BTW, I don't buy into the theory that the vimanas are using liquid mercury rotary engines. The descriptions of electrical power generators are often weird, and point to ancient knowledge about power generation, but so far I found nothing in the book that would support that theory.
As soon as I find time, I'll elaborate on the power generator and the gravity defying device.
It's completely unknown currently whether any of the vimanas or yantras would work as intended, b/c it'd be a tremendous effort to build them, and no-one seems to have invested the resources into doing so. One university in India is reported as having attempted to make two of the metals (apparently successfully), but so far I found no further information.
One of the bird-like crafts was apparently once built and test flown before 1923 (with the help of the guy who channeled the book). However, it is unclear whether it contained all of the yantras prescribed for the craft. Nor it is known whether it had the huge building and tower that is supposed to be on the top of the craft:
The tower can be hidden from view by one of the yantras that claim to make the craft invisible or partially invisible.