Etymologically speaking, we pronounce ‘helicopter’ incorrectly.
Most people assume it is a compound word from ‘heli’ and ‘copter’ but in fact its compound roots are ‘helix’ (adapted to ‘helico’ for compounding) and ‘pter’ (from ‘wing’, just like ‘pterodactyl’, or ‘winged finger’ in its Ancient Greek roots), for the compound of ‘helix wing’ or ‘helical wing’ as denoting the rotors.
Like many English words, however, it did not arrive directly from Ancient Greek but from French, hélicoptère, coined by Gustave Ponton d’Amécourt in an 1861 patent filing, but still from the Greek roots.
While pronunciation has moved significantly in the past 2000 years, the original pronunciation could be said to be closer to elix-tehr. Helix in the original Greek, έλιξ - does not have the leading H sound, and pter - πτερ - wouldn’t voice the p, and would have the e shortened for a sort of eh sound.