General > General Technical Chat
Mess with your minds: A wind powered craft going faster than a tail wind speed.
electrodacus:
--- Quote from: Brumby on August 31, 2021, 05:30:12 am ---OK.
Since you claim to have all the answers with the physics knowledge and maths to support your conclusions, I put this challenge to you....
Take all reasonable references from the blackbird example and generate the maths to describe its operation. Once you have done that, the run time limitation of such an experiment - which you claim will occur - should naturally fall out of those equations. Please tell us what that time limitation will be, so that it can be validated experimentally.
The onus on doing this falls on YOU - not everybody else here - as
(1) You are the one claiming what was demonstrated in the Veritasium video is incomplete
and
(2) Those of us who understand the principle as it was presented agree that the demonstration proves the point.
You need to prove your point.
--- End quote ---
I started to do just that but the guy that designed Blackbird is to secretive about the spec. I used some approximations to get the amount of energy needed by blackbird to get to that speed record and was around 6Wh witch is almost nothing but since I used so many approximations all of them will add up to quite a margin of error.
Also wind speed in real world has huge fluctuations that will significantly impact the results like in that record they did I saw the wind speed data and wind fluctuated during the test between 8mph and 15mph while they decided the record was when vehicle got to 28mph and wind speed was lowest in that period thus the 2.8x record but I could say more realistically the record was maybe 2x they just took the best case as they completely ignored energy storage.
There is a much easier proof and that will be to push the blackbird or smaller model to a minimum design speed around 10 to 12mph in a day with no wind and see how vehicle will continue to accelerate way past the push speed.
(1) The explanation is not incomplete is completely incorrect.
(2) The explanation there made absolutely no sense and was wrong mathematically (modified formula to seem like it matches the results).
To disprove the current explanation all I need is a single phrase.
A vehicle power only by the wind driving in the same direction as the wind will be 100% efficient when it is at same speed as the wind (possible only in theory) and from this since no claim can be made that above 100% efficient device was ever proven higher than wind speed is impossible if powered only the the wind and no energy storage.
Seems super intuitive for me as no air particle can get to a vehicle that travels at or above wind speed in same direction so is clear wind can not power this.
The explanation to get around this was that vehicle can extract energy from the difference between the two mediums that is ground and air but that is disprove by my simple wheels only diagram as there treadmill has a different speed than ground and vehicle can not use that energy to move forward from left to right.
So my diagram is complete analog to the treadmill propeller cart since there wind speed is zero and treadmill represents the ground moving under a stationary vehicle.
The only reason propeller version works and wheel version is not working is the energy storage part.
I also made a diagram where I added energy storage to the wheel only version see diagram below where to motor wheel I added a spiral spring (similar to what you will find in a tape measure) and that connects the motor to the wheel.
So when you put the vehicle on the treadmill with generator wheel and the motor wheel on the fixed red box the spring will be compressed to a max level after that the motor wheel will just slip as it is the case for the propeller in air and so when you let go the vehicle will move forward as on top of the insufficient power provided by the generator wheel you have now spring stored energy to add the difference and some extra to move the vehicle from left to right.
So my explanation not only makes sense and breaks no rules but predicts exactly what the test results show
--- Quote from: Brumby on August 31, 2021, 05:40:50 am ---Or, you can pull a number of of thin air - just make sure the error bar is included.
For example if you say "5 minutes" and the experiment runs for 10 minutes at higher than wind speed, then don't turn around and say - "Oh, it must be longer".
Pick a number and stick to it.
--- End quote ---
Is not so much about time is about the distance same as in the above analogy with spiral spring. So if spiral spring is winded say 100x wheel rotation and wheel is say 2m circumference the vehicle can travel for 200m before it will start to slow down.
So on the treadmill model if you stop the vehicle with hand from going forward the energy storage will be again fully charged up and as long as you keep your hand there you just have a treadmill powered fan. You will observe the exact same thing with my spring based model and it will be easier to see.
cbutlera:
--- Quote from: electrodacus on August 31, 2021, 05:58:08 am ---To disprove the current explanation all I need is a single phrase.
A vehicle power only by the wind driving in the same direction as the wind will be 100% efficient when it is at same speed as the wind (possible only in theory) and from this since no claim can be made that above 100% efficient device was ever proven higher than wind speed is impossible if powered only the the wind and no energy storage.
--- End quote ---
I think this is at the heart of the problem. As I understand it, your claim is that if Blackbird is travelling faster than wind speed, then for every joule of its kinetic energy taken by the wheels, only about 70% of that energy is returned by the propeller, so the vehicle will slow down until it is travelling below wind speed, and therefore could not exceed wind speed unaided.
The flaw with this argument is that it is not taking account of frames of reference. The wheels are extracting kinetic energy from the vehicle in the frame of reference of the ground, so the number of joules extracted by the wheels will match the number of joules of kinetic energy (with respect to the ground) that are lost. The propeller is imparting kinetic energy to the vehicle in the frame of reference of the air, so the number of joules provided by the propeller (less transmission losses) will match the number of joules of kinetic energy (with respect to the air) that are gained. The gain in kinetic energy with respect to the ground will be greater than this, and can easily be greater than the amount originally lost, even with the transmission losses.
A thought experiment may help to clarify this.
Imagine that I am a passenger who weighs 70 kg, seated in an aircraft cruising at 200 m/s (about 450 mph). Suppose now that I stand up and walk forwards along the aisle at 1 m/s (about 2.2 mph) with respect to the aircraft. In the frame of reference of the aircraft, I have increased my kinetic energy from 0 to 35 joules (mv2/2), and that energy has been provided by my muscles. However, when the same event is viewed from the frame of reference of the ground, I have increased my speed from 200 m/s to 201 m/s, so I have increased my kinetic energy from 1,400,000 joules to 1,414,035 joules, a difference of 14,035 joules. No laws of physics are being broken, because the aircraft engines had to work a little harder to maintain the aircraft speed while I was accelerating, so they provided the additional 14,000 joules, to add to the 35 supplied by my muscles.
What you are doing in your explanation of Blackbird, is the equivalent of the ground observer in my thought experiment, thinking that my kinetic energy in his frame of reference can only have increased by 35 joules, because that is how much energy my muscles have generated. So he concludes that if I started with 1,400,000 joules of kinetic energy, I can now only have reached 1,400,035 joules. Therefore my speed can only have increased to 200.0025 m/s, or 0.0025 m/s with respect to the aircraft. The ground observer can see that this is obviously wrong, and so there must be a flaw in his reasoning.
Now imagine if Blackbird plus driver weighs 200 kg, and is travelling at 11 m/s, with a 10 m/s tailwind, both with respect to the ground, so Blackbird's speed in the frame of reference of the air is 1 m/s. With respect to the ground, Blackbird has a kinetic energy of 12,100 joules (mv2/2). Suppose that the wheels now extract 1,000 joules of this energy, reducing the kinetic energy (with respect to the ground) to 11,100 joules and therefore the ground speed to 10.54 m/s. With respect to the air, Blackbird now has a speed of 0.54 m/s and a therefore a kinetic energy of 29 joules. Suppose that the propeller, after losses, manages to add 700 joules to this kinetic energy, increasing it to 729 joules. The speed with respect to the air will now be 2.7 m/s, so the speed with respect to the ground will be 12.7 m/s. This is greater than the starting figure of 11 m/s. So Blackbird can indeed accelerate beyond wind speed without breaking any laws of physics or requiring energy storage.
Kleinstein:
--- Quote from: electrodacus on August 31, 2021, 05:58:08 am ---To disprove the current explanation all I need is a single phrase.
A vehicle power only by the wind driving in the same direction as the wind will be 100% efficient when it is at same speed as the wind (possible only in theory) and from this since no claim can be made that above 100% efficient device was ever proven higher than wind speed is impossible if powered only the the wind and no energy storage.
--- End quote ---
This is not true, as going at 100% the wind speed with a sail is not 100% efficient, as least not as energy efficency. You actually use no wind-engy at all. So at best you could call it a zero divide by zero to get 100%, but math does not approve this.
If one finds a clever way to harness the wind power even when moving faster than the wind, there is problem using this power to go fater than the wind.
The point is that there is such a clever way.
So your arguing is that it should not be possible to go faster because you assume that it is possible to get power from wind when going as fast as the wind or fast. One can not assume something from your intuition to prove it. This is not how logic works.
--- Quote from: electrodacus on August 31, 2021, 05:58:08 am ---Seems super intuitive for me as no air particle can get to a vehicle that travels at or above wind speed in same direction so is clear wind can not power this.
The explanation to get around this was that vehicle can extract energy from the difference between the two mediums that is ground and air but that is disprove by my simple wheels only diagram as there treadmill has a different speed than ground and vehicle can not use that energy to move forward from left to right.
--- End quote ---
The effect is a bit complicated, so one should be care ful with the intuition - this is why the thread title is "Mess with your minds: ..", as this is an effect that may conflict with simple intuition.
You idea with getting energy from the difference in velocity is perfectly correct and the wheeled model on the threadmill is also perfectly fine. However the interpretation of the wheeled vehicle on the treadmill is different. One can look at it in 2 ways and both ways are perfectly valid.
In any case if you don't find a case covered in the experiment (e.g. circular motion or enery storage) the experiment tell absolutely nothting about it. So it can not prove that this is not possible. It can not even prove that it is impossible to go backwards in time - though we all know we can't.
The first way to look at it as to take the band as the wind. In this picture the vehicle is morving in the direction against the wind. This is also working with real wind, as a windmill driving a vehicle with the windmill aginst the wind. I don't think there is a controversy about this.
However the same experiment can also be interpretet in a differnt way. One can consider the band of the treadmill as the earth and the outer frame of the treadmill as the moving system. This is a Galilean transformation ( https://en.wikipedia.org/wiki/Galilean_transformation) and a well know method in thought experiments.
In this picture the "wind" goes from left to right and the vehicle going from left to right is this moving faster than the "wind" / moving frame. Just immagine a camera moving with the belt.
So the experiment with the belt diven vehicle proves that it is possible to move faster in the direction of the driving system (e.g. wind).
So the idea of using the speed difference works. The simple wheels only mode proves that it is possible. The ideal wheels only case can also be calculated
and shows that is work. Going from the wheeled case to the prop drive need the extra technical hurdly to get the prop efficient enough. So this one "needs" the experiment and as shown on the treadmill it works.
The same identification is also used in the treadmill experiment with the prop driven vehincle. The wind inside is the still standing air. Which is the same speed as the frame of the treadmill. The belt represents the ground the vehicle is drivind on.
Brumby:
--- Quote from: electrodacus on August 31, 2021, 05:58:08 am ---Seems super intuitive for me
--- End quote ---
In other words, you've already made up your mind.
--- Quote --- as no air particle can get to a vehicle that travels at or above wind speed in same direction so is clear wind can not power this.
--- End quote ---
It's not just wind. It's wind plus something else.
--- Quote ---The explanation to get around this was that vehicle can extract energy from the difference between the two mediums that is ground and air
--- End quote ---
Careful ... you're almost getting it!
--- Quote --- but that is disprove by my simple wheels only diagram
--- End quote ---
Utter rubbish. Your "wheels only diagram" is not relevant - never has been - but as you continue to insist it is, you are condemned to never understand correctly.
Brumby:
--- Quote from: IanB on August 31, 2021, 05:52:46 am ---The interesting thing about the treadmill experiment is that they missed an opportunity to show something more. They let the model vehicle fly forwards and drive off the end of the treadmill. But if they had simply anchored it in place with some thread, they could have shown it pulling against the thread indefinitely while the treadmill was running.
--- End quote ---
Now THAT is a brilliant idea. Put a tension measurement device on that thread and you could generate data of tension vs speed with alternative gearing and propeller parameters.
You could chart the results and get a very good visual understanding of the behaviour.
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