Flat winged foamies do have a proper airfoil -make no mistake about that - needing a special set of curves to become an "airfoil" simply is not so.
Actually the flat wing on foamies is the best "airfoil" for that application.
3D- simply having enough thrust to do sustained maneuvers at up to and including vertically oriented maneuvers.
That's what a chopper does
Don't get lost in "airfoil" types as to which is best - that is an entirely different thing
fat airfoils are less sensitive to AOA.
Look at the logic
a flat plate generates huge amounts of lift at extremely low AOA
now visualise the airfoil getting fatter and fatter until it is round .
The amount of lift vs drag gradually changes places till you have only drag --which can be made to lift also but in a different manner -
Thick airfoils do NOT make more lift -making a better 3D model -they simply desensitize AOA and are easier o make strong due to cross section.
if you are designing your own "perfect 3D flyer " try to get the lightest model with the most static and dynamic thrust and use surfaces which react easily and predictably to prop airflow .
The best wingloading is subjective -but I favor pretty low loadings for slow stuff.
for fast tumbling stuff - smaller wings -