I saw this on youtube last night because I subscribe to that channel. Guy's a good teacher. The parallels to sailing are interesting, so am guessing Doug Riley could weigh in on this with a better practical understanding than most folks. Great experiment.
I think Chuck is on the right track re: wind gradient.
To extract energy from moving air, we must change either the air molecules' direction or their speed (or both). If our vehicle is travelling directly downwind, then changing the air molecules' direction is out; they are standing still relative to the vehicle, so we'd have to add power from somewhere to deflect them. If the vehicle is also moving at the same speed as the wind, then changing the air molecules' speed is also out; there's no energy (relatively speaking) to extract from them.
Sails on sailboats are remarkably sophisticated wing systems. They employ slotting, variable camber, variable incidence, variable area, variable center-of pressure location in two axes, and variable twist. That last item is used all the time -- to allow for wind gradient between the wind direction and speed near the water, and at the top of the boat's mast. The direction of the relative wind on a sail is noticeably more a-beam at the top of the mast than down low, so we twist off the head of the sail a controlled amount to achieve optimum angle of attack up and down the span of the sail. Twist too much, and the top of the sail loses all lift; twist too little, and the top of the sail stalls. The control devices that enable us to manipulate sail twist vary from boat to boat, but usually include the traveler, the boom vang and the jib-car tracks.
David "Birdy" Bird claimed that he could feel the wind gradient between the top and bottom of his gyro.
BTW, there's an early flying contraption displayed at the Owls Head Transportation Museum in Maine. Conceptually, it's similar to the desert windmill cart in the video. It's an ordinary fixed-wing glider, except that there's a mast and sail mounted above the fuselage. The designer seems to have believed that he could extract thrust from the sail to propel the plane and make the wings work. Even the best sail won't propel a vehicle straight into the relative wind, though -- and that's what wings want.
I don't think it has anything to do with wind gradient, the key to the phenomenon is the term VMG (velocity made good) used at about 7:26 into the video. VMG is the speed at wich a sailboat approaches its destination while tacking before (or against) the wind. As explained at 7:30 to 7:50 in the video a boat can move faster downwind than a balloon, drifting with the wind speed. A short video on VMG is here:
As explained at 1:38 in the video VMG is the speed of the boat times the cosine of the angle between the course of the boat and the destination (or the bearing of the destination you read on the boat).
You can watch VMG at work in the Americas Cup races. I have selected the first race of the semi final (US vs. Italy, Italy won to become the contender) which you find here: https://www.americascup.com/en/relive/prada-cup/semi-final/11_Semi-Final-Day-1
At the beginning of the film it is explained that the race can only be started if the wind speed is below 21 knots, so our wind speed is taken to be 20 knots (the broad blue arrow on the left of the drawing). All arguments of course apply equally well to speeds below that.
The attached drawing depicts the situation on the first downwind leg of the race. At 18:28 the speed of Prada Pirelli, the Italian boat, is 47.7 knots while its VMG is 39.4. The course angle is thus 34°. As we see, a boat, tacking before the wind, can attain a speed away from the wind which is almost twice the true wind speed. This is made possible because the force on the sail is perpendicular to the apparent wind (purple arrow), with the red component of F being the driving force which is in equilibrium with all drag forces on the boat (this is explained at 7:16 in the cart video).
The cart makes use of this phenomenon via its propeller. As is explained at 8:09 in the above video the propeller blades are exactly like a boat tacking downwind, only that the boat is moving on a cylindrical surface. It is therefore the propeller blades which attain a VMG greater than the wind speed and since the cart is attached to the propeller the cart also attains that higher speed. The propeller blades keep "tacking downwind" due to the transmission from the wheels which makes them spin. What a beautifully shrewd idea....;-)
PS: One might ask, why we need that tail wind. Well, if the tail wind were zero the angle between the course of the boat and the apparent wind (purple) would vanish and then we would no longer have any driving component of the sail force.
PPS: I think the final explanation of the pilot is still quite a bit off the mark...;-)
Tacking downwind is different than running directly before it -- which is what I thought we were discussing. If running directly before the wind, the the sailcart's angle to the wind is zero and the cosine of that angle is 1.
If OTOH you zig-zag, your sails become true wings instead of mere drag devices, and boat speeds higher than wind speed are indeed possible.
This fact came dramatically to light when iceboats were invented in the 1800's. For awhile, iceboats were the fastest vehicles built by man to date.
I'm confused about the prop vs sail part.
At the start of the video it says that a tracking sail spinning about an axis is how you can go faster than the wind. So they built a two bladed spinning sail but said it's the same as a prop. However in the video he says the wheels are powering the prop which is propelling you forward, not the prop powering the wheels.
This doesn't make sense to me.
When the chain comes off for the second time at 12.20 you see the prop speed up fast due to the wind and no resistance. So the prop is propelling the wheels using the efficiency of an airfoil to go faster than the wind.
Something else that's confusing me is
at 5.18 the prop is spinning clockwise whilst going forward
at 5.48 the prop is spinning anticlockwise whist going forward
at 5.57 the prop is spinning clockwise whist going forward
at 6 .00 the prop is spinning anticlockwise whilst going forward
Quote: the wheels are powering the prop which is propelling you forward /Quote
No contradiction here. The wheels are powering the rotor to make it spin and increase rotational velocity as the cart's speed increases. This is just to generate that relative wind on the blades which would be created by a boat's speed. It is the relative wind acting on the blades that drives the craft, not the wheels. The wheels are a bit like an autogyro prerotator, you got to make it spin to make it work.
As for the different ways of turning at 4:09 he says that he is controlling the propeller pitch with the lever on the left hand side (that's when the two chaps disagree on which way to push the leaver to break). The propeller pitch indicates that the prop is turning clockwise, seen from the front, when it looks like the prop is reversing it is probably at the speed when you see a wheel turning backwards due to the 1/18th sec lag in human vision.
At least they knew pretty well what they *wanted* the cart to do when they conceived it. One should keep in mind that in engineering the tiniest detail may be crucial. If, e.g., you get the transmission ratio between the wheels and the propeller wrong your cart will not attain the desired speed or might not move at all so, finally, they did a pretty good job making a pretty freaky idea work. Thumbs up for the team!