Some will hate this!!!

[markwbird]

The propeller is driven by the wheels so the anology is not accurate. It is a fan blade pushing the car forward, just like if the propeller had a motor, only the motive energy comes from the wheels, not a motor.
Think about the car not moving, with a tail wind. The propeller doesn't move because there is a direct drive between the wheels and the propeller. The propeller presents a bluff body to the wind which creates a force pushing the car forward. As the car begins to move, the propeller starts to turn. The tricky part is what happens when the car approaches wind speed. Apparent wind drops as you approach wind speed going downwind so a bluff body would have less force from pressure differential and rolling resistance provides losses which would prevent you from ever reaching wind speed. BUT you have a propeller being turned by the wheels which increases the pressure differential which pushes the car forward (thrust or force pointing forward). With no losses this would create a limitless speed increase, however in the real world there are inefficiencies which limit the speed.

Yes. I know the wind is pushing the cart forward and the wheels are spinning the blades. That does not negate my explanation. Quite the opposite. This is how the blades are made to move in the correct direction to make the physics that I am explaining work. If the wind were just spinning the blades it would not work.

 

[srimes]

The statement is that you cannot sail "dead down wind faster than the wind speed" which is true in all cases. This vehicel is "traveling" dead down wind and if it did not have anything else could not exceed wind speed. The trick here is that the two blades are NOT trevling dead down wind. They are on a continuous jibing angle that wraps into a helix.

The vehicle is powered solely by the wind. Sounds like sailing to me. As for "sailing dead down wind," sailing direction refers to the direction of the vehicle, not the sails. Look at kite sailing where they fly figure 8s. If you ask what direction they're heading they'll answer with the direction of the boat, not the momentary direction of the kite.
I call it sailing dead down wind faster than the wind speed.

Here's a follow-on video:

Great video, but I feel like they left out the crucial component of propeller lift/drag ratio (maybe it was covered in the 1st vid, I didn't watch). The propelling force is the airfoil lift, and airfoil drag is perpendicular to lift, so in this case rotational, which gets transferred to the wheels. It's easy to get l/d ratios way over 1, but you have to add in transmission losses (gearbox, bearing drag, rolling resistance, etc). As long as lift/combined drag is > 1 you'll go faster than the wind.

 

[Hayden Watson]

which gets transferred to the wheels.

This is the revearse of the mechanics. The rotaion of the wheels is what drives the rotation of the propeller. That rotation of the propeller / sails creates an apearent wind that keeps them on a broad reach which allows them to develope lift which propels the craft down wind.

 

[srimes]

This is the revearse of the mechanics. The rotaion of the wheels is what drives the rotation of the propeller. That rotation of the propeller / sails creates an apearent wind that keeps them on a broad reach which allows them to develope lift which propels the craft down wind.

"The propelling force is the airfoil lift, and airfoil drag is perpendicular to lift, so in this case rotational, which gets transferred to the wheels."

Yes the wheels make the prop spin. The airfoil drag makes the propeller want to not spin. This drag force gets transferred to the wheels.

 

[Tedd]

I feel like they left out the crucial component of propeller lift/drag ratio...

I guess that depends what you mean by "crucial." Any propeller with anywhere near the optimum pitch is going to have an L/D much greater than 1. It would be pretty much have to be deliberately designed to fail not to. So pitch is the issue, not L/D, and they do have some discussion of that in the first video.

There's also a video by the woman who built the demonstration model that goes into a bit of detail about getting the pitch right, and shows some failed attempts where they used the wrong pitch. There's even a bit of theoretical discussion about what range of pitch will work.