Originally Posted by hawk37
HazelNuts, does the above explain the very slight variation of speed for max L/D between mach .2 and mach .8 shown in your graph?
The speed for max L/D varies with weight and altitude. I suppose you mean the very slight variation of max L/D between mach .2 and mach .8 ? No, its the other way round. the P.S. is based on the variation shown in the graph. At typical cruise altitudes for this airplane the max L/D will occur in the mach range of 0.7 to 0.75, shifting to the left as the altitude reduces.
I cannot explain why the effect of Mach is as shown, except that I would expect the max. L/D to reduce near the aerodynamic ceiling beyond M.8. Compressibility changes the pressure distribution around the airplane. I would be surprised if the effect on drag would be identical to the effect on lift at any given angle of attack.