Nice model. I pulled out the Legos and made one of my own, which made the whole concept much easier to understand.
By the way, is that a medlar in your photo stream? How was it?
That reminds me of the little trick with a spool of thread. Lay the spool on its side with a length of thread extending from the underside. Pull on the thread and the spool will roll towards your hand that’s pulling the thread, and it will do it faster than you’re pulling the thread.
Sure is - they’re pretty good - there’s a thread about them here:
Oy. Another counter-intuitive concept, but once again, it turns out that the universe is stranger than we suppose. 
CurtC, there are many, many modern sailboats that can cross the river in under two minutes on a reach. The latest America’s Cup boats can cross it in less than half that time. The best ice-boats and land yachts can do it in about 30 seconds.
There are open ocean racing boats (such as the Jules Verne Trophy holder Groupama III) that can maintain such a pace for days on end.
[quote[Now applying this to the Blackbird, the propeller really is going faster than the wind. It could cross the same distance in under two minutes. So I don’t see how the tacking explanation helps here.[/QUOTE]
Once you wrap your head around the first part above, you will learn that the explanation not only makes sense, but is the correct one.
As long as you’re here, what is the correct pronunciation of Rick Cavallaro’s last name? (I would say Kav uh yar oh, unless he is from Texas)
Thanks for the links.
Kav uh lair oh
JB
Hi I just want to see if I’m understanding this right: are there 2 conditions in this process?
Condition 1: slower than wind - wind spins propeller, wheels move car forward
Condition 2: faster than wind - ground turns wheels, propeller moves car forward
I dont think so, or else there would be a weird transition.
I couldn’t figure out the transition either. How about: it’s always in condition 2, initial acceleration comes from the force of the wind on its silhouette?
As far as I can tell (in layman’s terms, which may not be scientifically accurate):
The wind pushes the whole system along the ground, turning the wheels
The wheels turn the propellor
The propellor pushes back against the wind
Another way to think of this - if you’re water-skiing behind a boat, you’re travelling at the same speed as the boat. If you ‘climb’ the tow rope, you’ll be travelling faster than the boat. It would be possible to create a little propellor-driven gearbox built into your water skis that would wind in (your end of) the rope for you - and thus you could be travelling faster than the rope that’s pulling you.
(that’s the same thing my Lego model does. I shot some video of it too, but I can’t get to the video from here to post a link)
That’s about it. When not turning, the prop is a sail (not a good one - initial acceleration is poor as you will see in the video). As the prop turns faster, the thrust from it increases.
Eventually equilibrium is reached when the amount of shove extracted from a prop blowing into a tailwind is counterbalanced by the friction of the cart and the losses in the system, but – and it took me a while to swallow this – there is no particular reason why the cart can’t be travelling faster than the tailwind before this happens.
Very interesting video. I like it because I am an aircraft engineer. Look closely at the propellor. All propellors have a leading edge and a trailing edge. The leading edge of the prop is usually, but not always, straighter than the trailing edge. What is always the case is that the leading edge has to be stronger than the trailing edge, and is therefore supported by being as near as possible to a line drawn through the hub. The trailing edge, not having to be as strong, can be unsupported some way back from that line. Notice the orientation of that propellor. The leading edge is towards the front of the vehicle, the trailing edge is towards the rear. Because of the pitch angle (pitch can be viewed as a screw thread, as the prop turns, it screws itself through the air), if that propellor were to be (motor)driven anti clockwise (when viewed from the front), and that motion transferred to the wheels, it would propel the vehicle forward. Look closely at the near blade as it is pointing at the camera, the pitch can clearly be seen, the leading edge is further forward and higher than the trailing edge. If the wind were to blow through the propellor from behind it, which is what is happening here, the wind would hit that blade and be deflected upward. The blade would react by moving downward and the prop would rotate clockwise (when viewed from the front). The prop is rotating anti clockwise. THE PROP IS NOT BEING DRIVEN BY THE WIND.
You have been hoaxed, Hamster King.
Even if this setup was for real, as the relative wind got less and less, the prop would be driven slower and slower until the vehicle (nearly) reached windspeed when it would stop accelerating. Also, if the wind turned the prop which drove the wheels, it would in no way be similar to what goes on in a particle accelerator. The vehicle would be active - that is it would be part of the driving force. The particle in the particle accelerator is passive and is not part of the driving force.
That’s right. The prop is being driven by the wheels - AGAINST the wind - which exactly explains everything you described.
It’s not a hoax.
You’ve completely mistaken the way in which this system works - it’s not surprising you’ve arrived at the wrong conclusion. Did you not actually read the discussion in this thread?
Here’s the video I mentioned upthread. This is a small vehicle made from Lego, geared in such a way that it moves faster than the thing pushing it.
Substitute wind for the racked rod and you’ve got essentially the same system as the Blackbird
Considering that you don’t seem to have a firm grasp on Newtonian mechanics, I don’t think you should be quite so confident that you understand relativity better than the experts.
Tell me, tomh4040, does 0.99999… equal 1?
I posted this thought on the Blackbird site, but here is another way of understanding how this thing works:
If air is moving relative to the ground, (the wind is blowing, IOW) you can extract some of the kinetic energy contained in that air by slowing it down relative to the ground. This is what all sailing craft, and all wind turbines do…no magic, and few would argue with this. It is notable that these machines are immersed within a mass of moving air. They are not just “hit by the wind” from the windward side, but contained in a field of moving air that exists on every side except downward. This is a subtle difference in viewpoint, but very significant in this case.
What blackbird does is take air that was moving, and slow it relative to the ground, just like any other sailing craft or wind turbine. This is it’s power source. What is different is that the propeller and wheel gearing arraignment allows it to pull in air from in_front of the propeller, and slow it down, leaving the wake of slowed air behind the prop and the car. It is not “outrunning it’s power source.” It is running into its power source. You could also say that it is constantly catching up to it’s power source, and extracting the power as it overtakes it.
Kevbo, thats an excellent way to describe it. Good work.