John Freestone
Graduate Poster
Well done, ynot, and congratulations on a successful demo. I suppose I understand you being disappointed no-one else built one and you had to do it on borrowed time, but I guess a lot of us are in similar circumstances. In my own case I don't even have a garage or suitable work area other than standing outside in December. But I don't have time anyway, or I would have experimented with either my 'pipe-racer' or some other variant on the wheel-driven 'Galilean transformations' (boosts or inertial frames).
I see your concern over the dissipation of stored kinetic energy, but that was one I never shared. It's interesting how we all pick up on different complications and issues of this question.
That's fairly close to 'dead cert' if I remember my school maths.
I'm impressed with your design, build and testing. There seems little need for more turntable versions, but more, of course, will equal better. Larger ones, and of more refined design, might follow, reducing some of the possible influences. However, as in most engineering problems, the unwanted influences are almost always in the direction of something NOT working, so if it works with more drag and suchlike, it'll work with more refinements, as I'm sure you will show in time.
I've been pondering on another alternative, which is just to turn your turntable vertically (or find a large rotating wheel that's suitable - fairgrounds, etc.?) and hang the wheel (or two, like an upside-down bike, for more stability) on top of the lower reach of the driving circle. A rather small version could be made with two bike wheels fastened together and the cart's driving wheel could then run in the depression formed where the two bike rims are fastened together (one having no spokes, just a bare rim). The cart's driving wheel would drive its prop, fastened below on the frame in the usual way. Now the interesting thing about this is that the cart will move forward from its resting place at the bottom as the 'fairground ride' circle is rotated, but the backward, tangential component of its weight will increase as it does so (similar to increasing the angle of the treadmill), until it balances in a permanent climb up the circle. This is possibly just an interesting meditation, and it's more impressive to watch yours, I'm sure, just endlessly going round against the direction of its 'road'.
An interesting thought about my pipe-racer is that if we blew a close fitting, boring shaped missile out of an air-gun (like they use for chucking marrows, I believe, in some places, for fun apparently), it would come out at near airspeed, but my pipe-racer would come out at some multiple of that. Have I just invented a more powerful air-rifle bullet? Well, possibly, but they haven't got long to accelerate, and bullets with props are a bit expensive!
In answer to my own stupid question of a long time ago: would such a device fall through a vertical tube faster than it does in freefall? The answer is no (I'm fairly sure), for the same reason that the DDFTTW technology doesn't mean we can drive a prop off our car's wheels (I mean your Porche) and go faster in still air. It's another of those little oddities. I imagined that if another forward force was driving the device (like an engine, or the weight of a pipe-racer falling through a tube), that would provide similar enough conditions to the tailwind, but I don't think it's similar enough. If anyone disagrees, that would be useful to hear. If you're driving your car in stationary air, just taking a similarly geared prop drive off the front wheel doesn't increase the speed, does it? If you push one of these test carts along the floor, does it take less effort with the prop attached? I would have thought so. Yet I don't imagine we could save petrol if we had props geared to our car wheels.
