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| Mess with your minds: A wind powered craft going faster than a tail wind speed. |
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| electrodacus:
--- Quote from: gnuarm on December 21, 2021, 05:59:34 am ---What brought it home for me was when the video talked about the difference in ground speed and air speed. The wheels are working at ground speed while the propeller is dealing with the wind speed. The higher ground speed means the "gearing" (including the propeller pitch, et. al.) allows the faster ground speed to create less resistance than the force generated by the prop. As others have pointed out the propeller blows air backwards from the car reducing the wind speed and lowering the wind's energy content conserving energy. That should cover it, no? --- End quote --- No that will not cover it and is a completely wrong explanation. Not only the explanation is wrong what it implies never actually happen in reality. It is enough to look at my example with vehicle below speed A, vehicle at wind speed B and vehicle above wind speed C to see what happens and why their explanation can not work. Look at C and notice that the way that both road and wind moved in the vehicle reference frame there is no chance for vehicle to move from left to right. This examples are just there to eliminate the magic of propellers and air that people seem to not understand and also to show that without energy storage no wind power vehicle can exceed wind speed. That also means that Blackbird will be above wind speed for a few minutes at most then start to slow down as the stored energy is used up. |
| electrodacus:
--- Quote from: gnuarm on December 21, 2021, 06:14:17 am ---Here's a thought experiment. Instead of wind and a propeller, what if the wind were a conveyor belt and the prop were another wheel on that conveyor belt? Now the wheels can be geared so the ground movement applies a force to the conveyor belt. Can the car move forward faster than the conveyor belt? Yes, as the conveyor ramps up speed the car would be propelled faster than the conveyor from the start. Give the wheels a 2:1 gear ratio and the car will move at twice the conveyor speed. Like with the propeller, the ground wheels turn faster, so the conveyor wheels can exert more torque resulting in a net forward force. This is stirring a memory of some toy I had years and years ago. It also is making me wonder if that could be used for something practical. --- End quote --- I already did just that see the earlier replay. I used treadmills but conveyor belts as the same thing. No the vehicle can not move faster than the conveyor belt unless you have an energy storage device. See the end half of my video for details on how energy storage is used for a direct upwind vehicle |
| gnuarm:
--- Quote from: electrodacus on December 21, 2021, 06:12:57 am --- --- Quote from: gnuarm on December 21, 2021, 05:49:25 am --- Power is in units of work per time, J/s. The 10W already includes the time element as it is joules per unit time. Joules is not defined with any regard to time. It can be 10 W for one second or 1 W for ten seconds. 10 W is 10 J/s. So that could be 10 joules in 1 second, or 100 joules in 10 seconds, but it is 10 joules per second. Interesting that neither work nor power are considered SI base units, but that's not important. I don't know if this matters either, but NIST shows work (energy) as derived from force and distance. They show power as defined by work per unit time (J/s). NIST Diagram Does that help? --- End quote --- Sorry not sure where you have this wrong definition of what power and energy is. Power is measured in Watt Energy measured in Joules Applying a power of 10W for one second will result in 10J or what I prefer 10Ws. So the 10W includes not time is just a rate of work nothing to do with time while Energy includes time thus why I prefer to use Ws instead of Joules. One Joule is power of 1W applied for 1 second but can also be 10W applied for 100ms or 1kW applied for 1ms. You get the idea that power has nothing to do with time it is the rate at witch work is done. Not sure why Power and Energy are so misunderstood and the way they are defined by NIST is juts super bad and understandably confusing. Why will you ever define Power using Energy ? No wonder why so many are confused about how blackbird works. --- End quote --- There is nothing wrong with anything I've said. All the examples you give agree with my post entirely. The error is in your thinking that somehow time is associated with energy. You can pervert the units as much as you want, but a joule is a unit of energy or work that has no time factor in it. I see you talk about "rate" of work. That is the same thing as J/s. In this context rate implies time. So work is independent of time as shown by your examples above. The same work results from different powers with different times, but the products must always be the same. Why? Because power is a rate, a function of time and must be multiplied by time to get work... to eliminate the time factor in work. Do you think the same way about speed and distance? Speed is analogous to power and distance is analogous to work. Speed times time is distance, power times time is work. Do you consider distance to be a function of time? The only difference is we usually don't have a particular name for speed, rather it's distance per time like miles per hour, although they do use knots. So is distance Knot·hours? I suppose none of this is going to matter. You seem to be pretty entrenched in your thinking. That's fine. We can agree to disagree. |
| electrodacus:
--- Quote from: gnuarm on December 21, 2021, 06:30:07 am --- There is nothing wrong with anything I've said. All the examples you give agree with my post entirely. The error is in your thinking that somehow time is associated with energy. You can pervert the units as much as you want, but a joule is a unit of energy or work that has no time factor in it. I see you talk about "rate" of work. That is the same thing as J/s. In this context rate implies time. So work is independent of time as shown by your examples above. The same work results from different powers with different times, but the products must always be the same. Why? Because power is a rate, a function of time and must be multiplied by time to get work... to eliminate the time factor in work. Do you think the same way about speed and distance? Speed is analogous to power and distance is analogous to work. Speed times time is distance, power times time is work. Do you consider distance to be a function of time? The only difference is we usually don't have a particular name for speed, rather it's distance per time like miles per hour, although they do use knots. So is distance Knot·hours? I suppose none of this is going to matter. You seem to be pretty entrenched in your thinking. That's fine. We can agree to disagree. --- End quote --- There is nothing wrong in what you said but. While your example with speed and distance is a bit strange distance can not exist without time or more clearly space time are one single thing and not separate. While speed is a rate same as power tho power includes speed so is a bit of a strange analogy. What I was saying is that problem can be solved using power only and the result will be a net power that will also have a direction of action so 1d vector. But doing the calculations this way without including time will give you just the rate of change and you need to add the time if you want more info on what happens with the speed over time for example or net power over time. In any case the point I try to make is that no wind only powered vehicle traveling direct down wind or direct upwind can do that without using energy storage and it will need to use something other than the vehicle kinetic energy as that can not be used for this particular case where direction is either direct upwind or direct down wind. For the direct down wind Blackbird the pressure differential energy storage is used and for direct upwind version small internal elastic storage or gravitational storage (depends on design) is used plus the stick slip hysteresis witch is the trigger to release stored energy multiple times a second. That also means that for the direct down wind blackbird the vehicle will only be above wind speed for a limited amount of time. |
| gnuarm:
I don't think I'm willing to watch a 25 minute video that is hard to understand when I find an error in the first four minutes. You define the power from the wind as some stuff * (w-v). You claim this is the reason why a propeller vehicle can't exceed the wind speed. This ignores the rotation of the propeller which is the whole point! I also skipped ahead to 19 minutes where you claim the geared wheels and stick are not being analyzed correctly. I don't see how you can say anything of the sort. The analogy is not perfect between the propeller car and the gear car because the gear ratio of the gear car makes it move instantly while the propeller car has to accelerate to beat the wind speed. Maybe I'll watch the rest of this video someday, but for now it's just too much work. So how far would they need to drive the propeller car for you to say it's not running on stored power? |
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