In my message I defined the limitation of the length of the torque transfer shaft as a question (see above). But it may be that I was below reality, as shown by the analysis of the rotating reel system which is a TRPT AWES. The Fig. 22.18 reproduced below summarizes said limitation:
The limitation is severe.
Of course it is a bit different for TRPT like Daisy because both ground and flying rotors are parallel unlike those of rotating reel. As a result the shaft length / rotor diameter ratio will be higher than for rotating reel system, before still increase by the rigid torque rings.
We speak about a single rotor. A stack of rotors could probably push the limit further for both systems (with always an advantage for the Daisy-like system).
So much for a few points of theory. Now in practice please let me know when TRPT or other torque transfer installations exceed say 20 meters in height: I haven’t seen anything like this for years now.
And I think that the possible height is a fundamental parameter to reach high altitude winds which justified the research on the AWES (see the work of C. Archer). Competing with HAWT at low altitude is a decoy.