Originally Posted by
JohnDixson
Crab wrote: “Normally the designer goes to some lengths to avoid any natural frequencies of the rotor system “. Yes and then preforms mechanical stability tests with mis-serviced landing gear struts/tires and main rotor dampers-at different weights and centers of gravity..
R.W. Prouty - Helicopter Aerodynamics
Resonances in the airframe components at the various excitation frequencies are fairly common when a helicopter first flies. It is not yet possible for the dynamicist to successfully guide the designer in his placement of stiffness and weight elements to obtain an absence of the coincidence of structural natural frequencies with frequencies of possible exciting forces. The late Bob Wagner used to give guidance to the designers by telling them to design everything in resonance as a way of ensuring that everything would be out of resonance when it was built.
Avoiding all of the troublesome frequencies is like trying to throw rocks through the gaps in a picket fence.