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Downwind Faster Than The Wind Cart

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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 :)


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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danielwhw2 years ago
Ok only got to watch the video with sound
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...
wiredcav (author)  danielwhw2 years ago
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.

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.
Speakup2 years ago
Highly amplified tack.

http://en.wikipedia.org/wiki/Tack_(sailing)
wiredcav (author)  Speakup2 years ago

http://en.wikipedia.org/wiki/Tack_(sailing)

And... http://www.arvelgentry.com/magaz/Downwind_Tacking.pdf
cody3052 years ago
Sorry if you already answered this, but here goes: Once the cart has exceeded the wind speed, the wind would no longer be pushing the vehicle. If the wind is not pushing the vehicle, (1) where does the energy come from to turn the propeller faster to continue increasing the vehicle speed? It seems that once you reach the speed of the wind, that would be the limit. Also, (2) have you had this tested by a reputable skeptic to certify there are no hidden batteries or motors, etc, that could be providing the extra energy needed to exceed the wind speed? The video was very impressive, but (from my engineering background) it looks like the laws of physics are being broken here (cold fusion?), and I would like this to be confirmed by an independent testing lab. If this really works as you say, then the applications could be enormous!
wiredcav (author)  cody3052 years ago
Once the cart has exceeded the wind speed, the wind would no longer be pushing the vehicle.

Correct.

If the wind is not pushing the vehicle, (1) where does the energy come from to turn the propeller faster to continue increasing the vehicle speed?

The wind is no longer pushing it, but the tailwind can now be seen as a reduced headwind.  The reduced headwind makes it possible for the propeller to produce the same amount of thrust using less power to do so.  In other words, the cart is moving over the ground at 20 mph, but only moving through the air at 10 mph.  This means the wheels can harness twice as much energy as the prop requires - for the prop to produce the same thrust that the wheels produce drag in turning it.

Keep in mind - power = speed x force.  We're going over the ground twice as fast as we're going through the air.  And the air we leave in our wake has been de-energized by the cart (i.e. slowed down).

It seems that once you reach the speed of the wind, that would be the limit.

That's what our intuition tells us - which is what makes this such a fun brain-teaser.  But high performance sailboats can tack to a point directly downwind and beat a balloon floating with the wind.  We're doing the same thing here, only our sails (prop blades) are tacking in a circle rather than back and forth.

have you had this tested by a reputable skeptic to certify there are no hidden batteries or motors, etc, that could be providing the extra energy needed to exceed the wind speed?

In more ways than you might guess.  We demonstrated it to the satisfaction of the North American Land Sailing Association when we set the world record for a wind powered vehicle going directly downwind.  But I've also posted these construction plans so that anyone can reproduce our results with a minimum of cost and effort.  I strongly encourage you to be one of the skeptics that tests our claim.  Many others have.

The video was very impressive

Thank you.

but (from my engineering background) it looks like the laws of physics are being broken here

Nope.  We take the laws of physics pretty seriously around here.  I wouldn't think of breaking them.  Now applying them in a counter-intuitive way - that's sort of a hobby.

I would like this to be confirmed by an independent testing lab.

You can see our raw data for the record run at www.nalsa.org along with our submission report and their observer report.

If this really works as you say, then the applications could be enormous!

There are some interesting applications we've discussed.  
I have since searched the internet on this and found more info that helped me understand how it works. The key thing for me was finding out that the propeller is pushing against the wind, and so the velocity of the propeller's air stream gets added to the wind velocity, so the cart moves at the combined wind-plus-propeller's airstream velocities. So now I am convinced, and I thank you for "teaching" me a new concept that I never would have imagined could be true.
wiredcav (author)  cody3052 years ago
The key thing for me was finding out that the propeller is pushing against the wind,

I'm glad you're developing an understanding of this.  But we have to be careful not to fall into the common misconception that the prop thrust is somehow creating a wind-buffer that the tailwind is pushing against.  When going faster than the wind, the propeller is operating exactly like a propeller on a Cessna - effectively pulling the aircraft into the new air in front.  There are no converging air masses behind the propeller.

However, the air coming into the propeller from in front is entering at a much lower speed (relative to the cart) than the speed of the cart over the ground.  This is what allows the propeller to effectively get more "bite" on that wind.
djimdy2 years ago
[Please correct me if I'm wrong, because I'm embarrassed to say that I don't totally get the energy explanation, but here is how I am looking at it mechanically]:

-Assume a 10 mph tailwind, correctly geared setup, a prop-shaft split in two, and car already at speed (10mph)
-Like the wind vane and antenna flag in the video, the propeller would not be turning, because at 10mph, it experiences no net wind. Its half of the shaft would likewise NOT turn.
-Wheels on car would be going like mad, as would its half of the prop shaft.
*If the shaft were to suddenly be connected, the propeller would turn, chew up air and throw it back, providing a net forward propulsion.
-that propulsion boosts the car
-which turns the wheels and therefore the propshaft even faster, thereby turning propeller faster, thereby producing more thrust.

If this is right, it seems to be what's happening in the treadmill demo, only without the benefit of the mechanical disconnect.
wiredcav (author)  djimdy2 years ago
-Assume a 10 mph tailwind, correctly geared setup, a prop-shaft split in two, and car already at speed (10mph)
-Like the wind vane and antenna flag in the video, the propeller would not be turning, because at 10mph, it experiences no net wind. Its half of the shaft would likewise NOT turn.
-Wheels on car would be going like mad, as would its half of the prop shaft.
*If the shaft were to suddenly be connected, the propeller would turn, chew up air and throw it back, providing a net forward propulsion.
-that propulsion boosts the car
-which turns the wheels and therefore the propshaft even faster, thereby turning propeller faster, thereby producing more thrust.

If this is right, it seems to be what's happening in the treadmill demo, only without the benefit of the mechanical disconnect.


I'd say that's surprisingly accurate.  A lot of folks refuse to believe it because it seems like a runaway positive feedback loop - for free.  The truth of the matter is that it is a feedback loop of sorts, and just like an audio feedback loop, it has an external power supply - the wind in this case.  

Try pulling the plug on the amp when the microphone is feeding back.  It stops immediately.  Not nearly so mysterious as this might seem.

Nicely done.
djimdy wiredcav2 years ago
Thank you, sir!

As for pulling the plug, I'm not sure I could visualize what would happen if the tailwind would suddenly stop or switched to headwind, however.

Based on the treadmill example, which substitutes gravity for drag, it seems that one could add a frontal wind at the propeller nearly up to the speed of the car, it would lose speed, but not stop until the frontal wind exceeded the speed of the car, which would then cause it to stop and then (with symmetrically shaped vanes) it would go the other way.
Such a nifty post. I will reread all the commentary at least twice and perhaps I will understand more. I did research into Land Yach's or Sand Yatchs several years ago.

The concept of a wind mill on a vehickle was a very old one indeed. The Chinse came up with one about 1500-2,000 years ago (I believe). A boat was conceived in the1800's as machinery was starting to make it's mark in America, (all things steampunk). When Conestoga Wagons were at their height of use there was a company started that proposed to make a rolling cart EXACTLY like the boat and this little car, to cross the plain states. Except this little thing called the railroad had evolved enough to wipe out the idea.

I had lots of research and even found several books in my local (gasp) library, some even had nice pictures. Due to the disabilities act my library threw out half its books and I can't even go back to try to find the tomes. And that was also 2 computers ago so I no longer have the bibliography.

ta=hanks fer making this instructable I hope one day land yaching returns to Long Island NY. Looks like a hoot.

here is a wiki with a picture proving it was done in the heartland of America first
http://en. wikipedia.org/wiki/Land_sailing
brooklyn

Please sire, May I have another?

thanks
wiredcav (author)  spark master2 years ago
The concept of a wind mill on a vehickle was a very old one indeed.

Indeed.  But this vehicle is not powered by a wind mill.  The big spinny thing is a propeller which is pushing the cart along faster than the wind. The wheels are turning the propeller - not the wind.

Thanks for the comment.
Based on my post above and I realize alot of this is based on your perspective. But I feel im onto something here.
And to follow my line of thought you have to consider the prop a sail with a twist (literally ). Sails rotating on around 1 a point.
The wind first of all is doing all the work that is being acted on.
In turn rotating the sails/prop which leaves the sail/prop esentialy slower then the wind so it can continue to act on it.
nice job on this by the way.
I may just have to build one to experiance it first hand.
ChicoHope...
wiredcav (author)  danielwhw2 years ago
you have to consider the prop a sail with a twist (literally )

That's exactly right.

The wind first of all is doing all the work that is being acted on.

This gets to be a very subtle matter indeed.  Like velocity, energy (or work) is frame dependent.   Whether the wind is doing the work, or the ground is doing the work, or whether it's a combination of the two is truly a matter of the reference frame you choose for your analysis.

In turn rotating the sails/prop

This part is less philosophical.  The torque that turns the propeller does in fact come through the prop shaft - from the wheels.  The wind is obviously a key player in how this happens, but the actual torque is not from the wind.

nice job on this by the way.

Thanks!

I may just have to build one to experiance it first hand.

I hope you do.  It really is a fun little bit of science you can do on your own treadmill.



HI,

I have tried reading as many posts at a sitting as my but allows and I can see I will need to reread yet again.

The original acceleration is just the effect of wind on a wheeled object. That continues until the speed of the wind (or a high percentage of it (upper limit of sorts) is reached. then as the wheels turn the whirley bird thing, it creates a back pressure for the wind to push more/harder? This allows the faster then light speed, errrrrr... faster then wind speed(sorry captain I could not resist)?

To make the prop push the wheels do we need to reverse the pitch?

I need to see my local HS fisics teachers for simpler explain.

thanks

wiredcav (author)  spark master2 years ago
The original acceleration is just the effect of wind on a wheeled object. That continues until the speed of the wind (or a high percentage of it (upper limit of sorts) is reached.

Yes, but the transition from being pushed as a bluff body to being propelled by the thrust of the propeller is a continuous and gradual change.

then as the wheels turn the whirley bird thing, it creates a back pressure for the wind to push more/harder?

Yes and no.  The wheel spin the propeller which begin to produce thrust, which pulls the cart into the wind ahead of it.  It's a common misconception that the prop is producing some sort of buffer of air that the tail wind pushes against, but this really isn't the case.

To make the prop push the wheels do we need to reverse the pitch?

If we want the vehicle to go upwind rather than downwind, we would simply increase the propeller pitch beyond the critical point.  Now the battle between torque and bluff body force when sitting still in a tailwind will go the other way - and the cart will go directly upwind.  But we'd really be much better off replacing the propeller with a turbine - which is exactly what we're about to do.

I need to see my local HS fisics teachers for simpler explain.

I should warn you that there's a very high likelihood of him getting it wrong.
I tend to think that its never really thrust that propels it forward since that implies power from nothing. but rather the prop is acting as a bluff body (infact a relative bluff body moving slower then the cart I'll explain that more) being pushed by the wind.
when the cart reaches wind speed the prop surface as a bluff body at the air contact point is moving slower then the wind.. this point is where the wind sliding past the prop moves backwards enough to overcome the cart speed relative to the wind allowing the (relative bluff body) to be continually pushed until I expect a point where the wind resistance to the cart in not overcome.

when the cart is at rest the torque acted on the wheels to go backwards just need to be overcome by the bluff body effect of the combined prop and cart body
so too much prop torque at stand still may act against the desired forward motion. So I expect rotational props would be a great benefit to this cart.
props flat or even rotated back to make the torque forward would benefit forward motion till some point approaching wind speed where rotating the props back to the current angle then would give you more relative bluff body effect.

keep in mind this is not my area of expertise. and now to read the whole thing and watch the video with sound now that I'm on my computer. :)
ChicoHope...

ps hope I didn't make any more stupid mistakes like spelling debate wrong...
wiredcav (author)  danielwhw2 years ago
I tend to think that its never really thrust that propels it forward since that implies power from nothing.

It's really thrust - in exactly the same way a spinning propeller on a Cessna produces thrust.  We're not getting that power from nowhere - we're exploiting the energy of the wind relative to the ground - and slowing it down to take it's energy.

when the cart reaches wind speed the prop surface as a bluff body at the air contact point is moving slower then the wind.. this point is where the wind sliding past the prop moves backwards enough to overcome the cart speed relative to the wind allowing the (relative bluff body) to be continually pushed until I expect a point where the wind resistance to the cart in not overcome.

Nope - when we're going faster than the wind, the prop no longer acts as a bluff body at all.  At this point each blade is generating lift exactly as a sail or wing does.  But of course that lift is horizontal - so we call it thrust.

when the cart is at rest the torque acted on the wheels to go backwards just need to be overcome by the bluff body effect of the combined prop and cart body so too much prop torque at stand still may act against the desired forward motion.

True.

So I expect rotational props would be a great benefit to this cart.
props flat or even rotated back to make the torque forward would benefit forward motion till some point approaching wind speed where rotating the props back to the current angle then would give you more relative bluff body effect.


Yes - with two caveats...
1) It would only help if the prop were allowed to drive the wheels.  We have ratchets that prevent that.
2) At and above wind speed, the prop acts like - well - a prop; not a bluff body.

keep in mind this is not my area of expertise.

No worries.  If you find it interesting I'm sure you'll be able to understand.  But it does take some head banging for most folks.
hmm on a side note if you do make one with props that rotate let me know if you can reverse the pitch and head back up wind. maybe thats just old technology but just curious.

and yes you are watching me evolve my thoughts on this real time with out study first LOL. Hope you can ignore that part.
wiredcav (author)  danielwhw2 years ago
let me know if you can reverse the pitch and head back up wind.

Yes you can.  But it's more efficient to replace the propeller blades with turbine blades to go upwind.  I've built a set of turbine blades for the Blackbird and hope to set the upwind record in the next few months.
smorin2 years ago
Congratulations!!
Why don´t you publish a video about the model working?
wiredcav (author)  smorin2 years ago
Congratulations!!

Thanks!

Why don´t you publish a video about the model working?

We have a bunch of them.  Check out "spork33" on YouTube.
rippa7002 years ago
Wiredcav - thanks for your excellent explanations throughout. Very clear and concise even when fending off oddball comments. I was struggling to understand how this worked from looking at the videos but you have enlightened me - a fabulous concept - well done and thanks.
wiredcav (author)  rippa7002 years ago
Thanks so much for the compliments! I'm glad you liked this little brain-teaser.
danielwhw2 years ago
What am I missing that makes the prop apear to be powered by a head wind.
granted I have no degree.
but based on the shape of the prop I would have expected it to rotate the other direction.
ChicoHope...
wiredcav (author)  danielwhw2 years ago
The wind is not turning the prop. It is geared to the wheels - so they turn it in the opposite direction the wind would.
Ok when I get to my computer ill have to look it over.
as I understand what your saying the wind is hitting (depate aside on the mechanisim) the back of the prop pushing the vehicle forward therefore turning the prop counter to the expected direction.
ah and now that you mention it I can see how the any given point on the blade relative to the air is now moving backwards at a rate that esentialy remains slower then the wind.
Ok this is off the top of my head so ill do more then watch the video with no sound and even read the text LOL
ChicoHope...
dropkick2 years ago
While I didn't read all the comments those that I did read are missing a basic knowledge of how sails, propellers, aircraft wings, etc. actually work.

None of them are "pushed" by the wind.

The wind/air travels across them and due to the curvature of one side of the (sail, propeller, wing, etc.) the air travels faster on one side. This creates an imbalance in the air pressure which causes the AIR PRESSURE (not the wind) to push on one side of the (sail, propeller, wing, etc.) which propels the item.

So it is actually atmospheric pressure that moves the vessel not the wind.

Note: air or atmospheric pressure is not wind, it is the weight of the air.

The only role the wind plays in your vehicles movement is in the wind's speed over the surfaces increasing or decreasing the imbalance - and with a propeller even that isn't straight forward, as the movement of the propeller also changes the air flow across the surfaces which increases or decreases the push of air pressure.
wiredcav (author)  dropkick2 years ago
While I didn't read all the comments those that I did read are missing a basic knowledge of how sails, propellers, aircraft wings, etc. actually work.

I'm not sure which comments you're talking about, but I've got a couple of degrees in aerodynamics, and have a pretty fair idea of how wings work.

So it is actually atmospheric pressure that moves the vessel not the wind.  Note: air or atmospheric pressure is not wind, it is the weight of the air

So you think sailboats are powered by atmospheric pressure - and not wind?  I fear you may need a refresher on some basic aero.

The only role the wind plays in your vehicles movement...

I'm actually pretty well aware of the role the wind plays w.r.t. this vehicle.  I'll be happy to help describe it to you if you care to understand it better.
vov35 wiredcav2 years ago
imagine infinite small particles. imagine a current of infinite small particles. This is your wind. Now imagine a solid, angled plane passing through this current, sure, it causes collisions and creates an overpressure or backwards wind. Yes you could call it "pressure", all of the collisions of particles with the prop, or you could call it wind, but in the end it's the same.

Gasses are difficult to intuitively understand, yes, but to argue that "wind" versus "pressure" drives the vehicle is as silly as saying that a propeller "pulls" versus "pushes". Once a proper intuitive understanding occurs, limiting it by the technicalities of proper terminology is ridiculous.

This individual has a decent understanding, he just named the idea differently than you did.

air is particles!
wiredcav (author)  vov352 years ago
This individual has a decent understanding, he just named the idea differently than you did.

Keep in mind, this individual is telling me that I don't have a basic understanding of how sails work.  I do in fact. 

I can also tell you that your model of the many particles hitting a plane is not at all accurate for describing how lift works on any sort of traditional wing in our normal atmosphere.  http://www.grc.nasa.gov/WWW/k-12/airplane/wrong2.html
vov35 wiredcav2 years ago
nonetheless, you do understand that air IS particles. it's the molecules of gasses trapped by the earth's gravity. And yes, it's earth's gravity that compresses them to the density we've come to know and love.

You're right, though, JUST modelling hitting a plane is inaccurate, one should also factor in collisions amongst the particles themselves, the force of gravity on all of the particles, and the random movement caused by thermal effects. These molecules also have momentum, mass, volume, and all other fun physical properties of matter.

Combined, all of these factors /do/ explain the bernoulli effect and thus lift upon a traditional airfoil. The third paragraph of the wiki article on that will explain it better if it doesn't make sense.

I still stand by the statement that he understands the concept (as do you) but has decided to use different terminology. It's silly arguing over that context. Admittedly I did not thoroughly read the OP at first, so I sprung to defending him, but evidently he's also been quite offensive about it.

"The only role the wind plays in your vehicles movement is in the wind's speed over the surfaces increasing or decreasing the imbalance." well... that's pretty much how wind works, it applies pressure to objects... you know what, I'm going to stop before I get lost in terminlogy too. 'Pressure' and 'wind' are terms we've devised to describe the behaviour of huge numbers of particles. What I'm saying is that which term one decides to use is trivial, because centrally the idea is the same.

OP, you're also causing an argument where there needn't be one. ANYWAY, I'm gonna get off of this before I realize that I'm arguing on the internet again
wiredcav (author)  vov352 years ago
nonetheless, you do understand that air IS particles. it's the molecules of gasses trapped by the earth's gravity.

Indeed I do.  And I understand that the claims that both you and dropkick made are simply not correct.

 I still stand by the statement that he understands the concept

His post indicates that he clearly does not.

What I'm saying is that which term one decides to use is trivial,

Agreed.  But we're not talking about semantics.  We're talking about a fundamental misunderstanding of what's happening here - and frankly about two people that are both being condescending - while incorrect.
vov352 years ago
What if you could vary the pitch of the prop to function as a sort of transmission?

then you'd be able to power the wheels for additional acceleration up to the speed of the wind, turn the prop into a flat "sail" as you hit the windspeed, and then continue to convert the vehicle into what it's doing in this instructable... Plus you'd be able to go upwind in the same vehicle.
wiredcav (author)  vov352 years ago
You're exactly right on all counts. We added variable pitch after our initial tests, but we don't allow the prop to drive the wheels for reasons relating to the rules of the record run.

Unfortunately, a propeller running negative pitch is much less efficient than a proper turbine. Thus I've built a set of turbine blades and hope to go directly upwind faster than the wind in the next 2 or 3 months.
What about reversing the blades completely and reversing the direction of spin (through a gearbox,) for running the prop as a turbine?
vov35 skrubol2 years ago
I considered this before suggesting a variable pitch prop. I do believe a mechanical gearbox could be designed to perform the task, but the variable pitch prop turned out to be a more elegant solution. Two major factors pointed me in this direction

Firstly, a variable pitch prop functions as a transmission capable of attaining ANY gear ratio within it's range. This allows the whole system to be maintained in an optimal position to maximise power transmittance. A traditional transmission would mean several gear ratio jumps which would try to approximate the ideal ratio.

Secondly, the prop/turbine has rotational inertia. Given the size of the prop relative to the cart, I'd assume that said inertia is quite notable. Thus, mechanically reversing the direction of rotation, or even shifting gears, would likely cause a large loss of energy for the cart. The change in prop RPM that would result from any traditional transmission shifting would likely produce adverse effects.

Come to think of it, the rotational inertia may be so great as to roll the cart if suddenly stopped or reversed.
skrubol vov352 years ago
I meant transmission only as a forward/reverse selector and still use a variable pitch prop to change the ratio.
The transmission would only be used when going from with wind to against wind, and the prop would have to be stopped when its direction is changed.
tartan5 skrubol2 years ago
I wonder if a D Drive could be put to use?

http://youtu.be/F6zE__J0YIU
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