Flight in the free-to-download game C-evo becomes available (as with most other Civ-type games) in mid-game, a little while after you or cooperative friends have researched Chemistry, Explosives, Physics, and Refining. You may have become able to build a glider earlier by building Leonardo's Workshop. Unless really keen to do long-range scouting or blocking of enemy units, by adding only fuel modules, you may be wise to delay air unit development until you research Advanced Flight.