Sometimes very active discussions about peripheral issues overwhelm a thread, so this is a permanent home for those conversations.
This is also a continuation of previous Sandbox threads (1) and (2) that have fallen victim to the dreaded page bug.
The Skeptical Zone
"I beseech you, in the bowels of Christ, think it possible that you may be mistaken."
Knowing there are many knowledgeable people here, I wonder if someone has already pondered the Blackbird phenomenon, where a wind-powered vehicle does the seemingly impossible and overtakes the initial tailwind and reaches a speed of over twice windspeed.
Here’s the video. Note the direction of rotation and pitch of the prop when the vehicle starts to move.
It’s tacking. I’m not smart enough to analyze all the vectors, but if it is travelling faster than the wind, it is doing the equivalent of tacking.
Quick and dirty thought. Imagine the blades don’t turn. In which case, they act like sails. The wind starts the vehicle moving forward, just as it would with a land sailboat.
The wheels turn, driving the blades. At this point my brain freezes.
First thing to note is the prop is geared to the wheels but when it starts off, the prop rotates against the wind and moves downwind despite the prop, not because of it. The vehicle is moving at first only due to the whole vehicle acting like a fixed sail.
Once it has passed windspeed it is effectively in a headwind and gears that up to be able to accelerate to perhaps twice windspeed, according to the observers. What I can’t work out is how the cart gets from just below windspeed, being driven directly by the following wind to beyond windspeed when it is being powered by the wind on the prop driving the wheels.
petrushka,
Glad I’m not the only one!
Exactly! And beyond wind speed there is wind energy available to turn the wheels if the pitch and gearing are right. It’s where the power for the transition comes from that’s baffling me.
Another possibly bad analogy. Consider a clock pendulum. The pendulum drives the escapment, and the escapement drives the pendulum. Of course there is energy leaking in from falling weights.
I can follow that, and I suspect with the sail car the blades continue to act like sails. I just don’t follow how.
MarkCC’s attempt
But see the comment at the bottom.
I have not studied Mark’s post, but your question led to those I-remember-reading-about-that-somewhere-a -few-years-ago moments. Luckily, it only took one guess to track it down. (I’m sure Keith could have quickly found it somehow with a Google search).
MarkCC’s Earlier Attempt He Retracts in Above
It appears that tacking takes advantage of friction to amplify motion. A clear violation of Sewell’s Second Law. Send in the thought police.
BruceS,
Thanks, Bruce. Great link! I see Mark initially made the same mistake I made about the prop turning against the wind, on start-up.
I thin there is a tendency to think of Newtonian reaction in a vacuum, whereas what we seem to have here is two streams of fluid (wind) pushing against each other.
As the car travels forward, we tend to think of the wind pushing against the blades, but the wind is pushing against the wind from the blades.
two streams pushing against each other. Not something that would work in a vacuum.
Need some seriously frictionless gearing.
I’ve seen explanations that make that point. Imagine the rotor blades as sailboats tacking at 45° round a cylinder. I can convince myself that, within frictional and wind-resistance limits, sustained above-wind-speed is possible but I can’t see how the vehicle gets through the dead-zone of no apparent wind.
Alan,
I think the problem is that you’re intuitively equating two conditions:
a) A condition in which there’s no wind and the vehicle remains stationary.
b) A condition in which the vehicle is traveling at wind speed, which means there’s no apparent wind with respect to the vehicle.
They seem equivalent, because in both cases there is no apparent wind and thus, intuitively, nothing to push the vehicle faster.
But the important thing is not whether there is an apparent wind relative to the vehicle — it’s whether there is an apparent wind relative to the propeller blades.
When the wind is blowing, there is always an apparent wind relative to the propeller blades, including in situation (b) above. Remember, in condition (b) the vehicle is moving relative to the ground, which means the wheels and the propeller are turning. That’s different from condition (a), in which they aren’t.
So in condition (a) there is no wind relative to the propeller blades, but in condition (b) there is. That makes all the difference in the world, and it’s how the vehicle gets through the “dead zone” you described.
I bet a GA could solve the problem.
petrushka,
No, there’s just air interacting with the blades.
No, you just need to make the propeller big enough to overcome the frictional force, however large it happens to be.
Programming in R for Data Science
Starts 4/19
Free
Nick Matzke is an R savant.
This is Chernobyl day, 30 years after.
http://ersimages.com/chernobyl/chernobyl.html
One for the scientists:
http://www.nature.com/ncomms/2016/160425/ncomms11328/full/ncomms11328.html
“Spontaneous formation and base pairing of plausible prebiotic nucleotides in water”
Do I detect a pulse in the body of OOL research?
Or have we just seen the birth of more gaps?
Hmm?
What is moving the cart initially? The wind blowing the cart as a “bluff body”? The prop turns against the wind driven by the wheels? Does the prop take energy from the wind at this point or not? It’s acting either as a propellor or a turbine but not both, surely?
When the cart is above windspeed, assuming rolling and air resistance are low enough and wheelgrip is high enough, correct gearing could extract enough energy from the wind via the prop acting as a turbine to propel it at above windspeed, agreed? The prop is acting like a turbine and the ultimate power source is air moving differentially than the cart.
The force on the cart due to the wind on the “bluff body” will diminish to zero as the cart approaches windspeed, no? The (static)* thrust produced by the prop being turned by the wheels cannot exceed the power supplied by the turning force derived from the wind on the “bluff body”, can it?
? 
? Where does the additional force come from to accelerate the cart past windspeed?
ETA *static as the prop and wind are moving at (nearly) the same relative velocity. The prop is effectively in still air.
Go back a look at the two fans posts.
Alan,
The propeller never operates as a turbine. It always provides forward thrust to the vehicle. Likewise, the wheels never drive the vehicle forward; rather, the forward motion drives the wheels, the wheels drive the prop via a mechanical linkage, and the prop pushes the vehicle forward.
Consider the four stages of motion:
1. At rest.
When the vehicle is at rest, there is a substantial tailwind acting on the prop, pushing it forward. The vehicle starts to move.
2. Moving below wind speed.
The prop is acted upon by a combination of the (diminished) tailwind and its own “wind” due to rotation. The prop continues to push.
3. Moving at wind speed.
The effective tailwind is now zero, but the prop is spinning at a good clip and is still generating thrust. The vehicle continues to accelerate.
4. Moving faster than wind speed.
There is an effective headwind. The prop is now acted upon by a combination of the headwind and its own “wind” due to rotation. The prop continues to push.
At all four stages the prop is providing thrust and the wheels are imposing drag.
Anyone want a pop at this one? : http://blueprintsforliving.com/molecular-phylogeny-prove-evolution-false/
Obviously a conspiracy theorist. Perhaps sal will bite.
Rich,
Off to a bad start. He got the helix backward on his home page.
-10 points.
Left-handed DNA is the least of his problems.
You’re being chiralish.
Yes it can. In the same way that an airliner’s forward speed is provided by the engines but the lift force is provided by the airflow over the wings.
The propellor blades are aerodynamic surfaces and so will generate a force acting in the direction of the cart. This force produced by each blade will be proportional to the product of half of the air density, the square of the velocity of the leading edge cutting through the air and the area of the forward surface of the blade. So obviously doubling the rpm of the propellor will quadruple the lift (thrust) generated. I would think that at high rpm and an optimal angle of attack the propellor would be capable of producing a fair amount of thrust.
Just to note that today I posted on Panda’s Thumb a rebuttal to the Evolution News and Views pieces by and about an argument by Wolf-Ekkehard Lönnig. He argues that the random deaths of the many offspring of highly fertile species will create a random noise that will overwhelm natural selection. I point out that he may be a retired plant breeder at the Max Planck Institute for Plant Breeding Research, but he doesn’t know enough theoretical population genetics. And being a creationist, he is happy to proclaim his ignorance from the rooftops. And being the Discovery Institute, the owners of ENV think his argument is just great.
(In particular note the ID The Future podcast where he holds forth, and the host of the episode, Paul Nelson, keeps summarizing his argument as a completely different one about rarity of function in protein spaces. Which it isn’t — it instead argues that even once you’ve found a favorable allele, natural selection won’t effectively favor it.)
LoL! As if theoretical population genetics is actual evidence. Maybe someday you will have some actual science and evidence to support your claims. That day is not today
Lönnig thinks population genetics is evidence — that’s why he wants to argue that it shows that natural selection doesn’t work. That’s why I refute him using population genetics — those are the terms he’s accepted.
Frankie,
Behold, -6 sigma challenges +6 sigma.
http://www.bbc.com/news/blogs-news-from-elsewhere-36226141
Can’t resist thinking of Ray Comfort and kirk Cameron.
Push? At above wind-speed, the cart can receive no “bluff-body” impetus. Indeed, the cart is experiencing some air resistance as well as friction. There is plenty of available energy in moving air if you can harvest it. How is the kinetic energy in the wind being transferred to the cart? How is the propellor being turned? To overcome the forces retarding the cart, there has to be energy transfer from the slowing of air molecules. If the prop is accelerating the air molecules to provide thrust, that must require power. Unless I am missing something and the prop can simultaneously extract energy from the moving air by slowing it while also accelerating it to provide thrust.
Yet you say:
A propellor that provides the force to accelerate an aircraft (or cart) needs to be connected to an engine (or other source of energy: rubber band, electric motor) to provide that thrust. I think propellors can manage to convert up to 80% of the turning force supplied by the engine as forward thrust.
keiths:
Alan:
It doesn’t need to. The propeller is pushing it. Remember, a turning prop generates thrust, so even though the body of the vehicle is not being “blown” faster by the wind, there is still a driving force.
You need to let go of the idea that the vehicle is simply being blown downwind, like a leaf. It’s not. Something much more interesting is going on.
There is. Air molecules are pushing the prop forward. By Newton’s third law, that means that the prop is pushing backwards, with equal force, on those same air molecules. The air was already moving at wind speed. The backward force from the prop decelerates the air. Kinetic energy is transferred from the air to the vehicle.
Alan, to CharlieM:
In this case the propeller is driven by the vehicle’s motion over the ground. If you move the vehicle, the wheels turn. If the wheels turn, the propeller turns.
The prop is “gripping” the air by spinning and it’s spinning because the wheels are being turned due to the wind blowing the cart downwind as a “bluff body”.
Yes indeed, I was missing something! The prop is “gripping” the air while the wheels are gripping the road and the airmass is moving in relation to the road. The pull of the prop is the sum of the pull transferred from the wheels plus the additional force because the airmass is advancing in relation to the road.
Alan:
No, that’s not it. As I warned above, you’ve got to let go of the idea that the vehicle is simply being blown downwind. That explanation doesn’t work. Something much more interesting is going on.
When the vehicle is moving downwind faster than the wind, air resistance is slowing it down, not speeding it up. That’s why it has a streamlined design. Take a look:
keiths,
Facts (that I at least think are clearly demonstrated):
1, Blackbird demonstrated itself capable of travelling at over twice windspeed travelling downwind.
2, The vehicle’s only source of power is the wind.
The solution to what seemed to me a paradox is gearing.
You should let go of the idea that you know what I think. The ultimate source of the power to propel Blackbird is the wind.
I think it might be useful to ask oneself why a tacking sailboat sails into the wind.
Alan,
I’m going by what you wrote:
That’s not right.
It might help if you’d visualize the forces acting on the vehicle, along with their magnitudes and directions in various operating regimes.
1. When the vehicle moves downwind faster than the wind, it is actually moving forward through the air. When it moves forward through the air, the body of the vehicle experiences aerodynamic drag opposite to the direction of motion.
2. Thus, when the vehicle moves faster than the wind, drag tends to slow it down, as does friction. What keeps the vehicle moving forward is the thrust generated by the propeller. If the thrust is greater than the retarding forces of drag and friction, the vehicle accelerates. When the thrust is balanced by the retarding forces, so that the net force is zero, the vehicle maintains its current speed.
3. When the vehicle moves slower than wind speed, it is moving forward over the ground but backward relative to the air. The body of the vehicle still experiences aerodynamic drag, but now the drag is in the direction of motion over the ground. Friction still acts in the opposite direction.
4. In other words, in this slower-than-wind-speed regime the vehicle is being driven partly by the propeller and partly by aerodynamic drag, with friction opposing the motion.
5. When the vehicle moves exactly at wind speed, its speed is zero through the air. The body of the vehicle experiences no aerodynamic drag at all. The propulsive force comes entirely from the propeller, and the retarding force comes entirely from friction. Drag has no role.
6. Only at time zero — the instant before the vehicle begins to move and the prop begins to turn — can we say that the vehicle is truly being blown downwind. That is, only at time zero is the propulsive force entirely due to aerodynamic drag. Once the prop begins to turn, its contribution to the propulsive force becomes nonzero.
Alan,
This is also wrong:
The propeller never acts as a turbine. The wheels drive the prop, never vice-versa.
In downwind mode (and only in downwind mode) the blades are indeed in propellor mode. However though the prop is geared to the wheels, the energy to drive Blackbird is ultimately deriving from the wind.
It might help you if I point out that my first post in this thread was just after stumbling across the directly-downwind-faster-than-the-wind (DDWFTTW) claims in another forum. Prior to that, I had never personally come across the claim and the various experiments and explanations. So my first comment on April 16th was me in fairly skeptical mode. Over the next month, I sporadically followed discussion and during that time realised I had fundamentally misunderstood the downwind run, overlooking the prop turning against the wind on startup.
There was no shortage of people offering helpful and unhelpful analogies, including members of the Blackbird team and finally both JB (John Borton) and Rick Cavallaro pitched in. So I’m fairly happy I now have a grasp of what is going on.
In downwind mode the cart is initially set moving by the wind pressure on the whole object, including the prop blades acting like sails. Wind energy transmitted from wheel drag via the bike chain turns the prop, allowing it to generate a little thrust but the cart initially picks up speed very slowly. Prior to experiment, theorists were doubtful carts would self-start and might need a push to get them going.
The key to seeing where the force comes from to accelerate the cart with respect to the ground is that the airmass in which the prop is turning is moving forward and as the prop starts to generate some thrust, it also gains from slicing into that moving air, “gripping” as I put it. It is the “cotton reel” effect.
There are only two that matter. Thrust and drag.
Correct.
But there has to be something driving the prop to generate thrust. Ultimately, the required energy is derived from the wind. Because the prop is pulling in air that is moving differentially to the ground, on which the wheels are dragging, there is leverage.
Indeed. If the prop were unconnected to the wheels and locked, the vehicle would eventually attain a steady state speed at some percentage of wind speed depending on the retarding frictional forces. A released toy balloon will quickly accelerate to wind speed and become stationary WRT the moving air.
The only driving force is the wind.
Correct.
There is wheel drag. The prop is geared to the wheels, remember. I guess you mean there is no air drag.
There is no source of energy to drive the cart other than the wind.
Alan:
I’ve pointed out two serious flaws in your thinking. Until you understand that your statements are wrong — and why they are wrong — you still don’t have a grasp of what is going on.
Here are your erroneous statements again:
And:
Do you agree that both of your statements are wrong? Do you understand why?
The first statement was poorly worded but basically correct.
The second statement is wrong and dates from my initial misunderstanding regarding how the wind energy is transferred to Blackbird when in downwind configuration. However the last bit
is correct. This is why conservation of energy is not violated.
You mentioned forces earlier. Do you agree that there are only two that matter: thrust, and drag? Do you agree the energy required for DDWFTTW travel must ultimately derive from moving air molecules interacting with the vehicle and losing kinetic energy in the process?
In the above sketch, you see I’ve indicated prop thrust and tension in the restraining thread which is pulled by a fixed anchor. Do you agree this is analogous to the Blackbird in zero apparent wind? Do you agree that for Blackbird to accelerate from this point there needs to be more prop thrust than total drag?
ETA correct text to displayed diagram.
Here’s another sketch intended to illustrate the point that for a windcart to accelerate past zero apparent wind, there has to be a net forward force – thrust must exceed drag.
In the above setup, would you agree that changing the mass of the descending weight or the diameter of the drum rotating the prop (and indeed pitch and diameter of the prop), we could demonstrate whether it is possible to achieve a net forward thrust compared to the force applied by the descending weight and that would not violate the conservation of energy?