The vimana-as-UFO myth is a case of "alternative" scholars misunderstanding the development of Hindu mythology while accepting without question false texts as true. Discussed chronologically, the mystery of the vimanas vanishes.
The word vimana means "that which was measured out." The word was first applied to the palace of the monarch, and from that to temples. Because temples are the houses of the gods, the Hindu gods' palaces could also be called vimanas. Thus, the large, flying chariots of the Hindu gods could also be vimanas. In the oldest texts, the Vedas, these were drawn by flying horses, like the chariot of the sun god in most Indo-European cultures (see, for example, the myth of Phaethon). In later epics, beginning with the Ramayana (c. 500 BCE), the chariots lost their horses and were depicted as flying on their own. The very first of these is the flying chariot of the earthly king Ravana called Pushpaka. By the time of the Mahabharata (c. 400 BCE), these flying chariots had grown in size--one was now described as 12 cubits in circumference--but they never lost the large wheels that marked them as derived from earthly horse-drawn chariots.
The concept of these flying chariots as UFO-style airships originates in a fraud, the Vaimanika Shastra, allegedly an ancient Sanskrit epic, but one "channeled" from the astral realm by a Hindu psychic in 1918. No evidence of this text exists prior to 1952, and even the "translator" of the text makes explicit that it was channeled from the spirit world between 1918 and 1923. The fake text specifically compares the vimanas to modern aircraft, describing their propulsion systems and other modern technological achievements.
Alternative theorists and ancient astronaut theorists embraced the text as a genuine, ancient text and thus Childress, in his Vimana Aircraft of Ancient India & Atlantis take it for proof of ancient aeronautics. Careful scholar that he is, he also missed the fact that it was a "channeled" "astral" text and instead claimed that it "was first reported to have been found in 1918...the genuineness...cannot be ignored" (pp. 37-38). He dated the text to between 400 BCE and 1000 CE. For ancient astronaut theorists, this was pretty close. By the time of the Anti-Gravity Handbook (2003), cited above, this treatise had morphed into "a fourth century B.C. text" "rediscovered in a temple in India" in "1875" (p. 131).
As for that UFO from the quotation that started this blog post: Guess what? Childress says it's from the Ramayana, but it's actually a poor paraphrase derived from a 1914 summary of the Ramayana in Myths of the Hindus & Buddhists, as copied by James Churchward in The Children of Mu (1945)--one of Childress's favorite sources:
"When morning dawned, Rama, taking the Celestial Car Puspaka had sent to him by Vivpishand, stood ready to depart. Self-propelled was that car. It was large and finely painted. It had two stories and many chambers with windows, and was draped with flags and banners. It gave forth a melodious sound as it coursed along its airy way."
As always, reality is a little different. Here is one passage describing Pushpaka from the actual Ramayana in a standard translation:
And the mighty monkey ascended the splendid car Pushpaka, containing figures of wolves,—made of Karttaswara and Hiranya; graced with ranges of goodly pillars; as if blazing in splendour; throughout garnished with narrow secret rooms and saloons, piercing the heavens, and resembling Meru or Mandara, and like unto the flaming Sun; skilfully reared by Vicwakarma; with golden staircases and graceful and grand raised seats, rows of golden and crystal windows, and daises composed of sapphires, emeralds and other superb gems; embellished with noble vidrumas, costly stones, and round pearls, as also with plastered terraces; pasted with red sandal, like unto gold, and furnished with a sacred aroma; and resembling the sun new risen. (Sundarakandam 9)
Elsewhere it is described as being filled with fruit trees, and sometimes it is drawn by geese.
Do you know many UFOs with "plastered terraces" and red paint?