I don’t know why you need the wind to drop to nothing, the cart is going to get up to wind speed and then beyond it so you’re going ot end up with a relative headwind whether or no. It is an inertial effect in that the continued spinning of the propeller is requiresd to exceed the wind speed, but an object with that much radius has a lot of rotational inertia.
Thanks, that’s what I assumed and I have no idea how much energy is being stored in the prop, anything other than that and my brain breaks. More to the point, in a perfectly steady wind speed it will never exceed or reach the speed of the wind (because friction.)
Rotational inertia is pretty counterintuitive, but size (distance from axis) matters more than mass in that context.
Oh, I’m all about the flywheels son!
There ya go. But yeah, if your underlying point is “that’s not endlessly sustainable” you’re 100% right by my understanding.
Reminds me of when I was maybe 12 and my brother was 16 or so and he was trying to blow my mind by conservation of momentum, and I asked, “Can’t you put a car in neutral and turn it and still go back behind where you did that?” Neither of us appreciated stored energy translated to friction forces.
A morphing machine you say?
Now, combine this thread with the one about killer robots over in P&R and…
Smart clothes has been an idea floating around for a while, this looks like some actual progress. Bet it ain’t cheap any time soon, though.
The pinned comment on Youtube has more detail. The prop isn’t free-spinning in the wind—it’s geared to the wheels. The wind pushes on the vehicle to start it. The wheels turn, spinning the propeller. The prop turns, generating a thrust. In the car’s reference frame, the air behind the prop is moving backward, even if it’s moving forward (more slowly than the car) in an external reference frame.
The thought experiment 8 minutes in was pretty good, I thought. A sailboat switching from broad reach to broad reach (i.e. 135° to the wind) can make progress downwind faster than a sailboat running before the wind. I believe the world record for sailing speed is something like 50 knots in an 18-knot wind. Wind speed is only the speed limit when you’re sailing directly with the wind, because apparent wind is a vector sum: wind speed plus your velocity. If you’re sailing across the wind, some component of your speed is added to the real wind speed to determine the wind speed experienced by your sails.
To return to the illustration from the video, turn the world into a cylinder, and picture the sailboats on a broad reach, sailing around the outside of it. That’s a propeller.
Or, to skip the metaphor, the speed of the propeller through the air isn’t limited by the speed of the wind, because the propeller blades are set at an angle to the wind, and so the relative wind they experience is a vector sum of the real wind and the component of the vehicle’s forward velocity in line with the chord of the blade.
e: of course, it’s not exactly like a sail because it’s geared to the wheels, so most of that doesn’t matter. A very confusing vehicle either way.
Version 1.1 will allow spinach to teach the fibers to send emails.
Am I just oblivious or is that incredibly crucial piece of information not mentioned in the video?
I was sufficiently nerdsniped by the first half of the video that I never got around to watching the second half, but I don’t remember them mentioning it on camera in what I saw.
Obligatory xkcd for those unfamiliar with the term:
CraigM
1843
It is explained in the video, in fact in great depth. The entire second half of the video goes into the mechanics of the gearing and how this in turn enables the craft to increase speed beyond wind speed.
depressing but cool podcast, Easter island history, heads, colonial exploitation, etc.
People always underestimate people. Doesn’t even have to involve ancient artifacts, either. The French at Dien Bien Phu thought the hills around the base were too rugged for vehicles to haul artillery and ammunition. They were right; the Viet Minh used people to do it.
Interesting follow-up to the Veratasium story on Blackbird, the cart that goes faster than windspeed downwind:
