One Man’s Quest to Outrace Wind (2011)
wired.comThe last time I read about this [1] was while working at a Silicon Valley startup with a small team of bright software engineers.
Most were perplexed by the problem as the URL spread, and the article, in my opinion, did more to confuse than explain. The newer article linked here, which I only skimmed briefly, looked to probably make the situation worse judging from its visual aids.
I thought it was obvious and simple to understand (and explain) how this worked:
The wind pushes the vehicle via simple drag forces, this alone can accelerate it, eventually approaching the speed of the wind.
The vehicle also drives a propeller via its forward motion through a mechanical drivetrain.
By spinning, the propellor acts as a forward motion compensator, and the wind will exert force against it even when the vehicle travels at speeds >= the wind.
Introducing sailboat-based analogies does not help understand any of this in my opinion, it only serves to confuse the audience. The folks behind developing this vehicle come from a sailing context, which explains why they're approaching it from that perspective. For the general public, sailing is not a natural model for reasoning about this stuff.
[1] https://www.wired.com/2010/06/downwind-faster-than-the-wind/
I don't know if your explanation is simple or obvious either, except maybe in a very hand-wavey sense that may or may not correspond to the truth.
The sailboat thing isn't really an analogy—it's a direct comparison, and the operating principle is the same. There are a lot of resources explaining the phenomenon of "apparent wind" and how it's possible for a sailboat's downwind velocity component to exceed the environmental wind speed. If you understand vectors at the level of a college freshman, you can understand how that works; you don't have to be a sailor. From there, the "cylinder Earth" thought experiment provides the intuitive leap to the operating principle of this dead-downwind vehicle.
They do touch on the concept of a faster-than-the-wind VMG downwind, but they probably should have emphasized it more in the explanation.
Introducing sailboats doesn't simplify explaining anything in my experience, they just pollute the mental model with more complexity.
There's value in the ground vehicle model for explaining to someone how sailboats can travel faster than the wind propelling them. But I don't think sailboats are a useful teaching aid in the opposite direction; explaining how the ground vehicle works.
So unless the goal is to teach specifically about sailing, I feel it's best to leave it out entirely. Except maybe at the end as a passing mention like "BTW, this is how sailboats manage to sail faster than the wind as well."
I have to disagree, because a sailboat is a pure manifestation of the model that must be understood to understand how this vehicle works: a foil held at a fixed angle in a moving fluid, constrained to move only along a straight line at a fixed angle wrt. the motion of the fluid.
Understanding this vehicle requires heaping more complexity (the spiral motion) onto that same model, so in fact I think a sailboat is a great teaching aid here, as it's a clean waypoint on the journey to full understanding. Sailboats also exist, and have non-controversially demonstrated the surprising ability to sail with a downwind VMG higher than the environmental wind speed, which will help skeptics to engage with the problem.
When I was a kid I used an erector set to make a car that moved directly into the wind using a similar technique. Many people don't think that is possible either, but with proper gearing, it works fine.
It's also possible to use a propeller sail to go directly upwind in water. You don't need gearing in water if you pick the right pitch on your underwater propeller relative to the pitch of your sail/fan.
Classic. I already knew about this, but the "cylindrical Earth" thought experiment is new to me and as a sailor (lapsed) with a physics degree I have to say I find it immediately convincing. It's not controversial that you can sail faster than the wind on some angles, so with a propeller in the mix I don't understand why so many smart people were willing to make absolute statements that this is not possible.
If you can go downwind 3x faster than the wind, can you turn and go perpendicular to the wind using this mechanism? Can you turn again and go directly upwind?
At a multiplier of 3x windspeed, it seems like the fact that there's wind at all is irrelevant other than to get off the starting line. You're generating your own wind power by pushing the propeller through the air. Is this correct?
Because if this works other than directly downwind, you have a perpetual motion machine - scale up, drive in a circle, and hook a dynamo to a wheel. Which makes me think I've neglected something somewhere...
Edit - from the Wired article:
"Skeptics think that the wind is turning the prop, and the car is turning the wheels, and that's what makes the car go," Cavallaro said. "That's not the case. The wheels are turning the prop. What happens is the prop thrust pushes the vehicle."
OK, now I'm really confused. I thought the prop rotation was screwing through the air slower than the relative airspeed and driving the wheels - now you're telling me that the wheels are spinning the prop to produce forward thrust?
If he can go from sitting still: wind speed -10, ground speed 0, to W0/G10, and accelerate up to W20/G30, what happens if the wind drops to 0 mid-run? Can he keep going at W20/G20, isn't that easier than W20/G30?
Here is a simplified picture.
The propellers are being pushed into a spin by the difference between the airspeed and the groundspeed. (Note that the propellers are locked to the ground by the wheels.) As they spin, the blades generate lift. That lift then pushes the craft forward.
Take away the difference between airspeed and groundspeed, and nothing is keeping the blades spinning. Therefore the system will run to a stop due to friction.
The connection between spin and lift is mediated by aerodynamics, not a mechanical linkage. And aerodynamics allows the vehicle to be pushed faster than the spin. Which is why, if you get friction low enough, it can go faster than the wind. And not slightly faster, but a lot.
The wheels drive the propeller.
Place the vehicle on a moving treadmill with the wheels rotating at the speed of the treadmill belt so it is unmoving in the middle of this treadmill. The wheel gearing would drive the propeller. Notionally, the propeller adds thrust to the vehicle more than it adds resistance to the wheels and causes the vehicle to move forward on the treadmill.
But consider, if you put an ideal toy car on a treadmill with a friction less axle etc, it would obviously never exceed the treadmills speed. At best it would stay mid treadmill for ever.
The idea is that some combination of additional gears and propeller would allow that toy car to continuously to speed up, theoretically infinitely. Understandably there is skepticism about this.
It's not a perpetual motion machine. It harnesses the differential between wind and ground to propel itself. How fast it can go depends on how much power is available, how efficient it is, how aerodynamic it is, etc.
So my question is, given the cylindrical Earth thought experiment (which I do find quite compelling), why bother with the weird drive train involving the wheels? Putting sails of the right proportion on a propeller shaft should be enough.
Because the cylindrical Earth experiment neglects the reaction forces of a sailboat's keel, or the land-boat's wheels. The propeller shaft would just rotate, you need that cylindrical Earth to have, essentially, a ballscrew spline.
You can't generate the differential with clever aileron wings, either - has to be in a different medium, otherwise the thing would just tumble.
I think I get it. I would have thought plain, free-spinning wheels could serve the role of the keel (I guess they do in a normal sail car), but you need to transmit that keel action from the wheels on the ground to the sort of virtual cylindrical Earth the propeller lives in.
In a normal sail car you'd have to tack, but otherwise the same forces apply. The clever thing about the propellor is you get a sort of cork-screw-tacking for free allowing your motion to be in-line with the wind instead of having to be at an angle.
I remember reading this in the physical magazine. Super interesting article around physics and harnessing the wind.
Spoiler: They went almost 5x faster than the wind propelling the craft
i think sailboat based explanation is useful only to the point. Airplane wing provides better and simpler model outright. It becomes more intuitive when you think how this craft goes straight into the wind (that is what sailboat can't do).