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video Downwind Faster Than The Wind Cart
This instructable will take you through the construction of a working model of the wind powered cart that answers the question that sparked so much controversy across dozens of internet forums. It resulted in tens of thousands of posts of heated debates, on-line and print articles in Make, Popular Science, Wired, Der Spiegel, and many other magazines and blogs around the world, and has appeared on the Discovery Channel's "The Daily Planet".
My buddy and I built a full-sized manned version of the vehicle (see the video of the "Blackbird" above) and established a world record for direct downwind speed of a wind powered vehicle. We've spoken at NASA, AIAA, Stanford, SJSU, and a number of other venues on this controversial topic, and won the Editor’s Choice award at the 2011 Maker Faire.
The question: Is it possible to make a wind powered vehicle that goes directly downwind, faster than the wind, steady-state?
Despite the relatively straight-forward analyses and explanations, we were assured even by professors of physics and aero that it simply can't be done. But after setting a world record with our full-sized cart, most of the naysayers have come around.
Now for about $40 in parts you can make your own working model and demonstrate this novelty for yourself.
Learning objective:
Critical thinking! I originally conceived of the downwind cart when a friend asked me whether a sailboat could tack downwind and beat a free-floating balloon to a point directly downwind. I wasn't sure, so I did a quick vector analysis. Somewhat surprisingly the answer was "yes". Being a huge fan of brain-teasers, I asked myself how to make this little tidbit even more twisted. By making a wind powered vehicle that could beat the wind DIRECTLY downwind, I suspected it would go against most folks' intuition - and that proved truer than I would have ever guessed.
The reality is that one can analyze this with high school level physics and math. As engineers, we rely heavily on our intuition for problem solving, but we have to remember to use that intuition to guide us - NEVER to replace rigorous analysis (or worse yet - observed results). Much to my surprise there have been a fair number of professors of physics, NASA aerospace engineers, and aerodynamicists that assured me this could not work - even after it was demonstrated. These folks relied on their intuition when it was time to carefully consider the simple analysis.
The secondary learning objective is to remind us that we don't stop learning when we start teaching.
Note: I later learned that I was not the first to have conceived of such a cart. An engineer by the name of Andrew Bauer built one in the 1960's. He had learned of the concept from a student paper written some 20 years earlier. We've never found that student's name.
The reason we built the cart was to settle the long running debate across a number of internet forums. I was clearly not going to convince many people with my analyses. The matter is settled now to about the same extent the moon landing is settled. The difference being - you can build your own cart in an evening or two and prove it to yourself :)
My buddy and I built a full-sized manned version of the vehicle (see the video of the "Blackbird" above) and established a world record for direct downwind speed of a wind powered vehicle. We've spoken at NASA, AIAA, Stanford, SJSU, and a number of other venues on this controversial topic, and won the Editor’s Choice award at the 2011 Maker Faire.
The question: Is it possible to make a wind powered vehicle that goes directly downwind, faster than the wind, steady-state?
Despite the relatively straight-forward analyses and explanations, we were assured even by professors of physics and aero that it simply can't be done. But after setting a world record with our full-sized cart, most of the naysayers have come around.
Now for about $40 in parts you can make your own working model and demonstrate this novelty for yourself.
Learning objective:
Critical thinking! I originally conceived of the downwind cart when a friend asked me whether a sailboat could tack downwind and beat a free-floating balloon to a point directly downwind. I wasn't sure, so I did a quick vector analysis. Somewhat surprisingly the answer was "yes". Being a huge fan of brain-teasers, I asked myself how to make this little tidbit even more twisted. By making a wind powered vehicle that could beat the wind DIRECTLY downwind, I suspected it would go against most folks' intuition - and that proved truer than I would have ever guessed.
The reality is that one can analyze this with high school level physics and math. As engineers, we rely heavily on our intuition for problem solving, but we have to remember to use that intuition to guide us - NEVER to replace rigorous analysis (or worse yet - observed results). Much to my surprise there have been a fair number of professors of physics, NASA aerospace engineers, and aerodynamicists that assured me this could not work - even after it was demonstrated. These folks relied on their intuition when it was time to carefully consider the simple analysis.
The secondary learning objective is to remind us that we don't stop learning when we start teaching.
Note: I later learned that I was not the first to have conceived of such a cart. An engineer by the name of Andrew Bauer built one in the 1960's. He had learned of the concept from a student paper written some 20 years earlier. We've never found that student's name.
The reason we built the cart was to settle the long running debate across a number of internet forums. I was clearly not going to convince many people with my analyses. The matter is settled now to about the same extent the moon landing is settled. The difference being - you can build your own cart in an evening or two and prove it to yourself :)
http://en.wikipedia.org/wiki/Sailing_faster_than_the_wind
and
http://adventure.howstuffworks.com/outdoor-activities/water-sports/sailboat4.htm
And I might add that first one talks about your cart.
I was left wondering about the correlation to the transfer of energy from wind to the cart in your explanation.
in reading the above I think my suggestion was close but not quite right.
the sailboat moves in the wind by the two principals described both the lower pressure on the back of he sail and the higher pressure on the front.
and as the boat moves forward at an angle the moving air across wind side of the sail builds pressure, while the air lowers pressure in forward side drawing it forward.
and when it moves faster then the wind I think that the forward moving wind meets the air moving across the wind side of the sail increasing the pressure therefore putting the energy into the system and the only reason the wind can meet that wind is due to the barber pole effect as coined from our previous conversation. basically the air is displaced by the sail back towards the wind enough to meet the wind even though the sail is moving faster then the wind
and from there we just look at your video to transition to the propeller and we have your cart moving dead downwind faster then the wind.
time for me to read down and see if any new posts were added since we last spoke :)
Daniel Hope.....
I don't recall why I stopped on that particular run. Obviously you have to stop all runs at some point. On a few occasions I broke the chain, but it doesn't look like that was the case here. More than likely I was simply approaching the portion of the lake-bed that wasn't suitable for a speed run - so I backed off on the propeller pitch and got on the brakes.
You mean that the cart is running downwind and that it does so not by being pushed by the wind but by taking energy from the wind through its mill that gives enough power to have the cart running faster than the wind ?…
Then of course, ""scientists" (I give quotes because a true scientist must always be unsure about his science) told you that you cannot spend more energy that what you create therefore the amount of energy to run the cart must at best equal the amount received. Then one must allow for friction, losses of energy for running the syste, etc … all which to less energy available for running the cart. Therefore the latter cannot go faster than the wind. Do I guess right ? I
'm only guessing.
But you should answer to your scientists a very simple fact : all those high performance and competition catamarans and trimarans can run faster than the true wind because they create their own wind. That is why you'll never see one of those sea monsters running downwind !!!… They simply cannot !!!… Their speed wind forces them to broad reach !… This is where they get their fastest speed, although it has been thought that the fastest speed on the water was reached by running downwind (which is true for classical boats).
But I'm somewhat getting off the subject.
Back to my question : did I understand right and, if not, could you tell me what is the meaning of all this ? …
Congratulations for your performance anyway !!!…
it seems hes set it up to continue to use the energy of the wind.
as I see it the rotating prop surface relative to the wind is still moving slower then the cart allowing it to continue to use the energy of the wind .
Hmm I still don't think I explained that very well LOL.
I need to make an animation or something for this.
That's correct. We're simply harnessing wind energy (which is sort of free I suppose).
it seems hes set it up to continue to use the energy of the wind.
as I see it the rotating prop surface relative to the wind is still moving slower then the cart allowing it to continue to use the energy of the wind .
That is true in a sense. Of course the propeller itself is moving forward faster than the wind - along with the cart. The the mathematical surface it defines can be seen as moving slower than the wind in the same sense that the stripes on a barber pole can be seen to be going down continuously - even when they never get any lower.
I need to make an animation or something for this.
I'd love to see it. Here's a really great animation one of our friends made:
Great post, BTW!
If this can help.
I noticed that as well. If you double click on it, it will come up full-screen and not cut-off. But here's the link:
http://www.youtube.com/watch?v=UGRFb8yNtBo
You can also get this link by right clicking on the video and selecting "copy video url".
The reason we can do this is that our cart is designed to exploit the energy of the wind relative to the ground - not the wind relative to the vehicle itself. It is very important to understand that the wind does not turn the propeller - ever. The propeller is geared to the wheels, and is turned by the wheels as the cart is initially pushed forward by the wind. As the cart begins to gain a bit of speed, the thrust from the spinning propeller begins to become significant.
You correctly point out that high performance sailboats don't run directly downwind, but instead tack downwind. What a lot of people don't understand is that these boats can actually reach a point directly downwind (by tacking) much more quickly than a balloon floating freely in the wind. That is to say that their downwind VMG (velocity made good) can be significantly higher than wind speed.
Our cart is designed to to the same thing - in a slightly different way. If you think of each of our propeller blades as a sail, you will notice that they never go directly downwind. Because the propeller is spinning, each blade is going both cross-wind and downwind at the same time. In other words, each prop blade is on a continuous spiraling downwind tack - just like the sail of the high performance sailboats.
We can also look at it from an energy conservation point of view and make a pretty simple argument that shows we do in fact have plenty of excess energy - even after frictional losses and imperfect efficiencies. Let me know if you'd like to see that explanation.
Just a little vocabulary nit to pick -
Tacking is turning the bow of the sailboat through the eye of the wind, done when working your way to windward (or upwind)
Jibing is turning the stern of the sailboat through the eye of wind, when working your way downwind.
But tacking is used in several ways, while jibing really only has one meaning. One can be "on a tack" (such as a broad reach). For example, when kitesurfing we have the right of way when we're on a starboard tack.
Tacking can also mean the process of changing directions (as you've described) or the process of getting to a destination through a series of sailing segments that are not individually directly in that ultimate desired direction.
The cart isn't tacking the propeller blades are, that's what causes the resistance that simulates a sail but not directly against the force of the wind.
What I'm offering is perhaps the angle off perpendicular (as applied to the blades) while spinning allows more force to accumulate.
Kind of like slipping off while resisting a force.
Example: your knuckles when the wrench slips, they have a tendency to speed up right before the yelling starts.
IMHO
Correct. That's the point of this thing. The cart goes directly downwind. The blades of the prop are on a continuous broad reach.
What I'm offering is perhaps the angle off perpendicular (as applied to the blades) while spinning allows more force to accumulate.
The prop blade is simply doing exactly what a sail does when a high performance sailboat is on a broad-reach with downwind VMG faster than wind speed.
Believe an old salt like me who's been wearing off his oilies since 1962 (good God ! … Time goes fast !…).
To be on a tack means having the direction of the wind come on one side of the boat or the other (starboard tack : the windward side of the boat is starboard / opposite for port tack).
Tacking has two meanings and only these :
1) Turning the bow of the sailboat through the eye of the wind AND changing tack (going from the starboard to the port tack, for example).
It is NOT - as ttraband thinks- going upwind and staying on the same tack. Eg. going from a broad reach to a close reach is by no means tacking : the boat simply gets "closer to the wind" as she stays on the same tack.
2) It follows that "tacking" means following an upwind course so as to sail the boat to the ultimate upwind direction by doing a series of tacks .
Hence, it follows that although she is not tacking, a sailboat running downwind is always on a tack : starboard or port tack. It has been agreed since way back then (12th, 16th, 18th century ???…) that a boat in such a position is said to be on a starboard tack when her boom is on her port side (conversely she's on a port tack when the boom is on her starboard side).
Now, for jibing, this is an altogether different matter, although this is related to tack.
Jibing means that while following a dead downwind course the boom's position goes from one side of the boat to the other (from starboard to port or vice-versa). This means that when you jibe, you change tacks. However by no mean you can say that the boat is tacking as she is still running dead downwind. In other words she has changed tacks but whe was not tacking.
As for kitesurfing / sailboarding / etc … I understand that regulations has been taken from Rules for Safety at Sea in which the main point is that a sailboat on a starboard tack has the right of way over others.
If these petty sailing vocabulary niceties are still a little bit cloudy to you (and if you're not already fed up by them !!!…) I suggest that you hop over her. I live in Le Havre, French side of the Channel, my 33ft sailboat is getting ready for the coming season. If all goes well she should be launched in a few weeks. I still have no plans for this summer cruise… but I'll find one and you are welcome to share the niceties of tacking on our way to England, be it the Isle of Wight or the Scillies ( then again the magics of Brittany …) and experience the joy of the first seasickness of the season (this not being mandatory !!…).
A nice week end to all !!…
Don't think so. I've been sailing for 40 years. I'm pretty familiar with the terminology.
LOL
Nah - I've had my say, and it's hard to get too excited about definitions. Maybe we can have it out over whether Pluto is a planet : )
Your video is great ant (almost) says it all.
I read the arguments for and against at length before Christmas 2010. I convinced myself of the facts, spinning blades spiraling downwind tack, propeller pushing etc. then questioned all the dad's around at christmas. Some even had PhD's. It was very interesting that the engineers could open there minds to the concept but the Physics PhD father wouldn't even debate it.
It is a bizarre concept and a great exercise in thinking the problem through.
...although a few bright sparks do sometimes get through :O)
Sadly I think that's too often true - but I don't think it's the fault of education per-se. I think it's a human tendency. I'd love to see engineering programs include problems liken this now and then to remind people that you can't drop all problems into the neat template you expect it to fit into. This "feels" like a perpetual motion issue - and therefore can be immediately dismissed. The problem of course is that it simply isn't what it looks like.
Thanks for your comment.
I understand now. And let me tell you I'm not good at these things, so you mage a pretty good job at telling how and why as well as your initial engineering.
Thank you very much.
I'm very impressed by all that.
If you want to add an explanation on how you get plenty excess energy, YES ! ∞ I would be very pleased.
Thank you again.
Thus over a fixed period, the wheels will go over twice as much ground as the prop disk goes through air. This means that a retarding force of F on the wheels will produce twice as much energy as is required by the thrust force at the propeller. After accounting for internal losses and innefficiencies it's still possible to achieve more thrust than is caused by the drag of the wheels that are used to turn the prop.
In fact, there's a very interesting result that comes out of this. A propeller or turbine on a moving platform such as this can actually harvest FAR more energy from the wind than a stationary windmill of the same diameter. In simplest terms, this is due to the fact that the moving platform is able to move through much more air, harvesting that energy.
He lost.
Do you think it would have worked?
From his point of view it would have been a high risk strategy though.
That's a very tough question. It can definitely be done, and has been done full scale - but never going directly downwind faster than the wind on water - yet. The only way I've seen it done was to use the rotor as a turbine turning the underwater prop. That can be effective, but cannot get you above wind speed (when running directly downwind.
Using a configuration like the Blackbird, in which the underwater prop (actually turbine) turns the propeller in the air, it's theoretically possible to go directly downwind faster than the wind on water - but it would be challenging.
I hope I answered your question. I'm not trying to be cagey.
I would guess driving the prop under water would be to expesive for drag and like the ground its going faster rel to the boat where the wind relative to thecart boat is slower.chicoHope...
Of course, it's like the joke where two guys are being chased by a bear, and one says, "Why are we running, we can't outrun the bear?" and the other says "I don't have to outrun the bear, I only have to outrun you!". ;-)
He wouldn't actually have to go faster than the wind, only faster than the square sailed catamarans. He would be producing more energy, but then suffer more losses due to the turbine, and it would be presumably heavier which would give more drag.
Now only one question remains : is wearing diving goggles and tuba while operating the turbine mandatory, or are you just looking forward to the next step which would be running this turbine on water ???…
;D
I should read some definitions of some of the words im using like bluff body as I don't think I have ever run across it before and im reusing it based on your use with LOL.
ok now another attempt to explain myself based on the new input.
I will preface this with all the energy into this system comes from the wind speed and then is trasformed in different ways.
im just going to look at this system after it exceads wind speed.
I will agree that the prop rotateing creates a low presure forward of itself therefore creating lift on the surfaceassisting in forward motion.
and again the higher pressure aft of the prop (I assume this is the thrust you mention)which I think is part of the mechanism to create the relative bluff face that travels slower then the wind giving the system a way to continue taking advantage of the wind energy.
so the faster the cart moves the faster the relative bluff face moves backwards relative to the cart maintaing a slower then wind bluff face
chicoHope...
Again, the wind is not pushing against any sort of virtual bluff face when the cart is going faster than wind speed. The propeller is acting exactly like the prop on a Cessna flying along on a no-wind day. Any instrumentation you use in front of, or behind, the propeller would show you exactly the same sort of flow you'd see from a propeller on a small plane.
http://en.wikipedia.org/wiki/Tack_(sailing)
http://en.wikipedia.org/wiki/Tack_(sailing)
And... http://www.arvelgentry.com/magaz/Downwind_Tacking.pdf