Ynot, I am well impressed with your have-a-go attitude, and would love to know more about how you've made your circular windtunnel. It looks a bit heath-robinson, but there's a place for that kind of engineering. I imagine that this method is quite unique in the history of DDWFTTW. The moving turntable in still air isn't, but making a windtunnel of 'infinite' length to test the cart in "real wind"
may be. Of course, detractors might say that it's not a real wind anyway, it's an artificial cyclone or something, and rant incoherently about friction and centrifugal force, but making a cart beat the tailwind in such a circular windtunnel is a good demonstration, combining the aspects of energy input to the air instead of the surface with easily demonstrated steady state. A knowing nod should go to the treadmill testers, who know that these aspects are not relevant to their results with a suitable understanding of mechanics, but for those with less education in the subject, the energy input at the belt, "still" air, and the need to prod the cart with a spork, tie or hold it back, or put it on a slope were recurring objections.
Something like this idea had occurred to me - I wondered about a circular pipe for my pipe-racer - but I didn't think of a suitable way to drive the air or other fluid round the pipe or trough without getting in the way of the cart. Another method I wondered about was to use a water-cart sunk in the bottom half of a circular trough, with paddles above moving the water round. That would work with air, too, and I imagine that your design must be something similar with the driven air below the turntable. I look forward to seeing more about it.
I suppose you have thought of this, but you might find that the air is directed even better by constructing upper and inner walls for the cart, although you'd have to leave a slot for the tether arm. The outer wall could be continued into a lid {ETA: some of this would need to be removable and preferably transparent}, integral with a partial inner wall coming down not quite as far as the arm. Maybe that's quite unnecessary though, and you seem to have had success with it already.
