Gravity.
Edit Reaction of ground against gravity (to stop the cart falling through the treadmill/road
Resolving horizontally:
friction from tyres = horizontal component of rotor thrust
Resolving vertically
gravity and vertical component of thrust = reaction at tyre contact
Friction from tyres causes a couple (monent or torque) in the axle.
That torque is transmitted through the linkages and gears to propeller.
Now this is where the wind plays it's only roll. The thrust generated by a given static propeller ie one that is rotating in still air is proportional to the angle of attack. Angle of attack is the angle between the cord of the aerofoil section (cross section of propeller blade) and the relative airflow. A static propeller induces air through the propeller (that's what we feel from a desk top fan). The air that a blade feels as it rotates is not in the plane of the rotor disc, it is angled slightly. That is the relative airflow. What is demonstrated here is that the propeller is rotating such that the relative airflow generates sufficient thrust to overcome the friction at the tyre/ground interface which would otherwise cause the cart to slow or fall off the back of the treadmill.
To go faster than the wind, more air will flow through the propeller, causing the angle of attack to reduce and conversely if the cart moves slower than the wind, the angle of attack will increase. Now change the propeller for one with a greater overall pitch angle and the propeller will generate more thrust from the same speed and will tend to accelerate the cart in the direction it is travelling. But there will be a narrow band of speeds that a given propeller would be able to demonstrate this since movement of the cart relative to the air changes the relative airflow and therefore thrust from the propeller.
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