Author Topic: Mess with your minds: A wind powered craft going faster than a tail wind speed.  (Read 147423 times)

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Online fourfathom

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Having said all of that, it has been regularly asserted that a sailboat can tack downwind faster than a balloon blown down wind.  If this is indeed true the advantage must be small and dependent on perfect execution and perhaps on boat configurations which perform poorly in other conditions.  I say this because racing yachts consistently set spinnakers and run nearly directly downwind rather than tacking to get there faster.  These people spend millions of dollars to win, they wouldn't ignore any consistent advantage.

It wasn't until recently that sailboats have been able to reduce drag sufficiently to be able to beat a balloon DDW.  Iceboats can easily do this, but traditional "displacement" sailboats (such as mine) just can't overcome the drag to be able to do it.  When I fly a spinnaker in heavy wind my best angle is DDW, not jibing, because I am limited by my "hull speed".  In lighter wind I will jibe back and forth, using the spinnaker, and make better progress than I would by sailing DDW.  The more modern "ultralight" boats have reduced drag to where they can beat the balloon by jibing.  Foiling boats (such as the Americas Cup catamarans that I posted the polars for) have even less drag and can very comfortably beat that balloon, and not by a small margin.  These boats don't even carry spinnakers as, even downwind jibing, the AWA is so far forward that the spinnaker would not be an efficient sail shape.

(AWA = Apparent Wind Angle, DDW = Dead Down Wind)
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Offline electrodacus

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Electrodacus is in love with his model.  A common problem with simulation people.  Simulations are wonderful things.  They can provide insight that is hard to obtain with actual physical tests, allow measurements that are literally impossible with physical tests and often save tremendous amounts of time and money.

But simulations have two fundamental flaws, that are often difficult to recognize.  First, they are all approximations and do not provide comprehensive information on when the omissions are important.  Second, usually a small problem but huge here, is that simulations no matter how large and wonderful do not inherently match the problem being simulated.  The math is all correct but doesn't represent the physics of the situation. 

In Electrodacus case there is a pretty obvious problem which he is overlooking.  He is not intrinsically wrong with his (10m/s -0.001m/s) formulation.  That is one way of presenting the problem.  But he is overlooking the fact that 0.001 m/s second of incremental velocity requires only a trivial amount of power.  Regardless of source.  In his own thought process there is zero drag in the condition of interest since the vehicle is moving at wind speed.  So the only consumer of power is the increase of momentum due to making the vehicle move faster.  The flawed thinking is ignoring the power required to maintain the initial 10 m/s velocity.

Having said all of that, it has been regularly asserted that a sailboat can tack downwind faster than a balloon blown down wind.  If this is indeed true the advantage must be small and dependent on perfect execution and perhaps on boat configurations which perform poorly in other conditions.  I say this because racing yachts consistently set spinnakers and run nearly directly downwind rather than tacking to get there faster.  These people spend millions of dollars to win, they wouldn't ignore any consistent advantage.

Not quite sure you understood the conditions of the problem.
There is no 10m/s vehicle speed condition that is the wind speed relative to ground the vehicle speed relative to ground is zero.
So question was how much such a vehicle can generate when driving directly downwind (relative to ground) at 0.001m/s and the other question how much power the vehicle requires to be able to drive upwind at 0.001m/s relative to ground.

So vehicle in the problem was not at same speed as wind speed.  With this information corrected what do you think ?

Offline electrodacus

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Having said all of that, it has been regularly asserted that a sailboat can tack downwind faster than a balloon blown down wind.  If this is indeed true the advantage must be small and dependent on perfect execution and perhaps on boat configurations which perform poorly in other conditions.  I say this because racing yachts consistently set spinnakers and run nearly directly downwind rather than tacking to get there faster.  These people spend millions of dollars to win, they wouldn't ignore any consistent advantage.

It wasn't until recently that sailboats have been able to reduce drag sufficiently to be able to beat a balloon DDW.  Iceboats can easily do this, but traditional "displacement" sailboats (such as mine) just can't overcome the drag to be able to do it.  When I fly a spinnaker in heavy wind my best angle is DDW, not jibing, because I am limited by my "hull speed".  In lighter wind I will jibe back and forth, using the spinnaker, and make better progress than I would by sailing DDW.  The more modern "ultralight" boats have reduced drag to where they can beat the balloon by jibing.  Foiling boats (such as the Americas Cup catamarans that I posted the polars for) have even less drag and can very comfortably beat that balloon, and not by a small margin.  These boats don't even carry spinnakers as, even downwind jibing, the AWA is so far forward that the spinnaker would not be an efficient sail shape.

(AWA = Apparent Wind Angle, DDW = Dead Down Wind)

In this particular conditions kinetic energy is used as energy storage device.
Thinks about ideal case where say you accelerate at 2x wind speed at an angle thus you can now change direction directly downwind and since there is no friction losses you can maintain 2x direct downwind forever.
A vehicle always driving directly downwind can not take advantage of the kinetic energy storage. So a vehicle always driving directly downwind requires some other form of energy storage and in the case of Blackbird that is the pressure differential created by the propeller.

There are many ways to prove that energy storage is used including driving until you get to peak speed and see how speed will decrease all the way down below wind speed. But the easiest way to prove if with the correct equations that show clearly there is no wind power available to any type of wind powered vehicle when that vehicle is at wind speed direct down wind or above (for above there is but that is negative meaning deceleration not acceleration).

Derek's proof was done using (vehicle speed - wind speed) and that is just demonstrably wrong as correct equation will have (wind speed - vehicle speed).
Same equation will need to apply to all conditions meaning when vehicle below wind speed and when vehicle above wind speed as you can not just change the equation when vehicle is at a different speed.

Offline Kleinstein

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Derek's proof was done using (vehicle speed - wind speed) and that is just demonstrably wrong as correct equation will have (wind speed - vehicle speed).
Same equation will need to apply to all conditions meaning when vehicle below wind speed and when vehicle above wind speed as you can not just change the equation when vehicle is at a different speed.
Recongnizing that the equation should conver different cases without changing the equation is a good point: this also applies to looking at the vehicle moving a snails speed aroung zero. The correct equation incudes this case.

The slow speed case does not directly apply to the vehicle going faster that the wind. However it clearly shows the the equation proposed by electrodacus is flawed and leads to obvious problems there. Looking at this point may also help him correcting his understanding of force and power.  Doing the calculations only with power can be tricky if there are processes with less than 100% efficiency (e.g. like the sail).

Having an equation with (w-v) or (v-w) is not such a big difference, it is just the sign and this may be different depending on how one defines the direction of force or an axis.  One has to look not just at the one line with the formula, but also the explaination fo the symbols.

I don't think Derek's proof has a major mistake in the equations, as it leads to the right conclusion - just getting a result that is not obvious to everybody is no proof that there is an error. Usually math is way more reliable than intuition.
 

Online fourfathom

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I say this because racing yachts consistently set spinnakers and run nearly directly downwind rather than tacking to get there faster.  These people spend millions of dollars to win, they wouldn't ignore any consistent advantage.

Just to emphasize this, racing yachts do *not* generally run nearly dead downwind.  Unless limited by hull-speed (iceboats, ultralight boats, and foiling boats are not hull-speed limited), a sailboat trying to get directly downwind will jibe back and forth for a better "speed to mark" or "speed made good" (which is the boat speed times the cosine of the angle of the course sailed relative to the course directly to the mark).

This doesn't apply to the land-vehicle, but if you're interested the only time a racing boat will sail directly downwind is when it is limited by hull-speed, which is approximately [1.34 * sqrt(boat waterline length in feet)] (speed result in knots)  This isn't a brick-wall limit, but above hull-speed it requires dramatically more power to move the boat.  I don't know the curve, but it's extremely steep.  When sailing at hull-speed there is no longer an advantage to sailing a longer course (jibing back and forth), since you can't go any faster.
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Offline electrodacus

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Recongnizing that the equation should conver different cases without changing the equation is a good point: this also applies to looking at the vehicle moving a snails speed aroung zero. The correct equation incudes this case.

The slow speed case does not directly apply to the vehicle going faster that the wind. However it clearly shows the the equation proposed by electrodacus is flawed and leads to obvious problems there. Looking at this point may also help him correcting his understanding of force and power.  Doing the calculations only with power can be tricky if there are processes with less than 100% efficiency (e.g. like the sail).

The equation I use perfectly matches reality. For some reason you think that is not the case but it can fairly easily be proven.
If your vehicle has no mechanical brakes and you want to keep the vehicle from moving in 10m/s wind with a 1m^2 sail then you will need to apply 600W to that motor (all ideal case so ideal motor and no friction losses).
If you apply no power to the motor the vehicle will accelerate towards wind speed.
If you want to go directly upwind with 0.001m/s then you need to supply the motor with 0.5 * 1.2 *1 *(10 + 0.001)^3 = 600.18W
If you want to go directly downwind with 0.001m/s then you can use the motor as a generator and take out 0.5 * 1.2 *1 *(10 - 0.001)^3 = 599.82W (you can just waste this as heat and you can maintain that low speed).


Having an equation with (w-v) or (v-w) is not such a big difference, it is just the sign and this may be different depending on how one defines the direction of force or an axis.  One has to look not just at the one line with the formula, but also the explaination fo the symbols.

I don't think Derek's proof has a major mistake in the equations, as it leads to the right conclusion - just getting a result that is not obvious to everybody is no proof that there is an error. Usually math is way more reliable than intuition.

Not sure why you think a change in sign is not a huge difference.  It is the difference between being able to accelerate instead of decelerate.
That is the problem the conclusions are totally wrong. The vehicle is not powered by wind directly but by stored energy.

Offline PlainName

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Does anyone have access to a treadmill and propeller model? ISTM the way to settles this is to get that model doing its thing, but have it tethered by some string so it can't move forward. Either the string is kept taught and energy storage is rubbish, or it goes loose and some store has clearly run out of puff.
 

Offline IanB

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Does anyone have access to a treadmill and propeller model? ISTM the way to settles this is to get that model doing its thing, but have it tethered by some string so it can't move forward. Either the string is kept taught and energy storage is rubbish, or it goes loose and some store has clearly run out of puff.

It doesn't matter if you do that. Our friend will just find some way to reject the experiment or the results. So it would be a waste of time to try.
 

Offline bdunham7

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If your vehicle has no mechanical brakes and you want to keep the vehicle from moving in 10m/s wind with a 1m^2 sail then you will need to apply 600W to that motor (all ideal case so ideal motor and no friction losses).

That's the point of contention.  I'm saying that this statement is not true.  What is your theory/proof/argument/evidence that supports it?
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Offline electrodacus

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If your vehicle has no mechanical brakes and you want to keep the vehicle from moving in 10m/s wind with a 1m^2 sail then you will need to apply 600W to that motor (all ideal case so ideal motor and no friction losses).

That's the point of contention.  I'm saying that this statement is not true.  What is your theory/proof/argument/evidence that supports it?

So what do you think is needed if not 600W to prevent the vehicle from being pushed by the wind ?
We can do this test relatively easily.

The equation showing that is simple and it is this one you can find everywhere for ideal case  0.5 * air density * area * (wind speed)^3
So 0.5 * 1.2 * 1 * 10^3 = 600W

Here I found an image supporting my case ciclist frontal surface area around 0.5m^2 and 10m/s = 36km/h notice that requires about 300W


https://ridefar.info/bike/cycling-speed/air-resistance-cyclist/
« Last Edit: December 15, 2021, 08:47:43 pm by electrodacus »
 

Offline bdunham7

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So what do you think is needed if not 600W to prevent the vehicle from being pushed by the wind ?
We can do this test relatively easily.

The equation showing that is simple and it is this one you can find everywhere for ideal case  0.5 * air density * area * (wind speed)^3
So 0.5 * 1.2 * 1 * 10^3 = 600W

An arbitrarily small amount of power is needed to hold it in place because power is force x speed.  Because the speed is zero, you can use as much gear reduction as you like.

If you can do the test easily, go ahead.

Your equation (the second one--I'm not commenting on the first) doesn't make any sense.  The motor output power is the force produced by the motor multiplied by the speed of the motor.  It doesn't know or care how fast the wind is blowing to produce the force that it is working against.
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Offline electrodacus

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An arbitrarily small amount of power is needed to hold it in place because power is force x speed.  Because the speed is zero, you can use as much gear reduction as you like.

See my edited comment above.

You try pedaling at 1km/h direct upwind with 35km/h wind and let me know how much power you need to do that.
« Last Edit: December 15, 2021, 09:08:13 pm by electrodacus »
 

Offline bdunham7

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You try pedaling at 1km/s direct upwind with 35km/h wind and let me know how much power you need to do that.

1 km/s is going to be spectacular, with or without the headwind. 

Seriously, what sort of argument is that?  If your motor setup is stupidly inefficient, it could take any amount of input power--even much more than 600W if you like--but at zero speed the output power is zero, so if you improve the design you can make it as efficient as you like, thus my 'arbitrarily small' statement.  For the .001m/s upwind example, you can make it arbitrarily close to 60mW because that is the output power.
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Offline electrodacus

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1 km/s is going to be spectacular, with or without the headwind. 

Seriously, what sort of argument is that?  If your motor setup is stupidly inefficient, it could take any amount of input power--even much more than 600W if you like--but at zero speed the output power is zero, so if you improve the design you can make it as efficient as you like, thus my 'arbitrarily small' statement.  For the .001m/s upwind example, you can make it arbitrarily close to 60mW because that is the output power.

It was a typo is obviously 1km/h (0.28m/s)

Just take a bicycle if you have one and try to drive direct upwind at as slow of a speed as you need to maintain balance (maybe 1km/h is a bit slow) and you will see you do not need mW but hundreds of watts with 10m/s (36km/h) as clearly seen in that graph I posted.

Offline bdunham7

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Just take a bicycle if you have one and try to drive direct upwind at as slow of a speed as you need to maintain balance (maybe 1km/h is a bit slow) and you will see you do not need mW but hundreds of watts with 10m/s (36km/h) as clearly seen in that graph I posted.

A human on a bicycle is not efficient in this configuration so the input power is high even with a low output.  And the 60mW was for 1mm/s.  Your chart is not relevant, it shows the energy input? for for a moving bicycle against air resistance.  A (nearly) stationary bicycle with a wind is a different problem with different results.
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Offline Kleinstein

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Going with 1 km/h against a heat wind of 35 km/h would need the force of going 36 km/h with no wind (irgnoring wheel friction and similar other power needs) with only 1/km/h instead of 36 km/h this would be 1/36 of the power needed for 36 km/h with zero wind. So taking the graph below to get a number, this for be bit below 300 W for 36 km/h and some 0.8 W for 1 km/h.

Other than for a rough number the chart is not really helping.
 

Online CatalinaWOW

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Having said all of that, it has been regularly asserted that a sailboat can tack downwind faster than a balloon blown down wind.  If this is indeed true the advantage must be small and dependent on perfect execution and perhaps on boat configurations which perform poorly in other conditions.  I say this because racing yachts consistently set spinnakers and run nearly directly downwind rather than tacking to get there faster.  These people spend millions of dollars to win, they wouldn't ignore any consistent advantage.

It wasn't until recently that sailboats have been able to reduce drag sufficiently to be able to beat a balloon DDW.  Iceboats can easily do this, but traditional "displacement" sailboats (such as mine) just can't overcome the drag to be able to do it.  When I fly a spinnaker in heavy wind my best angle is DDW, not jibing, because I am limited by my "hull speed".  In lighter wind I will jibe back and forth, using the spinnaker, and make better progress than I would by sailing DDW.  The more modern "ultralight" boats have reduced drag to where they can beat the balloon by jibing.  Foiling boats (such as the Americas Cup catamarans that I posted the polars for) have even less drag and can very comfortably beat that balloon, and not by a small margin.  These boats don't even carry spinnakers as, even downwind jibing, the AWA is so far forward that the spinnaker would not be an efficient sail shape.

(AWA = Apparent Wind Angle, DDW = Dead Down Wind)

Thanks for the input.  I haven't watched the cup races for a couple of decades now and wasn't aware of the new reality.  I lost interest when the rules committee seemed more interested in controlling the winner than in providing a competition.
 

Online fourfathom

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You can push against a brick wall with lots of force, but no work will be done, so there's no power.
If the wind pushes against a vehicle with a the wheels locked and not rolling, no work will be done, so there's no power.
But if the wind pushes against a vehicle and a motor used to keep the wheels from rolling, there's definitely force, but the motor will also burn Watts to keep the wheels from rolling.  Watts is power.  I suppose the motor is operating at 0% efficiency.

And this actually has nothing to do with the reality of the demonstrated sustained DDWFTTW vehicles.
« Last Edit: December 15, 2021, 09:47:58 pm by fourfathom »
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Offline bdunham7

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But if the wind pushes against a vehicle and a motor used to keep the wheels from rolling, there's definitely force, but the motor will also burn Watts to keep the wheels from rolling.  Watts is power.

Yes, but arbitrarily small power depending on the design of the system. 

Quote
And this actually has nothing to do with the reality of the demonstrated sustained DDWFTTW vehicles.

No, it has to do with the specific misunderstandings regarding 'conservation of energy' that are not allowing Electrodacus to understand how and why they work. 
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Offline electrodacus

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A (nearly) stationary bicycle with a wind is a different problem with different results.

That is where you are wrong. The problem is basically the same.  Why do you think 300W are needed constantly to maintain that speed ?  Is the air drag and you have that if you are cycling at 36km/h with no wind or at 1km/h with a 35km/h head wind. Same drag so same amount of power required.

Offline electrodacus

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Going with 1 km/h against a heat wind of 35 km/h would need the force of going 36 km/h with no wind (irgnoring wheel friction and similar other power needs) with only 1/km/h instead of 36 km/h this would be 1/36 of the power needed for 36 km/h with zero wind. So taking the graph below to get a number, this for be bit below 300 W for 36 km/h and some 0.8 W for 1 km/h.

Other than for a rough number the chart is not really helping.

You need 300W to cycle at 36km/h with no wind and also 300W to cycle at 1km/h with a 35km/h head wind.

Offline electrodacus

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You can push against a brick wall with lots of force, but no work will be done, so there's no power.
If the wind pushes against a vehicle with a the wheels locked and not rolling, no work will be done, so there's no power.
But if the wind pushes against a vehicle and a motor used to keep the wheels from rolling, there's definitely force, but the motor will also burn Watts to keep the wheels from rolling.  Watts is power.

And this actually has nothing to do with the reality of the demonstrated sustained DDWFTTW vehicles.

If you have no energy storage (like a spring between you and the wall) plus mechanical brakes you will need the power to maintain that force on the wall.
Yes if the wind pushes against a vehicle and vehicle is not moving no work is done but if it moves even at super low speed work will be done.

All of you seems to have some misconception about this that is why I insist. Once you get this you will understand why direct downwind faster than wind is impossible without energy storage. Same as direct upwind without energy storage is also impossible tho that is a fairly different problem.

Offline Kleinstein

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The low speed case has no direkt relevance for the fast downwind case, but is shows that the way electrodacus thinks about power and force is totally screwed up. I still had hope he may realize that the 600 W for moving at a snails speed in the wind is obviously wrong.

Even for someone more used to electricity it should be clear that the mechanical power is speed times force.
For the vehicle to move it is about overcoming the opposing forces. There is no opposing power, as the power has no geometric direction.

You need 300W to cycle at 36km/h with no wind and also 300W to cycle at 1km/h with a 35km/h head wind.
That is absoulutely nonsense. It may be bit windy to drive a bike, 1 km/h is more like very slow walking and walking against the wind is still relatively easy.

Instead of using a bike, maybe consider a sail pulled by a rope. How about the power when using pullies ?  Is there anything magic in the fore caused by the wind that makes it different from other forces ?
 

Offline bdunham7

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That is where you are wrong. The problem is basically the same.  Why do you think 300W are needed constantly to maintain that speed ? Is the air drag and you have that if you are cycling at 36km/h with no wind or at 1km/h with a 35km/h head wind. Same drag so same amount of power required.

The relative wind speed produces, as you say, the same force.  That force, multiplied by the speed of the bicycle, is the power required.  The power required is not the same in each case.

Quote
All of you seems to have some misconception about this that is why I insist.

The people that have this 'misconception' comprise a pretty large group.  Ask some people you trust as experts, you'll be shocked at how many are in that group.

Why don't you 1) consult a physics textbook  2) ask someone qualified that you will actually believe  3) write to a few physics professors and see if they'll reply or 4) devise some sort of experiment that either you can do or a workable example where the results of your calculation are falsifiable?

I've tried to explain this to you many different ways and it just doesn't sink in.  I assure you I'm correct here--something I rarely assert definitively.
« Last Edit: December 15, 2021, 10:06:46 pm by bdunham7 »
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Offline thm_w

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You need 300W to cycle at 36km/h with no wind and also 300W to cycle at 1km/h with a 35km/h head wind.

Not according to this: https://www.omnicalculator.com/sports/cycling-wattage
Its ~10W at 1km/h vs ~300W at 36km/h
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