I swore I would not get drawn into this again, but I cannot help myself....
Indicated airspeed (IAS) is not a real speed, but a parameter defined as True Air speed (TAS) times the square root of the normalized (that is, divided by sea level air density) air density. One can always solve this definition for either the TAS or the density, and substitute the result in an equation. For some equations, in particular the lift of a wing and the parasitic drag, which in general depend on TAS and density, such a substitution results in a fortuitous cancelation: the density disappears, and only IAS is left. Of course the lift still depends on density, it's just "hidden" inside the definition of IAS.
Now for aeroelastic flutter, the situation is much more complicated. Many modes of vibration are possible, with different dependencies on TAS and density. Making a substitution of IAS (and density) for TAS does not, in general, result in any fortuitous complete disappearance of one of the other variables. But if we know the functional form of the flutter equation for any given mode, then the following statements are ALL TRUE:
1. In general, the onset of flutter depends on density and TAS.
2. In general, the onset of flutter depends on density and IAS.
3. In general, the onset of flutter depends on IAS and TAS (this is awkward to use, but is nonetheless true).
Now this is awkward, so the next logical question is:
1. Can I look at all densities from sea level to the maximum operating altitude of an airplane, and specify a single TAS which, if not exceeded, will be safe? The answer is YES.
2. Can I look at all densities from sea level to the maximum operating altitude of an airplane, and specify a single IAS which, if not exceeded, will be safe? The answer is YES.
What do the FARs say for normally certified aircraft? The answer is option (2), the manufacturer is to specify a single IAS which is safe at all altitudes the plane is certified to operate;OR, a chart of density (expressed in density altitude) and IAS which is not to be exceeded must be placarded.
The reason for specifying IAS to be used is clear: that information is readily available in every airplane. The FAA does not think it reasonable that pilots be expected to pull out an E6B and spin the wheel, prior to every descent during which high speeds may be encountered. I note that the vast majority of certified aircraft meet this requirement without resorting to placards.
What about E-AB aircraft? Well, as usual, it is up to the builder, not Vans, to determine safe limits. But finding Vne requires dangerous testing, or very sophisticated analysis, not available to the majority of builders, so most use Vans' numbers. I note that Vans is under no obligation whatsoever to furnish these numbers, and I am grateful that as a responsible manufacturer he has chosen to spin test, load test, etc. his kits. However, I do find it strange and very inconvenient that (at least per a single article written by Ken and since referenced many times) he has claimed a single TAS number. This is contrary to what is done on every certified aircraft, and therefore is bound to cause confusion. It is also dangerous, because the average pilot does not think that way. Ken's article about turbocharging is a red-herring. In my plans-built 10 I certainly cannot exceed 200 KTAS at 18,000'. BUT, I can easily nose it over to come down, and watch the airspeed creep a bit into the yellow (not red) arc. But wait! I just exceeded 200 KTAS! At the published service ceiling of 22,000' I can do this while still within the green arc on the airspeed indicator. Do we really expect those with steam panels to be wheeling away on their E6Bs, prior to commencing a descent? I am sure more than one RV pilot has exceeded the red line if it is a TAS number because they were unaware of this "not the usual" stating of Vne. I really wish Van would release all his test data, and be more forthcoming of what combination of airspeeds and densities are considered safe, and what are not, and find a way to express them consistent with the FARs for certified aircraft - just because that's what pilots are use to using.
In the mean time, as always, E-AB builders are themselves responsible for setting safe limits; hopefully those who wish to expand the envelope will have the means and skills necessary to insure they do so safely.