52 2.6.2 Blade Twisting Effect. 54 2.6.3 Flapping and Feathering Interchangeability 62 2.6.4 Stick Vibration..... 65 2.7 SWASHPLATE CONTROL 73 2.7.1 The E.Burke Wilford Rotor S System .74 2.7.2 Pitch-Roll Coupling With the Wilford System..... 77 2.7.3 The Raoul Hafner Rotor System-Part .81 2.7.4 Gust Response With Hafner Rotor System ....... 89 2.7.5 The Raoul Hafner Rotor System-Part