@PierreB In principle what you are talking about can be done like this. I expect this is feasible for enough distance to make sense energy production wise. Launching though can be an issue, because the ground distance required for AWE would be prohibitive.
You can add more pulleys back and forth, but that would add too much friction
@tallakt, Thank you for this rope drive system which takes account of reel-out/in phases, if I understand correctly, and following the discussion about having heavy parts at ground if I remember correctly.
That said, Garrett provides a suitable advice by writing:
With a motor on (2) and (2) this could possibly work. Though I expect there could be some losses concerning that the reeling in motor needs to have some force to maintain tether tension on the second (slack) tether, and while in theory that energy is regenerated on the reeling out motor, there would be losses.
Also a brake would be necessary to maintain zero speed efficiently. Otherwise, definitively not impossible. Though I’m not sure if I’m looking at an effective design here. I guess time will tell.
Unless the pulleys, or some of them, were powered and they were only engaged during takeoff and landing. The two outer pulleys would stay static and you’d have a row of pulleys above the rope drive you’ve drawn and one below. To retract the rope drive, the top row of pulleys would move down and the bottom row would move up, giving the rope drive the appearance of laced shoelaces.
The pulleys being powered allows you to add an unlimited number of them.
Or, thinking about it more, you could perhaps do it in 3 dimensions, if you needed to save space:
You’d have the original 2 pulleys, say along the y-axis, the pulleys angled or vertical so that one side of the rope drive is above the other, along this same y-axis a bit to the side of one of side of the loop you’d have a static row of pulleys (with enough space between the pulleys so that the moving pulleys can fit between the static pulleys), to the other side of the loop you’d then have the moving row of pulleys, which then moves along the x-axis to result in this one side of the loop catching on all the pulleys and lots of rows of lines. Next would be to have a static grid of pulleys under or above these rows of lines, and a movable grid of pulleys moving up or down.
That looks like work. But also perhaps a somewhat viable way to extend and retract a rope drive hundreds of meters long, while it was spinning.
Or perhaps instead of this 3d idea you could have, instead of only the one side of the rope drive being engaged by the pulleys, have both sides engaged by pulleys to create two planes above one another, with the pulleys for both planes mounted on the same moving and static arms.
Automatically wrapping and unwrapping the rope drive on and off a drum is probably easier and cheaper. But still probably not very good.
You’d only do this if the rope drive absolutely had to spin all the time. If it only had to start spinning once it was fully extended there are much simpler options.
Hi Garrett, I wonder if higher lift coefficients (CL for a given spin ratio) than those you mention on the poster could be achieved by towing the Magnus kite in a straight line at the same speed as its speed during crosswind flight, so as to avoid all the turns and U-turns that could be detrimental to aerodynamic efficiency and smooth rotation. What are your thoughts on this?