Originally Posted by
CliveL
The low supersonic "sloping" Vmo/Mmo was derived from performance considerations. The performance obviously depends of the variation of drag with airspeed and Mach No, together with the thrust variation with altitude and Mach. The objective was to choose a flight path which maximised the increase in energy height per lb of fuel consumed. From these sums one could generate a nominal boundary which gave the basis for a scheduled Vm
Having flown the ‘baby Concorde’ (i.e. Mirage III), the slope of the low supersonic Mmo boundary immediately reminded me of the optimum energy climb profile. CliveL’s post above confirms that. So it’s not a physical limit of the aircraft, just a performance limit. There’s little point in going any faster as you would just be wasting fuel. EXWOK’s comment that pilots simply climbed on the barber’s pole suggest that the lower supersonic Mmo boundary is related to this optimum energy climb profile.
For more information on how the optimum energy climb is determined, web search the term ‘Rutowski Climb’.