I've been interested in these for a while now, ever since the big splash (heh) from sharrow marine a few years ago, heres a link to some data they've provided comparing their prop vs a standard one:
https://sharrowmarine.com/blogs/performance-reports/project-...
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
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
Do we know what mechanism causes efficiency loss at mismatched RPMs?
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?
This series of videos from Rcestflight is some nice testing of different RC boat propeller designs. It was a competition with a prize and people could send their designs to be 3d printed. There was at least one toroidal design as well. They could show this stuff at schools...
They seem to make the ride smoother, but they don't seem to help much with distance/runtime per battery charge. Like a minimal/marginal improvement in that regard, vs a standard prop.
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