Originally Posted by Willy Miller
What about the reducing TAS!
red line FL180 284 IAS = 360 TAS
red line FL250 245 IAS = 340 TAS
why the lower TAS
(props not reaching Mcrit, tips travelling at 350 kts ish at 850RPM)
360 KTAs at 18,000ft is ~ M0.58
340 KTAs at 25,000ft is ~ M0.57
(Speed of sound drops by ~20 kts between those altitudes, so the Mach number is near constant).
That seems to indicate it's a Mach Number line, not a speed dependent line. But M0.60 is pretty slow for a critical Mach even for a straightish wing like the dash8.
Also, if the aircraft is doing 360 KTAS the prop tips are certainly going faster than that. If that 350kts is based simply on prop diameter and revs, which I think it is (gives a 13ft dia prop, which sounds about right) then you really need to combine the tip rotation speed with the aircraft forward speed.
If you do that, then at 350kts rotation plus 360kts forward you get about 500 KTAS - which is about M0.83. Starting to become important.