Peter wrote:
what will be the first to break
The design specs ? 
corrosion, gust loads or too high control input, thermal weakening of the structure
What on earth have corrosion, gusts, control inputs or temperature of the airframe got to do with Vne? The last one excluded if we are at Mach 2 in an aluminium aircraft.
Are you saying that the airframe corrodes faster as it moves through the air faster, or that a Piper Cub at 122mph is getting too warm?
The faster you go, the less you can move the stick before the aircraft is overloaded. At some point even a millimeter will overload the aircraft, but I would think it will have fallen apart long before that point. The same goes for gust.
ortac, as I have written, weakening of the structure may lead to a simple break up dur to overstressing the structure beyond its carrying capacities. Those loads can be result of gusts or control input. Plus it could weaken the torosional strength and influence the divergance speed or safe flutter speed. I never said too high speed lead to corrosion or that a Cessna 172 would suffer thermal weakening of structures. But there is more out there than 172s.
I don’t see how compromised structural integrity is relevant to the original question. If an airframe is damaged then it could fail at Vne, Vno, Va or even Vs.
And above Vne the airframe is liable to fail with no gusts and no control inputs (ie at 1G), as shown on a V-N graph.
The forces on the structure increase with flying speed. So a damaged structure might hold below V_A, but not above V_NE.
If something breaks by aerodynamic forces at V_S, you are not able to fly at all.
Neil wrote:
There is also the question of tailplane load at high speed
Many crashes happening during spiral dive (graveyard spiral) seem to begin with the tail breaking/separating. This would be due to overspeed, not the high G involved. The wing’s moment increases proportionally to the indicated speed squared, and the horisontal tail has to counter that. (After the tail has failed, the wing moment will push the nose violently “down” forcing the wing to fail in the downward direction.)
Of course flutter is the Vne limiting factor in many designs, but I have the impression that the tail structure is limiting in many of the rest.
An indication that flutter is limiting is that Vne has lower IAS values at high altitudes. This is because flutter is a function of true airspeed, whereas static structural loads and moments are related to indicated airspeed.
huv wrote:
This is because flutter is a function of true airspeed, whereas static structural loads and moments are related to indicated airspeed.
Well. Flutter is a function of St number. St number is a function of true airspeed and Re number. Re number is a function of true airspeed, density and viscosity. Viscosity is a function of temperature. Flutter also changes with altitude, but it is not directly proportional to IAS (or momentum), I guess. But higher density will also restrict the movement of a body, so flutter would be more likely with less density, higher up.
What a pitot actually meters is the differential force over some area due to mass flow multiplied by true airspeed (the momentum). All the structural loads will of course be directly proportional to the pitot meter (IAS), because they are a function of the momentum of the air. I have often wondered why the derived IAS is used instead of momentum directly. I’m am sure birds use momentum as a “meter” and differentiate between this and velocity 
(a bit nerdy tonight)
LeSving wrote:
I have often wondered why the derived IAS is used instead of momentum directly.
If you wish, you could consider IAS (well, CAS, actually) to be momentum expressed in funny units. It’s true that using “proper” momentum units would not make any difference at all as far as piloting the aircraft is concerned as V-speeds could just as well be expressed as momentum. OTOH it won’t help either and using airspeed units simplifies the relation between CAS and TAS. TAS ≈ CAS increased by 1% × density altitude/700. So even at FL100, they ASI will only show about 14% less than the TAS.
