Do Alaskan bush pilots generate a Clmax of 5 with 90° flaps?
The following graph is from Cahill, Summary of Section Data on Trailing-Edge High-Lift Devices. As you can see, for a plain flap, with gaps fully sealed, the $C_{L_{max}}$ maxes out at around 60deg.



When I played around with xfoil, things became unreasonable beyond 40deg. Probably some assumptions in the boundary-layer equations broke down. Since Javafoil utilizes the same computational approach as xfoil, I would expect the same behavior.

On the Alaskan Bush pilots, are we sure they are 90deg deflection, and not just some high deflection that looks almost vertical? With multi-element, slotted flaps, you can do better than plain flaps.