Consider that the double axe has at most 2 sq ft of airfoil with a radius of maybe 4-5 ft, while human-powered helicopters need (at 16-18 Str and probably 16+ Con) multiple 20' radius rotors operating in ground effect to manage a minute of hovering at max; total area is something over 300 sq ft. (That is, they're gaining a substantial boost by never going more than 2 ft off the ground.)
Dragons arguably fly partly by innate low-key magic (much like the magic that holds undead or constructs together); also, they're designed to fly, and have big enough wings to manage a small margin (i.e., enough to perform wing buffets).not being able to do anything while flying defeats the purpose of flight. poor maneuverability requires forward momentum, unless you spend yet another feat on hover. making the feat weaker for the purpose of realism is a bad way to go, especially in a world where dragons fly, and can still make wing attacks in flight.
Humans (or elves, or dwarves, or other substantially similar races) are not built for flight; our power-to-weight ratio is horribly low in general. However, if I wanted to be mean I'd say make the feat give nothing but hovering; actually moving anywhere, or getting more than 15' off the ground, requires another feat and still higher ability score prerequisites.