# Downwind Faster Than the Wind Cart

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## Introduction: 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 :)

Building the working model cart - Part 1 of 3:

Building the working model cart - Part 2 of 3:

Building the working model cart - Part 3 of 3:

If you want to take on a significantly larger project, you can follow our blog that takes you through the build of the full-scale Blackbird.http://www.fasterthanthewind.org/

It would be fun to see someone break our record and go downwind at 3X wind speed or better.

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## Questions

How about some solid testing on a dyno in a wind tunnel? Colored smoke streams might provide some interesting results in there, as well as how much power is derived from the "feedback".

Hi. I've followed this effort for the past few years and, yes, excellent brain teaser. Thanks you for spending the effort to share this information with all of us. And for the thoughtful patient explanations.
Initially, the difficult understanding for me was the cart transitioning through 0 relative wind. Relative to the cart that is. Then I realized if I were racing my fast catamaran down wind, the event of the boat exceeding the downwind speed of a free floating balloon (time made good) is trivial to comprehend. Cool.

Any progress with the upwind attempt? And, is there a blog or other site to follow it?

>> Any progress with the upwind attempt?

Yup. We set the downwind record at 2.8X wind speed in 2010. We set the direct upwind record at 2.1X wind speed in 2012. The downwind blog is www.fasterthanthewind.org and the upwind blog (much more brief) is here: http://fasterthanthewind.wordpress.com/

We're done with the vehicle now and ready to give it away. You want it?

It seems to me the wind should turn the prop the other way

Indeed! The wind tries to push the cart downwind and it tries to turn the prop the other way. Because the prop is geared to the wheels, it's trying to turn one way, while the wind tries to turn it the other way. The gearing between the wheels and the prop is such that the wheels win this tug-of-war. Utimately that's what allows the car to reach and maintain a downwind speed faster than the wind.

I'm not sure I'm getting it right.
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 !!!…

14 replies

Yes, the cart runs directly downwind, and it can achieve steady speeds of 3X wind speed or more (we've unofficially done runs at 3.5X wind speed). Intuition might suggest this is impossible because once you reach wind speed, there is no wind at all over the cart. But we can and do continue to accelerate beyond that point.

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.

this a fantastic summary, nicely put

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.

Just goes to prove that "education squashes imagination".

...although a few bright sparks do sometimes get through :O)

>> Just goes to prove that "education squashes imagination".

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.

Thanks very much. It's been a lot of fun (and frustrating at times!)

Very interesting idea, and I love that the theorists keep trying to argue with the real-world results, just like when PhD's try to tell us that it's impossible for a bumblebee to fly. Lucky for us that the bees don't listen to the PhD's!

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.

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)

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.

Not trying to be any kind of expert here, just trying to explain my reasoning.

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

The cart isn't tacking the propeller blades are,

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.

Folks sorry to say that you're both on the wrong path on this specific point of seamanship vocabulary.
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 jibingthis 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 !!…

Folks sorry to say that you're both on the wrong path on this specific point of seamanship vocabulary. Believe an old salt like me who's been wearing off his oilies since 1962.

Don't think so.  I've been sailing for 40 years.   I'm pretty familiar with the terminology.

ah ah !!!… here comes a battle of vocabulary niceties !!!…

LOL

ah ah !!!… here comes a battle of vocabulary niceties !!!…

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 : )

Totally agree… I was just overtaken by kick I take from pleasure with words.

Your video is great ant (almost) says it all.