Is an English translation/reference to the German publication available? It refers to work by Dr. V. Raghavan Emeritus Sanskrit Professor of Madras University.
It appears are detachable flying parts of temple Gopurams.
Four strong liquid mercury vessels must be installed in the inner structure. When heated by Sun's rays or by other means the Vimana (aircraft) develops a powerful thrust from the mercury.... etc.
Several related You Tube videos (Hindi) are appearing nowadays , we have no means to authenticate them:
Update on Vimanas regarding similar wider context of Srirangam and Padmanabha temple cultures:
( 6 min 00 secs to 11 mins 58secs )