What's the speed of propeller airplanes?
Thrust is not equal to speed... Flow rate at high speed is the only index of speed. For example, I personally flew 64 culvert and culvert 55, 64 bypass 55 ducted thrust is quite much, but in the same plane, the 64 is not 55 fast culvert culvert...... When it comes to the propeller speed, actually turboprop and piston in terms of speed and speed without what gap, as for the "propeller aircraft speed, use a larger pitch propeller type with close to the limit (piston or turboprop engine) + Design of an ultra low wind resistance model, propeller aircraft broke the sound it is entirely possible.
As for the precise values that break through the speed of sound, it's not clear... So, there's no doubt that propeller planes can do anything that breaks the speed of sound.