Toroidal Propeller
en.wikipedia.org
en.wikipedia.org
The issue, as far as I can deduce, is that the design of the prop needs to be almost perfectly tailored for the expected flow regime. Tweak the RPM, power, and velocity variables just a bit, and the hyper efficent, silent prop becomes detuned, negating any extra benefit it might have had. (In the link above, you can clearly see that the prop was designed for peak performance at around 2500 engine RPM). Contrast that to a typical prop, where you can get decent thrust and efficiency with just about any old scooped blade with a pitch angle.
Rctestflight on youtube gathered some fairly insightful, albeit limited data on this topic, would recommend! https://youtu.be/oXgp7IGnvKA
https://www.savree.com/en/encyclopedia/controllable-pitch-pr...
But these use actuators to mechancially adjust the pitch rather than relying on deformation.
Twisting a few props about a their axes is a lot simpler than the geometry changes that are required acros the control surfaces of a toroidal prop, however. I don't think that would even be feasible
What if multiple props are arranged in a series, akin to how turbo engines have stages with different compression ratios, but instead of a fixed shaft with varying blade geometry we have varying differential rotation speeds between them adjusted based on RPM?
There are all sorts of youtube videos for DIY/3d printed, including metal printing.