Blackbird faster-than-wind vehicle and it's that time to defend basic scientific principles (again)

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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:

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 yourself (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:
https://www.scienceforums.com/topic...ysics-debate-about-that-wind-powered-vehicle/

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 https://www.scienceforums.com/topic/38721-the-1969-blackbird-wind-turbine-sailing-craft-concept/ )

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 be respectful, 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

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: - if you think that analogy is not valid to demonstrate that the speed of a power source is not the same thing as the power 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 power out of the wind than a simple sail does, by having a way to leverage 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: https://www.mh-aerotools.de/airfoils/propuls4.htm

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, good job educating people about something! Also good job to Veritasium for pushing this public. The amount of backlash is very interesting, and very revealing.

-Anssi
 
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  • #3
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  • #4
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Yeah, Monty Hall Problem is indeed exactly what comes to mind from all of this, and I find it absolutely fascinating for the exactly same reasons. When I first heard of it, it gave me a brief pause but I didn't find it difficult to reason why it works like it does. I remember thinking simply "okay, my first choice has 66% chance of being wrong, and if it is, the host will reveal the other wrong choice for free." Done.

That's the entirety of the solution, yet there are otherwise very intelligent people who not only make a blunder, and insist on being right even when repeated being explained the very simple mechanism here. So why that occurs?

Isn't it absolutely fascinating that the more a person is knowledgeable about a topic (such as statistics or physics), the more prone they are to fall victim to a houlier-than-thou attitude, and assuming everyone else are simply much dumber than them and that's why they are fooled. This is always done in the name of "defending science"; when in fact the attitude is exactly as anti-scientific as can be. The key word there was "knowledgeable". Not necessarily "understanding".

I mean, what makes some people just much more prone to fall victim to the logical blunder? To my experience, you can roughly divide people to two types of learners; those who tend to memorize things, and those who tend to understand things. I've always been heavily leaning to the latter camp. Did terrible in school if anything had to be memorized, but usually aced everything that could be answered by reasoning about it. In fact I compensated for my dislike to memorize things by figuring out how to reason about things even when they were not taught that way, and sometimes realized I ended up understanding things more deeply than my teachers. I remember many incidents of teachers getting confused by me solving things in completely different manner than the textbook suggested. Some teachers liked it, some didn't.

What is interesting is that people who memorize things can do very well in the education system, and end up with large credentials, even if they are absolutely useless at reasoning about anything logically themselves. I had a math teacher (also by far the best teacher I have ever had) who was always acutely aware of this problem, and specifically laid down traps for people who had not formed any understanding of the topics, but merely memorized things. Some people who used to be the best ones in class suddenly became absolutely useless and confused just about everything. It was so revealing. In the meantime suddenly I scored top scores of the entire school - without ever even making an attempt to remember anything. I always wanted to rather just reason the answers on the spot from having formed understanding of a topic and thus would not fall prey to any "memory traps". This was all completely due to a teacher who wanted to make sure her students understood things, instead of just remembered things.

So the point is, I think these falling for these brain teasers reveal that tendency to "memorize over understanding" in people. This is why it's so surprising that you can have physics professors or even Nobel physicist who just do not get it, and rather rationalize to themselves that they are simply smarter than everyone else.

Or that's how it certainly seems to me 🤷‍♀️

-Anssi
 
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  • #5
AnssiH
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We have some existing threads on this topic. You may want to join one of those. Here is one that popped up on the forum search:

https://www.physicsforums.com/threads/ddwfttw-looking-for-the-least-confusing-explanation.896869

Unfortunately, we have no influence over that site. All we can do is keep things more factual here.
Yeah I saw a number of threads about the topic here, and seems like for the most part people here are "getting it" pretty much straight (with occasional kicking and screaming ;) )

If you look at the thread at the other forum I think you can see that the problem is probably not the way I'm explaining it (tried quite many different perspectives), but rather the attitude of the other party that completely prevents them thinking about it. It's the houlier-than-thou attitude that we all sometimes get when we feel like we just don't have time for non-sense. It just gets a rather extreme form here, and the irony becomes incredibly thick when that attitude is posed as a defense of science (when it's in fact the exact opposite of scientific thinking). You will see the other party posting even quotes of Feynman warning about this exact tendency.

Anyway, the sad part of it is that scientific thinking often gets confused with "believing in science", and locking threads with misguided physical understanding only promotes that confusion. This leads into some of the most damaging confusion against scientific thinking, I think.

Btw that moderator just told me in PM that it's a crackpot idea to even suggest that force could be leveraged into larger force. 🤦‍♂️ So I think I should probably do the responsible thing, and approach other moderators of the forum with a suggestion that a person who does not understand basic leverage of force should perhaps not attempt to moderate a physics forum. I mean.... am I wrong?

-Anssi
 
  • #6
Baluncore
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I think it is interesting that British engineering has an affinity for 126 mph;

The wind-powered vehicle, Greenbird, weighing 600 kg, set a speed record in 2009, of 126 mph.

The blue steam locomotive, Mallard, set a speed record in 1938, of 126 mph. But Mallard weighed 167 tonne, and was running down Stoke Bank, a 1 in 200 descent, with a train behind it.
 
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  • #7
hutchphd
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That's the entirety of the solution, yet there are otherwise very intelligent people who not only make a blunder,
I first heard this as a"puzzler" on "CarTalk" (a great show formerly on public radio hosted by MIT-trained auto mechanics) and said "this can't be right!" So later I got a piece of paper and 5 minutes later had the decision matrix in hand and an amazed smile on my face.
Incidentally "belief in science" is an oxymoron....!!
 
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  • #8
OmCheeto
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... So anyways, now there's no education happening because...
OceanBreeze/Moderator from sciencforums.com said:
[of] no math or logic

I've been involved in a bunch of such discussions here at PF. My guess as to why they keep popping up is that the problem never gets solved. And I think OceanBreeze hit the nail on the head, as to why.

ps. I may or may not have said the exact thing a decade ago.
 
  • #9
AnssiH
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Incidentally "belief in science" is an oxymoron....!!
Yeah, I wish more people would realize that.

I've been involved in a bunch of such discussions here at PF. My guess as to why they keep popping up is that the problem never gets solved. And I think OceanBreeze hit the nail on the head, as to why.

ps. I may or may not have said the exact thing a decade ago.
Do you mean you think no education is happening because there's never any math or logic in the presentation as to how this works?

I would disagree with that, because in order to convince oneself that this does not violate the basic laws of physics, all that's needed is to calculate the power input from the wheel, the implied braking force, and how much thrust force you can get for that same power. All the required math to calculate that is those two simple equations in my OP. There is as much reason for a scientific paper, as there is reason for scientific paper proving Monty Hall paradox. Or a gearbox. Or a pulley.

You see, Monty Hall Paradox can also be explained with extremely simple math. The problem of some very educated people not getting it is not that the math would be somehow too complex - it really isn't (see Wikipedia article for this). It's that their broken intuition tells them that math just cannot possibly be right, so they dismiss any and every attempt to show this as math as "flawed somehow, and I'll figure out how one day". Same as denying any video, or any other demonstration of the principle as valid. (And there lies a possibility to show with baby steps how some things they accept as valid actually map exactly to the operating principle of this vehicle)

The real way towards understanding it is to take a step back from those assumptions that cause the initial blunder, and work out how it works until one realizes how it aligns with basic laws of physics, and leverage. That lego video really captures the entire principle completely. And once people get it, they also get why no one is bothered of submitting a scientific paper.

The blunder is really very simple, it is to intuitively equate wind speed with harvestable wind power. To convince oneself that the "speed" of a power train does not tell you how much "power" you get from it, that lego video demonstrates it already. As does a truck engine and its gearbox. Or one just needs to think about, is it enough to know the "speed" of wind to calculate how much power can be harvested from it. Of course it is not.

Then there's of course the more basic blunders of analyzing a propeller as if its a sail and coming up with an answer that it cannot move faster than wind (of course with a sail it could not), and all the other different ways to explain oneself how any demonstration of the principle must be simply mistaken. Just like with Monty Hall Paradox.

But to me the most amazing aspect of all this (especially in Monty Hall case) is how willing some people are to rationalize to themselves - without blinking an eye - that they simply are one of the few people who understand how the proposal is fundamentally flawed. And how that rationalization is so strong that they never feel the need to seriously examine their own assumptions. I would get that sentiment if the "apparent problem" didn't seem so damn obvious. But since it is so obviously "seemingly impossible", I would think it would give people a hint.

-Anssi
 
  • #10
OmCheeto
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...

Do you mean you think no education is happening because there's never any math or logic in the presentation as to how this works?

I would disagree with that, because in order to convince oneself that this does not violate the basic laws of physics, all that's needed is to calculate the power input from the wheel, the implied braking force, and how much thrust force you can get for that same power. All the required math to calculate that is those two simple equations in my OP. ....
I fail to see how your two equations solve this problem.

AnssiH said:
P_harvested_by_wheel = F * ground_velocity = 10 N * 10 m/s = 100 Nm/s = 100 Watts
...
Thrust = 100W / 0.5dv
 
  • #12
AnssiH
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I fail to see how your two equations solve this problem.
P_harvested_by_wheel = F * ground_velocity = 10 N * 10 m/s = 100 Nm/s = 100 Watts
...
Thrust = 100W / 0.5dv

Those are indeed the two I was referring to! 👍

So if it doesn't immediately jump out, I'll explain in detail. Please think this through;

We want to know how much power we can get from the wheel, how much it would be causing the vehicle to decelerate, and how much thrust we could gain in exchange. Right? Don't assume you just know the answer intuitively.

First, the more power we harvest, the more we are causing the wheel to brake. As you might know, power is the time derivative of work ("rate" of work). The more "newtons" you can push the further at the less amount of time, the more "power" you have. 1 watt is literally defined as "1 newton meters per second". With 1 watt you can push 1 newton object, one meter, within one second.

In other words, if you are harvesting power with a wheel whose circumference is moving at 10 meters / second against the ground, with a harvester that is causing 10 Newtons of decelerating power, then you are harvesting - by definition - 10 N * 10 m/s = 100 Nm/s of power. You may be harvesting that power into a flywheel, or collecting it into a battery, or pushing it straight into a propeller. Either way, you are harvesting 100 watts of power (minus losses, but let's do the fundamentals first).

Okay so that's the first equation done, and I hope you have now an intuitive image in your head of a momentary snapshot in time, where a vehicle is moving at the speed of 10 m/s, and harvesting 100 W of power at the expense of 10 N of deceleration force. We are talking about a leverage mechanism, so very important to not mix up the different types of quantities accidentally ;)

Next we want to know how much thrust force 100W of power can generate. In particular, we are interested of answering "can the vehicle reach faster than wind speeds directly downwind". So if we set the situation such that at 10 m/s the vehicle is moving exactly at wind speed (wind is also moving at 10 m/s), then all we need to answer is, can we generate more forward thrust force than 10 Newtons with 100 Watts of power. If we can, then we know the thrust force is larger than deceleration force at this moment, and in the next moment the vehicle is moving faster than wind, and we will have answered our question.

So that second equation is a simple version of power to thrust by propeller, according to momentum theory of thrust. See equation 4 in https://www.mh-aerotools.de/airfoils/propuls4.htm and equation 1 in the same page for defining v and dv

So, ignoring lossess:
Power_from_engine = Thrust * (air_speed + air_mass_acceleration_by_propeller / 2)

In other words, to solve for Thrust;
Thrust = Power_from_engine / air_speed + 0.5 * air_mass_acceleration_by_propeller

Since the air_speed is 0 at the moment of interest, it reduces to:

T = P / 0.5dv
Which is what I wrote.

T is thrust force, P is engine power and dv depends on our propeller design - the larger the propeller, the smaller the amount of air acceleration we need for producing given amount of thrust force (because we are dealing with more mass, it's just basic Newton's laws we are dealing with here). Incidentally, as you can see from that very relationship, the larger the propeller (the smaller the dv), the more efficiently we generate thrust for given amount of power.

So now if you understand that relationship exactly, I think we are ready to crack this mystery by placing our available power into that equation, and deciding some reasonable number for dv. Let's say 1 m/s of air mass acceleration for easy calculation (and dramatic effect):

T = 100 Nm/s (a.k.a. watts) / 0.5 * 1 m/s = 200 Newtons.

200 Newtons of force pushing us forward out of spending 10 Newtons of force on decelerating. Impossible? Think again. What does a leverage do? It converts force into larger force. The power harvesting was happening against ground momentum, while the power deployment was happening against wind momentum - that's the leverage. The implied power is coming out of the difference of momentum between two mediums that we have placed our vehicle in-between, and from the fact that only our propeller size and speed limits our ability to collect force from larger amount of mass. Exactly like that lego demonstration - substitute wind momentum for the upper beam, and propeller for the gearing system against that mass, and there you have it.

Does that mean it's free energy? Of course not. No wind, no difference in momentum between mediums, and no way to leverage anything at all.

Does it mean the vehicle would accelerate forever? Of course not. As you gain more air speed, you also start to lose your ability to power against it effectively. On other words, in theory (mathematically) you can always mathematically increase your leverage forever, but realistically at some point you run out of ability to effectively get a hold of one or the other medium. And ultimately your entire energy reserve is capped to the total momentum difference between between the two mediums (i.e. even if you could harvest it all, at some point Earth would stop rotating and all wind would cease to blow (so to speak)).

(Note also that with V of 10 m/s, - if the vehicle was working against 10 m/s headwind - with the same equations, we would get an ideal thrust output of little less than 10 Newtons, as one would expect since our thrust efforts would not be helped by tailwind - there would be no leverage involved)

And there you have it, those two simple equations encapsulate the whole system from physics fundamentals point of view. Yes, in real case there are losses involved depending on our engineering prowess so we are not going to get really that high numbers (but we will easily get more thrust than deceleration still). But the main point to grasp is that physics fundamentals imply this system works. In fact, if it didn't work, now that would be weird 😱

It should also be clear at this point that it is possible to leverage momentum in headwind direction by changing the leverage other way around. Also exactly as implied by physics.

(Yes that Olympiad exam also has got simple equation, but I find my version even easier to comprehend intuitively...but maybe that's just me 🤷‍♂️ )

-Anssi
 
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  • #13
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I think what intrigued me about this is that I often think when the wind hit a windmill, the blade turn with the wind. It baffles me when the blade turns the opposite of the wind. So here I drew up a system of how we can make the blade turn any direction.

we have blades and a large wheel. When the wind hit the blades, the blades wants to turn with the wind, however, the large wheel anchored to the ground produce a large torque. The torque transmit through gears to turn the blades in any direction desired. There we have it.
AC04F5D2-FAEC-4712-A72E-416C6DDC8992.jpeg
 
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  • #16
A.T.
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When the wind hit the blades, the blades wants to turn with the wind, however, the large wheel anchored to the ground produce a large torque. The torque transmit through gears to turn the blades in any direction desired. There we have it.View attachment 293825
Yes, considering extreme cases often helps, and your huge wheel makes it obvious.

Alternatively you can consider what happens when you change the pitch of the rotor blades, as this directly controls how much downwind force vs. torque they produce. All the possible cases are in the table below. For downwind faster than the wind you need CASE C, which spins opposite with wheels on the ground, than it would when held in air, off the ground.

rotor_carts_01.png


This animation can help to make sense of the diagram above:

 
  • #17
AnssiH
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You guys are starting to restore my faith in humanity.
 
  • #18
hutchphd
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I wish someone could do that for me!!
Between COVID and 🐘 mine has evaporated.
 
  • #19
OmCheeto
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Reading your post hurt my brain.
...
10 Newtons of decelerating power
...
Power_from_engine = Thrust * (air_speed + air_mass_acceleration_by_propeller / 2)
...
Also, it's a general rule of mine that I stop reading after the first mistake. I gave you the benefit of the doubt and looked for a second one, which didn't take long. Hence, I ignored the rest of your post.

I also went back and reviewed some of the more than a decade old threads on the topic and decided there's no logical reason for me to go through that again, so I'll be dropping out of this discussion. Cheers!
 
  • #20
Baluncore
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So I have a hypothetical land yacht with a pylon and two blade rotor. The blades and the ground wheels have generator / motors, connected by a reversible, infinitely variable ratio, converter / gearbox.
Given a flat area with a specified steady wind, I select two points, a starting point and a destination.
I need an algorithm that will solve;
What path between the points gives the minimum travel time ?
How should I configure the rotor and gearbox ratio ?
How do I transition to, then accelerate on, a new leg of the course ?
 
  • #21
AnssiH
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Reading your post hurt my brain.

Also, it's a general rule of mine that I stop reading after the first mistake. I gave you the benefit of the doubt and looked for a second one, which didn't take long. Hence, I ignored the rest of your post.
I feel privileged to have been allowed not one but two whole mistakes. 😌
But sorely disappointed that you decided to not explain what those mistakes were. How am I supposed to now learn anything?

Shall we try scientific method instead?

Best,
-Anssi
 
  • #22
Baluncore
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But sorely disappointed that you decided to not explain what those mistakes were. How am I supposed to now learn anything?
that is causing 10 Newtons of decelerating power,
Dimensional analysis makes it possible to verify the equation by equating the units. That fails immediately you mix Newtons and power. Is it a power in watts, or a force in newtons? It is an inconsistency that makes it very difficult to follow because there are many interpretations.

When the unit is named after a person, the unit is not capitalised, the units newton and watt are named after the people, Newton and Watt. But the symbols for the units are capitalised, so the units newton and watt become N and W.

Power_from_engine = Thrust * (air_speed + air_mass_acceleration_by_propeller / 2)
You have got me there.
Energy = force * distance; Power = force * velocity.
(air_speed + air_mass_acceleration_by_propeller / 2)
Is ½ · (air_mass_acceleration_by_propeller) a velocity ?
You should not add things that do not have the same units.
 
  • #23
A.T.
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So I have a hypothetical land yacht with a pylon and two blade rotor. The blades and the ground wheels have generator / motors, connected by a reversible, infinitely variable ratio, converter / gearbox.
Given a flat area with a specified steady wind, I select two points, a starting point and a destination.
I need an algorithm that will solve;
What path between the points gives the minimum travel time ?
How should I configure the rotor and gearbox ratio ?
How do I transition to, then accelerate on, a new leg of the course ?
That's a complex optimization problem. The optimal strategy will depend on how the various efficiencies compare to each other at different conditions. This is already complicated for a conventional sail craft, and even more so for a rotor-sailcraft. I guess you would need numerical optimization, that basically simulates the different options.

Also, for anything that is not too close to directly downwind / upwind, you will likely find that a conventional sailcraft with a vertical wingsail of comparable size is more efficient.
 
  • #24
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Even experts are human. The Monte Hall problem is an extreme example since it should routinely lead an expert to apply a simple Bayes Rule calculation to get the correct answer and to identify where people are going wrong.
In the "faster than wind" problem, it seems to me that a person who can accept that a sailboat can even tack against the wind should also be able to accept the "faster than wind" answer.
In fact, a sailboat can go faster downwind than the wind without the use of internal moving parts, gears, or other simple machines -- just a keel and a sail pointed correctly.
 
  • #25
A.T.
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In fact, a sailboat can go faster downwind than the wind without the use of internal moving parts, gears, or other simple machines -- just a keel and a sail pointed correctly.
Yes, this analogy is used in the Veritasium video and also visualized here:

 
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  • #26
AnssiH
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Dimensional analysis makes it possible to verify the equation by equating the units. That fails immediately you mix Newtons and power. Is it a power in watts, or a force in newtons? It is an inconsistency that makes it very difficult to follow because there are many interpretations.
The very definition of a watt is 1 nm/s.

It's defined as "the rate of work when an objects velocity is held at constant 1 m/s against a force of 1 newton". So I don't really see a dimensionality problem here as that's what I'm literally calculating, speed against a constant opposing force. Thus P = Fv and the dimensions are well defined (newtons and meters per second). Thus we get very conveniently into watts.

Or do I completely misunderstand your argument? 🤔

When the unit is named after a person, the unit is not capitalised, the units newton and watt are named after the people, Newton and Watt. But the symbols for the units are capitalised, so the units newton and watt become N and W.
Good point! Never really thought of that, nice to learn new things :)

You have got me there.
Energy = force * distance; Power = force * velocity.
(air_speed + air_mass_acceleration_by_propeller / 2)
Is ½ · (air_mass_acceleration_by_propeller) a velocity ?
You should not add things that do not have the same units.
I cited the source which defines the quantities:
https://www.mh-aerotools.de/airfoils/propuls4.htm

TL;DR "air_mass_acceleration_by_propeller" is their dv which is the "additional velocity, acceleration by propeller" (their words), in other words the speed difference implied by the air having been disturbed by a rotating propeller. It is indeed probably a bit misleading for me to have used the word acceleration there as it's not really.

But with that mistake fixed, you can see from there, it comes out as fully valid dimensional analysis. It comes from the idea of doing a momentum analysis on the air mass that is being disturbed by the propeller (hence, "Momentum theory"). It's very crude approximation of the real thing ignoring inefficiencies etc, but it's plenty good enough for demonstrating the simple fact that propeller must be capable of giving plenty of thrust even at 0 air speed, since it's thrust depends on ground speed. So, when people think about that for a while, it should dawn on them that actually if the propeller is able to give more thrust than power generation is losing, then this does indeed represent a way to leverage more thrust from the air mass, than a sail could.

And that really is the only thing that needs to be demonstrated to understand the operation principle, which was my point about "these are the only two equations you should need to convince yourself of the validity of the operation principle"

Thanks,
-Anssi
 
  • #27
Baluncore
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The very definition of a watt is 1 nm/s.
I sight read your first line as 1 nanometer per second.
To be accepted you need to be obsessive about correct case and punctuation; 1 N·m/s.

But with that mistake fixed, you can see from there, it comes out as fully valid dimensional analysis.
I have not gone back to check and correct what you wrote. It should not be necessary to back-track and analyse every line, in order to work out what you actually meant when you wrote it.
Pseudoscience involves making terminology soup from scientific text. It is accuracy, dimensional analysis, and fact checking that makes the difference.

I am interested in the subject, but I don't want to waste my time. You usually have only one chance to make your point clearly. To put it simply, if you do not use the crystal-clear language of physics and math correctly, your work will be rejected in frustration.
 
  • #28
AnssiH
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I sight read your first line as 1 nanometer per second.
To be accepted you need to be obsessive about correct case and punctuation; 1 N·m/s.
Lol, yeah you are right!
Sorry about that :)

I have not gone back to check and correct what you wrote. It should not be necessary to back-track and analyse every line, in order to work out what you actually meant when you wrote it.
That's perfectly understandable, and truth be told I feel like even this analysis is overly complicated to point out the simple fact that laws of physics are not being broken by something being able to gain more speed than the speed of its "powertrain", which air mass effectively is in this case. I mean coming from fundamental concepts, we are talking about something not much more complicated than a gearbox. But when people get into calculating efficiencies and realistic propeller aerodynamics, I think we are no longer talking about "is this violating laws of physics", but rather "can this be engineered".

Thanks again!
-Anssi
 
  • #29
AnssiH
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So that thread is closed again, and this time the closing argument is that yes, propellers rotate and thus generate thrust, but only if the rotation is done by "an engine".

The story doesn't say how air detects the means by which the propeller is rotating, to somehow circumvent the propeller when it is powered by ground momentum. Would have loved to hear the explanation, but instead I was just told my (rather patient) persistence of pointing out blunders is a sure sign I have a mental illness.

1639604755312.png

Quickly someone, restore my faith in humanity again.
 
  • #30
Baluncore
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The story doesn't say how air detects the means by which the propeller is rotating, to somehow circumvent the propeller when it is powered by ground momentum.
An “engine” is any device that changes energy of one form into another.
It could be as simple as a lever, a winch, a gearbox, or a motor.
A motor is a particular engine that motivates something, it causes movement.
An “engineer” is someone who arranges or operates any such engine or device.
The “engineer” employs “ingenuity” to arrange such things, hence the job title.
The term “engine” is NOT derived from “energy”.
https://www.etymonline.com/word/engineer
 
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  • #31
AnssiH
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Thank you, I'm feeling it coming back again.
 
  • #32
Baluncore
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Quickly someone, restore my faith in humanity again.
That is all part of the free counselling service we offer here at PF.
 
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  • #33
hutchphd
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Quickly someone, restore my faith in humanity again.

I have an alternative to our service. Instead I suggest a mantra that one quietly says as one learns to ignore the fools and miscreants (with apologies, it works for me when all else fails):

"I am up to my a**hole in midgets.
I am up to my a**hole in midgets.
I am up to my a**hole in midgets. "


Usually three times is sufficient. During the past five years I have needed it more and more. Peace.
 
  • #34
hmmm27
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What's funny is that a "directly into the wind" vehicle isn't too difficult to understand, whereas the "downwind faster than the wind" one is... but they're exactly the same thing except upside-down, each other.

Quickly someone, restore my faith in humanity again.
Manatee1[1].jpg

How's this ?
 
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  • #35
A.T.
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What's funny is that a "directly into the wind" vehicle isn't too difficult to understand,
For some it is. I have seen some denial of that too.

whereas the "downwind faster than the wind" one is... but they're exactly the same thing except upside-down, each other.
Yes, they are symmetrical counterparts:



 
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