Do frequency response / Bode plots for an aircraft or helicopter come from modeling or empirical testing? I was intrigued while reading an article by this snippet:


  Examination of the V-22’s frequency response plots (Bode plots) in
  2000 showed that the aircraft failed to meet the basic stability
  requirements mandated for rotorcraft and was indeed PIO-prone.
  Since 2000, the Navy and Bell/Boeing have stated that the frequency
  response of the aircraft has been modified to remove this tendency.
  Updated Bode plots have not been made available to date.


My question: How does one go about obtaining frequency response plots for an aircraft? Is it a modelling exercise or an  empirical one?

http://www.g2mil.com/V-22safety.htm
The answer is both modelling and/or empirical methods are used.

For aircraft with a closed loop Flight Control System then the clearance requirements before flight requires that sufficient gain and phase margins exist for the system. For example with Military Aircraft this was defined in MIL-F-9490D. This would involve building mathematical models of the aircraft and control laws and calculating margins from the frequency response of the model. This allows the engineers to analyse the robustness of the system under various tolerance and failure states before flight.

It is possible to analyse stability margins from flight data, but note even the RTSM software referenced in the comments talks about comparing the flight data calculated margin against an a priori model.

Also note that Handling Qualities requirements, i.e. saying it is PIO prone, are additional requirements over and above the basic stability requirements. This introduces another feedback path with a pilot in the loop. For this there are additional frequency response parameters to meet, for example average phase rate. Again examples in MIL-Spec requirements MIL-F-8785C, these will all be assessed using mathematical models.