Originally Posted by
ZnCrO4
I agree with your first comment, though I believe the second one is more aircraft based. Some aircraft do in fact have a Vmo that will decrease with alt although it decreases at a slower rate as to become higher than Mmo at some transition alt. For instance in the HS125 I believe the Vmo to Mmo is somewhere in the neighborhood of fl330 depending on the day. Vmo starts to decrease above 12k if i remember right. The reason for this I am not entirely certain although I think it may have to do with flutter.
thats all I have to add...
I think we are all talking the same language. Airflow around an airplane is not a constant; it varies with location. For example, the tip of a propeller could theoretically go supersonic while the prop near the spinner was still subsonic because the tip is moving a lot faster, even though it is the same airplane moving through the same airmass (don't know if that is actually possible, it is just the simplest example I can think of).
So, as you climb, the pressures created by your EAS/Mach vary from wing root to wing tip, from the nose to the tail, etc. A decreasing Vmo as you climb prior to reaching Mmo is probably being dictated by the Mach effects on some part of the airplane, but it is being presented to the pilot as an airspeed.