With your explanation of the blackbird faster-than-wind mechanism. Could you test it by monitoring the speed of the vehicle and the air pressure behind the propeller?
Since the vehicle should be oscillating between being slower than the wind and faster than the wind. Just a thought. Would a heaver vehicle change the rate of speed oscillation? just some thoughts.
There are two very different version of Blackbird.
One is for direct downwind faster than wind and that will use relatively large energy storage capacity the pressure differential that propeller creates so vehicle will use that energy to accelerate well above wind speed for many seconds even a few minutes then when stored energy is used up the vehicle will slowly decelerate as it is using up the stored kinetic energy and will slow down for some other minutes depending on amount of friction losses untill it gets well below wind speed.
The experiments done where all only a few seconds and ended just before stored energy was used up.
The wind is super variable in real plus the large black bird has propeller with adjustable pitch angle so the data from that is fairly useless.
But the treadmill model is fairly easy to prove since it is inside so zero wind speed and while the model is kept with the hand on treadmill the energy storage is being charged (pressure differential created on each side of the propeller) and when released the energy starts to be used up so if a video from the side is made you can see how the vehicle acceleration rate drops as the stored energy is being used up but the treadmill is way to short and so the experiment can not be done until vehicle uses all stored energy maybe a minute or two and starts to move backwards.
For the direct upwind model yes you can see that vehicle is slower and faster a few times a second. In that case the energy storage is in part of the vehicle like the belt or tiers or even chain and is a very small amount of stored energy so typically only good for a fraction of a second and is best observed with a high speed camera.
But I already showed that effect with my toy car in the video. The second part of the video talks just about direct upwind vehicle.
The two direct upwind and direct down wind use a very different energy storage so they work different in that sense.
For example you can not have a wheels only analog of direct downwind just an analog of direct upwind because air is compressible and so to emulate that you will need to have one wheel on a super stretchy road to store energy in that.