That's partly where you're going wrong. Your euqation shows the wind speed at which power is zero. It also shows the power available at any particular speed. But... so what?
All it's showing is the possible power, but how much does the vehicle need to move? When you look up on t'web how much power something needs to do, say, 15mph it's not relevant to this because most of that power will be overcoming drag. There isn't any drag here. Actually, it is negative drag, which is what's pushing the thing along. So you have all this power available but where is it going? It's accelerating the vehicle.
Now, if there were a little less power available, perhaps because something is sucking it off, would that make the vehicle go backwards? Of course not! It will just accelerate a little less. It is literally a brake on the wheels, but there is still enough to power available from the wind to drive the vehicle forwards.
The vehicle, without the prop, would never get to wind speed. It would be very very close but there is friction, so it's not quite there. Your formula will tell you how much power you have at that 'very close to wind speed' speed, and I'll bet you it's a surprisingly small amount because there is no drag to overcome. So just think how much extra power would be available if the vehicle was a little bit slower than that.
So, that formula tells us how much power we have and when it would theoretically run out. It doesn't tell us how fast we go, how much we can divert for other usage, how quickly we can accelerate, nothing.
Yes available wind power is accelerating the vehicle plus it covers frictional losses (but we can ignore those if you talk about ideal case where there is no friction just to get the best case scenario).
If the power you take at the wheels is less than available wind power the vehicle acceleration rate will just decrease.
If you take more power than available from wind then vehicle will decelerate as in the example I gave before when 4.8W of wind power was available with vehicle speed at 4m/s and and 16.2W of braking power was applied to wheels in order to power a 16.2W incandescent lamp. In that case the vehicle continued to decelerate until vehicle speed dropped to 3m/s and at that point wind power was 16.2W exactly covering the power taken at the wheels for the lamp.
And yes the slower the vehicle speed the more wind power is available but if you can not store energy then that power will just accelerate the vehicle witch is a form of energy storage (kinetic energy) but not the type that will allow you to exceed wind speed.
I think I mentioned before but I can (anyone can) build a vehicle that uses no propeller and exceeds the wind speed exactly the same way Blackbird is doing and actually be more efficient.
All that is needed is a sail (collapsible will be best to get rid of the drag when above wind speed) the propeller is doing this in a natural way).
Then 3 or 4 super capacitors 3000F 2.7V as each can store about 3Wh so in the same range as the large Blackbird.
The capacitors will be fully charged well before the vehicle gets to half the wind speed and from there the wind power and sail are no longer needed as the stored energy allows a 300kg (same weight as Blackbird) to be accelerated to about 3x the wind speed maybe even 4x will low enough friction losses.
To accelerate a 300kg vehicle from say 3m/s (half the wind speed of 6m/s) to 13m/s (a bit higher speed than Blackbird record of around 12.4m/s).
Vehicle kinetic energy at 13m/s is 0.5 * 300 * 13
2 = 25350Ws = 7Wh to this some frictional losses will need to be added but it will not be much.
So if I charge 4x 3Wh = 12Wh super capacitors (way smaller and less dangerous than a huge 20m
2 swept area propeller) I can easily exceed blackbird speed record.
I don't want a 20m
2 sail as that will be to crazy but with say a very manageable 2m
2 sail 10x smaller than blackbird swept propeller area I will need to spend:
Say charging when vehicle is at 1m/s that is 6-1 = 5m/s wind speed relative to vehicle 0.5 * 1.2 * 2 * 5
3 = 150W so I will say generator is just 80% efficient still 120W available to charge the supercapacitors.
I need 12Wh to fully charge that is 43200Ws so I need 360 seconds (a bit slow if nobody pushes the vehicle like it was the case with Blackbird) and if there are no wind gusts above 6m/s to help still 6 minute charge time is reasonable then just a few minutes to accelerate to top speed of 13m/s maybe even more dempensing on friction losses.
Or I can increase the sail size to 20m
2 and then I can charge the capacitors in just over half a minute.