The "Modern" flight model option is what should always be used with aircraft in MSFS - it's the new, MSFS flight modeling system. The "Legacy" flight model option is FSX flight modeling, and is not what MSFS-designed aircraft are meant to be used with. Unfortunately, some folks who recently bought the JustFlight Piper Arrow, which has some of the most accurate flight dynamics developed for MSFS to-date, thought that the flight modeling was off and was uncontrollable, where in fact they were using it with the "Legacy" option, not the "Modern" option as it should be and was designed for/to be used with. Modern flight model activated, and it will fly just as it should/is intended.