The Mystery of Blackbird: Exploring Wind Power Leverage

In summary, the Veritasium video on Blackbird and Down Wind Faster Than The Wind principle was informative and showed that people can be ignorant of physics concepts.
  • #36
A.T. said:
For some it is. I have seen some denial of that too.Yes, they are symmetrical counterparts:


I was going to post my version of this simple experiment. This guy did it better and with A-V aids! Perfect.
It is then trivial to show $$v=\frac {v_{track}} {(1-\alpha)}~~where~~~\alpha =gear~ratio $$
The rest is simply detail...
 
Physics news on Phys.org
  • #37
hutchphd said:
I was going to post my version of this simple experiment. This guy did it better and with A-V aids! Perfect.
It is then trivial to show $$v=\frac {v_{track}} {(1-\alpha)}~~where~~~\alpha =gear~ratio $$
The rest is simply detail...

Exactly, he made several variants of this gear box, but the formula is the same, including for an idealized version of the wind powered one.

2OVvW6Q.png








 
  • Like
Likes hutchphd
  • #38
hmmm27 said:
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.
I was an early skeptic/denier when this subject first appeared on PF. The fact that upwind is much easier to design/achieve and involves fewer steps in startup makes a big difference for understanding/accepting it. You can pick basically any gear ratio above a minimum torque and a craft will start from a stop and accelerate upwind. But a standing start is very difficult with downwind because it's a different operating mode/region than DWFTTW.
 
  • Like
Likes hutchphd
  • #39
russ_watters said:
You can pick basically any gear ratio above a minimum torque and a craft will start from a stop and accelerate upwind.
And if you pick a gear ratio sufficiently below that minimum torque the craft will start from a stop and accelerate downwind.

russ_watters said:
But a standing start is very difficult with downwind because it's a different operating mode/region than DWFTTW.
Starting downwind in general is much easier than starting upwind. Even a tumbleweed can start rolling downwind. But the specific mode the Blackbird starts downwind, is the symmetrical counter part to starting upwind. They are both equally "difficult".

Compare "START UP" for "CASE C (DOWNWIND PROPELLER)" with "CASE E (UPWIND TURBINE)" in the diagram below. They are on the opposite side of the stalled "CASE D" where the forces from air & ground approx. balance.

rotor_carts_01-png.png
 
Last edited:
  • Like
Likes hutchphd
  • #40
russ_watters said:
But a standing start is very difficult with downwind because it's a different operating mode/region
And, if you forget to put the parking brake on when you stop for a pee halfway through a downwind run, you could find yourself chasing the vehicle as it rolls merrily upwind, back in the direction you came from.
 
Last edited:
  • #41
AnssiH said:
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 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

It’s well known in sailing you can travel faster than the wind speed by tacking, though you would have to travel at an angle to the wind not directly downwind.

Under low friction scenarios such as ice sailing you can go 5 to 6 times faster than the wind:

https://en.wikipedia.org/wiki/High-performance_sailing#Iceboats

Robert Clark
 
  • #42
RobertGC said:
It’s well known in sailing you can travel faster than the wind speed by tacking, though you would have to travel at an angle to the wind not directly downwind.
It’s not widely known, even among sailors, that you can achieve a directly downwind velocity component (downwind VMG) greater than windspeed. That's because most recreational sailboats cannot to this. Those that are into high performance sailing, land-yachts or iceboats are more familiar with this.

According to GPS measurements, some iceboats can achieve apparent wind angles of 6-7°:

https://www.nalsa.org/Articles/Cetus/Iceboat Sailing Performance-Cetus.pdf

This corresponds to a downwind VMG of 5 times windspeed. Here is an interactive vector diagram that visualizes the relationship between apparent wind angle the achievable velocity vectors:

https://www.geogebra.org/m/tj5qf3w2

Here the vectors for maximal downwind VMG given an apparent wind angle of 6°:

AWA6_max_downwind_VMG.png
 
Last edited:
  • Like
Likes RobertGC and hutchphd
  • #43
Thanks for that. Quite interesting.

Robert Clark
 
  • #44
Slightly tangential, but I remember the furore when Bernard Smith published his '40kt Sailboat' book back in ~1963: Proa configuration, air-foil, water-foil, whoosh...
"No, can't be done, utter nonsense etc etc."
Never mind that ice-yachts and sand-yachts, their terrestrial equivalents, worked very well.
Aquatic version subsequently proven to work, to point where ruddy Americas Cup designs run 50+kt, ~100 kph...
So, try the prop-sailer, it works, unscramble the math...
 
  • #45
RobertGC said:
Thanks for that. Quite interesting.
In a recent video Steve Mould shows a mechanical model for sailing faster than the wind, but unfortunately he misses the chance to demonstrate the key aspect relevant for DDWFTTW: downwind VMG greater than windspeed.



So I recreated his model to show exactly that aspect:

 
  • Like
Likes Michael C
  • #47
If you have some education and memory, somewhere along the line you learned that the power of a fluid flow is proportional to the cube of the fluid's velocity. And so, you will eventually come to the thought that when an object being propelled by a fluid flow matches the velocity of the fluid, power will go to zero, and you will think with no additional power being supplied, no further increase in velocity is possible. Ipso Facto, DWFTTW is impossible. QED. And you will think a little bit about vectors, and say that tacking at an angle to the wind may yield forces that could possibly allow speed faster than the wind, but that DDWFTTW is out of the question.

I think the error comes because of the frames of reference. It is natural to think of the vehicle and what is propelling it as all being in the same frame of reference, and therefore a zero power point will comes as inevitable. Once you break that association in your mind, then it can all start to make sense (but until you ride in such a craft and experience the wind, you may not <believe> what you <know> to be true.

Consider a cart tied to a kite flying in the wind. Release the brakes and the cart will begin to accelerate downwind until some velocity where the various forces of friction match the force exerted by the kite string. Now you will think that moving will lessen the force exerted by the string. When you let go of a kite string, it falls out of the sky after all, and isn't moving the equivalent to letting go of the string? IT IS NOT. The air still flows, and is still developing power proportional to the cube of its velocity, but you are no longer harvesting any of it when you let the string out. The power and the force are there and remain unchanged, but you can no longer feel or measure them once you let go of the string.

Now consider sitting on the cart and reeling in line at a constant velocity. The velocity of the cart would be equal to whatever velocity the wind supplied until forces balanced, plus whatever you supplied and you have no problem believing that you could yank in line fast enough to move the cart faster than the wind speed. And you have no problem believing that your yanking on the line changed the force the kite exerted on the line by not one iota.

Now, instead of the wheels turning freely, and you yanking on the line, the line is attached to a windlass driven by the wheels at, say, a three to one ratio and the kite IS big enough to pull the cart without skidding the wheels. Yup, the cart will move faster, just as it did when you hauled in the line. And if it was geared right, the cart might even haul in the line fast enough that the kite remains stationary to its initial ground position--and you would never argue that this is 'impossible.' Nor that it depends on the length of the string. 10/100/1000 metres, that doesn't matter. Nor that you could switch the direction the windlass is turning and move the cart upwind instead of downwind.

A propeller driving a gearbox, rather than a kite and windlass, makes the string infinitely long, but other than that, changes nothing--except our perceptions. We perceive the propeller and cart as being in the same frame of reference, and therefore, somehow or another the wind velocity will go to zero robbing us of power, force and energy. But that just ain't so. In any situation where you can gain mechanical advantage over the force provided by a flowing fluid, you can generate velocity faster than that of the fluid.

I <know> it, but man is it hard to <believe>
 
  • #48
N1206 said:
I think the error comes because of the frames of reference. It is natural to think of the vehicle and what is propelling it as all being in the same frame of reference, and therefore a zero power point will comes as inevitable. Once you break that association in your mind, then it can all start to make sense (but until you ride in such a craft and experience the wind, you may not <believe> what you <know> to be true.
Yeah, pretty much. The kite example is pretty good. Of course for people who are not willing to examine their beliefs, no argument does anything. The most remarkable aspect to me is just how anti-scientific some people get when the feel the need to defend science.

I guess a fairly general pattern is that a person realizes the wheel-propeller mechanism is supposed to do "something", but because they have a simple wind equation in mind they have already decided that no matter what, "it can't create additional power". This is the thought they use to convince their own mind to avoid thinking about it any further. It's a classic short-hand to avoid a rational analysis.

This makes them see anyone who implies otherwise as a "crank who obviously doesn't even understand basic physics". So even when it's pointed out it's just a leverage mechanism, that's never given any thought because it's "coming from a crank anyway".

The denial of various real-world demonstrations of the same principle is where it really gets hilarious, and tends to be where these people usually reveal their poor ability to reason about things. Almost without fail they will offer an explanation that would violate the known laws of physics.

And when their argument is reduced to some obvious absurdity, they usually stop responding to it (or lock a thread). It's not that they realize they were wrong. It's just that they realize they have no explanation - at least not yet - but they also are still convinced they are absolutely correct "somehow", and get back to the comfort of that initial reasoning allowing them to avoid thinking altogether.
 
  • #49
Simple analogy: Two geared wheels of same size, and one gear of smaller size. Attach the small gear to one large gear, then use the other large gear to drive the small gear. The first large gear spins much faster than the second large gear. The first gear is the car and the second gear is the wind. How does the wind make the car go faster?
 
  • #50
Kosdon3200 said:
Simple analogy: Two geared wheels of same size, and one gear of smaller size. Attach the small gear to one large gear, then use the other large gear to drive the small gear. The first large gear spins much faster than the second large gear. The first gear is the car and the second gear is the wind. How does the wind make the car go faster?
Not sure I can follow your analogy without a diagram. But there are 3 different gear analogies in post #37, just above:

https://www.physicsforums.com/threa...entific-principles-again.1009943/post-6576716

Do you have one of these in mind?
 
Last edited:
  • Like
Likes hutchphd
  • #51
AnssiH said:
Yeah, pretty much. The kite example is pretty good. Of course for people who are not willing to examine their beliefs, no argument does anything. The most remarkable aspect to me is just how anti-scientific some people get when the feel the need to defend science.

I guess a fairly general pattern is that a person realizes the wheel-propeller mechanism is supposed to do "something", but because they have a simple wind equation in mind they have already decided that no matter what, "it can't create additional power". This is the thought they use to convince their own mind to avoid thinking about it any further. It's a classic short-hand to avoid a rational analysis.

This makes them see anyone who implies otherwise as a "crank who obviously doesn't even understand basic physics". So even when it's pointed out it's just a leverage mechanism, that's never given any thought because it's "coming from a crank anyway".

The denial of various real-world demonstrations of the same principle is where it really gets hilarious, and tends to be where these people usually reveal their poor ability to reason about things. Almost without fail they will offer an explanation that would violate the known laws of physics.

And when their argument is reduced to some obvious absurdity, they usually stop responding to it (or lock a thread). It's not that they realize they were wrong. It's just that they realize they have no explanation - at least not yet - but they also are still convinced they are absolutely correct "somehow", and get back to the comfort of that initial reasoning allowing them to avoid thinking altogether.
I think intuition, and our education fail us. We think of this like collisions. 'I throw a ball at 20 m/s and hit a motionless object sitting on a frictionless surface, how fast does the object I hit move?' But if I throw a ball at 20 m/s at an object moving at 20 m/s, how much do I change its velocity? Well if they are both vectored the same, the ball never hits the object. But wind is NOT like throwing balls! It is NOT like throwing infinite balls of infinitesimal mass. And so we suffer a failure of imagination.

We have similar failures when someone thinks about lift on aerofoils and starts in on Bernoulli, and upward pressure on the wings and how is THAT supposed to work? But if you think about it as balance-of-forces, then all of a sudden things begin to make sense. A hovering helicopter is pulled down by a force equal to mass times g. It has to apply an equivalent force to a mass of air in order to hover. The aerofoil shape forces air to accelerate in a vectored way. THAT makes sense -- but it is not generally how lift is taught to people. And I think very, very few people actually get formal education on fluid flow--but think they can extrapolate from common sense and what they have been taught.

Fluid Flow.
A physicist is quoted as saying he'll have two questions for God. "Why relativity?" and "Why turbulence?" :)
 
  • #52
My intuition for this whole thing was "huh, they're taking advantage of the energy gradient between the motion of the wind and the motion of the ground to do work, neat" and I moved on.

Their demonstration on a treadmill is literally just reversing the direction of the gradient by moving the ground instead of the air. The physical equations to represent it didn't seem necessary to explain it - maybe the obsession is a result of paper representations superseding demonstrations and experiment in the education system.
 
  • #53
InkTide said:
My intuition for this whole thing was "huh, they're taking advantage of the energy gradient between the motion of the wind and the motion of the ground to do work, neat" and I moved on.
That is the essence of the thing. The differential velocity of two materials provides the source of energy. The various possible engineering solutions to energy extraction, and the propulsion reference, simply cloud the issue.
 
  • #54
InkTide said:
My intuition for this whole thing was "huh, they're taking advantage of the energy gradient between the motion of the wind and the motion of the ground to do work, neat" and I moved on.

Their demonstration on a treadmill is literally just reversing the direction of the gradient by moving the ground instead of the air. The physical equations to represent it didn't seem necessary to explain it - maybe the obsession is a result of paper representations superseding demonstrations and experiment in the education system.
If you immediately see that the cart running on the treadmill is doing exactly the same thing as the cart moving along the ground in the wind, you are a whole leap ahead of many people who can't/won't accept this, equations or not.
 
  • #55
This system is much simpler than a lot of people are making it out to be, including Derek.

a) Wind can press forward on any surface moving slower than the wind.
b) A surface moving at an angle to the wind is moving slower than it in one dimension. (that's why a sailboat can sail fast at an angle to the wind)
c) A surface sliding across the wind can move forward as fast as the point of the wind it is being pushed by - not the point of the surface. That's why a sailboat can tack back and forth fast enough to beat a balloon: because the forward part of the sail is replaced by the backward part of the sail (closer to the source of wind) as it moves.

Beating the wind in this boat, duplicating a boat tacking on a cylindrical Earth as in Derek's lovely video, requires a, b, and c.

Thinking of the propeller as a "fan moving the boat forward" is in my opinion true but confusing. Its point isn't to fan the air backward but to fan itself backward to meet the air. The air is still moving forward against the blade (referenced to the ground) as the vehicle moves forward. It is pushing the vehicle forward, and some of that forward momentum is siphoned to keep up the leverage that allows it to go forward.

It's not really any more confusing than the idea that humans can run forward, pushing against the ground even though they are moving forward, by using their feet. We're just not accustomed to the system.
 
  • #56
Yeah, there seems to be many very intuitive ways to understand this. For me I just instantly started to think about two objects, like a rope moving inside a pipe, and thinking whether it would be possible to latch on to the rope, press a wheel against the pipe, and then gain leverage to move faster than the rope. The answer is very obviously yes.

But clearly different people have different kinds of intuitive sense, so what works for one person might not work for another. I think the biggest obstacle some people have must be that they simply try to convince themselves for reasons to not have to think about this further, and that escape is offered by the thought "no matter how you cook wind momentum, you can't turn it into faster speed", which as we know is patently false.

So that's why when you say the magic word "leverage", it does nothing because it sparks no motivation to think about the system.

On that topic, I attached a picture of yet another very simple way to think about the fact that no laws of physics are being violated. It's a long screw that is driven by a wheel. As soon as the screw is partially inside the wood beam, surely it's possible to push the wooden beam along, and have the wheel roll against a ground in such a way that the screw is moving faster than the wood beam.

It should be pretty obvious that that is analogous to gaining leverage from the momentum between two mediums. And it should be obvious that it must be physically possible for the screw to indeed push into the wood. And it should be obvious that propellers and screws are quite similar - there's just more slippage for the propeller. But that should not be a problem when we discuss about the theoretical possibility of moving faster than the medium you are "powered by" (in semantical sense).
 

Attachments

  • screw_downwind.png
    screw_downwind.png
    3 KB · Views: 125
  • #57
Mike S. said:
...because the forward part of the sail is replaced by the backward part of the sail (closer to the source of wind) as it moves.

Yes, the intersection of a streamline with the sail/blade is moving slower than the air along that streamline so the air can still push the sail/blade forward:





 
  • #58
 
  • #59
Hasn't this been covered already?

Yes, the thing is wind powered and 1) goes straight upwind, and 2) goes downwind faster than the wind.
This isn't a violation of any physics, however unintuitive it may seem.

I didn't watch the video, but I recognize the blackbird.
 
  • Like
Likes russ_watters
  • #60
Halc said:
Hasn't this been covered already?
Yep. So has pulling on a bicycle pedal making the bike go in the opposite direction. Etc, etc...
 
  • Like
Likes Halc
  • #61
Averagesupernova said:
Yep. So has pulling on a bicycle pedal making the bike go in the opposite direction.
Ah, but when it's going downwind the wheels do the energy input (exert drag) and the prop does the propulsion, sort of like the bike wheels making the pedals go instead of the other way around.
 
  • #62
Thread closed for Moderation...

Thread reopened while we search for the previous thread(s)...
 
Last edited:
  • #63
Halc said:
Hasn't this been covered already?
Multiple times, and I'd like to merge this new thread into the latest one, but I'm having trouble finding it. I seem to remember it was based on a good video about some bet that somebody won doing a test with a prototype. Anybody able to find it? Thanks.
 
  • #64
berkeman said:
Multiple times, and I'd like to merge this new thread into the latest one, but I'm having trouble finding it. I seem to remember it was based on a good video about some bet that somebody won doing a test with a prototype. Anybody able to find it? Thanks.
Isn't that the video linked to in the OP of this thread?
 
  • #65
vela said:
Isn't that the video linked to in the OP of this thread?
You think it was the same video? I'm not sure how to search on that. Plus, I seem to remember that the still picture for the video in the previous thread showed the vehicle sailing downwind on a desert flat of some kind, not faces...
 
  • #66
  • Love
Likes berkeman
  • #68
Last edited:
  • Like
Likes berkeman
  • #70
AnssiH said:
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.
What do we mean by the word "understand"? Hard to say in the case of an on-going discipline like Science. To understand the present state of knowledge doesn't mean your picture will be the correct one in the future. It can only mean that you have arranged all the facts (at the time) in your mind in a way that's self-consistent and which allows you to extrapolate reliably and to predict accurately what will happen in new situations / experiments.

The depth to which the individual feels they can do that will vary. Many people feel that they understand something on the grounds of what they have merely memorised it but how many of us fully apply the criterion above before feeling we have understanding? (Some smart Alec on PF can come along and shatter that understanding so easily.) o:) o:) o:)

Basic Maths is a field where I believe that we can justify saying that we 'understand' because we can repeat endless arithmetic problems and always get the right answer. But that's because it deals with axioms, I guess.
 

Similar threads

Replies
15
Views
2K
  • Classical Physics
2
Replies
40
Views
4K
  • Aerospace Engineering
Replies
3
Views
2K
  • Classical Physics
Replies
5
Views
1K
Replies
21
Views
1K
Replies
34
Views
2K
Replies
271
Views
41K
  • Other Physics Topics
Replies
30
Views
3K
Replies
9
Views
492
Replies
8
Views
934
Back
Top