Performance Qs
 

(Done the thread search - found no answers)
I fly the Q400. Vmo changes quite a bit during climb. The bird strike-Vmo-limit below 8.000 I understand (not an aerodynamic limit). What I don't understand, is why the Vmo increases slightly (4 kts) from 10.000ft to 18.000ft, and then starts to drop so steeply up to 25.000ft - from 286KIAS to 248KIAS in just 7.000ft?
At M.58, the compressibility effects cannot possibly be that pronounced? Or? Could it be due to the propellers? What am I missing?

I have no familiarity with the Type in question but, in general ...

At lower levels the barber pole looks at Vmo which will be EAS-limited .. but it's showing IAS .. hence the increase with height.

At the crossover Hp the Mmo limit takes over which accounts for the reasonably steep IAS reduction with Hp above crossover height.

Answered previously here

Also here and there's some more general discussion of prop design issues here

Old Smokey's reply in the first thread linked above is both technically the answer and very prescient:

All my questions answered! Guess I'l have to refine my search parametres... Thanks, guys! :)