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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

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    $\begingroup$ Practical example at Nasa: In-Flight Stability Analysis of the X-48B Aircraft $\endgroup$ – mins Jan 16 '16 at 23:19
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    $\begingroup$ @mins So, it was generated on the fly? $\endgroup$ – Sanchises Jan 17 '16 at 17:40
  • $\begingroup$ @sanchises: Yes, the excitation inputs are generated on the flying aircraft. The aircraft is remotely piloted from the ground station (flight test pilot and engineer). The analysis is done in quasi real time on the ground, using a set of measures downloaded from the aircraft, a new set of loop excitation commands is uploaded, etc. The stability margins are computed using a RTSM software. $\endgroup$ – mins Jan 17 '16 at 18:13
  • $\begingroup$ @mins Yeah, I was just going for the pun there, sorry. Thanks for the link though, because coincidentally it describes something quite relevant to a project I'm doing. $\endgroup$ – Sanchises Jan 17 '16 at 18:22
  • $\begingroup$ @sanchises: Shame on me! Yet a good one. $\endgroup$ – mins Jan 17 '16 at 18:31
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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.

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