Aerodynamics question

[Snapper77]

Just a quick question, why does Vmo decrease with altitude?

[Baldricks Mum]

Its got to do with the Constitution, Its Mabo, its the vibe of the thing.

No really it has to do with Density of the air over the aerofoil or something like that...

Sorry I'm not much help.

[lineboy_nz]

Vmo appears to drop with altitude because the A.S.I. will underread with increasing altitude.

[Capt Fathom]

As you climb, a fixed Indicated Airspeed (IAS) equates to an increasing Mach Number. Eventually, the rise in compressibility drag makes climbing at a fixed IAS untenable, so you continue to climb at a fixed Mach Number instead. This mach number is generally close to the mach number you will cruise at. This change over occurs around 28000ft.
When climbing, the temperature will be dropping (in the troposphere), and assuming you are maintaining a constant Mach number, the True Airspeed will decrease as the temperature decreases, and hence your IAS will also decrease.

I'm sure there are some other factors involved, but that's it in a nutshell.

[conch]

If IAS is kept constant TAS increases with altitude. At a certain altitude TAS starts to get that big that the airflow across the upper camber of the wing starts to approach the sonic range. The air gets compressible, a shockwave starts to develop and buffet is felt.
To avoid this, Vmo is kept constant up to a certain altitude and then the maximum speed is Mach number limited. Mach is the ratio of TAS/speed of sound, so to keep the Mach number constant TAS (or IAS on your ASI) has to be reduced constantly with increasing altitude.