Dash 8-400 Vmo vs altitude

[Sirijus]

Hello Dears,

my sleep at night is disturbed because I can not figure out the answer.

Vmo for the Q400:
245 KIAS @ 0-8000ft
282 KIAS @ 10000ft
286 KIAS @ 18000ft
275 KIAS @ 20000ft
248 KIAS @ 25000ft

Why does the Vmo rise between 10000 and 18000 feet before starting to reduce again? Is there something magical about about that altitude? :ugh:

[LimaFoxTango]

Was told 245kts from 0-8000ft is for the windscreen.

[Sirijus]

Thanks LFT,

that one is clear for me though :) It's the part at 18000 I don't get

[Tu.114]

That seems to be a Mach limit of about M0.58.

If You like, You can ask the FMS for a Mach reading (via one of the Data pages -> ADC1/2); although our books have nothing to say on this, I am under the impression that the limiting speed is always at the same Mach number.

[Amadis of Gaul]

If that's the only thing disturbing your sleep, you're still doing okay, I'd say.

[ElChasper]

Correct me if I'm wrong, but I believe the limiting factor is the propeller blade tip speed. The higher you climb, the lower the the temperature thus resulting in the tips approaching the speed of sound. You don't want your tips to be above the speed of sound, so VMO decreases above 18000.

[Another St Ivian]

245 KIAS is a windshield birdstrike limitation.

Beyond that...

286 KIAS @ 18000ft == Mach 0.599
275 KIAS @ 20000ft == Mach 0.599
248 KIAS @ 25000ft == Mach 0.599

Aviation Calculator (http://www.hochwarth.com/misc/AviationCalculator.html)

The reason for constant Mach above 18,000' is due to the propellers. Most turboprop aircraft have limiting mach numbers in the region of 0.60 for this reason.