Not just related to turbopropeller aircraft.
Was the same on the DC-6, DC-7, and Lockheed 1649A.
The latter two had Mach number limitations as well as CAS.
In the case of the DC6, was indeed related to aileron/elevator flutter at very high speeds. In the case of the elevator, an additionl hinge was added at the outboard end on the -6B models, and eventually added to the straight six as well.
When the DC6 was designed, it used the same wing as the DC4 (with added washout at the tip), but with much larger engines (ie: 60knots faster), so in descent with cruise power selected, flutter bacame a problem. Likewise with the DC7, the DC6 wing was used (slightly modified again), but with much larger engines (and 60knots faster still), compressability effects were noted.
A lot was learned in the late 40's/early 50's about trans-sonic airflow.
Of course, the wing on turbopropeller aircraft can be designed for very high speed flight (with big enough engines) but then there becomes a problem with landing speeds, and the rather complicated flaps to keep these within reason.
Turbojet aircraft as well have decreasing Vmo as altitude is increased...and as indicated mach number increases.
As altitude increases, TAS increases for a constant IAS/CAS.
At higher altitudes, trans-sonic flow can become a problem, so IAS is limited, to allow for this condition.
Last edited by 411A; 8th Dec 2003 at 23:42.