Question by Frst Grade Rocks! Ω: How can a wind powered vehicle go downwind faster than the wind?
Here is a video of a vehicle that is wind powered and can go downwind over twice as fast as the wind.
Best explanation.
You need to analyze how it works at below the speed of the wind, how it transitions from slower than wind speed to faster than wind speed and how it travels faster than the wind.
This is a gedanken experiment. Prefer independent thought over looking something up.
**************
Good start. Sorry, I realize that the cite I gave you did not include a schematic.
The propeller is tied into the drive wheels like this: http://blog.makezine.com/upload/2010/11/downwind_faster_than_the_wind_black/windCart_2.jpg
Two things to consider
1. You are correct that when the vehicle is going at windspeed, it generates no power
2. Watch the propeller in the video and both when traveling below windspeed and above.
**************
Hi John:
That’s a pretty cool vehicle you got there. Thanks for showing up here.
Based on the schematic, I figured you’d have a hard time transitioning through the at wind speed zone. Just seemed to me that the blade was using energy from the wheel to pull the vehicle through this transition and that later on, when in a headwind, it would be transmitting energy to the wheels to power the vehicle. (You are also transitioning from vehicular drag being a power source to it being drag) My presumption was that it would be normal variations in wind speed would be enough to get you through.
Guess I’ll have to read up on your project instead of trying to reverse engineer it.
Thank you again.
************
As an addendum, I once ran across a brainteaser about whether a boat with a wind turbine for a sail which powered a propeller below could sail directly into the wind.
My conclusion was that it could. There are overlaps with your project.
*******************
Al P., some of the confirming data is public. See: http://www.fasterthanthewind.org/2010/05/testing-graph.html (You can click on the graph to enlarge).
************
John, I found my thinking error and now agree with you that you would have a lot of power at windspeed. My initial thought was a design that would do great going up wind but would suffer at and below windspeed. (I was using the blade as a turbine to power the wheel instead of the other way around). I saw that you are planning an upwind attempt next summer. Good luck.
You must do a regular google search to find discussion (including this one) related to your vehicle.
Lastly, to anyone following this question, it comes down to the basic principle of aerodynamics. If the propeller is generating more lift than total vehicular drag (and efficiency losses), then you can channel the lift into powering the vehicle.
With this vehicle the lift is channeled into thrust of the propeller (and the drive wheel siphons off some of the thrust energy to spin the propeller). But I suspect that for an upwind vehicle, the lift is used to spin the drive train thereby using the wheel as the primary drive.
Given the efficiency of some of the propellers for wind turbines, I am not surprised that this vehicle can go so fast.
Best answer:
Answer by Scythian1950
I’ll think about this one more later, but first thoughts: 1) From the time it started from rest to speeds less than wind velocity, it uses wind power much like how a simple sailboard would, even though the energy is first converted into mechanical motion by the windmill, and then delivered to the wheels. 2) When the vehicle reached wind velocity, then the windmill experiences no net air movement, and therefore no extraction of energy from the wind. Other means of capturing the energy of the wind will have to be used, such as use of wings or sail. Details are hard to see here, but I would guess that it’s relying on forward momentum and/or flywheel somewhere, like in the blades of the windmill. 3) When the vehicle exceeded wind velocity, then it would be like if I were to tow the vehicle on a wind-dead day, and simply by getting it up to speed, it would then continue on its own. This would be like a perpetual motion machine, which is why I would dismiss the idea that this vehicle could continue indefinitely at “twice the wind velocity”. The only way it could capture such wind energy is if it employed some form of tacking as what good sailboats are capable of doing. But it wouldn’t continue in a straight line directly downwind.
But lacking details of this device, nonetheless I’ll give this one some more thought.
Edit: The link below talks about this vehicle, arriving at more or less the same conclusions I’ve given here.
Edit 2: We can disregard the first two cases, where the cart is going slower and at the same speed as wind velocity as being trivial cases. We look into the claim that it’s capable of generally going faster than the wind, say, twice as fast. If we allow “tacking”, i.e., where the cart doesn’t have to stay straight downwind, then it is possible to arrive a destination of arbitrary distance, because what it can do is to travel at some angle from downwind, and like what good sailboats can do, go faster than the wind. Then using this momentum, it can point downwind for some time, and repeat. Perhaps there is some optimal wavy track to do this. However, let’s suppose that the cart can only go straight downwind, as if on a track, and let’s suppose that it is already at a certain speed above wind velocity. Can it maintain that speed indefinitely? Well, we have to consider that it has to overcome not only “wheel resistance”, but wind resistance as well, because the apparent wind velocity will be impeding it. So if it were possible for the windmill to harvest enough energy from the wind to overcome both, then there would be nothing to keep from devising a perpetual motion cart that will happily travel arbitrarily far even on a dead-wind day. I don’t think I need to give a full explanation of why this would be impossible.
Edit 3: John, remember the words of Carl Sagan, “Extraordinary claims require extraordinary evidence”. What your Blackbird seems to be capable of is perpetual motion, so I would welcome your explanation of why it is capable of maintaining its “twice wind velocity” speed indefinitely even on a straight path downwind, instead of some other path. You might be onto something fundamentally important, so why not explain to the public?
Remember, I am not ruling out that it can arrive at a destination with twice wind speed velocity, but just not in a straight line to it.
Edit 4: The only difference between this device working on a “dead wind” day and when there’s wind is that the ground moves faster relative to the cart than the wind. Let’s say we have a single-prop plane flying with a tail wind. It does have more momentum (and kinetic energy) relative to the ground than relative to the wind. In an ideal perpetual motion machine, such a plane’s forward motion on a “dead-wind” day provides the necessary power to drive the propeller to make it go that fast, i.e., there is a power balance. But in this case, the plane’s forward motion on a windy day actually provides more than necessary power to drive the propeller “to make it go that fast”. Why? Because of the tail wind, the propeller doesn’t have to work as hard. How fast it can go does depend on propeller efficiencies as well as drag and wheel friction, but however counterintuitive this sounds, I believe it does and can work. I wouldn’t be placing a bet against it right now, the wiki article notwithstanding.
Edit 4: John, thanks, it was a nice brain-teaser, a pretty counter-intuitive result. I think I do understand it now, it just took a while to wrap my mind around this one. Your project has my vote of confidence.
Edit 5: FGR!, forget about transitioning to “hyper wind velocity” speed. Imagine that we have a single-prop plane flying with a tailwind, and there is a miles-long rack (linear) gear bolted to the ground underneath it, and the plane has a gear riding frictionlessly on the rack gear. We tow it to faster-than-wind velocity. If this small gear does work, then the plane slows down, the rate determined by power consumption. But where does this work energy go? Turning the single-prop, which restores its momentum, plus extra because wind resistance isn’t what should be expected, i.e. the ground is moving faster than the apparent wind. This little extra is sufficient to cover mechanical losses and other inefficiencies, so that it is not a perpetual motion machine. The trick is to compare this with an idealized perpetual motion device, which can exist in theory, just not in practice.
Edit 6: This idea of starting with a perpetual motion device and then putting some interesting twists to it as to make it a practical, yet counterintuitive, device, has many potential realizations. For example, let’s take the classic example of a electric motor and electric generator combo, whereby the motor drives the generator, which in turns powers the motor. In theory it is possible, but in practice it is not because of unavoidable energy losses. But let’s put the combo on a kart, which runs on 2 pairs of tracks. The electric motor powers the wheels which run on one pair of tracks, and the electric generator is driven by wheels running on the second pair. But instead of both tracks being immobile, let’s say that the tracks on which the wheels powered by the electric motor runs on is actually like a moving track, moving away from the 2nd pair. Suppose the cart is speeding faster than the first pair of tracks, so that it still requires electromotive power to drive the wheels forward on the tracks. However, the wheels running on the 2nd pair of tracks are spinning FASTER, thereby generating greater electric power than is being used by the electric motor. This excess can be used to overcome real world energy losses, such as aerodynamic drag. This works at all because the two pairs of tracks are moving relative to each other.
There are many other possible creative variations of this same concept. This is sure to spur the “perpetual motion industry”, even though perpetual motion machines are still, in practice, impossible.
Add your own answer in the comments!
7 comments
No ping yet
John says:
March 19, 2013 at 2:45 pm (UTC 0)
You say this is a ‘gedanken experiment’. In the spirit of that experiment, it wouldn’t be fair for me to submit an answer in this case as I am the one two primary designer/builders of the vehicle in the video you open your question with.
I would however like to say that point #1 in your “Additional Details” section is incorrect as it relates to our vehicle. The Blackbird is still harvesting energy at exactly windspeed and generates copious amounts of thrust from that energy. That is how it continues to accelerate to a speed much greater than wind speed.
I know this is not a discussion forum, so I will not explain further unless asked.
EDIT:
Sythian, I am happy to explain it, and I’ve done so countless times but I don’t think that’s what the originator of the question had in mind when he asked for people to post their answers — I believe he posed it as more of a brainteaser and since I have the answer to the brainteaser it’s not polite to shout it out right at the start.
It’s not perpetual motion, and if you believe that it can be done on an angle to the wind, then you should be able to figure out by watching the Blackbird videos how we do it going DDW (hint: not all of the Blackbird is going DDW).
Our construction blog: http://www.fasterthanthewind.org
Explanations: http://dwfttw.blogspot.com/
EDIT #2
AI P — NALSA placed and observed 18 recording sensors placed on the Blackbird itself, on a chase vehicle and on the surrounding desert floor for the world record runs. This data was used to ensure the process was indeed ‘steady state’ and not some ‘playing tricks with the wind’ stunt. NALSA will happily share data with you — just contact them through their website. And thanks for the kind words — much appreciated.
@AI P — Yes we have all the data, but it does nothing for the project’s credibility for *us* to release data from the record runs. We’ve made that mistake before and simply been accused of screwing with the data before releasing it. There is a verfiable chain of custody if you get the data from NALSA because they made sure we didn’t screw with it when it was collected. NALSA has data they formatted available on their site via download link, and I know for a fact that if you wish they will also give you raw data upon request.
@Sean Rupert:
No Sean, there is no computer control necessary or used. In fact we ran the prototype with a fixed pitch head with no change in airfoil angle at all from dead stop to well over 2x the speed of the wind.
http://www.youtube.com/watch?v=EEuAqq8FINw
@Sean Rupert:
I already did.
“Explanation: http://dwfttw.blogspot.com/“
Al P says:
March 19, 2013 at 3:12 pm (UTC 0)
@Anyone
You need, for starters:
On track vector anemometers that transmit three
dimensional vector wind speed data f(t) as the
vehicle breaks the anemometer’s “laser coordinate”.
At this time, the vehicle’s onboard microcontroller
will store this data for future correlation with:
vehicle speed, gravitational potential, and a bunch
of other crap if this were my project.
“Can a vehicle go downwind faster than the wind?” is
a political question. A better question might be:
At what points in time did the vehicle go faster/slower
than the wind as measured in the lab frame for example?
The wind speed suddenly drops to 10 mph while the
vehicle is at 30mph. Vundabar! Timely verifiable
data tables are required. Only then can I answer the
question: When and where did the vehicle go faster than
the wind and how does effective wind speed compare with
effective vehicle speed.
Show me the actual raw data, the beef. Webcast it
while Mr. Rogers is onsite;) Save your words and
graphs for the blog. Just mentioning the word “perpetual”
might drive the scientific community away from your camp;
great for headlines.
@John
In conclusion, I know from experience the work required
to bring such a project to life. If you poured creative
energy (and capital) into this, you have my respect.
@FGR
I have gedanken’s, but I will not answer.
@John
You’re welcome.
Hopefully, as one of the designer/builders of the
vehicle you should have access to the data, above
all others, I believe. Post it on the web. This should
add much credibility to your project. I have no qualms
with regard to the physics involved here.
@John
I would post my data and their data.
Post link(s) to their data on your site when possible.
In time others should duplicate or surpass the official
data record vindicating your claims. Of course it
will increase the project’s credibility in time
regardless of the actual results. To hell with the
nattering bloggers. Empirical data that stands the
test of time is the final judge. Best wishes.
@Scythain
I don’t have time to respond to your “perpetual
stuff” right now, but you’re lighting a fire under
my butt.
Edit:
Me on the motor/generator combo:
Well, a dc motor is also a generator.
One of the most energy efficient uses
of this “combo” is electromagnetic
braking. Part of the kinetic energy that
was transmitted to the motor/kart/system by
the source is returned to the source during
the braking phase via polarity reversal for
example. The second law of thermodynamics
reasonably extracts a price for this action.
To date, the law of conservation of energy
returns all the energy to entity and challenges
us to put humpty dumpty back together again.
Very intuitive.
@designer/builders
Do onboard computers dynamically optimize airfoil
angle attack via sensors or is this not allowed?
Edit:
First grade, I did study references, saw
the graph, and read the blogs. There is no
problem with the physics. For me, it just
a matter of seeing the raw data and fully
understanding construction details.
Edit
As of Today:11/17/2010
For the record, 3 days ago I asked the following question:
Do onboard computers dynamically optimize airfoil
angle attack via sensors or is this not allowed?
2 days ago, the following contributor cloned my question.
http://answers.yahoo.com/activity?show=JkD4TgERaa
Currently: “Sean Rupert”
Less than 1 day ago, the following contributor answered
my cloned question.
http://answers.yahoo.com/activity?show=G46G1RNjaa
Currently: “John”
******************
And now I quote John:
“…I have the answer to the brainteaser it’s not polite to
shout it out right at the start…”
“Sythian, I am happy to explain it, and I’ve done so countless times…”
******************
Well, explain it countless+1 more time before the end.
OlO says:
March 19, 2013 at 3:58 pm (UTC 0)
I have a simple answer of this question.
Do u know about PASCAL LAW ( hydraulic force)? similar case is here. some pockets in the vehicle accumulate the energy of wind and then this accumulated energy helps the vehicle to move faster than the wind speed. i hope u got it what i said!
Jayesh says:
March 19, 2013 at 4:27 pm (UTC 0)
good
balmiki says:
March 19, 2013 at 5:12 pm (UTC 0)
no idea dude
spork says:
March 19, 2013 at 5:27 pm (UTC 0)
JB and I designed and built this thing from scratch. I drove it. The North American Land Sailing association inspected it, instrumented it, witnessed our runs, and ultimately awarded us a world record for going directly downwind at 2.8X wind speed, steady-state. I’m guessing the point of these questions is not to have people make random guesses as to how and whether it works.
It does work – as described in countless places on the internet.
Sean Rupert says:
March 19, 2013 at 6:25 pm (UTC 0)
Do onboard computers dynamically optimize airfoil
angle attack via sensors or is this not allowed?