When were airfoil cross-sections first used in turbine blades? When did this become standard practice? Did people understand the connection to aircraft, or did heavier-than-air aircraft exist at all yet?
Good description in the link, but curved surfaces were in use as sails around 800 years ago and Bernoulli came up with his formula , Ludwig Prandtl was testing sections in 1017 in his large wind tunnel, the first naca section was tested in 1935 .... http://www.flyingmag.com/technicalities/technicalities-short-history-airfoils
And this link to Sir Frank Whittle is also very illuminating as a design error had the shape of the blades incorrect during testing around December 1937... https://books.google.ch/books?id=a_YTYwP4aS4C&pg=PA120&lpg=PA120&dq=turbines+whittle&source=bl&ots=aEH_MZYiEQ&sig=SVPqca_ONdeuOQ3rAXVZHRDm2o4&hl=en&sa=X&ved=0ahUKEwi4guii1MjWAhUEcRQKHQHFAncQ6AEInAEwFg#v=onepage&q=turbines%20whittle&f=false