I must say i am fully and even 200% in line with spork analysis.
And let me rephrase my "rotating spinacker" story, i was unclear in my analogy, sorry:
- i agree , it is abolutelly not possible with a classic sail boat or a kite , even by looping the kite or whatever (to loop, the kite need wind comming from backward, so direct down wind is not possible. Definitelly.
- i was just refering to a spinacker because a spinacker is just "pushed " by the wind fully perpandicular, hence absolutelly no mention to L/D ratio is possible here. In other word, it is like a door in front or the wind that wants to open... no AoA at all, or say 90 degree if we can say like that...
Nothing to do with apparent wind (in the sense of vehicule speed). Nor inertia, nor momentum...
Blade are turning with an angle just to compensate part of the vehicule movement and BAD influrnce of apparent wind , by still "presentinh a surface" on which the wind coming from the back of the vehicule can still push the blade likeca door (exactky as it does it at the start of the vehicule, easier to understand at that 0 instant).
Lets see it like this, theoretically (and once "visualuzed, it is very easy to understand):
- imagine the angle of blade is such that in the blade surface goes backard exactly as much as the vehicule goes forward during one turn of the wheels (it is just a mattter of choosing the good angle).
- then, you will agree, at start , 0 speed, the wind pushes on the blades, immobile. So the vehicule starts moving downwind.
- then at very low speed, the whhels impose the mouvement of the blades (with no or very few counter reaction (*) ) , and it still works the same: the wind pushes the surface of the blade, even if the blade turns (but we just do not care that it turns, it it still a surface that "opposites" to the wind, amost perpandicularly, even if rotation is not mechanically used and necessary at that stage.
- (*) explanation here : the blade surface rotating exactly at the speed of the vehicule, there is no additional draft of the blade due to apparent wind (negative) of the vehicule. On contrary ... anyway.
- when the vehicules reaches wind speed : i do not see any change in the principle nor on the forces in place (only the structure of the vehicule stops beeing pushed by the wind): the angle of the blades is still the same = calculated to go "backward" at sale speed as the wheel of the vehicle = if you take now the ground as reference, the blade surface is immobile ! So it still catches the wind from backward (and not from frontward like would a classic rotor do).
- so in theory there is no limit and the vehicule can accelerate always and always ...
- but you have frictions that you cannot totally emiminate (wind frontward on the structure, wheels, mechanical drivings, wing thickness surface hat cuts the air), so you stop when they become too high
This example of angle exaclty compensating the speed is for illustration. It also works at lower angle, it just leads to better performance at low speed (higher acceleration, easier to tackle frictions), but final speed will be limited . If angle is only 50 % of wheel, then the assymptote of speed will be twice the wind...
Please let me know if i was clear. And very interested by Spoke comment on this vulgarisation