Accelerating a vehicle from a stop to 1m/s in 100ms is an acceleration of 10m/s2 (slightly over 1g) and would take 1000 newtons of force. What were the forces calculated earlier, 53N at zero speed and 46.8 at 1m/s? So maybe an average of about 50N? So I'd say that is going to take more like 2 seconds.
It will be 60N at zero vehicle speed
Vehicle at 1m/s has a kinetic energy of 0.5 * 100kg * 1m/s^2 = 50Ws (same as 50 Joules)
So since average was around 500W available over that period about 100ms (0.1 seconds) are needed to transfer that amount of energy from wind to vehicle kinetic energy.
This is ideal case no friction in real world with some friction added it will be a bit more than 100ms but not by much.
I know we take very different routes in calculating the same thing but results needs to be the same
In 2 seconds vehicle will be at over 3m/s
That isn't actually all that crazy, but in order for that to be a workable theory you would have to demonstrate that there is a reasonable place to store the energy and that, in fact, the Blackbird was using that energy during its above-windspeed run. The only place where any energy can be stored is in the propeller. Energy storage and on-demand recovery in uncontained air is something you're going to have to affirmatively demonstrate experimentally before anyone believes that--and I don't think you'll succeed. Anyhow, as for the propeller, it does indeed store some energy but now you have to calculate how much, and more importantly, show that the Blackbird is actually using that energy by slowing the propeller down. You can easily put an upper bound on the energy in the propeller by estimating its mass and the tangential velocity of the tips, then positing that all of its mass is at the tips. So throw some numbers at that and see what you get. Does the video have a high enough framerate to determine the rotational speed of the propeller?
Even if all of that worked out and you can demonstrate that the Blackbird stores some amount of energy in the propeller, that doesn't mean that the stated principle (stated by the rest of us) isn't valid or that the Blackbird will slow down as this stored energy is exhausted.
I will see things the other way around. I demonstrated that using correct equations an ideal 100% efficient wind powered vehicle can not exceed wind speed without energy storage.
But of course I did calculated how much energy needed to be stored for blackbird to get to 28mph with 10mph average wind speed (real wind speed average was larger but I was generous).
Blackbird is around 300kg including the driver the wheels are bicycle wheels so super low rolling resistance.
Kinetic energy of the vehicle at 28mph (12.5m/s) will be 0.5 * 300kg * 12.5^2 = 23437Ws = 6.5Wh
To get there even with no wind to start with is not that much of a problem even 3 small 3000F 2.7V super capacitors can store that about 3Wh per capacitor.
Even a cell phone battery can deliver this energy twice it just can not deliver that fast but supercapacitors can charge and discharge in seconds.
Now the propeller is just massive at 5.3m diameter 20m^2 swept area is larger than the floor area in my livingroom and not much pressure differential is needed to store this sort of energy needed to accelerate to 28 or 30mph even without the initial help from the wind.
So I did looked at all the numbers to make sure they all fit correctly.
People just imagine that there is a much larger energy involved when that is not the case.
The treadmill model is even more ridiculous as for the vehicle to accelerate to 1m/s (way more than it was demonstrated in the video) it will have needed just 0.5* 0.5kg * 1m/s^2 = 0.25Ws that is 0.07mW and so a cell phone battery could do that test over 200000 times with power at the wheel and 100000 times with a 50% efficient propeller. There are no typos there is just how small the amount of energy needed is for that 500g or so model.