No sweat - hard to tell emotional content if you don't know someone. I've got thick skin. Which I now apparently need, as I get needled for not knowing more about flutter, even though I worked for Scaled, although not as an aerodynamicist . And a bunch of searching that I did previously was reasonably opaque to me, hence my question. Yeah, and since the last time I futzed with ODE's was... 37 years ago, I might as well be staring at martian. So I appreciate the pointers - although I'm not totally clear on the deeper math, I do understand more than previously. Let me try to summarize what you've said and what I gleaned from the docs you pointed to so we can see if I actually understand it. Flutter is a function of many parameters - air density, TAS, TAS^2, and structural stiffness in multiple modes, and more. The interactions of these parameters are exceedingly complex and non-linear. GENERALLY, the effect of altitude (density) on flutter speeds can be approximated by using the average of the EAS (IAS, at low Mach #'s) and the TAS as the flutter speed limit, but this is only a rule of thumb - it is NOT an analytical limit. While that will USUALLY be a reasonable limit to use for approximating how fast one can go before things flutter, given a known flutter limit at a known altitude (density), it is by no means a guarantee, and even using a straight TAS line as the flutter limit, no matter what the density, is not necessarily a conservative position. Due to the fact that flutter is an interaction of multiple modes of vibration, even GVT and analytical calculations are not always accurate in predicting flutter speeds at all altitudes, and only real flutter testing can determine whether or not the analysis is accurate. So, in conclusion, using the EAS+TAS/2 limit, due to the density dependence of the driving frequencies, will usually give a reasonable limit for flutter speeds. Using TAS only will give a more conservative limit (but NEITHER are guarantees of flutter resistance). My error, then (and the error of the Van's article to which I and others have pointed) was in claiming that flutter was a function of TAS, without acknowledging that it's more complicated and not truly dependent on either EAS or TAS, but on density and the other factors mentioned above. Close?