Going Faster Than the Wind In a Wind-Powered Cart

Shawnconna writes "Can a wind cart travel faster than the wind? A group of makers say, 'Yes!' Make: Online has published a story about the Blackbird wind cart that just set a record. This is a follow-up to an earlier story in which Charles Platt built a cart based on a viral video where a guy claimed he'd built a wind-powered vehicle that could travel downwind faster than the windspeed. Charles built one and said it didn't work. Heated debates broke out in forums, on BB, and elsewhere on the Net. In the ensuing time, a number of people have built carts and claimed success, most principally, Rick Cavallaro. He got funding from Google and JOBY to build and test a human-piloted cart. They claim success, with multiple sensor systems on board, impartial judges and experts in attendance."

Comments

store and release energy?

[BBrown]

I probably am just thinking about this too simply, but can't something go faster than the wind if it stores some of that energy and uses it later?

Re:store and release energy?

[whoever57]

When this happens, the speed of the wind relative to the vehicle is zero (as if there were no wind) and the propeller is rotating, just like in the first scenario. So, as long as wheel friction and internal friction are small enough, the vehicle will accelerate to go faster than the wind.

Congratulations, you just invented a perpetual motion machine!

In the example described in the article, the author overlooks one huge fact -- the treadmill is a source of energy, so assuming that a treadmill in a room with no wind is equivalent to traveling over a road with a wind from behind is fundamentally flawed.

To critique your explanation: why should the propeller continue to turn? There is no energy being gathered from the wind, so the only reason that it might turn is that the kinetic energy of the overall cart is converted into the turning the propeller. The propeller will not add forwards force equivalent to the rearwards force from the front wheels (as they extract kinetic energy), hence the cart slows down.

Re:store and release energy?

[Cecil]

That actually depends entirely on which frame of reference you choose to view the problem in. It is therefore both subjective and inaccurate.

Re:store and release energy?

[Alsee]

First lets imagine zero wind, the car traveling 100 mph, and lets assume that all components of the car operate with 100% efficiency.

We tie a perfect generator to the wheels to extract 100 watts of power. Conservation of energy says it will apply a force slowing the car down. We pipe the 100 watts running a perfect propeller. It applies a force speeding the car up. If all components are perfect, conservation of energy says the car will go at a constant 100 mph forever. Perfect balance.

Now lets break that perfect balance. Imagine there's a 50 mph wind. That wind is slower than the car, but now the car only sees a 50 mph headwind instead of a 100 mph headwind. Everything else the car sees is identical. In effect we just added 50 mph to the propeller's thrust. The car is going to accelerate. The wind is slower than the car, but by breaking the balance between wheel and propeller that wind is effectively adding energy to the car.

Now of course we go back to reality where the drive system is less than 100% efficient. If the energy being effectively added by the wind is greater than efficiency losses then the car will speed up.

The wheels travel a large distance over the ground, but because of the wind the propeller sees itself traveling a shorter distance through the air. The key point is that energy = force * distance.

The wheels experience a small force * large distance.
The propeller experiences a large force * small distance.

small force * large distance = large force * small distance

Power at the wheels = power at the propeller.
Large propeller thrust - small wheel drag = net acceleration.

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Re:Duh?

[Psychotria]

Well, it depends on the traffic, obviously.

It's funny you should say that because the first thing I noticed was the dust being generated by the vehicles alongside the contraption to film it; i.e. dust being raised by the vehicles alongside creating air currents that appear to be heading towards the contraption

The reason that I don't believe it.

[7-Vodka]

The reason that I don't believe this claim, is because physical demonstrations can be rigged. I want to see the mathematics. Is it too much to ask? I mean, they build lots of models, including expensive ones, they wrote articles claiming they can do it, they posted numerous videos on youtube claiming they can do it.... Where is the fucking math? Why it so hard to post it?

The main reason nobody believes these clowns, is because they're not good at explaining how it works. I don't even see an attempt at it. Until then, what am I supposed to believe? My gut instinct or my lying eyes?

Re:The reason that I don't believe it.

[Anonymous Coward]

You don't have to be an accomplished mathematician to build a working model of something. It's quite simple to build something and have no idea how to accurately describe the math behind it. Just because there's no formula's doesn't mean they are lying.

Re:The reason that I don't believe it.

[John Hasler]

How about if you post your math showing that it cannot work?

Re:Here is how it works...

[Anonymous Coward]

You fill your posts with technical terms, but you don't know what the hell you're talking about.

A propellor has very little drag. That's the whole point of a propellor.

If propellers had such a magical property, they would hang propellers on boats instead of sails. The difference between the two is not "drag" but the fact that propellers are moving with respect to the wind.

There is no connection the ground

Not true. Sailboats have fins that effectively constrain motion is one direction, much like wheels do on a cart.

Re:SImpler; just what sailboats do

[sznupi]

What you again fail to understand - the sail moves sideways (sure, a quite specific case of sideways - it rotates; but there is no difference from the perspective of the wind)

Re:All depends

[Software Geek]

Why is it that you are comfortable with a device that travels upwind at 3 times the wind speed, but think that one that travels downwind at 3 times the wind speed is a perpetual motion machine?
Clearly, any wind powered vehicle that travels faster than the wind in ANY direction must be harvesting energy from the velocity difference between the wind and the ground, not the velocity difference between the wind and the vehicle, or it would be a perpetual motion machine.