Redpill, one simplistic way to visualize what it happening when the vehicle is exactly at wind speed is to imagine a telephone pole standing upright on (not in) the ground. Place a sail placed halfway up the pole and let the wind blow the pole over from the force of that sail.
Since the sail is only halfway up the pole, as the wind begins to blow the pole over, the top of the pole will be moving much faster than the middle of the pole where the sail is located. It’s easy then to imagine the top of the pole moving faster than the wind while the sail is moving slower than the wind and functioning in the expected manner.
The Blackbird is merely a combination of mechanical and aero devices which roughly replicate the above but on a continuous rather than momentary basis.
From a conservation of energy perspective, I can only see a couple of possible contraptions that would, with contingencies, work. The sole source of energy input into the system as a whole is the wind–I agree that the ground is never providing a net input of energy into the system, though it can mechanically provide a transfer of energy (e.g. converting rolling energy into stored energy in a battery). This energy wasn’t added by the ground, though, it was transferring kinetic energy within the system that was earlier added by the wind.
Case 1: A pulsed system could be created that momentarily exceeds the wind speed, but the long term average is still necessarily below the wind speed due to friction. Say…put on your emergency brake, have a propeller charge up a battery for a while, then release your brake and have your motor accelerate past wind speed. Of course, you will expend that energy eventually and have to slow again.
Case 2: The “car” as a whole is made out of an unobtainium that has a density less than the surrounding air medium and can somehow reach a steady state where its Car mv^2 == Air Volume mv^2 with mass being lower and velocity being higher due to the density difference.
The force of the wind can act on the propeller and be transmitted to the body. It really doesn’t make any difference. In fact, I’m pretty sure the thing relies on the sail area of the propeller to get moving.
Here’s a thought experiment purely from a basic-is-this-at-all-possible-without-violating-the-laws-of-the-universe perspective:
Suppose you built a car out of super-light super-frictionless material with such a clever design that it had basically zero air resistance. Basic Newtonian physics tells us that once you pushed it up to a particular speed, it would take zero energy to maintain that speed.
So you’re out on the salt flats and there is no wind. You push your thing up to 50 miles an hour in some direction. It coasts along. Now you say “hmmm, I’ve got these wheels spinning at 50 miles per hour… can I grab energy from them somehow and use it to make me go faster?”. That is where you run into all the paradoxes people are talking about here… you’d be effectively pulling yourself up by your own boostraps, creating a perpetual motion machine.
However, suppose there’s a 30 mph breeze (for purposes of this thought experiment, we don’t care what direction it’s in). You again coast up to 50 mph and are zipping along and want to go faster. So what you can do is take a tiny little super-efficient windmill electric generator and plop it into the ground by the front of your car. It will generate a few watts of power. When it gets to the back of the car you pick it back up again. Repeat times a whole bunch. You get a small intake of power. Use that power to drive an electric motor that accelerates you. Repeat until your velocity is infinite.
Now, obviously that’s a comically impossible way to ACTUALLY do things, but the key point is the difference between hypothetical A and hypothetical B. In neither case are you using the wind as a driving force to propel your vehicle, but in hypothetical B you can take advantage of the (effectively) infinite amount of available energy that comes from having wind blowing past a stationary surface to achieve acceleration. In practice, though, it’s done with clever mechanical devices, not tiny windmills.
The real reasons this does not violate any laws of physics:
A certain amount of energy is available from a given volume of moving air
A certain amount of energy is required to move a vehicle
There’s just no reason at all why a vehicle cannot intercept a volume of moving air that happens to deliver sufficient energy that is more than enough to move it faster than that volume of air.
No, because if it pushes the vehicle forward, that makes the propeller turn in the opposite direction to the one the wind would like to turn it (this is in fact the whole reason the thing works.
It doesn’t matter if the force is applied to the propeller blades - in the case of my Lego model (which is an analogue of this vehicle), all of the force to drive the thing is applied to the gear that is driven by the wheels, but the thing still moves. (I’m uploading a video at the moment that I hope will make this clearer.
OK, here’s the video of my little Lego car that moves much faster than you push it:
A few notes:
The toothed rack is analogous to the wind
The small gear on top is driven backwards by the forward motion of the wheels - analogous to the propeller
Obviously, the mode of delivery of energy is different for this vehicle, but it’s operating on the same principles as the Blackbird - in completely layman’s terms: it’s using its own forward motion to generate a ‘push’ back against the force pushing it.
One more thing - this is just mechanical advantage. Although the vehicle moves much further than the rod pushing it, the force that has to be applied to the rod to make this happen is greater.
If the gearing is increased further, it should in theory mean the vehicle moves even further/faster forward for a shorter/slower push, but in practice, what happens is that the gears stall, locking the wheels and they skid along the ground surface.
Let me point something very important here – the ratchet has absolutely nothing to do with normal operation of this craft. The ratchet has not once released and gone ‘click, click, click’ during the running of the vehicle.
Here’s the purpose for the ratchet on the prop shaft – if the driver of the Blackbird has to make a panic stop, the spinning mass of the propeller would try to keep it spinning while the brakes (on the drive axle) try to bring it to an immediate stop. For safety reasons, the brakes are much more powerful than the propulsion system (just as on any car). We wanted strong brakes (for obvious reasons), but we don’t want our transmission to break just because the driver had to slam on the brakes.
Without the ratchet, the mass of the prop would backdrive the transmission and overload it. With the ratchet, in a panic stop the vehicle can be brought to zero quickly, while propeller can at its own pace coast to a stop independent of the vehicle movement, just going ‘click, click, click’ until it RPMs are zero.
As an additional plus, the ratchet allows us to tie the prop down and push the vehicle around by hand (backwards) without the prop spinning and getting in our way or hitting something.
Again, since under normal operation the torque on the shaft is always in the same direction, the ratchet remains engaged the entire time during and run. We’ve never had to make a panic stop so it’s never even been used for it’s primary purpose.
The rub with this is that everytime you pick up your generator, you have to accelerate its mass to the velocity of your car, slowing your car each time you do so. This could never be an ideal massless generator either, because if it is storing any energy at all it necessarily possesses mass when you again pick it up due to mass/energy equivalency. At best, your thought experiment could give my “Case 1” pulsed system as result.
I don’t think the original windcar video was even actually travelling faster than the wind at all, I think there was just turbulence or vortices caused by the shape of the car and the fan blades that gave the streamer a misleading flutter. If they had mounted the streamer to the front of the car, it would be more of a substantive demonstration. Otherwise, the video is inconclusive.
That’s right. I certainly believe that it is possible to exploit mechanical advantage to drive yourself faster than the wind for a non-sustainable period of time, but I interpret their claim as being able to do so perpetually given a long enough road.
So then, do you believe that the videos with the model on the treadmill are faked? When NALSA certified the speed record of the Blackbird, did they have the wool pulled over their eyes, or are they in on the deceit?
You interperet our claim correctly in principle – obviously things would wear out so “perpetual” would not be a valid claim.
“Steady state” is the actual claim. It’s somewhat of a semantics difference however as given an endless wind, endless road and a vehicle made of wear proof unobtanium, it would harvest wind energy perpetually.
Through a carefully worded set of NALSA rules, along with instrumentation and observation provide by the BOD of NALSA, “steady state” was clearly demonstrated.
I believe you’re reaching this conclusion based on an inappropriate application of the laws of thermodynamics.
You can’t get out more energy than you put in, but there’s no reason why it should take more energy to move a given vehicle at (say)50mph than can be extracted from a given volume of air that is moving at (say)30mph.
