Hi, This thread is similar to one in the Misc Section....
Do you find that your options are limited by the 'Coffin Corner' such that speeds must be kept between 370-500 Kts at FL350 and even less than that at higher FLs.
Flying in the Coffin Corner?Air France Flight 447 | Guest Blog, Scientific American Blog Network
Apparently the U2 could have a speed range of just 5 Kts at maximum altitude.