Author Topic: Mess with your minds: A wind powered craft going faster than a tail wind speed.  (Read 147457 times)

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Offline IanB

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you do not have an extension cord to a stationary location

You do have an extension cord to a stationary location. It is the wheels of the vehicle, which are sitting on the ground, which is stationary.
 

Offline electrodacus

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you do not have an extension cord to a stationary location

You do have an extension cord to a stationary location. It is the wheels of the vehicle, which are sitting on the ground, which is stationary.

Wind power is applied to vehicle body (including propeller or anything that has a surface facing the wind) that will be the area in the formula and then part of that applied power (you can not get all) can be taken from the wheel and transferred to the propeller.

So say at some point in time 100W are available to the vehicle as wind power then you can chose to use all that to accelerate the vehicle (increase kinetic energy) or you can decide to only use 50% for vehicle acceleration (it will take longer to get to a certain speed) and use the other 50% to power the propeller by taking 50W from the wheel basically this 50W from the wheels are braking.
Now that propeller may be 50% efficient so you will end up with 25W of thrust. This will be fairly useless and wasteful in more cases but air is a compressible fluid so most of this energy will get stored by increasing the pressure differential on each side of the propeller.
This stored pressure differential is what allows blackbird to exceed wind speed even by significant margins but this will last for at most a few minutes depending how low friction the vehicle is then vehicle will start to slow down as there will no longer be enough stored energy to cover the losses and to accelerate.

Offline fourfathom

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electrodacus, perhaps you should ponder the demonstrations with the small vehicles on the treadmill.  I recall that you dismissed these demonstrations, but do reconsider.  In these there is no appreciable energy storage, and the vehicle is propelled faster than the treadmill speed for extended periods of time.  In these, the frame of reference has shifted, with still air and moving ground.  Perhaps this will help.

If the vehicles work, but your models and equations don't, you should suspect that your models and equations are wrong.
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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electrodacus, perhaps you should ponder the demonstrations with the small vehicles on the treadmill.  I recall that you dismissed these demonstrations, but do reconsider.  In these there is no appreciable energy storage, and the vehicle is propelled faster than the treadmill speed for extended periods of time.  In these, the frame of reference has shifted, with still air and moving ground.  Perhaps this will help.

If the vehicles work, but your models and equations don't, you should suspect that your models and equations are wrong.

Not sure why you think I did not agree with any experiment.  The treadmill model showed in Derek's video works exactly according to my explanation.
While you set the vehicle on the treadmill you need to keep it there until all energy storage sources are fully charged up then you can release.
During that period you charge all flywheels (vehicle wheels and propeller so all spinning mass) and the important part you are also charging the pressure differential as tis is what will power the vehicle against the treadmill travel direction.  The thing is that this stored energy is large enough that the treadmill is way to short to demonstrate how vehicle will start to slow down.
If you try to interact with the vehicle by keeping it moving forward you are just charging the pressure differential back to the same initial level.
So keeping the vehicle forced in place will result in you having an inefficient treadmill powered fan.


Offline fourfathom

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The treadmill model showed in Derek's video works exactly according to my explanation.
While you set the vehicle on the treadmill you need to keep it there until all energy storage sources are fully charged up then you can release.
During that period you charge all flywheels (vehicle wheels and propeller so all spinning mass) and the important part you are also charging the pressure differential as tis is what will power the vehicle against the treadmill travel direction.  The thing is that this stored energy is large enough that the treadmill is way to short to demonstrate how vehicle will start to slow down.
If you try to interact with the vehicle by keeping it moving forward you are just charging the pressure differential back to the same initial level.
So keeping the vehicle forced in place will result in you having an inefficient treadmill powered fan.

When you are holding the vehicle in place on the moving treadmill (relative airspeed zero) the vehicle is pulling on the string.  Using your equation there should be zero force, but there obviously is.  Remember, if you swapped the reference plane, this is identical to the vehicle rolling across the ground at windspeed.  In your equation, where is this stored energy coming from when (vehicle speed = windspeed)?

Hint:  There is no appreciable stored energy.  There is potential energy that can be extracted.
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Offline Kleinstein

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Now that propeller may be 50% efficient so you will end up with 25W of thrust.

The thrust of the propeller is not measured as power, but as a force. The difference between force and power is important here. The force can be quite high, when the wind speed relative to the propeller is low (e.g. zero speed if the vehicle moves with the speed of the wind). A fixed efficiency number does not work here as this would lead to infinite force.  On the other hand that same force when applied to the wheels that slow down the vehicle will create power proportional to the vehicle speed. With enough speed of the vehicle, which means enough speed of the wind, there can be more power available than needed (a constant power for a given force) to power the propeller.

The design calculation would be about like this: Chose a prop speed where is works good.
In standing air the prop needs a certain power to create enough thrust, let's say 2 x the force needed to overcome the vehicle friction. A constant friction force independent of speed is reasonable as there is no wind drag at the speed of the wind. So half that force would be available to work against the wheels and from that power drive the prop.  The required wind speed for the vehicle to reach the speed of the wind would than be the required prop power divided by half the prop thrust. The quality of the prop (and the friction losses) determine the needed wind speed.

When driving at the speed of the wind, the prop sees standing air around it, and can thus produce the thrust (force) like standing in still air. The force is good to produce a power of wind speed times thrust. In the example half is needed to drive the prop and the rest is available power, e.g to accelerate or overcome friction. So you see, that it is possible to extract energy, even when going at the speed of the wind. It does not work with a simple sail, but it does with the prop to produce a force even with zero speed difference.

For the wind power available to a sail it would be proportional to (w-v)² for the force and w for the speed. So somehwat in between the formula given by  electrodacus and IanB.  The prop can improve on the factor (w-v)² and still produce some force at V=W. With the prop I am not sure the full area would be effectiv, but that would only be some costant factor, like a different sail shape that may also lead to a factor a bit off 0.5.
 

Online bdunham7

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25W of thrust

I don't want to rehash our much earlier discussion, but you saying this just another iteration of the fundamental misunderstanding that I was unable to get you to see then.  This misunderstanding--and your statement here--is exactly where you are repeatedly going wrong.  It's not a typo, lack of clarity or your self stated inability to explain things clearly.  It is the fundamental error that is preventing you from seeing reality here.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline electrodacus

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The difference between force and power is important here.

This is exactly what most seems to struggle with.

This is a wind powered vehicle  and as such the most important question is what wind power is available to the vehicle (not force) but power.
Vehicle kinetic energy (and thus speed) can not increase if power is not available.
Vehicle can stay with brakes on for the entire day and significant force can act on your vehicle from the interaction with air particles / wind but that will make no change to vehicle kinetic energy / speed.

Both vehicle kinetic energy and propeller are powered by wind so you can chose the ratio of power distribution but that is about it. The main point of the propeller in this design is to store energy so that it can be used to accelerate the vehicle above wind speed.

So most important question to answer for any wind powered vehicle is what the wind power available to vehicle is?

Offline electrodacus

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When you are holding the vehicle in place on the moving treadmill (relative airspeed zero) the vehicle is pulling on the string.  Using your equation there should be zero force, but there obviously is.  Remember, if you swapped the reference plane, this is identical to the vehicle rolling across the ground at windspeed.  In your equation, where is this stored energy coming from when (vehicle speed = windspeed)?

Hint:  There is no appreciable stored energy.  There is potential energy that can be extracted.


Sorry not quite sure what you are saying.
The treadmill model is not powered by wind is powered by the treadmill (as in initial condition as treadmill charges the vehicle energy storage) then that stored energy is what powers the vehicle once you let go to the vehicle.
You can call that stored energy potential energy and yes that will be correct.

Offline electrodacus

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25W of thrust

I don't want to rehash our much earlier discussion, but you saying this just another iteration of the fundamental misunderstanding that I was unable to get you to see then.  This misunderstanding--and your statement here--is exactly where you are repeatedly going wrong.  It's not a typo, lack of clarity or your self stated inability to explain things clearly.  It is the fundamental error that is preventing you from seeing reality here.

The lack of understanding is coming from you.

Online bdunham7

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The lack of understanding is coming from you.

Really?  Do you not see the inherent fallacy in the term "25W of thrust"?
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 

Offline electrodacus

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The lack of understanding is coming from you.

Really?  Do you not see the inherent fallacy in the term "25W of thrust"?

There is absolutely nothing wrong in the way I presented thrust and you will understand that if you knew what power is.

Offline IanB

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The main point of the propeller in this design is to store energy so that it can be used to accelerate the vehicle above wind speed.

No, the main point of the propeller in this design is the change the effective wind velocity at the vehicle. If the vehicle was travelling at wind speed with a sail, the difference in speed between the wind and the sail would be zero. No force available. But if the vehicle is travelling at wind speed with a rotating propeller, the difference in speed between the wind and the moving surfaces of the propeller can be greater than zero. Hence, with a propeller, the wind can still provide force.
 

Offline IanB

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There is absolutely nothing wrong in the way I presented thrust and you will understand that if you knew what power is.

Thrust has dimensions of force and is measured in Newtons.

Power has dimensions of rate of doing work, and is measured in Newton-meters per second, or Watts.
 

Offline electrodacus

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The main point of the propeller in this design is to store energy so that it can be used to accelerate the vehicle above wind speed.

No, the main point of the propeller in this design is the change the effective wind velocity at the vehicle. If the vehicle was travelling at wind speed with a sail, the difference in speed between the wind and the sail would be zero. No force available. But if the vehicle is travelling at wind speed with a rotating propeller, the difference in speed between the wind and the moving surfaces of the propeller can be greater than zero. Hence, with a propeller, the wind can still provide force.

That speed at the propeller is needed in order to increase pressure differential so basically storing energy. If air was not a compressible fluid then having the propeller will have been very useless and vehicle will not have been able to exceed wind speed.
The same wind power equation applies to a direct down wind sail vehicle as it applies to this propeller based one.  The difference is that while wind power is available so below wind speed some amount of energy will be stored in an energy storage device then that stored energy is used to accelerate above wind speed.
Same thing can be had with a sail type vehicle just add an electric generator/motor and a battery and you can charge the battery while below wind speed then use that stored energy to accelerate above wind speed.

Offline electrodacus

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There is absolutely nothing wrong in the way I presented thrust and you will understand that if you knew what power is.

Thrust has dimensions of force and is measured in Newtons.

Power has dimensions of rate of doing work, and is measured in Newton-meters per second, or Watts.

You can have Force without having power means vehicle speed and kinetic energy will not change.
Power is a more than Force especially in this context.

Offline PlainName

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Quote
Same thing can be had with a sail type vehicle just add an electric generator/motor and a battery and you can charge the battery while below wind speed then use that stored energy to accelerate above wind speed.

Contrary to your presumed intuition, having the propeller feathered until reaching wind speed would be better since it would present less drag. So no storage of anything before then to release after.

In fact, I think that's demonstrated with the treadmill models, which go from nothing to tread speed almost instantly.
 

Offline electrodacus

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Quote
Same thing can be had with a sail type vehicle just add an electric generator/motor and a battery and you can charge the battery while below wind speed then use that stored energy to accelerate above wind speed.

Contrary to your presumed intuition, having the propeller feathered until reaching wind speed would be better since it would present less drag. So no storage of anything before then to release after.

In fact, I think that's demonstrated with the treadmill models, which go from nothing to tread speed almost instantly.

It is not about intuition but the reality of how this universe works.
The treadmill model is being held not dropped on the treadmill.  During those few seconds the treadmill charges all energy storage devices available on the vehicle the multiple flywheel devices and the pressure differential.
That is a light weight vehicle so it will not require much energy to accelerate.

That treadmill is running at about 5.4m/s
Vehicle seems super lightweight maybe 0.5kg (just a guess)
Energy needed to get that vehicle from 0m/s to 1m/s (way higher speed that was ever seen in video) will be 0.5 * 0.5kg * 1^2 = 0.25Ws (0.25J if you prefer).
Do you have a sense of how much energy is 0.25Ws ?
That is about 0.07mWh
Typical smartphone battery is about 15Wh
You can take out your smartphone battery and accelerate that vehicle over 100000 times from 0 to 1m/s assuming 50% efficient propeller/motor combination.

Offline IanB

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If air was not a compressible fluid then having the propeller will have been very useless and vehicle will not have been able to exceed wind speed.

Air being a compressible fluid is not a means of energy storage, as the moving air in the propeller stream is less dense (lower pressure) than the still air in the surroundings. This can be learned in every physics or aerodynamics textbook as Bernoulli's principle.

There is no way that the lower pressure air can "push" from behind.

 

Offline PlainName

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Do you have a sense of how much energy is 0.25Ws ?

Where is this energy being stored. It ain't in a flywheel and it's not inertia of the model. It can't be the airstream because, as pointed out many times, there is nothing to retain it.
 

Online bdunham7

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You can have Force without having power means vehicle speed and kinetic energy will not change.
Power is a more than Force especially in this context.

You can have force without power, but do you understand under what specific conditions that can happen?
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Offline Kleinstein

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The correct explaination does not need energy storage, as it works in a steady state. I think the idea of energy storage is more like a futile attempt to cast doubt in what the experiments show. So it may still help to show, that there is no significant energy storage in the prop driven vehicle, that helps:

I don't see how the propeller would be a usefull way of energy storage: The energy in the flywheel action increase when the vehicle goes faster. So it can not be used to accelerate (unless you constantly change the gearing). It is actually slowing down the acceleration. The pressure field around the prop is also expected to get stronger when the prop goes faster - so also more like additional energy needed to accelerate.  In addition the pressure field would dissipate quite fast (at the speed of sound) as there is no back wall. So the energy is gone in fractions of a second. This also gives an idea that the energy in the compressed air would be pretty small, good for a fraction of a second at most.
Elastic energy in the drive train (e.g. the chain or twist of a shaft) would also up with more force and thus for most cases with higher speed. With a relatively solid drive train the stored energy is pretty small. With the force of the drive and less than 1 m of elastic range, thus would only power the vehicle for less than 0.5 seconds at a speed of 1 m/s (and correspondingly shorter at higher speed).

The vehicle with wheels (e.g. beam on top if the wheel version from Derek) work even easier than the ones with the prop - even less change to have some hidden energy storrage there. The vehicle with the rubber band does some slip stick (not slap-stick :-DD). I don't consider the rubber band helping, it is more like making it more complicated and not working as well. It would also work with a chain drive instead, and than without the slip-stick part. The rubber band does a little energy storage. Hoever the vehicle only oscillates between standing still (which corresponds to the speed of the wind) and going forward (corresponding to faster than the wind). So this shows that it is harvesting energy when going at the speed of the "wind".
 

Offline fourfathom

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The treadmill model is being held not dropped on the treadmill.  During those few seconds the treadmill charges all energy storage devices available on the vehicle the multiple flywheel devices and the pressure differential.

No, while the vehicle is being held on the treadmill it is reaching equilibrium.  If after release it was burning off stored energy it would instantly begin to slow down.  Instead, it accelerates.
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Offline electrodacus

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Air being a compressible fluid is not a means of energy storage, as the moving air in the propeller stream is less dense (lower pressure) than the still air in the surroundings. This can be learned in every physics or aerodynamics textbook as Bernoulli's principle.

There is no way that the lower pressure air can "push" from behind.

A propeller can create a pressure differential in air and that is where energy is stored.
See below graph   https://en.wikipedia.org/wiki/Axial_fan_design


Offline electrodacus

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No, while the vehicle is being held on the treadmill it is reaching equilibrium.  If after release it was burning off stored energy it would instantly begin to slow down.  Instead, it accelerates.

If it has stored energy say that 0.07mWh I calculated that are needed to accelerate that vehicle from 0m/s to 1m/s.
Of course those 0.07mWh of stored energy will accelerate the vehicle if is a slow acceleration or fast is irrelevant it will still have enough stored energy to get to 1m/s and in the video the vehicle did not get close to 1m/s.
It probably needs minutes at that slow acceleration rate to use all the stored energy.
Things can be changed to show deceleration before getting to the other end of the treadmill like adding enough weight to the vehicle.


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