The first of my three comments in Sulekha blogs on references to ancient flying using Vimana

Comment #1 (on 27th Oct. 2019 in Sulekha blog "Hamster on wheel (poetry)" by Geeta Kashyap: http://creative.sulekha.com/hamster-on-wheel-poetry_637684_blog):

Sometimes we come across words in ancient texts which probably were used differently long ago than used presently. 'Vimana', which you mentioned below, appears to me such a word.

I have seen the word Vimana, which these days is used for aircraft (flying machine), translated in some old Sanskrit dictionaries simply as a cart or chariot moving swiftly / fast (on the ground).

In that case, 'Pushpak Vimana' in the Ramayana implied really as a ground based flowers-covered vehicle (cart or chariot) traveling very fast on the ground (even as flying in air, according to the poetic symbolism of Ramayana poem). This makes sense because Vimana as a real aircraft during Ramayana time probably did not exist due to lack of supporting technologies for a flying machine, whereas Vimana as a speedy ground based cart or chariot (even a swift boat over water), covered with flowers also (as Pushpak), could be there easily for use by people during Ramayana's period.

There is another reason to think that 'vimana' was a ground vehicle (non-flying cart / chariot etc.) in the Ramayana.

Rama, according to the Ramayana, defeated and killed Ravana to free Sita in Lanka around the time of Dushera.

Then, taking Sita and Lakshman along with him in 'Pushpak Vimana', Rama reached Ayodhya on the night of 'Diwali' (people in Ayodhya celebrated his arrival by lighting Diwas or lamps), 20 days after Dushera and covering a distance of 2000 - 2200 km from Lanka.

Traveling 2000 - 2200 km in 20 days (or at a rate of about 100 km / day) is quite feasible in a ground based vimana, by having a cart or chariot that travels only 5 or 6 hours each day at an average speed 15-20 km/hr (a logical speed and daily operating time for a cart / chariot even long ago)

Needless to say, the flying kind of vimana was not required for Rama, Sita and Lakshman to reach Ayodhya in 20 days after killing Ravana in Lanka,  and they could easily instead complete the journey in a ground chariot (also a vimana).

Incidentally, the above also shows the historical and logical nature of events, places and times mentioned  in the Ramayana. Moreover, 'pushapak vimana' in the Ramayana could also be a reference to a light and small cart / chariot (made of light variety of wood and small in size), and not just a flower-covered vehicle.

Additional (original) related information (blog) on vimana (or Vimaan in item #10, and Jatayu in #4):
Subhash C. Sharma "Symbolism in the Ramayana" (2006), http://creative.sulekha.com/symbolism-in-the-ramayana_183925_blog