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Unlocking the Mystery of Blackbird: Exploring Wind Power Leverage
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[QUOTE="AnssiH, post: 6572366, member: 45441"] I see there has been threads about Blackbird and "Down Wind Faster Than The Wind" operating principle in general, and I see a good number of people (most?) in those threads seemed to pick up some understanding of how it actually works. So that's a good start and gives me hope. I had never heard of this thing (to my recollection) when I saw the recent Veritasium video about it, the first one of the two: [MEDIA=youtube]jyQwgBAaBag[/MEDIA] When I saw that, my thoughts were "ah, leverage over wind power, pretty clever", and moved on, mostly just puzzled by how could this be difficult concept to grasp for anyone with basic physics knowledge (you know, pulleys are not magic either). I get it, at first glance it's easy to mix up the concept of wind speed with wind power, but "surely anyone who is interested of thinking a little would get it quite easily", right? Imagine my surprise when I first heard about that bet between Kusenko and Derek on some news articles. I mean, a physics professor making a blunder mixing wind speed with wind power, and failing to consider leverage. "Wowza", I thought. So at that point I decided to post some thoughts, which were supposed to lead into discussion about the sociological phenomenon where otherwise astute people can confuse their broken intuition with physics and stick with that belief, and how hard it can be to make out the difference if you are not really able to reason and build actual understanding of a thing [I][B]yourself [/B][/I](And this really is not unique to Blackbird). And basically how people end up making arguments that are the very definition of "crank argument", against what they believe are "crank arguments". The irony is very thick there. Anyway, to my surprise the people running that forum felt this is some kind of magical "energy out of nothing" hoax, and despite calm explanations that thread is now locked because the forum moderators think it's a hoax instead an attempt to educate. The whole thread is rather amusing comedy of errors, with demands of balloon videos (not sure why balloons are the be-all-end-all of scientific proof) and claims that I really don't know anything about Galilean transformation if I think that it applies in this case too, and various equations of wind pushing against a sail. Go check it out: [URL]https://www.scienceforums.com/topic/38295-that-ridiculous-physics-debate-about-that-wind-powered-vehicle/[/URL] So anyways, now there's no education happening because anyone daring to suggest that leverage is real also with wind is getting shut down by the moderator who doesn't want their forum to be overrun by internet trolls. (For example [URL]https://www.scienceforums.com/topic/38721-the-1969-blackbird-wind-turbine-sailing-craft-concept/[/URL] ) But maybe there are reasonable people here who might want to drop in a reasonable PM to the moderator(s) over there, until perhaps they pay attention. Obviously [U]be respectful[/U], don't be a troll! I think they'll get it if they just get convinced enough to think a little. Okay, and for anyone who still might feel like this thing "obviously breaks the laws of physics but somehow now one else but me realizes it", or otherwise thinks someone is claiming magical free energy (no one is except your own broken intuition), then I think Veritasium did an okay job explaining how it works [MEDIA=youtube]yCsgoLc_fzI[/MEDIA] If in doubt, here's a little FAQ: "But how can an vehicle move faster than the wind that is powering it?" - The same way that the wheel of a car can rotate faster than the engine drive shaft is rotating; by using leverage via gearbox, or via propeller. I saw someone in this forum has made a perfect example already with nothing but legos: [MEDIA=youtube]Cz4sf-pZ8jk[/MEDIA] - if you think that analogy is not valid to demonstrate that the [B]speed[/B] of a power source is not the same thing as the [B]power [/B]of it, then you have something upside down in your own intuition, and I suggest first reading up on how an ordinary pulley can convert force into larger force without breaking the laws of physics. "But it's not possible to get more speed out of the wind by driving it through a wheel" - It is possible to get more [B]power[/B] out of the wind than a simple sail does, by having a way to [B]leverage[/B] the momentum of the wind with a propeller. With larger propeller and larger rotation speeds you are simply leveraging more air mass than a sail. It's nothing more fancy than that. "But what about those last equations in the thread before it was locked?" - They are mixing up ground speed and wind speed, and using dv as vehicle acceleration instead of air mass acceleration by the propeller - pretty basic blunders. In the end the equation amounts to describe power collected by a sail the size of a propeller. That would be a pretty poor wind vehicle indeed. The correct calculations are not that hard to do. I might as well drop simple versions here real quick. For simplicity, say 10 m/s wind, and we examine a moment when we are moving exactly at wind speed. Now we just want to know, how much the power harvesting at the wheel brakes the vehicle vs how much thrust we are getting for the same power (as the propeller is effectively in 0 wind). Let's say we choose to use 10N of force at the wheel for power harvesting. That amounts to 100 watts (ignoring lossess): P_harvested_by_wheel = F * ground_velocity = 10 N * 10 m/s = 100 Nm/s = 100 Watts And how much thrust can we get for 100 watts? The exact value depends on engineering parameters of course, but let's say we use fairly large propeller (larger ones are more efficient for the same power), then taking equation 4 from here and solving for thrust at 0 wind environment: [URL]https://www.mh-aerotools.de/airfoils/propuls4.htm[/URL] Thrust = 100W / 0.5dv This is ultimately just approximation based on the propeller impacting the momentum of air molecules, and Newton's third law (equal reaction). Here dv is the amount of wind acceleration by the propeller. The smaller this number, the larger the propeller, and the more thrust we get for the same power. If we do 1 m/s (very large propeller), then we get 200 N of force (minus lossess). For 5 m/s, we get 10 N. Accounting for realistic lossess, the propeller efficiency is going to be about 80%, and we lose some in general friction. Overcoming the 1N braking force is really not that big of a deal. And the main point is, this is now just an engineering challenge, not fundamental physics problem. Anyway, until this makes its way to all 5th grade physics books and suddenly "everyone knows it, duh", who wants to help educate the physics moderators over at Science Forums? Or we can just continue talking about the sociological aspects about this here (Monty Hall Paradox, anyone...?) I see the people who built these things are members in this forum - if you are reading this, [B]good job educating people about something![/B] Also good job to Veritasium for pushing this public. The amount of backlash is very interesting, and very revealing. -Anssi [/QUOTE]
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