Aerodynamically and physiologically, this feat makes my head hurt. A mere 20 Str (barely more than an unusually strong but otherwise mundane human) is by no means enough to lift, much less propel, a human with such a horrifically inefficient airfoil as a double axe.
For reference, the first human-powered airplane was flown in 1961, using composites and lots of careful aerodynamic design. A human-powered helicopter is even trickier; current attempts use even more sophisticated composites, and are ridiculously large (for example, 100' across, with 40' rotors). Also, they tend to make use of leg muscles, rather than arm muscles, for increased strength and endurance. Brute-forcing your way into the air with a makeshift airfoil and the strength and agility of your arms just isn't going to happen any short of 35 Str and 25 Dex, at the very least, never mind the endurance problems. (I wouldn't have any problems with an [epic] feat requiring 37 Str, 29 Dex, and 21 Con to pull this off, as a matter of fact.)
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