Susier,
I hope the Rapp.org article didn't give you the impression that Gulfstreams are poorly designed or prone to random stalls. They aren't.
What I was trying to convey with the post was that the G-IV test program suffered from a wing drop problem during OEI continued-takeoff testing whose cause was also attributed incorrectly by the engineers. In other words, it's not the aircraft that is at fault, but rather some of the assumptions made about it during testing.
Of course, that's exactly why testing is performed: to validate those assumptions. On the 650, they computed the wrong critical AOA in ground effect. On the G-IV, it's less clear about why the wing stall occurred (Mr. Johnson's account was given more than a quarter century after the test program ended), but they solved the issue with vortex generators.
My question was: give what happened with the G650, is it possible the IGE critical angle was also mis-computed on the G-IV? It seems that even Gulfstream didn't realize how different the critical AOA can be in ground effect versus in free air. I was not aware of that either. Nobody teaches you that the critical AOA is different, but if anything I would expect the angle to be higher, not lower, because performance is generally enhanced in ground effect.
Anyway, I hope that clears it up a bit.
