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I'm an aerospace engineering PhD student and for my research I'm investigating how to come up with a way to pick the most critical flight conditions for flutter. The main idea is to understand how it is currently done in industry and expand that approach for nonlinear cases.

I've been scouting the web to find information on how companies do that (Airbus, Boeing, Embraer etc.) but I was only able to find a paper from Dassault (A Review of Industrial Aeroelasticity Practices at Dassault Aviation for Military Aircraft and Business Jets) that briefly describes their practices for flutter, without really answering my question.

FAR 25 (hence same for EASA) in AC 25.629-1B (the "extension" of part 25.629, which is dedicated to flutter) states that:

For nominal conditions without failures, malfunctions, or adverse conditions, freedom from aeroelastic instability is required to be shown for all combinations of airspeed and altitude encompassed by the design dive speed (V_D) and design dive Mach number (M_D) versus altitude envelope enlarged at all points by an increase of 15 percent in equivalent airspeed at both constant Mach number and constant altitude. The figure below represents a typical design envelope expanded to the required aeroelastic stability envelope. Note that some required Mach number and airspeed combinations correspond to altitudes below standard sea level.

Aeroelastic stability envelope

So does anybody know whether designers only pick a few points at the boundaries of the envelope to actually check for flutter? Or something else is done? For example, in the case of the envelope above this would mean running a velocity-based linear flutter search for several mach numbers between C'' and B'.

Thanks everyone!

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    $\begingroup$ I think that the "State of the art in aircraft industry for flutter analysis" is proprietary of each company, not sure if you can find something more than simple and generic explanations... $\endgroup$
    – sophit
    May 14 at 18:48
  • $\begingroup$ @sophit I am not sure that is the case, because aircraft manufacturers have to document all their testing results and methodology as part of the certification process. $\endgroup$ May 14 at 19:08
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    $\begingroup$ I don't know if those documents are publicly available $\endgroup$
    – sophit
    May 14 at 19:22

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Airbus uses Simcenter Testlab to streamline and improve its flutter analysis process. Their flutter testing is divided into three parts:

  • Real-time: Live data is collected during the flight to check safety and continue the flight envelope
  • Near real-time: Rapid modal estimation is used to determine the flight's overall safety and the flutter test program
  • Offline: Recorded flight data is analyzed in detail and a final report is produced

Simcenter Testlab Flutter Analysis is a Siemens Digital Industries product that is part of the Testlab system. There is a case study on the Siemens web site for the product covering how Airbus used it on the A380 program.

(Full disclosure: I used to work for Siemens and I haven't received a dime from them in 42 years.)

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Military flutter testing philosophy and practice may be of interest, also. As a former flight sciences manager for high-performance aircraft, we built up to and then flight tested at several 'edge' points, yes. Of course, ground testing and analyses would cover more far more flight conditions and configurations.

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