Author Topic: Mess with your minds: A wind powered craft going faster than a tail wind speed.  (Read 147276 times)

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Offline Labrat101

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It's Christmas 🎄  maybe the answer is under the tree . Ask Santa if he can travel faster than tail wind !
Eat drink & think of something else .
 :popcorn:  :popcorn:
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Offline Kleinstein

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Asking how fast Santa has to ride his sled to get all around the world in 48 hours is a nasty one:
https://www.popularmechanics.com/military/aviation/a30317765/air-force-redesign-santa-sleigh/
 

Offline Domagoj T

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Here's a very rough design of a wind powered yoyo toy with an infinite string that uses the exact same principle as the previously described yoyo experiment.
Note that the teal batman wings are connected to the lower side of the purple string and they magically* fold like an umbrella when they reach the rear axle and unfold once they go past the front axle. The wings travel front to back in regards to vehicle, but forwards in regards to the ground, this means that the wings always travel slower than the vehicle, so there is a sweetspot where vvehicle > vwind> vwings. That sweetspot depends on combined friction of the system and wing efficiency, but it most certainly is there, if friction is minimal.


*that's a problem for the engineering department
 

Offline electrodacus

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I think the big problem for electrodacus is that at wind speed (and faster) there seems to be no relative wind to power anything. The only way to go faster than what's pushing is via some stored energy, ergo there must be some stored energy somewhere. Where? The only possible place, given that the prop is quite important, is the prop wash.

Which seems reasonable if you haven't seen this thing work, and all the explanations so far seem to run up against the problem that at wind speed there is no push (or even pull), so there is no obvious way this could be working.

So... how about an alternative explanation without the maths that no-one can agree on...

ISTM the most important thing to bear in mind is the prop is rotating and pushing air back. For a given rotational speed, the air will flow backwards at some derived speed - the faster the prop goes the faster the air flows back. You could use this to push the vehicle at a given speed by altering the speed of the prop, so we could say that the prop turning at xrpm is equivalent to airflow of ym/s. It works in reverse, of course - push the vehicle and the airflow will cause the prop to turn proportionally to the vehicle speed through the air.

So, back to wind speed and no push. But there is a push - the prop wash is effectively a wall (albeit quite flimsy!) moving at a negative speed relative to the vehicle and tied to the vehicle. If the vehicle is moving at wind speed, the prop wash is moving backwards at some speed relative to the vehicle and thus the wind. The wind can push against the prop wash to continue pushing the vehicle a little faster. In effect, the prop wash is an infinite length of the string we see with that yo-yo.

By what powers the  prop? It is still the wind because it is able to be pushing the vehicle still. With the prop wash moving backwards and effectively part of the vehicle, the vehicle is not at maximum speed relative to the wind, so the wind can keep pushing it.

What you are describing is the offset due to constant wind.
Do you agree air is a compressible gas ? Else this will just not work.

To remove air from equation you can just take the power from back wheel and apply that however you want to the front wheel of the vehicle so no propeller and more importantly no compressible fluid (air).
Now try to imagine wind pushes the vehicle you take part of that wind power from the back wheel and apply it to the front wheel. What happens when you get at or above wind speed ? What powers your back wheel to be able to power the front wheel ?

Same thing if you use a propeller under water so imagine Blackbird type vehicle driving at the bottom of a river so all is the same except air (compressible fluid) is replaced with water (incomprehensible fluid). Again it will not work so that vehicle will never be able to exceed stream speed powered only by the water stream. 

Offline electrodacus

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I draw attention to my earlier question:
Quote
Are you saying that energy is stored across the entire swept area of the propeller?

Yes the entire swept area of the propeller acts as a barrier keeping the low and high pressure volumes separate.

How does that work?  (I think this needs a diagram.)
Here it is. I posted this several times but people try to ignore it or say it is incorrect.
Propeller disc is the red one in the diagram. P2 is the pressure immediately after the disk down wind and is higher than PA that is ambient pressure while P1 is the opposite so lower than ambient pressure thus the exact mechanism of pressure differential energy storage.
Increase this pressure differential and you increased the amount of stored energy.
https://en.wikipedia.org/wiki/Axial_fan_design

Offline PlainName

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Quote
Do you agree air is a compressible gas ?

Yes, but I don't think it's relevant. Without air there would be no puzzle since there would be no wind to move faster than :)

I think the crux of it is that you're missing the effect of the wall of air. That wall could be a moving sail, being pushed back by the vehicle, except that there is a finite distance to which it could be pushed. With air, there is no limit because there is always air in front of you to push back. There is no infinite mechanism to do the same for a sail.

But let's assume for the moment that it is a sail, and the vehicle is infinitely long. The sail starts at the front and moves backwards at 1ms, say. There is a wind pushing the vehicle at 2ms. But that's pushing against the sail, so the wheel speed across the ground is actually 3ms - the 2ms wind plus 1ms sail.

That's how it works. The vehicle speed across ground is the wind speed plus the prop wash speed. Simple as that.

Ah, but the prop is powered by the wheels. Which is fine because the real vehicle speed will never reach the theoretical maximum. As has already been discussed, as the max is approached the wind power drops off and the speed stops increasing. So, in our example above, the vehicle may be doing 2.5m/s and can't go faster because of the lack of power as the vehicle-prop velocity approaches wind speed. But, as also noted, as the vehicle speed drops the apparent wind power will increase. Given the available wind power (acting against the diameter of the prop wash) is greater than the frictional losses of pushing the wheels to turn the prop, the realised speed can be higher than the winds ground speed (which is what all this is about).
 

Offline electrodacus

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Quote
Do you agree air is a compressible gas ?

Yes, but I don't think it's relevant. Without air there would be no puzzle since there would be no wind to move faster than :)

I think the crux of it is that you're missing the effect of the wall of air. That wall could be a moving sail, being pushed back by the vehicle, except that there is a finite distance to which it could be pushed. With air, there is no limit because there is always air in front of you to push back. There is no infinite mechanism to do the same for a sail.

But let's assume for the moment that it is a sail, and the vehicle is infinitely long. The sail starts at the front and moves backwards at 1ms, say. There is a wind pushing the vehicle at 2ms. But that's pushing against the sail, so the wheel speed across the ground is actually 3ms - the 2ms wind plus 1ms sail.

That's how it works. The vehicle speed across ground is the wind speed plus the prop wash speed. Simple as that.

Ah, but the prop is powered by the wheels. Which is fine because the real vehicle speed will never reach the theoretical maximum. As has already been discussed, as the max is approached the wind power drops off and the speed stops increasing. So, in our example above, the vehicle may be doing 2.5m/s and can't go faster because of the lack of power as the vehicle-prop velocity approaches wind speed. But, as also noted, as the vehicle speed drops the apparent wind power will increase. Given the available wind power (acting against the diameter of the prop wash) is greater than the frictional losses of pushing the wheels to turn the prop, the realised speed can be higher than the winds ground speed (which is what all this is about).

You just try to delude yourself with a crazy explanation.
What you call that wall of air is the wind (air particles moving in the same direction in average) and once your vehicle exceeds that the vehicle will face the opposite as not vehicle is no longer pushed by wind (but needs to travel with the equivalent of a headwind).
So not only there is no wind power available to vehicle but it has to deal with air moving in the opposite direction relative to the vehicle.

You all have this same insane explanation that you can output higher power at the propeller than you take from the wheels.
You are describing an over-unity device so the wrong explanation of how the vehicle actually works.

Offline IanB

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Do you agree air is a compressible gas?

No, I don't agree. All of the wind turbine power equations you have posted treat air as an incompressible fluid. All the equations you have posted have a constant density for air, which you state as 1.2 kg/m3.

If air were to be considered a compressible gas then the density would vary as a function of pressure, and this would appear in the equations. Since none of your equations have a variable density in them, you are stating that air is modeled as an incompressible fluid.

So if you try to say air is a compressible gas you are contradicting yourself.
 

Offline electrodacus

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Do you agree air is a compressible gas?

No, I don't agree. All of the wind turbine power equations you have posted treat air as an incompressible fluid. All the equations you have posted have a constant density for air, which you state as 1.2 kg/m3.

If air were to be considered a compressible gas then the density would vary as a function of pressure, and this would appear in the equations. Since none of your equations have a variable density in them, you are stating that air is modeled as an incompressible fluid.

So if you try to say air is a compressible gas you are contradicting yourself.

That equation I posted shows the available wind power the stored energy is a separate equation that makes no sense to talk about until you understand that vehicle has no wind power available once wind speed is exceeded.
In my video I show the way energy is stored in compressed air and how vehicle gets to exceed wind speed.  It is for ideal case so no losses but those can also be added for a more accurate real world result and to show the deceleration after vehicle got to peak speed.
I also linked to a proper calculator that is designed for the air pressure against a fixed wall or parapet and that clearly shows different pressure levels behind the wall thus clearly illustrating the air as a compressible fluid.
Here is the link again   https://eurocodeapplied.com/design/en1991/wind-pressure-freestanding-wall
And there is obviously one of my replays above sowing the pressure differential for an axial fan.

Offline Labrat101

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I gave the formula in reply #1071
key parameters in propeller design, the main ones being the power to drive the propeller,
 and the thrust that the propeller delivers.
    • The angle between blade chord and propeller plane is the geometric pitch (blade angle β
    • )
    • Angle of attack of the blade element α=β−Φ
    • The effective pitch angle is Φ=arctan(V/ω∗r)
. This is often given relative to rotational speed n = rounds per second of the propeller, and the propeller diameter R:
Φ=arctanVn⋅D∗1π⋅r/R
The advance ratio J of the propeller is
J=Vn⋅D
Induced velocity should be constant over the blade, implying that β
decreases linearly with increasing r: the propeller blade twist. Because the twist changes linearly, one point on the blade can be taken as the representative blade β
, and this is usually taken at either 70% or 75% of the radial distance.
It can be shown that for a given blade geometry, the power and thrust coefficients CP
and CT of the propeller are determined only by J and β0.75. If tip speed is below speed of sound and the blades are not stalled, Mach and Reynolds number effects are negligible.
CP=Pρ⋅n3⋅D5
CT=Tρ⋅n2⋅D4
"   All Started With A BIG Bang!! .  .   & Magic Smoke  ".
 

Offline PlainName

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Quote
What you call that wall of air is the wind (air particles moving in the same direction in average)

No! The 'wall of air' is the prop wash flowing backwards. The wind is a separate flow going forwards. The two meet at the 'wall'.
 

Offline Kleinstein

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That equation I posted shows the available wind power the stored energy is a separate equation that makes no sense to talk about until you understand that vehicle has no wind power available once wind speed is exceeded.

Wise words :  now I understand why some of the equations given by electrodacus make no sense.

The disagreement is about whether it is possible to get wind energy when already faster than the wind or not. So by definition we do no agree on this and can not use this in our arguments, as it would be a circular argument.
 

Offline electrodacus

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Wise words :  now I understand why some of the equations given by electrodacus make no sense.

The disagreement is about whether it is possible to get wind energy when already faster than the wind or not. So by definition we do no agree on this and can not use this in our arguments, as it would be a circular argument.

There is a big difference between wind power and stored energy. If vehicle was wind powered it could drive forever above wind speed (not possible) but if it is powered by stored energy then it will start to slow down once the stored energy is used up.

It is very important to understand that this direct down wind blackbird will slow down after stored energy is used up and it will not as you all claim maintain above wind speed indefinitely.

Offline PlainName

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Quote
What you call that wall of air is the wind (air particles moving in the same direction in average) and once your vehicle exceeds that the vehicle will face the opposite as not vehicle is no longer pushed by wind

OK, so if we ignore the prop and have an infinitely long vehicle with a sail moving from the front to back, don't you agree that the SAIL could be moved at wind speed and the vehicle would then move faster? Ignore for the moment the power needed to move the sail - we are just interested in if we are together at this point.

If you don't agree, please say exactly why not.
 

Offline electrodacus

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OK, so if we ignore the prop and have an infinitely long vehicle with a sail moving from the front to back, don't you agree that the SAIL could be moved at wind speed and the vehicle would then move faster? Ignore for the moment the power needed to move the sail - we are just interested in if we are together at this point.

If you don't agree, please say exactly why not.

Not quite sure I understand your infinitely long vehicle but you seems to say the sail will move directly down wind relative to vehicle and then that movement will be used to power the vehicle.
If that is correct then no the vehicle can not move faster than wind since at the point vehicle speed and wind speed are equal the sail will no longer move relative to vehicle body and no movement means no power.
But also when vehicle is above wind speed the sail will move in the opposite direction relative to vehicle thus will slow the vehicle down.

Offline PlainName

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Not quite sure I understand your infinitely long vehicle

OK, just very long then. Let's say it is 30m long. The sail is mounted on the vehicle just like Domagoj T's affair, but a single vertical sail. The sail starts at the front and stays there.

Now:

1. the body of the vehicle is streamlined so the only power transferred to it by the wind is via the sail. With the sail fixed in place and the wind blowing at 2m/s, are we agreed that the vehicle will move at 2m/s (or close enough)?

If not, why not?

2.  There is no wind. A motor turns the pulley moving the sail to the back at 1m/s. The power for that motor comes from a battery - we are not concerned with that at the moment, but it has nothing to do with the wind, the road, anything. Are we agreed that the vehicle moves forward at around 1m/s?

If not, why not?

3. There is a 2m/s wind, and the sail is moving backwards at 1m/2. Why cannot the vehicle be moving at ~3m/s when we know the sail will be pushed at 2m/s and the vehicle is pushed by the sail at 1m/s?
 

Offline fourfathom

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OK, so if we ignore the prop and have an infinitely long vehicle with a sail moving from the front to back, don't you agree that the SAIL could be moved at wind speed and the vehicle would then move faster? Ignore for the moment the power needed to move the sail - we are just interested in if we are together at this point.

If you don't agree, please say exactly why not.

Not quite sure I understand your infinitely long vehicle but you seems to say the sail will move directly down wind relative to vehicle and then that movement will be used to power the vehicle.
If that is correct then no the vehicle can not move faster than wind since at the point vehicle speed and wind speed are equal the sail will no longer move relative to vehicle body and no movement means no power.
But also when vehicle is above wind speed the sail will move in the opposite direction relative to vehicle thus will slow the vehicle down.

No! No! No!

The sail is on a carrier that rides on a track bolted onto an infinitely long vehicle.  The sail carrier is pushed by a worm-gear, turned by the motion of the wheels.  Or attached to a belt-drive conveyer belt, powered by the rotation of the wheels -- those details don't matter.  As the vehicle moves forward, the sail moves back (towards the rear, and the source of the wind).  When the sail is at windspeed, the vehicle is traveling faster than the wind.  How much faster is set by the gear ratios.

The vehicle doesn't need to be infinitely long, just long enough that you are satisfied that any stored pressure-differential energy has been used well before the sail hits the end of the vehicle.

You will then want to consider other ways that a sail can be moved through the wind.  For example, a spinning propeller.
We'll search out every place a sick, twisted, solitary misfit might run to! -- I'll start with Radio Shack.
 

Offline electrodacus

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OK, just very long then. Let's say it is 30m long. The sail is mounted on the vehicle just like Domagoj T's affair, but a single vertical sail. The sail starts at the front and stays there.

Now:

1. the body of the vehicle is streamlined so the only power transferred to it by the wind is via the sail. With the sail fixed in place and the wind blowing at 2m/s, are we agreed that the vehicle will move at 2m/s (or close enough)?

If not, why not?

Yes if ideal case no friction vehicle will move close enough to 2m/s

2.  There is no wind. A motor turns the pulley moving the sail to the back at 1m/s. The power for that motor comes from a battery - we are not concerned with that at the moment, but it has nothing to do with the wind, the road, anything. Are we agreed that the vehicle moves forward at around 1m/s?

If not, why not?

If vehicle has any weight then that will need to be accelerated thus vehicle will not be at 1m/s relative to ground. If vehicle is light enough it may get very close to 1m/s before the sail gets to the back of the vehicle.

3. There is a 2m/s wind, and the sail is moving backwards at 1m/2. Why cannot the vehicle be moving at ~3m/s when we know the sail will be pushed at 2m/s and the vehicle is pushed by the sail at 1m/s?

I will assume you now removed that battery from the vehicle. So what moves the sail backwards ?

Offline PlainName

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Quote
I will assume you now removed that battery from the vehicle. So what moves the sail backwards ?

No, the battery is still driving the motor. It is simply a combination of 1 and 2: wind blowing in the sail, sail being moved along the vehicle.

Do you agree the vehicle ground speed would be close to 3m/s?
 

Offline electrodacus

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No, the battery is still driving the motor. It is simply a combination of 1 and 2: wind blowing in the sail, sail being moved along the vehicle.

Do you agree the vehicle ground speed would be close to 3m/s?

Yes but this just proves my point that an energy storage device is needed to exceed wind speed.

Offline Brumby

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Yes the entire swept area of the propeller acts as a barrier keeping the low and high pressure volumes separate.

How does that work?  (I think this needs a diagram.)
Here it is. I posted this several times but people try to ignore it or say it is incorrect.

The diagram might be correct for what it's describing - but it is does not match what is happening with the Blackbird.

The first thing I see is that there is no confinement surrounding the Blackbird propeller - and that is clearly an important part of the diagram you presented.  Does this not compromise what you are saying?

 

Offline IanB

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No, the battery is still driving the motor. It is simply a combination of 1 and 2: wind blowing in the sail, sail being moved along the vehicle.

Do you agree the vehicle ground speed would be close to 3m/s?

Yes but this just proves my point that an energy storage device is needed to exceed wind speed.

Every time you dodge the question like this and try to change the subject, it just proves you are playing games.

Either answer the question, or stop playing.
 

Offline electrodacus

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The diagram might be correct for what it's describing - but it is does not match what is happening with the Blackbird.

The first thing I see is that there is no confinement surrounding the Blackbird propeller - and that is clearly an important part of the diagram you presented.  Does this not compromise what you are saying?



There is no confinement with that axial fan I used as example. You just misinterpreted the diagram.
That is exactly applicable to blackbird.
What you see there is the approximate boundary (fluid at rest and in motion) and no physical barrier.
And yes since there is wind the other fluid will also be in motion but at different speed.

Offline PlainName

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No, the battery is still driving the motor. It is simply a combination of 1 and 2: wind blowing in the sail, sail being moved along the vehicle.

Do you agree the vehicle ground speed would be close to 3m/s?

Yes but this just proves my point that an energy storage device is needed to exceed wind speed.

OK, so now we go back to scenario 2 and replace the moving sail with a static propeller. Recall that:

* There is no wind
* The propeller is powered by a battery

If the speed of the propeller wash is 1m/s, do you agree that the vehicle will move forward at around 1m/s?
 

Offline electrodacus

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No, the battery is still driving the motor. It is simply a combination of 1 and 2: wind blowing in the sail, sail being moved along the vehicle.

Do you agree the vehicle ground speed would be close to 3m/s?

Yes but this just proves my point that an energy storage device is needed to exceed wind speed.

OK, so now we go back to scenario 2 and replace the moving sail with a static propeller. Recall that:

* There is no wind
* The propeller is powered by a battery

If the speed of the propeller wash is 1m/s, do you agree that the vehicle will move forward at around 1m/s?

Replacing the moving sail with a propeller will not be the same thing if the propeller is not powered by something.
And since you do not have a battery and you do not accept there is an energy storage device (pressure differential) then there is nothing that can power the propeller other than the kinetic energy of the vehicle but taking energy from that will result in reduced vehicle speed. So propeller will still rotate but slower and slower same thing that actually happens after the vehicle used all the pressure differential and vehicle starts to slow down. 


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