[HN Gopher] A wind-powered vehicle that can travel twice as fast...
___________________________________________________________________
A wind-powered vehicle that can travel twice as fast as the wind
itself
Author : dutchbrit
Score : 163 points
Date : 2021-08-09 11:49 UTC (11 hours ago)
(HTM) web link (www.businessinsider.com)
(TXT) w3m dump (www.businessinsider.com)
| zamadatix wrote:
| For those that hate the way news articles are written I found the
| Wikipedia article a much clearer read
| https://en.m.wikipedia.org/wiki/Blackbird_(wind-powered_vehi...
| IgorPartola wrote:
| Like the professor in TFA I am also confused: how can the
| ground be imparting power to the propeller while moving
| downwind?
| LeifCarrotson wrote:
| The wheel friction and gearing allows the vehicle to tie the
| effective velocity of the sail (actually a propeller) to the
| velocity of the ground.
|
| Energy can be extracted from the interface between the ground
| at one velocity and the air moving at another velocity. A
| sail tied to the ground and extending into this wind will
| exert a force according to the drag equation:
| Force = 1/2 x drag coefficient x density x area x
| (differential velocity)^2
|
| Typically, when going downwind, sailors ignore the ground (or
| the water) and just extract some energy from the difference
| in speed between the vehicle and the wind. Obviously, as the
| vehicle accelerates, this velocity difference asymptotically
| goes to zero where there is no more force and no more
| acceleration.
|
| For a more static example, imagine an enormously long vehicle
| with a linear bearing running the entire axis from the front
| to the back. Assume for the sake of argument the vehicle is
| ultra-light and has very low rolling resistance, some kind of
| cross between a top-thrill dragster and a train,
| hypothetically a kilometer long. On the axial bearing, mount
| a wind turbine with a cable and an anchor that can be fixed
| to the ground. Start with the wind turbine at the front of
| the vehicle, then drop anchor to fix the wind turbine in
| place. There is a velocity differential between the ground-
| referenced turbine and the air that will impart force on the
| turbine blades, use this to generate electricity. Use an
| electric motor to drive the bottom part of the vehicle
| forwards. When the turbine reaches the back of the vehicle,
| lift anchor - remember we're in spherical cow territory, so
| assume it's foldable and super-light so this part takes
| little energy and can be done arbitrarily fast - and push it
| back to the front of the vehicle, where you can drop anchor
| again. In this system, the energy you can extract only
| depends on the difference between the ground speed and wind
| speed, not the vehicle speed.
|
| Blackbird is just like this repetitively anchored wind
| turbine concept in that the point of reference for the wind
| energy extractor is the ground instead of the vehicle. But
| instead of dropping an anchor, you're tying to the ground
| speed through tires, and instead of shuttling the turbine
| from the back to the front, you're continuously advancing it
| with the gear mechanism, and instead of driving with an
| electric motor you're directly using the drag force.
| IgorPartola wrote:
| In your static example how does the turbine get back to the
| front of the vehicle? Does the force of the wind push it
| there? If so, if the wind is blowing at 10 knots but the
| base of the vehicle is moving at 20 then as far as the
| turbine is concerned it is actually being pushed backwards,
| no? Or is the idea that when you lift the anchor the
| vehicle slows to below wind speed while the turbine gets
| pushed?
| LeifCarrotson wrote:
| For that 'spherical cow' example, assume that the vehicle
| has batteries to average out the duty cycle while the
| turbine is going back to the front. Fold the mast down
| flat and stow the blades so it has minimal air
| resistance, assume it has minimal weight, and let an
| electric motor zip it back to the front of the vehicle at
| 100 knots ground-speed or 90 knots into the wind, if
| you're moving forward at 20 knots this gives you an 80%
| duty cycle.
|
| The critical thing to glean from the example is that when
| it's anchored, the 10 knot ground speed wind is
| generating power at the turbine, and this has absolutely
| nothing to do with the speed of the vehicle.
|
| Yes, in the real world, there are all kinds of problems
| with actually building such a vehicle, and the folding
| and weight and efficiency and timing and air resistance
| and rigidity and friction and so on would make it hard to
| actually get it to work. But these are all purely
| engineering problems, the physics obviously work!
| efxhoy wrote:
| wind pushes vehicle. wheels spin. wheels drive propeller (via
| gears). propeller pushes vehicle.
|
| I first assumed it was the other way around with
| wind->propeller->wheels, which it is not.
| shagie wrote:
| The original Veritasium video https://youtu.be/jyQwgBAaBag
|
| The video of the bet and the further explanations -
| https://youtu.be/yCsgoLc_fzI -- at 13:28 is an example
| without aerodynamics.
| CarVac wrote:
| The wind pushes the propeller, the gearing of the wheels to
| the propeller cause the overall vehicle to advance faster
| than the blades on the propeller (as seen by the wind).
|
| It's an overdrive--the wind pushes the vehicle faster than
| the wind itself, but the vehicle gets less net propulsion
| force when you sum up the tractive force and the wind force.
| formerly_proven wrote:
| Wind does not drive the propeller.
| CarVac wrote:
| I did not say that. I wrote "Wind _pushes_ the propeller.
| "
|
| The vehicle is basically extracting energy from the
| difference in velocity between the ground and the air.
|
| The wind pushes the propeller forward, the ground pushes
| the wheels backwards.
| IgorPartola wrote:
| > Downwind, when the vehicle is traveling faster than the
| windspeed, the ground is the fastest-moving medium relative
| to the vehicle, so the wheels harvest the power and impart
| it to the rotor, which propels the vehicle.
|
| So the wind pushes the vehicle faster than wind speed to
| start with? How?
| CarVac wrote:
| The angle of attack of the propeller blades and the
| gearing relative to the wheels cause the blades to be
| seen by the wind as moving upwind relative to the ground.
|
| Alternatively, think of it as merely harvesting energy
| from the velocity difference between the ground and the
| air. It's connected to the ground by wheels, and to the
| air by a big prop.
|
| It doesn't matter how fast the vehicle itself is moving
| as long as the gearing is right.
|
| Ultimately, the top speed is limited to when the friction
| the vehicle encounters from drivetrain losses and aero
| drag exceeds the power it can extract from the
| wind/ground speed difference.
| sopooneo wrote:
| I agree that theoretic top speed is limited only be drag
| (and other resistive forces) if you are able to adjust
| propeller pitch, gear ratio, or wheel diameter on the
| fly.
|
| But if those values are fixed, as with Blackbird, then
| you are of course limited to a top speed that is some
| constant multiple of wind speed.
| vel0city wrote:
| Propeller pitch is not fixed on Blackbird, that is one of
| the few controls it has.
| gubby wrote:
| I think the key conceptual thing that's difficult to grasp is the
| propellor's interaction with the wind.
|
| Forgetting the speed of the car vs the wind for a moment, the
| propellor is arranged so it is trying to push air _into_ the wind
| (i.e. it is propelling the craft). So, whatever force the wind
| might impart on a simple flat disc of similar radius to the prop,
| the force the prop actually experiences is higher than that
| because it is pushing against the wind.
|
| For me, this realisation that the prop is pushing against the
| wind, increasing the overall force, unlocked understanding how
| this craft is possible.
| choffee wrote:
| Is this similar to sailing? Where the lift generated by wind
| passing over the curved sail or prop blade produces more lift
| allowing the boat or car to go faster than the wind speed.
| ehnto wrote:
| Yes, that's where the creator got the idea I believe. He was
| imagining two sails rotating around a cylinder, which it turns
| out, is a propeller.
| mumblemumble wrote:
| Yes. If I remember right, the angle of attack of the propeller
| blades when this thing is at speed ends up being about the same
| as that of the sail when the boat's on a beam reach.
|
| To extend the analogy further, I _think_ that the wheel-and-
| chain mechanism ends up functioning analogously to the boat 's
| keel.
| wccrawford wrote:
| I saw the Xyla Foxlin video about this (and her treadmill
| version) and that was a fun build to watch.
| rkalla wrote:
| This blew my damn mind 10 years ago when it was first written up
| in Wired - the way the article was written, the build up to the
| actual run... all of it was so inspirational and today I still
| refer to it in my mind whenever I think: "People do the
| impossible all the time and the guidance they get from everyone
| around them is that they are INSANE"
|
| I think these are the closest we get to moments of 'magic' in
| real life.
| IgorPartola wrote:
| This is slightly off topic but I have been using this trick
| lately to try to determine which part of a news article is likely
| to be BS: read the article in the movie announcer voice (you know
| the one: "in a world gone mad, one man..."). Whatever headlines
| or parts of the article sound boring are likely to be legit and
| whatever parts sound like they would fit in an action movie are
| likely overblown, underreseaeched, irrelevant, etc. Try it with
| this passage:
|
| > They even brought in several of science's biggest names,
| including Bill Nye and Neil deGrasse Tyson, to help decide who
| was right.
|
| This part just stands out like a sore thumb.
| pteraspidomorph wrote:
| Science's Biggest Names! It got a chuckle out of me while
| reading.
| dagw wrote:
| I mean they're not entirely wrong in the sense that if you
| asked a bunch of Americans to name some currently active
| 'scientists' then those names would almost certainly be at
| the top of list (I'd guess that those two plus Anthony Fauci
| would make up the top 3).
| ben_w wrote:
| In this case, your BS detector is giving you a false result:
| https://youtu.be/yCsgoLc_fzI?t=275
| IgorPartola wrote:
| Thanks for the data point!
|
| I would still argue that BN and NdT are only the biggest
| names in pop science (as in people who wear "I <3 science" t
| shirts) and not in science as in academia. But that's cool
| that they did weigh in on this subject.
| villgax wrote:
| Is there an equivalent for water/hydrofoils maybe?
| foxyv wrote:
| I think in his original video he described how a tacking boat
| can go faster than one going straight down wind. As I
| understand it, that was the inspiration for the propeller. Here
| is a good site explaining it:
|
| https://newt.phys.unsw.edu.au/~jw/sailing.html
|
| > How can boats sail faster than the wind? Lots of boats can -
| especially the eighteen footer skiffs on Sydney Harbour. Ask a
| sailor how, and he'll say "These boats are so fast that they
| make their own wind", which is actually true. Ask a physicist,
| and she'll say that it's just a question of vectors and
| relative velocities.
| xaedes wrote:
| "The physics of a push-me pull-you boat"
|
| http://202.38.64.11/~cxyu/AJP_pushmepullyouboat.pdf
| marcodiego wrote:
| This was posted about a week ago:
| https://news.ycombinator.com/item?id=28010133
| JoeAltmaier wrote:
| Any sailboat?
| thehappypm wrote:
| I think I have an analogy that actually explains how this works.
|
| Imagine one of those people-mover walkways like they have at the
| airport. Standing next to it on a skateboard, you grab the
| handrail. Even though the skateboard has energy loss to friction,
| you go exactly as fast as the handrail.
|
| Now imagine you put a little electric generator in the wheels of
| the skateboard. Now, not only are you moving at the speed of the
| handrail, you're also generating some electric power, sourced
| from the handrail.
|
| Now, instead of grabbing the handrail with your hand, yo hold
| another electric skateboard, and use the power from your feet's
| skateboard to power that, so you are actually being pulled along
| the handrail. You'd be inching along the handrail, going faster
| than it.
| rusk wrote:
| This apparently violates conservation of energy as you're
| claiming to take energy from one source, put it through some
| apparatus (i.e two skateboards) to produce more energy without
| putting anything additional in. Can you account for this?
| [deleted]
| ladberg wrote:
| The people-mover has to expend more energy to push you
| because you're applying a stronger drag force, so energy is
| conserved.
| rusk wrote:
| Okay so in this case the people mover is drawing the
| additional energy. Additional energy is being provided from
| somewhere i.e a diesel generator somewhere up stream has
| had to increase her fuel consumption to maintain the power
| on the network.
| tpoacher wrote:
| "I offered him another 10,000 bet" is such bullcrap though.
|
| For the guys making the vid this is win win due to youtube ads.
| Kusenko isn't exactly taking a cut from that.
| zamadatix wrote:
| It's one of the top YouTube channels, they make money off
| YouTube ads constantly whether or not the professor wants to
| double down or not. The idea was to point out Kusenko knew it
| wasn't just a technicality that lost him the original bet even
| if he didn't want to admit it when conceding.
| 1-6 wrote:
| I never had a good experience with my Joby GorillaPod. All the
| plastic cup pieces developed cracks and the friction just wasn't
| there. The structural contraption holding up the propeller looks
| shotty as well. I'm pretty sure Joby Energy and Joby Photographic
| equipment are separate entities but that name Joby doesn't
| instill any confidence.
| Jedd wrote:
| Sailing faster than the apparent wind[0] has been going on for a
| while, especially on v.high performance land and water yachts.
| [1]
|
| The physics involved still twist my brain, but there's nothing
| terribly new here.
|
| [0] https://en.wikipedia.org/wiki/Apparent_wind
|
| [1] https://en.wikipedia.org/wiki/High-performance_sailing
| nichohel wrote:
| No, the issue in question here is sustained sailing faster than
| the wind _directly_ downwind, which is not what water yachts
| and sailboats are doing. The connection (belt, gear) between
| the wheels and the propeller is necessary to do the former.
| tshaddox wrote:
| Yeah. Two boats could sail faster than the wind such that
| their center of mass is moving directly downwind faster than
| the wind.
| aaron695 wrote:
| This is not what they are talking about. It's always step one
| to get past to understand what's going on here.
|
| I don't believe DDWFTTW has been achieved on water.
| textman wrote:
| large ocean yacht racing boats can go much faster than the wind:
| https://www.kqed.org/science/8503/how-do-these-boats-sail-fa...
|
| Also this: "According to the World Ice Racing Circuit, ice boats
| can sail four to five times wind speed. In March 2009 a land
| sailboat reached 126 miles per hour on a dry lake bed in the
| Mojave Desert" from: https://www.straightdope.com/21344013/how-
| can-racing-yachts-...
|
| I think I read somewhere that an ice sailboat did 143 mph.
| paconbork wrote:
| The difference here is that the vehicle in the article is
| traveling parallel with the wind
| theli0nheart wrote:
| I thought the video below provided a good explanation for why
| this works. The physics are similar to those of a single
| sailboat going faster than the wind.
|
| Imagine two sailboats on a cylindrical ocean on diagonal
| tacks, on opposite poles, and rotating around the core with
| rods connecting them to the core. Now you have a propeller.
| :)
|
| https://www.youtube.com/watch?v=jyQwgBAaBag&t=404s
| dahart wrote:
| Muller's bet reminded me of the Mysthbusters episode where they
| put a fan in a boat that blows on it's own sail and moves the
| boat forward, which I'm pretty sure I thought wasn't possible
| before it aired.
|
| https://mythresults.com/blow-your-own-sail
|
| It also reminds me of the Brayton Cycle, and turbofan/turbojet
| engines where the output thrust of the blades is applied to
| subsequent blades that turns the shaft, causing the first fan to
| spin even faster.
|
| To me this feels similar to blowing on your own sails, and to
| Veritasium's Blackbird that blows air against the wind to go
| faster than the wind. These all feel counterintuitive, but they
| work.
| tootie wrote:
| Whether it's an electric fan or your own lungs, it's pulling
| energy from storage (a battery or your own chemical energy
| storage) and converting it to kinetic energy. It's no different
| than if you put the fan or your face in the water facing
| backwards to propel the ship.
| dahart wrote:
| That seems too dismissive of the part that's counter-
| intuitive, and I think this is legitimately counter-
| intuitive, which is why physics professors are losing bets.
|
| The fan on the boat is blowing air forward from behind the
| sail, and by it's very presence on the boat, it applies a
| force to the boat that is trying to move the boat in reverse.
| Without the sail, the boat does move in reverse. It's
| surprising that one can seemingly recover more force to push
| the boat forward than was already used to push the boat in
| reverse. IIRC Myth Busters confirmed the result but didn't
| come to a detailed conclusion as to why, which could involve
| the shape of the hull, directions of the hull & sail relative
| to the fan direction, or some aerodynamics of the air blocked
| by the sail.
|
| In any case, I think it's quite different from putting the
| fan in the water facing backwards to propel forward.
| jcims wrote:
| The thing I wonder about stuff like this is that a phenomenon we
| see in one domain will typically transfer to another domain. For
| example, the 'hydraulic analogy' [1] for electronic circuits is
| pretty damn good at least on a macroscopic level. I wonder if
| there are applications of this phenomenon in other types of
| physical systems (including electronics).
| rusk wrote:
| This seems to be highly domain specific though. It's reading to
| me like an example of what you can do when you have highly
| advanced knowledge of a domain to the point where you can do
| stuff that looks like magic, specifically because it apparently
| violates generalised principles established across domains.
| jayd16 wrote:
| This is like putting a turbocharger on a sailboat. I love it.
| rusk wrote:
| A turbo charger increases fuel consumption though. You can't
| just consume more wind.
| ladberg wrote:
| In this case they are actually "consuming" more wind. A
| normal sail "consumes" wind by taking moving wind and
| reducing its speed to 0. This "consumes" wind by taking
| moving wind and sending it back the other direction, so it
| creates more thrust per unit wind.
| rusk wrote:
| Sounds to me like it improves efficiency rather than
| consumes more
| ladberg wrote:
| Yep, I thought about it a bit more and you're totally
| right.
| everly wrote:
| The article (and wiki page linked in another comment) seem to
| indicate that this vehicle design already proved it can go faster
| than the wind over 10 years ago.
|
| Odd that the professor was so keen on betting in that case.
| xaedes wrote:
| $10.000 got donated and a lot of publicity created. I think it
| is just good marketing. Dunno if it is worth 10k but it got on
| HN a few times, so it undoubtedly works. And the topic itself
| is super interesting!
| Strilanc wrote:
| The professor's reasoning is near the start of the video about
| the bet [1]. Basically they thought the makers of blackbird
| were fooling themselves (e.g. measuring slower wind at the
| bottom of the car vs the top of the propeller, or getting
| pushed above the average wind speed by gusts then selectively
| reporting).
|
| 1: https://www.youtube.com/watch?v=yCsgoLc_fzI
| michaelt wrote:
| _> The secret to Blackbird, Cavallaro explained, is that once the
| wind gets the vehicle going, its wheels start to turn the
| propeller blades -- they 're connected to the blades by a chain.
| As the vehicle speeds up, its wheels turn the propeller faster
| and faster. The propeller blades, in turn, act as a fan, pushing
| more air behind the land yacht and thrusting it forward._
|
| That's a terrible explanation - no wonder people doubt the system
| would work!
| twobitshifter wrote:
| It's really not that complicated - wind speed is not the same
| as energy. The propellor is a device that pushes against the
| air to move forward, it takes energy from the wind, produces a
| force, and the force produces momentum in the vehicle. We know
| that a stationary turbine can continually take power from the
| wind. When that wind energy is stored in a battery it can power
| your car much faster than the wind. This just takes a shorter
| route and stores the energy in momentum.
|
| See sailboats faster than wind
|
| https://www.kqed.org/science/8503/how-do-these-boats-sail-fa...
|
| > The America's Cup sailboats are sleek and fast. The AC72, the
| type of catamaran used in this year's race, can travel almost
| three times the speed of the prevailing wind. On June 18th
| Emirates Team New Zealand recorded a speed of 50.8 mph (44.1
| knots), with a wind speed of about 18 mph (15.6 knots).
|
| When sailing directly in line with the wind, the apparent wind
| in a sail is equal to the actual wind, so there's no gain. A
| propellor reshapes the direction of the apparent wind to be in
| the same direction of the wind.
|
| More interesting is that blackbird can go 2 times as fast as
| the wind running directly_into_ the wind. That ought to help
| understand how this is possible.
| cblconfederate wrote:
| Sailboats can run faster than the wind because of lift (which
| is not involved here), but not when running away from the
| wind
| 6510 wrote:
| the blades move sideways
| cblconfederate wrote:
| if it sails away from the wind, the lift is pushing it
| sideways, not forward
| jjeaff wrote:
| And if you turn a sail sideways and put it on a spindle
| with several other sails, you have what looks a whole lot
| like a propeller.
|
| So instead of sailing sideways so that your air foil sail
| can move sideways through the wind, the Blackbird
| harnesses the power of sales moving sideways through the
| wind while pushing the craft directly into the wind.
| mypetgoat wrote:
| Exactly. The article does not understand this at all.
| [deleted]
| jasonwatkinspdx wrote:
| Propellers are lift devices. The lift vector is parallel to
| the axis of rotation.
|
| It all works the same downwind, and the crazy fast boats
| like the AC72s do in fact sail faster than the wind when
| running away from the wind.
| marcan_42 wrote:
| Here's another take:
|
| Step 1: a propeller is like a wall or (straight on, no wing
| stuff) sail moving forwards forever. When the wind pushes on a
| propeller it's like it's pushing against a sail or wall, but
| instead of moving forward it turns in place. Virtually, it's
| still like a wall moving forward. So it converts linear push to
| a rotational motion.
|
| Step 2: Wheels are the opposite for solid surfaces: they couple
| rotational motion to linear motion on the ground.
|
| Step 3: the propeller on the vehicle is connected to the
| wheels. The gearing makes the "virtual wall" of the propeller
| move _backwards_ as the vehicle moves _forwards_. This _cancels
| out_ the forward motion of the vehicle from the point of view
| of the air.
|
| So from the point of view of the wind, the propeller is like a
| wall or sail that _isn 't moving_ with the vehicle. It's stuck
| stationary on the ground, or moving slower.
|
| The reason why the wind can't push a vehicle faster than the
| wind is that once you get to wind speed, there is no more speed
| difference to impart a force on the vehicle. Here, even when
| the vehicle is at wind speed, there is still a difference
| between the (slower) virtual speed of the propeller and the
| wind. So it still gets pushed, and can further accelerate.
|
| TL;DR the vehicle uses the wheels and propeller to make itself
| appear to be running slower relative to the wind, and so it can
| continue to accelerate when its real speed has reached the wind
| speed.
|
| Another way to visualize it is: instead of a traditional
| propeller, imagine it like a water wheel, or an impeller, with
| vanes the air hits. As the vehicle moves forward, the vanes
| move backwards. With the vehicle at wind speed, the vanes
| moving backwards cancel out some of its speed, so that there is
| still a relative difference between the wind speed and the vane
| speed, so there is still a force exerted.
| marcodiego wrote:
| I really would like a simple explanation of how that is
| possible. AIUI, when speed is stable, the wheels turning
| provide power to the propeller. But if the propeller pushes the
| car, which in turn makes the wheels turn faster making the car
| faster than the wind itself, what makes it different from
| perpetual motion machines?
|
| I would love a diagram showing how the power flows in this
| system.
| lizardmancan wrote:
| imagine the thing has no wheels. the wind provides an energy
| source.
|
| now imagine a sail cart. you can put a dynamo on the wheel
| and extract power
|
| now imagine a sail boat sailing close to the wind. it can go
| much much faster than the wind speed
|
| now imagine a batter powerd propeller pushing a cart
| forwards.
|
| now imagine the battery powered prop is pushing against the
| wind. a+b
|
| now remove the battery and extract the power (a) from the
| wheels.
| tshaddox wrote:
| It's certainly not perpetual motion, because the vehicle
| would quickly come to a stop if the wind speed (relative to
| the ground) went to zero.
| otherme123 wrote:
| I felt this in a sail boat: when the boat is moving below
| wind speed, the wind pushes the sail and the boat. At some
| point, wind and boat equal speeds, so it feels like theres no
| wind at all except the sail is full.
|
| Some well built boats can generate pull (instead of push) in
| the sail, as if it was a plane wing. The air passing on the
| convex (forward) side travels faster than the air on the
| concave, making the pressure lower on the forward side and
| pulling the boat, and you feel again the wind but against
| your face (as if you were running forward). It's a bit
| counterintuitive, because what your eyes see is the sail
| pushing the boat. So it's not perpetual motion, because the
| wind needs to keep blowing or the boat slows again. What
| slows the boat is the friction boat-vs-water.
|
| In the case of the car above, the wind needs to reach the
| point of wind speed == car speed, and beyond that the wheels
| keeps rotating the "sails", and thus keeping the pull. If the
| wind stops, the car can keep going for some time, but as the
| speed of the wind through the sails is lower, the pull is
| also lower and at some point it stops. But in this case, the
| friction is wheels-vs-ground, much lower than the boat-vs-
| water above, so the car can keep going (same what happens
| with a bike when you stop pedaling in a flat surface, the
| bike can keep going for long, and even a faint pedaling or
| tail winds keep the thing going). This car is a clever
| construction to keep the wind at high speeds through the
| blades.
| ska wrote:
| > as if it was a plane wing.
|
| This is how nearly all sailing works. Excepting the case of
| sailing _directly_ downwind, the curve of the sail works
| like a wing, and the keel provides something to "push"
| against - the effective "lift" (if it were a wing) drives
| you forward.
|
| This is why you can sail into the wind at all...
| abakker wrote:
| Obligatory mentioned of the AC72 using rigid wing sails.
| - https://www.youtube.com/watch?v=hbPTnF6liF8
| eyko wrote:
| Veritasium did a video on the Blackbird (plus a bet to prove
| it does indeed work).
| https://www.youtube.com/watch?v=jyQwgBAaBag
| unanswered wrote:
| Where the power comes from is easy: the wind gets slowed
| down. Of course the nature of wind is such that you could
| never measure this, but the point is that when the propellor
| pushes against the air, that air's velocity relative to the
| ground is decreased while the vehicle's is increased. The
| vehicle leaves behind it an imperceptible region of less-
| energetic air and takes that energy with it.
| marcodiego wrote:
| Now I think I'm beginning to understand... At first, wind
| propels the vehicle, moving it forward and turning its
| wheels. Its wheels turn the propeller which slows down the
| air, even the air in front of it. The energy lost by the
| air is now the energy that moves the vehicle!
|
| Indeed! There is no requirement that the vehicle must be
| slower than the wind! I think understood it!!!
| unanswered wrote:
| Ah, but not so fast. It actually depends on your frame of
| reference.
|
| Suppose the vehicle is facing the +x direction. Suppose
| the wind is travelling at +10m/s relative to the ground.
| Suppose the (experimentally determined) maximum velocity
| of the vehicle under these conditions is +12m/s. Now
| consider an inertial reference frame which, compared to
| the ground's reference frame, is moving at +11m/s. (I.e.,
| pick the frame in which the ground moves at -11m/s.) Now
| let the vehicle run. There will come an instant when the
| vehicle is stationary in this reference frame (when the
| vehicle has accelerated to +11m/s relative to the ground,
| not yet having reached its maximum velocity).
|
| Consider the instantaneous change in kinetic energy at
| this instant. The air about to be pushed by the propeller
| has velocity -1m/s and will accelerate in the -x
| direction. The vehicle has velocity 0m/s and will
| accelerate in the +x direction. In both cases, the
| kinetic energy is actually _increasing_! So in this
| frame, where does the increasing kinetic energy of the
| air-vehicle system come from?!
|
| The answer is the ground. It has velocity -11m/s. When it
| pushes against the contact point of the wheels, it is
| therefore pushing the contact point in the -x direction.
| But this means the contact point is pushing against the
| ground in the +x direction. And therefore the ground is
| undergoing a minuscule +x acceleration. That acceleration
| is therefore _decreasing_ the kinetic energy of the
| ground -- by a lot, because of the mass factor.
|
| And this is why we say the energy is derived from the
| difference in velocity between the ground and the wind --
| because depending on your inertial reference frame, the
| kinetic energy might be coming from one or the other. In
| the frame of reference of the vehicle, it starts out
| coming from both, but when the (relative) direction of
| the wind shifts (so that relative to the ground the
| vehicle is travelling faster than the wind), the kinetic
| energy starts coming from the ground only, and the
| vehicle is forced to transmit some of it to the air to
| keep moving. But this might actually be the nicest
| explanation of _how_ the vehicle outpaces the wind: in
| the faster-than-the-wind regime, the vehicle is
| transmitting energy from the ground into the air --
| _which is exactly what you would expect to happen, given
| the mechanical linkage between the wheels and the
| propeller!_ -- but some of this energy bleeds off into
| increasing the velocity of the vehicle itself, because it
| 's on wheels and that's what happens when wheeled
| vehicles push against something behind them, no matter
| what the ground happens to be doing (such as moving
| backwards like a treadmill, in this frame of reference).
|
| Disclaimer: I did not do well in college-level Mechanics.
| But I think I have convinced myself of the above
| explanation, just barely.
| colechristensen wrote:
| You're harvesting the energy from the difference in velocity
| of the ground and the wind. Sure it is easy to think about
| that being possible when you're attached to the ground but
| it's perfectly possible when you're moving too. Regardless of
| how fast you are moving or in which direction there is always
| that difference been air and ground.
|
| To go faster than the wind you just have to be a bit clever
| in how you capture that available energy.
| munchler wrote:
| Here's a simple explanation:
|
| Let's assume that the windspeed is 10 km/hr. We all agree
| that the wind can push this vehicle so that it is now going
| 10 km/hr downwind, right? From the vehicle's point of view,
| the wind is now 0 km/hr. But its wheels are turning, which
| turns the propeller, which behaves like a fan, pushing the
| vehicle forward in the still air, so that it is now traveling
| faster than the wind.
|
| This isn't perpetual motion, though, because eventually the
| thrust generated by the propeller is insufficient to overcome
| the additional drag created by the vehicle's faster speed.
| mumblemumble wrote:
| Your explanation that the propeller works like a fan
| implies a sort of perpetual motion, though, since it
| implies that the source of energy that pushes it past the
| speed of the wind is somehow coming from inside the car
| itself.
|
| The truth is that the car is always getting its energy from
| the wind, and not expending any of its own. It's just that
| it's been set up in a clever way that breaks our intuition.
| We tend to want to mentally simplify the propeller down to
| a disk whose motion vector is the same as that of the car.
| But here you've really got to think about the rotation of
| the propeller and the helical path its blades travel along.
| The wind is interacting with surfaces whose momentary
| motion relative to it at any given moment in time are very
| poorly modeled by the disk abstraction.
| pkulak wrote:
| The problem with this explanation, at least to me, is that
| it doesn't require any wind at all. Why not just give it a
| push in still air and have to take off into the distance
| forever?
|
| EDIT: I think it's clicking. It's because the air and the
| ground are moving relative to the craft at different
| speeds. So if the ground and air were still, there would be
| no speed difference to draw energy from.
| tshaddox wrote:
| Why would you consider those two scenarios analogous? In
| one scenario, the wind is "giving it a push" constantly,
| and in the other scenario, you're giving it a push for
| only a brief moment.
| munchler wrote:
| Right. This mechanism doesn't actually require wind. You
| get the same effect by placing the vehicle on a treadmill
| in a windless room: It will move forward on the
| treadmill.
| pkulak wrote:
| Well, that's the same as having wind.
| tunesmith wrote:
| Yeah I think that if you put a fan in front of the
| treadmill, the vehicle would stop. Or is it behind the
| treadmill? In front, I think.
| yodelshady wrote:
| Aye, where do the wheels get the energy from? Actually, more
| seriously, is there any point where the net torque on the
| drivetrain is zero?
|
| Assuming you're happy that, so long as the wheels are low-
| friction, that's no difference between "Downwind faster than
| Wind" and Upwind, and that a single design of craft can do both
| Downwind and Upwind, then all you need for the "dead air" part
| is to store energy in a battery, spring or flywheel.
| namlem wrote:
| The energy comes from the air. The air molecules in the wake
| of the propeller have lower kinetic energy than the rest of
| the air.
| cfallin wrote:
| The way this clicked for me (it took some thought!) was to see
| it as a lever, or at least an analogue to one.
|
| Imagine a lever with the fulcrum on one end, attached to the
| ground. Wind pushes on the middle, and the car is at the other
| end of the lever. So wind exerts more force with less speed,
| and the car moves forward with more speed but less force.
|
| That's basically what the linkage and gear ratio between the
| wheels and propeller do; it's the counterintuitive power flow
| that makes it so confusing...
| jcims wrote:
| I think this is a good intuition about it, and Xyla Foxlin
| goes into this a bit in her video explaining the build for
| Veritasium's video.
|
| https://youtu.be/VUgajGv4Aok?t=432
|
| The essential 'gear' ratio is between forward motion of the
| vehicle and the propeller pitch (essentially how far the prop
| would screw through a tub of jello for a given amount of
| rotation). The vehicle has to move *faster* on the ground
| than the prop does through the air for this to work at all,
| and it sounds like it should be roughly twice as fast for it
| to work on something that is reasonably simple to engineer.
| So if your prop pitch is 5", your wheel size and gearing
| should allow the vehicle to move forward approximately 10"
| for each rotation of the prop.
|
| (Note that her video does not explain 'why' this is the case,
| just that it is a phenomenon that the Blackbird inventors had
| determined/discovered and is an essential design criteria).
| slowmovintarget wrote:
| And one of the Veritasium videos:
| https://www.youtube.com/watch?v=yCsgoLc_fzI
| slowhand09 wrote:
| Its actually the correct explanation tho.
|
| This article sucked by the way. It said "Any sailor worth their
| salt can tell you that a boat can travel faster than the wind
| by cutting zigzag patterns; that's called tacking."
|
| Any sailor worth their salt knows that is not true at all. This
| practically destroys any credibility on this topic by this
| author.
| fanf2 wrote:
| but it is true?
| https://newt.phys.unsw.edu.au/~jw/sailing.html
| https://www.boatdesign.net/threads/tacking-downwind-
| faster-t...
| chongli wrote:
| What's not true about it? According to Wikipedia [1] there
| are a bunch of speed records for sailing faster than the wind
| by tacking downwind.
|
| [1] https://en.wikipedia.org/wiki/High-performance_sailing
| Jefenry wrote:
| Not to be too pedantic, but tacking is taking the bow
| through the wind. So you're going upwind. If you're tacking
| downwind you are gybing by taking the stern through the
| wind.
| tcpekin wrote:
| I think it's just very poorly worded. Sailboats can go faster
| than the true wind speed, like in the America's Cup, and
| typically do it do it going upwind. The addition of tacking
| is not wrong, it is zigzagging upwind, but just very oddly
| worded.
| dahart wrote:
| > Any sailor worth their salt knows that is not true at all.
|
| You sure? Take a gander at this article
| https://en.wikipedia.org/wiki/High-performance_sailing
|
| You're right in the sense that tacking is not the word for
| sailing faster than wind, but it is true that sailors can
| tack faster than the _apparent_ wind, and that professional
| sailors know that. The "apparent" part is an important part
| of the discussion that the quote didn't include or clarify,
| but would be implicitly understood by the saltiest of them.
| sopooneo wrote:
| You're right that it falls far short, but it corrects one of
| the fundamental misunderstandings many people have about the
| vehicle: the wind does not spin the propeller, the wheels do.
| cblconfederate wrote:
| Gravity does
| jrs235 wrote:
| So, did/does the vehicle in the desert start with a push
| (from a force other than the wind) or did it really get
| started moving by the wind pushing it?
|
| Edit: FYI in the video he does sort of cover his claim even
| if it got started by a non wind force by stating the vehicle
| can maintain a speed faster than the wind pushing it. But I'm
| still curious, was a non wind force needed in the desert to
| get it started?
| Retric wrote:
| No, it's light enough for the wind to start pushing it.
| jrs235 wrote:
| Even with 100+ pounds (50+ kg)?
| Retric wrote:
| Yes, at the extreme end you can find videos of 3,000+ lb
| cars flipped by the wind.
|
| It looks quite aerodynamic, but by comparison I can start
| pushing a 3,000lb car on flat level ground. So this thing
| probably needs less than 10 pounds of force to start
| moving.
| mumblemumble wrote:
| Any wind that's strong enough to overcome static friction
| would do the trick, and I can't see why 10mph wouldn't be
| enough.
|
| The small models that you can build at home (sorry, can't
| find the plans anymore; this was 10 years ago now) do not
| require a push to get started.
| raisedbyninjas wrote:
| The fan blades are an airfoil, which have a minimum speed
| to create the lifting effect.
| mumblemumble wrote:
| Lift isn't really a part of how this gets started.
| Regular old pushing works on airfoils, too.
| [deleted]
| jrs235 wrote:
| Understandable that it doesn't take much wind for a
| vehicle well under a pound. But what about one that
| weighs well over 100 pounds?
| mumblemumble wrote:
| The 100 pound vehicle also has a much larger cross
| section to the wind. And its propeller is further off the
| ground, so it's getting cleaner wind than the little
| model version.
|
| I can get a boat that weighs more and has less sail area
| going on less wind than that. And that thing has to be
| dragged through the water on top of everything else.
| amluto wrote:
| In the water, there is no static friction and no
| velocity-independent friction. If you push a floating
| boat, it moves.
| mumblemumble wrote:
| Fair point. Though, I would guess that Blackbird's
| internal mechanisms are well-enough engineered that it
| has very, very little static friction where it counts.
| [deleted]
| wongarsu wrote:
| In the veratasium video [1] you can see it starting just
| from the wind (first attempt is around 5:20). It works
| just fine because as you scale up the vehicle you scale
| up the propeller, so there's plenty of surface for the
| wind to push against.
|
| 1: https://www.youtube.com/watch?v=jyQwgBAaBag
| dharmab wrote:
| My motorcycle with me on it can weight 700-800 pounds and
| has a much smaller cross sectional area than blackbird.
| And yet it still gets pushed around sometimes by
| crosswinds .
| karmakaze wrote:
| A better way of looking at this is to not think of 'the
| propeller' as being in one plane for the wind to 'push'.
| Since the prop is rotating, consider the 'airspeed' of the
| surface of a blade. You can have zero airspeed when the
| vehicle is moving forward faster than the wind. Similarly the
| airspeed of the blade is slower (actually negative) than the
| vehicle's airspeed which allows the wind to push the blade
| surface when the vehicle is moving faster than the wind.
|
| If you think of a longitudinal vertical plane you can
| calculate the speed of a point on the cross-section of the
| blade moving backward relative to the vehicle as a function
| of the rotation speed. As the vehicle goes faster, so too
| does the prop surface 'move' backward.
| Valgrim wrote:
| Then what is spinning the wheels?
| mumblemumble wrote:
| Asking what is spinning what is maybe framing it in a way
| that impedes understanding.
|
| A quick summary of the basic forces involved are: The wind
| pushes the propeller, the propeller pushes the wheels, the
| wheels push the ground, the ground pushes the wheels, the
| wheels push the propeller, and the propeller pushes the
| wind. Or perhaps it's better to say that everything is
| dynamically interacting with everything else.
|
| I think that what this all adds up to is that, by
| introducing this mechanical linkage, the cart is extracting
| energy from the difference between the wind speed and the
| ground speed. This runs counter to our intuition, which
| assumes that it should be getting its energy from the
| difference between the wind's speed and its own speed.
|
| That's what allows it to go faster than the wind. If it
| were based off the difference between the wind speed and
| the cart speed, then the forward force would go to zero as
| the cart's speed approaches the wind speed. But the
| difference between the wind speed and the ground speed is
| not related to how fast the cart itself is moving. So the
| forward force on it doesn't disappear as it approaches the
| speed of the wind, and instead it will continue to
| accelerate until the forward force balances with drag and
| rolling friction.
|
| A kite might be a good starting intuition pump here? Kites
| generally don't do much of anything useful unless they're
| mechanically tethered to the ground.
|
| From there move on to sailboats. The reason why this cart
| can go downwind faster than the wind isn't all that far off
| from the reason a sailboat can sail into the wind (albeit
| not directly into the wind), but, critically, only if it
| has a keel or centerboard.
|
| (Edit: Another detail to point out is that the linkage
| causes the propeller to turn in the opposite direction it
| would if it were spinning freely. And remember that
| Newton's third law works in both directions at the same
| time.)
|
| (Edit again -- Another observation that might help change
| one's intuition is that, while the cart as a whole may be
| moving relative to the ground, the part of the cart that's
| currently touching the ground at any given moment is more-
| or-less stationary with respect to the ground. And is also
| mechanically linked to the propeller.)
| [deleted]
| blueblisters wrote:
| Perhaps another way to think about it - tailwind.
| Aircraft can derive energy from the wind even when
| they're going faster than the wind-speed.
|
| The propeller arrangement is a glider that is being
| pushed by the wind. The glider is tethered to a cart
| whose wheels in-turn drives the propeller to become a
| "powered" glider. Without a tether, the relative speed of
| the glider with respect to the wind would be zero. With
| the tether powering the propeller, the relative speed of
| the glider is greater than zero, which makes the cart go
| faster than the wind.
|
| Thinking about the "glider" and the tether as two
| separate systems makes more intuitive sense, imo.
| gpderetta wrote:
| Very good explanation. Thanks. I was thinking of some
| other contraptions to explain this (to myself mostly),
| but the glider->powered glider made it click. Even if it
| might not exactly how this vehicle work, at least it
| demonstrates a system that can obviously go faster than
| the wind speed.
| jfoutz wrote:
| Sailboats are one I struggled with, especially sailing
| into the wind.
|
| I think my intuition there came from a boat going the
| same direction as the wind, say, north at 1 meter/sec.
| now the boat can't go faster than 1 m/sec, but if you
| turn 45 degrees to the east you can travel sqrt(2) m/sec,
| because you're still going north at 1/ms and the boat
| gets pushed along to the east as well.
|
| the closer you get to perpendicular to wind, the faster
| you can go.
|
| I still don't quite get sailing into the wind, but I'd
| imagine this wheel and propeller system works the same
| way. The vehicle motion restricts moving to just a line,
| the wind pushes along the x axis, so the vehicle can sort
| of zoom along in the y direction, much faster than moving
| along x.
|
| __edit__
|
| oh, I see, it's the difference between ground and wind. a
| sailboat couldn't do this but something clever with
| propellers and linkages probably could
| jasonwatkinspdx wrote:
| A boat absolutely can sail downwind faster than the wind
| speed. The crazy cutting edge boats like you see in
| America's Cup or the giant trimarans that vie for various
| world records do this somewhat routinely.
|
| The key to understanding this is: sails are devices that
| work like wings, via lift, not via drag (exception,
| spinnakers and such). A sailboat has two wings: the sail,
| and the keel (or centerboard). The boat is extracting
| energy via how the lift vector of the sail projects onto
| the vector the keel keeps the boat tracking along. As you
| sail downwind the apparent direction of the wind begins
| to rotate forward in response to the boat's forward
| momentum. With extreme performance boats, this process
| can continue to the point where the boat is experiencing
| a forward apparent wind while sailing downwind.
|
| This is all pretty counter intuitive but valid physics.
| Speaking for myself, the most easy way to get an
| intuition for it all is to rent a windsurfer for an
| afternoon. It'll all make sense after that.
|
| People have tried sailboats with propeller rigs like this
| cart, but they end up being more trouble than they're
| worth. The current best anyone has figured out is solid
| wingsails, which have anywhere from 3x to 7x the lift
| drag ratio of traditional cloth sails. Hence their use in
| the crazy billionaire bragging contest races.
| mumblemumble wrote:
| It's hard to really explain how boats do it using only
| text and no good way to draw force diagrams.
|
| I blasted through this video pretty quickly, but it
| appears to explain things well, including how it's
| possible for a sailboat to exceed wind speed:
| https://www.youtube.com/watch?v=jJtvGF8vZbE
|
| The cart adds some moving parts, but I am pretty sure
| that all they're doing is ensuring that the blades' angle
| of attack is about the same as it would be for a
| sailboat's sail when it's sailing into the wind, by
| turning the propeller at an appropriate speed.
| im3w1l wrote:
| I think sailing into the wind works because you divide
| the force of the wind into components _twice_. Dividing
| into components first one way and then another lets you
| pull some tricks.
|
| The first time comes from the sail, you divide the wind
| into a component flowing along the sail (doing nothing),
| and one that is perpendicular to it (pushing it)
|
| Then you take the perpendicular-to-the-sail part and
| divide that into a side-ways (drift) and a forwards
| component (propulsive). The side ways one is neutralized
| by the keel.
|
| Btw I kinda wonder if a rotatable keel tuned just right
| would allow sailing right into the wind...
| foerbert wrote:
| The wind, I think. My current impression is that when it is
| starting, the whole thing is basically just acting as a big
| sail. You could lock the propeller and prevent it from
| spinning and it would still move forward.
|
| After that is where I start to get more fuzzy on the
| matter. On the one hand it makes sense that by using some
| of the speed from the wind to do other things, you can
| extract more total energy from the wind. And by doing
| propulsion as that "other thing" it makes sense you get up
| to a higher speed.
|
| But then it seems to me like there would be a problem once
| you go faster than the wind, because it's no longer pushing
| you. I'm not sure if some other effect takes over, or if
| I'm thinking about the wind in an incorrect way, or if my
| whole line of thought about this is wrong.
| Joeri wrote:
| I may be wrong, but this is how I understand it. The
| propellers push against the wind. Effectively you're
| creating a faster wind by pushing back at it. The energy
| that drives the propellers still comes indirectly
| entirely from the wind, but wind across a larger volume
| behind the propeller so therefore carrying more energy.
| jasonwatkinspdx wrote:
| You've got it exactly right.
| wizzwizz4 wrote:
| The fact it's going forwards.
| Vvector wrote:
| And what's making it go forward?
| Retric wrote:
| The wind.
| wizzwizz4 wrote:
| The propeller pushing backwards on the wind uses up less
| energy than it gets from the wind blowing it - when it
| spins backwards to "keep pace with" the wind, that means
| the wind is faster than it and it can get boosts from the
| wind.
| DebtDeflation wrote:
| Yeah, my first inclination is that since the wind is turning
| the propeller and the propeller is turning the wheels via a
| gear system, the speed of the vehicle will be determined by the
| propeller speed, the gear ratio, and the tire diameter
| (ignoring friction and air resistance for a moment). The gear
| ratio is the critical component. It's not like the propeller is
| a simple sail. I could be completely missing something though.
| jcims wrote:
| Gear ratio *is* the critical component, specifically prop
| pitch vs. forward motion, but not exactly for the reason you
| describe. Xyla Foxlin built a working model and provides a
| bit more info here:
|
| https://youtu.be/VUgajGv4Aok?t=432
|
| In short the working model she built has approximately a 2:1
| ratio between forward wheel motion and propeller pitch. IOW
| if the propeller pitch is 5" per revolution, the wheels will
| require 10" of forward travel to spin it one time.
| chongli wrote:
| No, the wind is not turning the propeller, the wheels are.
|
| The reason the wheels are able to turn the propeller without
| slowing the vehicle down is that the speed of the wheels over
| the ground is greater than the speed of the propeller through
| the air thanks to the tailwind.
| bxbb wrote:
| The prop is spun by the wheel, not the wind. The tailwind
| provide push.
|
| Think of the wheels as a power bank: It convert and
| temporarily store linear force to rotational force. As long
| as it accumulated enough power to counter the rolling
| resistance, headwind drag, gravity, gearing loss, etc; the
| excess rotational force can be used to spin the prop. Which
| in turn generate additional push to be converted and stored.
|
| Within this "loop", the vehicle got two linear input,
| tailwind and propeller. This will allow continuous
| acceleration once the vehicle goes faster than tailwind up to
| the point where the additional force from the prop is
| completely negated.
| sopooneo wrote:
| Most critically: the wind _is not_ turning the propeller.
|
| If you watch the videos when the thing is first getting
| going, it is slow enough to see that the propeller is
| actually spinning in the _opposite_ direction it would be if
| it was acting as a windmill.
| catchmeifyoucan wrote:
| Actually, knowing it works now. Even internal combustion engines
| have only a 40% energy efficiency in converting gas to power, the
| rest is lost as heat.
|
| In this case, I'm trying to think how the wind energy is lost,
| and it seems that some of that might be lost to the static
| friction in turning the wheels, and also the attached propeller.
| However, the energy used to turn the propeller is outputted
| again, as thrust. Since the initial wind was enough to overcome
| the static friction, any additional power might be causing it to
| move faster than the wind. I'm no physicist, but this is pretty
| cool.
| dahart wrote:
| > Since the initial wind was enough to overcome the static
| friction
|
| I've seen this idea in a couple of comments here, but I'm not
| sure what it means, can you elaborate?
|
| Rolling wheels are still under static friction. They don't
| become dynamic friction unless the wheels are in a skid. Unless
| what you're talking about is the static friction of the oiled
| axle, but I assume that's not what you meant?
| sopooneo wrote:
| The treadmill demos made me think of another counterintuitive
| possibility that I wondered if anyone had pulled off: sailing a
| boat _indirectly_ up a river in no (bank relative) wind.
|
| Turns out, it has been done:
| https://www.youtube.com/watch?v=q2il8Fagbyk
| dd36 wrote:
| Can it be done with solar sails?
| sopooneo wrote:
| Not an expert, but I'm going to say no. There is no medium in
| space, moving relative to the solar wind, against which the
| "keel" of a solar sail ship could push.
| adolph wrote:
| Gravity? I guess something with enough mass to use gravity
| might be too large to use solar sails.
| Tuna-Fish wrote:
| Gravity does not actually provide a force in the sense
| that a sailboat would require. (It acts on all parts of
| the vessel equally.)
|
| There is some work on the idea of using both a photon
| sail and a magnetic sail on a single vessel. In many
| interesting locations, these would push in different
| directions, allowing you to sail.
| xaedes wrote:
| This is similar to flying in air.
|
| I suppose when your craft is big (or long) enough you could
| leverage differences in luminous flux (or intensity?) to
| generate some thrust. The differences will be tiny and the
| necessary craft size big. One would have to run the actual
| numbers, but it _could_ work in principle.
|
| Compressing air by moving through it to leverage the
| resulting difference in velocities sadly won't work =( The
| steady state is then just the speed of air. I guess for light
| this also applies in a similar way.
| goldenkey wrote:
| Solar sails only absorb enough energy for tiny crafts in
| space, a frictionless environment. You overestimate photon
| flux momentum from the sun.
| Akronymus wrote:
| Solar sails indeed only absorb a tiny amount of energy, BUT
| per given area.
|
| They can theoretically made to be extremely large without
| much weight relative to the size. Which in turn allows you
| to accelerate even quite large payloads in a reasonable
| amount of time.
| goldenkey wrote:
| The mirrors are extremely heavy and solar sails are
| exclusively used above our atmosphere, in space, where
| more flux from the sun is received.
|
| The flux even at earth orbit is a measly 47e-7 Newtons
| per square meter. That's 0.00016905632 pounds per square
| inch.
|
| You still stand by what you just stated?
|
| It's not feasible to do below clouds, in an environment
| with constant friction, air or water resistance.
|
| The only reason you can do it in space is because that
| tiny slow acceleration can accumulate over a long period
| of time. This precludes frictional losses that would
| normally be incurred within atmospheres.
| Akronymus wrote:
| It seems I misunderstood the thread. I somehow thought we
| were talking about sails in space rather than in atmo.
| goldenkey wrote:
| So what were you originally referring to? Solar wind in
| space? I'm honestly confused now.
| Akronymus wrote:
| I was actually. I rhought the parent I originally replied
| to was saying that solar sails aren't useful for large
| payloads in space.
| andrewflnr wrote:
| They overreached a tiny bit in reaction, but they were
| responding to someone who seemed to be proposing solar
| sails inside the atmosphere.
| noxer wrote:
| Very bad article. Watch the 2 videos instead.
| database_lost wrote:
| "A popular YouTuber filmed himself driving a wind-powered vehicle
| downwind faster than the wind itself. [...] Muller, the creator
| of the Veritasium YouTube channel, likes to break down funky
| science concepts for his 9.5 million subscribers."
|
| The "popular YouTuber" is Derek Muller, who studied Engineering
| Physics and has a PhD in Physics Education Research... It's so
| frustrating to see how the writer tried so hard to make as much
| as clickbait-y title as possible...
| dmurray wrote:
| Having 9.5 million subscribers on a science-based YouTube
| channel is definitely more notable than having gone to
| university, or indeed having a PhD. I think the writer picked
| the correct description here.
| database_lost wrote:
| I see your point, but the title and first paragraphs try to
| paint the picture of a "funky science" youtuber betting a
| UCLA physicist $10k and winning (which for me is very
| different to the truth: UCLA physicist bets another physicist
| $10k and loses)
| ehsankia wrote:
| He did study Physics but his main field is science
| education, I think it's fair to assume a physics professor
| would know a bit more. I also think it's a bit of your own
| preconceived notion at play here assuming that calling
| someone a "Youtuber" is meant as a negative. This is
| equivalent to saying "Science communicator vs physics
| professor", which is a fair portrayal.
| database_lost wrote:
| I really don't think being called a Youtuber is meant as
| negative. I take issue with how the writer chose to
| present this story. If this was about the intricacies of
| ad revenue on YouTube for example, great, no need to
| specify he also has a physics degree. But I'm
| disappointed to see them paint him as just a guy who
| "likes to break down funky science" (and made 10k off a
| physicist) when he is more than that, with relevance to
| the actual story. For someone who doesn't know his work,
| he might as well be one of the guys burning snow with
| lighters a few months ago to prove its fake snow...
| glitchc wrote:
| It's a sad indictment of the times when people feel that
| having a million subscribers (read: being popular) is more
| notable than having a PhD (read: expert training) in a
| relevant field.
|
| It's no wonder anti-vaccine material is so widespread.
| Someone famous on Youtube said it was bad, it must be true!
| danpalmer wrote:
| Subscribers do so for a reason. In this case not
| insignificantly because of expert training.
|
| I'd suggest that very few YouTubers are popular because
| they are popular, in fact I think this is fairly rare in
| general. I think it's more likely that they provide some
| value to their audiences. I think looking down on
| "celebrity culture" often fails to account for the types of
| value that one doesn't value themselves.
| glitchc wrote:
| Re the first statement, that's complete horseshit.
| Celebrity culture is all about being part of a cult
| following and finding like-minded people. Any correlation
| to objective fact or basis in reality is purely
| coincidental, and has no bearing on the quality of the
| person or the content of the movement being followed.
|
| I totally get that you value Gig Hadid's opinion on
| wardrobe whereas I don't, nor do I judge, to each his/her
| own. But, to claim that her opinion is more
| accurate/better/truthful to that of a respected fashion
| designer or industry expert is inherently flawed. Wearing
| clothes is not the same as designing them.
| [deleted]
| kube-system wrote:
| I think it's just a simple matter of the relative
| exclusivity of those accomplishments.
|
| If I asked you who Woodrow Wilson was, what would you
| respond with?
| glitchc wrote:
| That's a false dichotomy. Being a president is
| tangentially related to exceptional intelligence.
|
| A better example would be Brian May (of Queen). No
| correlation between academic credentials and artistic
| career. There, if the article were about black holes and
| May had an opinion, I would expect the reporter to cite
| his PhD, making it a (potentially) informed opinion.
| kube-system wrote:
| The people who have top YouTube channels also have
| exceptional intelligence. Maybe less so in logical
| intelligence or rote memorization, but more so in other
| intelligences.
|
| > A better example would be Brian May (of Queen). No
| correlation between academic credentials and artistic
| career
|
| I am not sure why that example is better -- Muller's
| academic credentials are very well related to his career.
| As were Wilson's.
| tzs wrote:
| If you believe that sailboats can beat a drifting balloon
| downwind by zigzagging, then there is a very good explanation in
| the original Veritasium video that shows that this is essentially
| the same thing. Here's a link to the section of the video with
| that explanation [1]. Watch about 2 or 3 minutes starting there
| and it becomes clear.
|
| [1] https://youtu.be/jyQwgBAaBag?t=404
| guy_named_matt wrote:
| I feel that the Business Insider title is really misleading and
| counterproductive; because Derek Muller is not 'a youtuber' --
| he's a serious physics educator, with a PhD in physics education
| research, and with probably the best youtube channel (or any
| channel, in any medium) in the world for explaining physics
| content; which has been running for a decade; and with nods from
| multiple famous physicists IN THIS VERY VIDEO saying that he is
| the best that there is. So the claim that 'A youtuber bet a
| physicist and won' just makes it seem that experts are wrong,
| whereas in reality, this was expert vs. expert. That being said,
| physics is such that if 'some youtuber' bet a physicist, and was
| right... physicists would concede.
| pengaru wrote:
| The propeller is being used a forward motion compensator.
|
| At a standstill, the wind just acts on the entire vehicle
| including the stationary propeller, and sets the whole thing in
| motion.
|
| Without the propeller, going directly downwind would be limited
| by the speed of the wind acting directly on the vehicle which
| otherwise lacks any relevant moving parts for the wind to act on.
|
| By adding the propeller, driven by the forward motion via the
| wheels, a part of the vehicle now moves backwards (imagine the
| cross-section of a propeller blade as it spins, its intersection
| with the wind travels backwards into the wind) to compensate for
| the forward movement, giving the wind something to act on even
| when the vehicle travels faster than the wind.
| jcims wrote:
| Derek from Veritasium asked Xyla Foxlin to build a functional
| model, and she goes through the trials and tribulations of that
| in quite a bit of detail in her video here:
|
| https://youtu.be/VUgajGv4Aok
|
| There's a key aspect discussed in 7:10 relating to the ratio of
| the propeller pitch to the vehicle forward motion that is
| necessary for this to work.
|
| (Xyla's channel is a great follow overall btw)
| haberman wrote:
| > Any sailor worth their salt can tell you that a boat can travel
| faster than the wind by cutting zigzag patterns; that's called
| tacking.
|
| This is a pedantic point, but when you're cutting zigzag patterns
| downwind it's called jibing, not tacking.
|
| I guess there are high-performance boats that can go faster than
| the wind upwind, and that are so fast that they can perform
| downwind tacking (since apparent wind stays ahead of the mast),
| but this is the exception, not the rule.
| sopooneo wrote:
| But what about once you are going downwind faster than the
| wind, and thus experience an apparent headwind? I'm not a
| sailor, but I thought I'd seen people using the term "tacking
| downwind" to describe that.
| haberman wrote:
| You will experience an apparent headwind when sailing on a
| broad reach faster than the wind, yes. But the jibe involves
| turning downwind (thus losing lift), as well as bringing the
| sails across (which will temporarily de-power them). It's
| been a while since I've been sailing, but I don't think most
| boats can maintain an apparent headwind through the entire
| maneuver, even if they are capable of faster-than-wind speed.
|
| Some can, which is why I mentioned downwind tacking for high-
| performance boats, but I think it takes a pretty fast boat to
| manage it.
| jcims wrote:
| Side note, this is my favorite mode of exhibiting pedantry. Let
| the conversation flow while still correcting terminology.
| Thanks for the info.
| jasonwatkinspdx wrote:
| State of the art in crazy expensive racing yachts (think
| America's Cup) is limited by cavitation on the foils to a bit
| over 50 knots. Ice boats are setting records over 120 knots. If
| someone figures out a magic foil, that gap may close, which is
| a thought that blows my mind.
| Ericson2314 wrote:
| Wow I had no idea, thank you.
|
| I was in a relative's ice boat once. I was pretty wonderfully
| terrifying -- awesome in all senses.
|
| To do that on water would be quite something!
| Someone1234 wrote:
| The most surprising thing here is that someone on Twitter made a
| bet, lost the bet, and then actually _paid_ said bet.
|
| Playing devil's advocate here a little: The whole machine is very
| counter-intuitive, and I can see how I myself may disbelieve it
| if not for the irrefutable demonstration(s)[0].
|
| [0] https://www.youtube.com/watch?v=yCsgoLc_fzI
| gibba999 wrote:
| It's not at all counterintuitive. It's just poorly explained.
|
| Energy is 1/2mv^2, and momentum (related to force) is mv.
| Anytime you have different speeds, you can arbitrage.
|
| If I am pushing against still air, I can generate nearly-
| infinite force for arbitrarily little energy.
|
| That's why you use wings instead of pointing jet engines
| downwards. You push a lot of air by a little bit (lots of mv,
| not a lot of 1/2mv^2, since v is small).
|
| If I am going an epsilon faster than the wind, with very little
| energy, I can push hard on the air.
| ses1984 wrote:
| Ok I have taken college level mechanics (after physics 1-2)
| and I have no idea what you're saying.
| gibba999 wrote:
| Perhaps I can't explain it well either, without pictures.
|
| But it's not all too complicated. With pencil and paper, I
| could explain to a high school student. If you've done
| college level mechanics, that's on me, or on the medium.
|
| Perhaps I need to make a video explaining this clearly,
| since people are running in circles around this and it's
| just not very hard.
| whatsakandr wrote:
| I think you're getting at it, but at the limit, you throw out
| the air, which Derek did in the follow up. Once you have two
| surfaces moving relative to each other, energy can be
| extracted. Doesn't matter how fast you're going relative to
| the two surfaces.
| gibba999 wrote:
| I actually find the limit to be somewhat less insightful.
|
| It sorta shows that it's possible, but it doesn't really
| show why it works in this case.
|
| 1. Air isn't solid.
|
| 2. There aren't two rollers.
|
| The analogy is a little bit distant.
|
| I just need to find time to make mine more eloquent.
| ehsankia wrote:
| To be fair, they both agreed to make the whole bet public. They
| went on video, with "science celebrities" like Neil Degrasse
| Tyson and Bill Nye, and I believe they even signed something?
| It's kinda hard to backup from that. It also was a UCLA
| professor, so not quite some random person on Twitter.
|
| EDIT: As an aside, I would also add that the original video did
| a poor job at explaining, and the demo had clear flaws which
| were the points the physics professor was making. More
| specifically, his 2 main points were
|
| 1. It's possible that when the picture of the flag was taken,
| the wind was slower than the car, but the car was still moving
| faster due to momentum
|
| 2. It's possible that the wind speed up at the propeller level
| is different from the ground level where the flag was
|
| Those were both very valid criticisms, even though they didn't
| actually matter in the grander scheme of things.
| Tuna-Fish wrote:
| The small demonstration that made it click for me is from that
| video:
|
| https://www.youtube.com/watch?v=yCsgoLc_fzI&t=812s
|
| This is exactly what the blackbird does, only with a propeller
| pushing air in place of the big wheel, and it's a much clearer
| and simpler demonstration of the same idea, which you can
| trivially verify.
| iamgopal wrote:
| This is simplest and best explanation, no doubt why he got
| his PhD in physics education.
| notJim wrote:
| How does a professor have $10,000 sitting around for a bet like
| this?? I am a nicely-paid tech worker, and $10,000 wouldn't
| ruin me, but it would be a major setback for that year.
| ladberg wrote:
| UC salaries are public[0] and it looks like he made $219k in
| 2019, which is pretty decent for a professor but yeah $10k
| would still definitely be a significant setback.
|
| [0] https://ucannualwage.ucop.edu/wage/
| neonate wrote:
| https://archive.is/FhZfN
| omega3 wrote:
| I find it extremely counterintuitive, especially this part:
|
| In 2012, Blackbird also demonstrated sailing directly upwind with
| twice the speed of the wind.[0]
|
| [0] https://en.m.wikipedia.org/wiki/Blackbird_(wind-
| powered_vehi...
| marcan_42 wrote:
| I posted an explanation for the downwind version above, but
| here's the gist of why all this is possible:
|
| To extract energy from the wind, you need a _reference_ that is
| slower than the wind. The ground is one such reference. The
| vehicle gets access to that reference via the wheels. It can
| therefore extract power from the wind _regardless_ of where it
| 's going or what it's doing.
|
| So it's like a wind turbine on the ground, connected to a car
| that has nothing to do with it. Of course the car can go faster
| than the wind in this situation; the wind speed doesn't matter,
| only the drag of the car and how big your wind turbine is (how
| much energy you can get out of it).
|
| This vehicle just makes the whole thing self contained with
| gearing and counterintuitive rotation of the propeller. It's
| touching the ground and can use that reference via the wheels,
| so it can pretend to be a stationary wind turbine on the ground
| this way. That means it can go downwind, or upwind; doesn't
| matter. As long as the wind is moving relative to the ground,
| it can harness that energy.
| ColinWright wrote:
| Other discussions:
|
| https://news.ycombinator.com/item?id=27695869
|
| https://news.ycombinator.com/item?id=27696621
|
| https://news.ycombinator.com/item?id=27707791
|
| https://news.ycombinator.com/item?id=27987321
|
| https://news.ycombinator.com/item?id=28000727
| smsm42 wrote:
| My first thought about it was that it's awesome. The sad thing is
| that my second thought about it was that if somehow physics
| became political, this guy would be banned from Youtube for
| "physical misinformation" - after disagreeing with a renown
| expert in the field - and maybe we would never found out about
| it.
|
| But, apparently, there's a way for people to disagree about
| science - without any of the sides being evil or stupid - and
| there's a way to find out who's right. Maybe we could learn
| something bigger from this?
| tkzed49 wrote:
| > Maybe we could learn something bigger from this?
|
| It seems to me that Derek did several things right here:
|
| - Choose a topic with which he has relevant experience
|
| - Collect experimental data supporting his hypothesis
|
| - Fairly address opposing viewpoints, in this case by providing
| a platform for a dissenting expert to present his evidence
|
| Are there examples of similar videos being removed from youtube
| for misinformation?
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