There is a thing called “ Whirl mode” seldom heard of recently. It destroyed two L188 Electras not long after they were first introduced. To keep it brief, when two rotating or oscillating components at different frequencies on a common frame are disturbed a problem develops if those frequencies eventually coincide or “ couple.” In the case of the Electras a wobble at the propeller shaft coupled with wing flex frequency in turbulence tore the wing off.