Despite what I said in my last post the sceptic inside still has some unanswered questions and I must retract my acceptance of the claim that a vehicle can travel downwind faster than the wind solely in the direction of the wind.
The two main issues are . . .
Can the thrust if the propeller ever exceed the rolling resistance caused to create that thrust? I don’t see how it ever can. If it could the vehicle would be able to be self propelled by it’s own inertia. When the vehicle has reached the speed of the wind or is placed on the treadmill in no wind, then wind can’t be considered as a factor as there simply isn‘t any wind relative to the vehicle.
I think a possible source of confusion is talking about "wind" instead of talking about "speed of the air". Saying "there isn‘t any wind relative to the vehicle" is equivalent to saying "the air is not moving with respect to the vehicle". This does not mean that the air no longer has any effect, or cannot be considered as a factor in the equation: the vehicle is still in contact with the air, so it can push against it.
When the vehicle is placed on the treadmill the thrust of the propeller is created solely by the motion of moving treadmill surface relative to the vehicle, and is being transferred to the propeller via the turning wheels and linkages. This is not free energy and there has to be a rolling resistance energy loss that is greater then the energy of the thrust developed by the propeller.
The movement of the propeller is indeed created by by the motion of the rolling treadmill surface relative to the vehicle, but this motion only continues because the propeller is thrusting against the air. Imagine what would happen if the propeller was not connected to the wheels: in this case, when the vehicle was lowered on the treadmill, the wheels would start spinning, but the prop wouldn't turn. The loss of energy due to friction would soon cause the wheels to spin slower and the vehicle would run off the left end of the treadmill (viewed as in Spork's YouTube videos). In fact, when the propeller is connected to the movement of the wheels, it thrusts against the air causing the vehicle to run off the
other end of the treadmill.
The essential thing is that it's not "the wind" that is the motive force, it's the
difference in speed between the air and the ground. If there's no difference, the vehicle won't run. If you go out on a day with no wind at all and put the vehicle on the ground, it won't do anything. If you now give it a push, there will indeed be a "wind" relative to the vehicle, but it won't help, since there is still no difference in speed between the air and the ground. In this case, the vehicle will run a certain distance but will be quickly slowed down by forces of drag and friction.
If there is a steady wind, the vehicle will accelerate up to the speed where the forces are in equilibrium, then just keep running. And it's important to point out that the point where the
forces are in equilibrium is not necessarily the point where the vehicle is stationary with respect to the air. When it is stationary with respect to the air, its wheels are being turned by the ground, the propeller is being turned by the wheels and therefore thrusting against the air.
The second issue is whether two opposing winds have a compounding effect or whether they somehow cancel each other out. The wind is moving relative to the ground at 10mph and the vehicle is travelling with the wind at 10mph. It’s claimed that some of the speed of the vehicle is coming from the thrust of the propeller. If say 3mph of the vehicles speed is coming from the propeller then only 7mph of speed is coming from the 10mph wind. What happens to the remaining 3mph of wind? As the vehicle is travelling at the speed of the wind it can’t be going past the vehicle at 3mph.
If there’s something I’m missing what is it?
If the vehicle is going at exactly the speed of the wind, then all the extra thrust of the propeller is doing is compensating for any speed losses caused by drag or friction. If the vehicle is going at 10mph in a 7mph wind, then we might consider that the thrust of the propeller is accounting for the extra 3mph. It's a simplification, but it may be a step towards understanding what's going on.