When you say 'mechanical flight' are you referring to heavier-than-air craft like airplanes and helicopters, or flying machines in general, like manned balloons and blimps?

Simon Newcomb is the famous example I’ve seen brought up before. Here’s an article he wrote around 1903 where he pretty much says heavier then air flight is practically impossible & would require the invention of new materials. Can’t say how widespread the opinion was though.