There's something wrong with that assertion.
Exactly, and this means that unlike the turbine, a propeller does not
*extract* energy from the air, it *imparts* energy to it.
There's something wrong with that assertion.
Exactly, and this means that unlike the turbine, a propeller does not
*extract* energy from the air, it *imparts* energy to it.@Ronald Raygun:
This is correct -- but only when considering energy relative to the device itself. In the case of the DDWFTTW vehicle, the propeller
*speed up* the air relative to itself ('cause that's what propellers do), but is *slows* the air relative to the ground (which is what has to happen if we want to harvest energy).Let's go back to the actively engaged skateboarder:
Let's suppose that the pedestrians on the sidewalk are originally moving at 1ms. Let suppose that that the skateboarder is moving at
2ms. Obviously in this scenario, before the the pedestrians are 'grabbed' by the skateboarder they moving at 1ms relative to the skateboarder AND 1ms relative to the ground. Once the skateboarder pushes them backwards, bringing them to a complete stop, there is a new set of relative numbers. Now, the pedestrian is moving 0ms across the ground and 2ms relative to the skateboarder.Pedestrian before contact: Relative to ground: 1ms Relative to skateboarder: 1ms
Pedestrian after contact: Relative to ground: 0ms Relative to skateboarder: 2ms
So, while the skateboarder *does* speed up the pedestrians (air molecules) relative to himself, it slows them down relative to the ground.
No one considers a wind turbine a PM device because we all know that in it's wake is left a volume of air that has been slowed relative to the ground. Wind that was going 10mph across the ground before it encounters the device is slowed and is only going 5mph after it encounters the device.
The source of the energy from the propeller driven device is absolutely no different from a physics standpoint -- just as the skateboarder leaves people in his wake going down the sidewalk slower than before, the Blackbird leaves a volume of air in it's wake going across the ground slower than it once was. Wind that was going 10mph across the ground before encountering the device is not only going
5mph across the ground.Folks, I apologize for mixing metric and US/Imperial units in my posts. I was attempting to stick to metric but I tend to automatically revert to US standards.
I'll try to be more consistent in the future. Thanks for tolerance.
Now do that without the chemical energy being used by your muscles!
Rubbish. It is the air that is gaining energy as you are pushing it. Any attempt to move air by the propeller must use energy by definition.
Nice set of brakes there.
Go on then.
@"dennis@home"
That's absolutely correct Dennis - and why I stated:
"To put it back in the skateboard example, the energy to power my arms has to come from somewhere ..."
You are again correct -- by definition, a propeller *does work on the air*(but only relative to itself) and it must be provided an input torque on it's shaft to do to so. But it's not rubbish in any way to assert that a propeller can be used to slow the wind down relative to the ground even when it's traveling at the speed of the wind -- and that's what the paragraph that you 'Rubbished" said.
And again, correct -- to extract any energy from the rotation of the wheels, there will of course be a resultant braking force. This braking force can be calculated as can the thrust of the propeller. Only if the thrust is equal to or greater than the braking force can any given speed be maintained or accelerated from.
I will do so, but give me a bit to put a post together. I'm off to lunch at the moment.
Feel free to be rude about my post from yesterday. I did drop some (metric!) numbers in there.
Andy
Alrighty -- following is a simple energy/force analysis for a small propeller equipped DDWFTTW vehicle. In this analysis, we show that in a 27.5ftsec wind, and with the vehicle traveling over the ground at
55ft/sec, the retardant force on the wheels needed to drive the propeller is less than the propeller needs to keep the vehicle at that speed.A:
1.0 HP =3D 550 foot-pounds per secondB:
1/2HP =3D 275 foot-pounds per second This means that at 27.5ft/sec, 10lbs of force can be produced by a lossless propeller consuming 1/2hp. (we'll deal with losses below)C: If the wind is blowing at 27.5ft/sec and our vehicle is traveling DDW at 2x the speed of the wind, the vehicle is traveling over the ground at 55ft/sec and through the air at only 27.5ft/sec.
D: Through the establishment of "A", we know we can pull 1.0 HP from the wheels of the vehicle if it's propelled by 10lbs of force, and through the establishement of "B" we can see that the propeller in the relative tailwind only needs 1/2HP input to produce that same 10lbs of force.
E: We subtract the 1/2hp that the prop needs to produce its 10lbs of thrust from the 1.0hp that the wheels can produce from that same 10lbs and you have 0.5hp left over for the system losses.(told you we would get to losses).
In the real world we don't have lossless components of course. If you consider an 85% efficient propeller (easy to achieve) and an 85% drive train (even easier to achieve) we've still got nearly 1/4 HP left over for the Crr of the tires (very low for high pressure bike tires) and aero drag (which is also very low as our relative headwind is slight).
Do the same calcs on a no wind day and it's easy to see that the wheels still produce 1.0HP at 55ft/sec, but now the propeller is force to do work at 55ft/sec rather than 27.5ft/sec and it now takes a full
1.0HP at the prop to produce the 10lbs of force. This of course means that there is nothing left for losses and since there are *always* losses, the vehicle simply can=92t motivate itself when there is no wind.For clarity, I would like to add two words to the first paragraph in my last post. I have wrapped these two words in *** to highlight the edit.
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Alrighty -- following is a simple energy/force analysis for a small propeller equipped DDWFTTW vehicle. In this analysis, we show that in a 27.5ftsec wind, and with the vehicle traveling ***directly downwind*** over the ground at 55ft/sec, the retardant force on the wheels needed to drive the propeller is less than the propeller needs to keep the vehicle at that speed.
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[cough]bullshit[/cough]
So on the one hand we have someone who's actually built the thing and shown that it works doing an analysis of it working, and on the other hand we have a random usenet poster saying it's bullshit for no reason. Who to believe....
Hmmm... tough call - particularly in light of the fact that I designed, built, and drove it.
Thanks for the numbers and your skateboarder examples, I think I can grasp it now, but I'd hate to have to explain it down the pub from scratch ...
You won't explain it from that description as its rubbish.
I can imagine it being done by moving the sail (propeller blades) in such away that they are effectively tacking across the wind as they would on a land yacht going sideways but that isn't what is being claimed here. They are claiming they can develop thrust by removing energy from the vehicle and that gives the vehicle more energy to make it faster, which is just daft.
No, they are claiming they can develop thrust by removing energy from the wind by slowing it down as it passes through their propeller, the reaction pushes the vehicle forwards, which spins the wheels, which spins the propeller ... I think!
The vehicle is going faster than the wind, so the air that passes through is not the wind, it is the wind - the vehicle speed which is some negative value as the vehicle is going faster than the wind.
How do extract energy from the wind when they are effectively going into the wind *they* are generating by moving the vehicle to make the vehicle gain energy and go faster?
This is in the same area as the numpties that think they can run a car on water by splitting it into hydrogen and burning it by using the "spare" power in the alternator. You can even see videos of them doing it (but none without the batteries being connected).
On Sep 30, 2:50=A0am, "dennis@home" wrote
I'm sure it looks that way to someone that can't understand it.
Of course it is. Can you honestly not see how our "sails" (i.e. prop blades) are going sideways due to the rotation of the prop, as the vehicle itself goes straight downwind?
Wrong
You can see us doing this without batteries attached. Moreover, we've posted detailed build videos on YouTube so you can build your own working model for about $40 and prove it to yourself. Do the perpetual motion nuts do that?
How do you think we managed to fool the aero departments at Stanford and SJSU? How did we manage to fool the official observers from the North American Land Sailing Association?
Perhaps it's time you consider that you simply have to try harder to understand what's being claimed and how it works. There's no magic (and no free energy) here.
Yes, but that is not enough, but you jumped on it without actually understanding what I said or how it works. Draw a vector diagram and you can work it out for yourself.
Well its been claimed you accelerate the wind in a negative direction using energy from the vehicle
Let me know when you publish detailed plans and someone duplicates it.
My apologies, I misinterpreted your "bullshit" comment as being yet another one of the people saying that since they couldn't immediately see how it worked, then it couldn't possibly work. I recognised the ThinAirDesigns name, but hadn't looked at the Team page.
Care to expand on your comment?
Have you seen the cr@p in this video?
The claim is you can take energy from the apparent wind and accelerate the cart into the apparent wind. If you can do this then you should be able to go out on a still day, push the cart to create an apparent wind and then go from that. If you do that then that would be pretty difficult to deny. I won't hold my breath.
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