BOAC,
I tried to put the chart here, but not able. If later I can, I will.
Input of Chart: Load Factor (margin), Weight, CG and Pressure Altitude. I believe they are all self evident for you.
The output comes as Mach No only, as the lowest and highest limit. And this makes sense to me.
Because:
The lowest limit practically leaves a margin to stall (by default 1.3 g limit). And we know at near Sea Level altitudes where compressibility effect (mach effect) does not kick in yet, the stall AOA is fixed and that is why stall speed as KCAS (dynamic pressure) is nearly independent from altitude. However at high altitude where compresibility takes effect, the stall AOA is variable (reduced as compared to SL) so stall KCAS greatly influenced for same conditions (e.g for same weight). Therefore expressing the Lowest margin as Mach is more accurate and more sensible for pilots because that is what we monitor at high altitudes.
The highest limit till cross over altitude (for 330 tahat is 30000 feet PA, where 0.86 mach equals to 330 KCAS and not effected by ISA DEV) is the dynamic pressure limit, that is why it is given as KCAS (or precisely KEAS). But what I know at high altitude the buffet reason is not the dymanic pressure (namely KCAS or KEAS) but the shock waves on the wing. Therefore expressing the highest margin as Mach is also more sensible for pilots.
Regards.
Last edited by JABBARA; 7th June 2009 at 14:46.