I have looked at a few of these calculations in these papers and I am aware the GPS method has been used to document PECs on helicopters. In the vector plots that I have seen used, the inherent sideslip that is common to most helicopters doesn't seem to have been factored in. At the higher helicopter speeds I would anticipate that it made little difference as the inherent sideslip is only of the order of 1 or 2 degrees. However, as the airspeed is reduced below 50 kts some helicopters have inherent sideslip of 10 degrees plus. I would have thought this would affect the results from the GPS method. Has anyone seen a paper that takes this into account.