No matter the construction of the machine, if the screw rotation speed to vehicle speed ratio were made low enough, you wouldn't need either of those.
Yes, Modified, that's right, I believe. This all applies to the rather different analogue I posted, and may not really be relevant to the main question of DDFTTW, of course, but addressing the point you're making - I get the idea that if the speed ratio were made 'low enough' for my gear system to turn nicely (i.e. have a very
high* pitch screw), then the balance of forces would reverse the rotation. When pushed at the thread location, instead of the whole thing being forced right along the surface, turning the wheels clockwise as shown (considered absolutely non-slip), the angle of attack would cause the screw to rotate the other way (upwards in the 'front' of the drawing), winding the machine against the input force, to the left.
*ETA to correct the bold, I put low first time round.
I think this is in keeping with what others have postulated - that with a low (or is it high?) enough gear, the cart version would respond to what
was considered a
tailwind and drive itself into it, backwards.
If the cart works at all as proposed, it would seem to depend on the relatively short pitch. This is what I - and many others it seems - get wrong when first looking at the road test video. We see a 'propeller-like' thing in a wind and instinctively know it should go in a certain direction (we've all held up little toy at the beach), and we also tend to imagine that if a machine is using wind-power as the headline says, then it will 'obviously' turn like a wind-turbine - power from that will be transfered to the wheels, etc. It takes a little while to figure out that the whole thing is working the other way round. (I'm sure you understand this, BTW, Modified, I'm just going through it for the sake of whom it might help, like me!).
The flatness, the relatively short pitch, of the propeller is necessary to make the machine get pushed like a cardboard box on a windy day, but its weight, keeping it in contact with the road, forces the wheels to rotate. This turns the propeller against all odds and the wind direction - like if we put electricity to an electricity wind-farm type generator - to force the wind back against itself. Then, I'm told, it is also the same short pitch that maintains the speed differential at all speeds between the cart's motion and the prop, which means that the driven surface of the prop is going slower than the cart, and hence can still be pushed by the wind after the cart has reached and overtaken the tailwind.
It all seems to depend on whether this force-feedback trick actually works in practice as claimed, and as can be seen, many consider it impossible without breaking the law of conservation of energy or whatever.
I hope to get satisfactory explanations of some of my questions, and, while some of the easier ones (or more equivocal ones) involving friction and 'apparent headwind', etc., have been replied to, there are some that I feel get to the principles more absolutely, which have not. This may be because they're taking more thinking about to explain to me, or because they expose more clearly the energetic impossibility.
I'll ask again in brief - if this works as an analoge of the tacking (or jibing, is it?) of a sail - then it suggests to me that if the vehicle gets up to windspeed (or is it even below this?), the rotating prop cuts through the same piece of air again and again. The reason I imagine that a yacht can increase speed moving across the wind up to arbitrary limits above windspeed is that it takes energy from a new layer of air all the time. It slows down a piece of air to gain energy and speed, but moves sideways wrt the air, so that loss of windspeed doesn't matter. The 'trick' of tacking with a rotation movement of the prop is perhaps ok in some circumstances, because the wind moves on forwards or backwards*, but at a certain speed, it seems it should slow a layer of air to extract energy from it, then meet that same air again at every pass. If a yacht tacks back and forth to gain energy, the wind has moved on for miles by the time it passes the same stream.
I've been wrong about plenty of this so far, and am just seeking solutions to my intuitive objections. The real world is more complex than all our little diagrams and most of our abilities to imagine. It just occurs to me that perhaps the prop could somehow employ the air from a wider area around it by some vortex-like force. Freely being pushed at airspeed is quite intuitive - we can just imagine a piece of paper blowing in the wind. What seems 'impossible' about faster than the wind must in some way relate to the amount of energy in that particular column of air in which the 'paper' or cart is travelling. If somehow it could extract energy from air around it, a piece of paper pushed by the wind could accelerate beyond it.
Unfortunately, I don't know of any way any system (paper or propelled cart) can suck energy from the surrounding air like that without an equal expenditure of energy somewhere else. I still cannot comprehend how this can be extracted differently just because we have the ground to help us, as has been suggested.
My other thought experiment was to imagine this machine only having access to the fluid its prop passes through, the pipe-travelling version I posted earlier. Now if I can indeed understand how a little gizmo like that in a pipe with water flowing in it could screw itself forward faster than the water - given the lower speed at the prop surface than the gizmo travels along the pipe - but I'd like to know where the extra energy is coming from, or if my intuition that it needs extra energy is wrong. I imagine putting more and more of these in a pipe and now we've got more and more momentum (even if they are the same density as the fluid, and are travelling faster than it). All instinct tells me I have to turn the power up to the pump driving my water through the pipe at some point. Same problem as rotating the prop blades through the same bit of fluid, in a way.
*Even in this cart scenario at other speeds, the prop is slowing a bit of air and then rotating round to meet the
same column of air (ignoring vortex-like weirdness). Even if that column of air has moved on forwards, this means that the machine is not flowing in 'windspeed' air, but something it has slowed. What puzzles me is whether this actually means the whole thing has to be balanced somewhere below 'carpark' windspeed, or if the machine still moves forwards through it (still a headwind for Michael's imp passenger).
Roll on the expert windtunnel tests - if NASA have shot down Bernoulli, can't they be persuaded to put Terry and the rest of us out of our misery?
Does any engineer know about something similar to my pipe gizmo? I can't see what it would be useful for, but hey, we can pop one in a pipe and it will reach the other end before the fluid.
While I'm on a roll, here's another thought experiment - turn this vertically. Drop one such gizmo in empty air, another into a vertical tube. Does the one falling through the tube fall faster than the one in freefall?
