@PierreB I hope you dont mind me quoting the data you posted elsewhere:
Blockquote My observations and my question concerns the power consumption of AWES based on Magnus effect but also the Sharp rotor as they scale.
To begin some informations:
The chapter 12 of the AWEbook 2018 mentions in page 290 that experiments with a small-scale system (Magnus rotor radius = 0.047 m and length = 0.45 m) the motor power consumption " is much larger than the power produced by the system due, among others, to the significant effect of frictions. For larger scale systems, frictions become less important compared to aerodynamic forces."
Omnidea’s curves on https://collegerama.tudelft.nl/Mediasite/Play/e51a679525fe491990de3a55a912f79d1d show that the motor power consumption is 1/3 or 1/4 the power produced by the system.
“Low C for the High Seas Flettner rotor power contribution on a route Brazil to UK Figure 1: One of the first rotor ships, the Buckau (left), and the present-day rotor ship E-Ship 1 (right) Source: wikipedia.org (accessed on 10 August 2012, right photo by Carschten). The Flettner rotor modeled here is 35 m tall and 5 m in diameter. Key to its aerodynamic performance are the lift, drag, and moment coefficients, respectively. They determine the lift force l , the drag force d , and the power pME that is needed to drive the rotor.The rotational speed ratio is set to α=3.5. The aerodynamical coefficients are set to cL=12.5, cD=0.2, and cM=0.2, respectively.”
So for Magnus large scale systems the power consumption ratio would be relatively lesser.
The numbers C_L = 12.5 and C_D = 0.2 seem very good and I would assume a best case scenario that could not be expected in real life without further R&D into building such wings.
From what I gather by reading stuff here and there putting in 1/3 or 1/4 of the energy is not uncommon for magnus effect.
I think much of this is due to skin friction, and we cannot expect this to reduce with scale, because, well, skin scales linearily with the projected area of the wing.
Furthermore, the problem for single tether AWE if getting the energy from the kite to the ground, assuming tether strength/mass/diameter to be the main limiting factors. Thinking this way, it doesnt really matter too much if you need to spend that energy locally on the wing.
If a C_D of 12 was realistic, a magnus wing with wingspan 10 m would replace a traditional wing (rigid wing of an airplane) with wingspan 22 meter (the aspect ratio being similar for the two). Though the magnus wing has a much lower glide number and is even more complex to build, I am not sure which is the better option now.
Now to add a few negative aspects og magnus: Because tether is your limiting design factor, you want the kite pull (and thus speed) to be constant. This is not easy to achieve. For magnus wings I cannot see an efficient way to control the power of the wing. You could change the rotational speed during a loop, but that induces further losses (breaking torque) and more weight for the increase in rotor motor power.
Of course you also need a quite hefty power generator for the rotor motors, causing further drag and mass penalty. The power generator probably would introduce a tail to the kite, as now the kite needs to be facing the wind (something that was not really important for just the manus wings)
Let me add, like you did, gyroscopic forces during a loop, along with no straightforward way to control roll and yaw… at least not in a millisecond feedback control sense.
Some of these issues (control issues in particular) would be mitigated by putting the magnus wings in a network, giving then more constant flying speed and reducing the need for power control.