why,when taxiing, the prop is making such a distinctive sound when you want to brake WITHOUT entering into reverse ( once again, the pilot told me he didn't used reverse, only power lever up to gate, and not over ).
some people call "reverse" anything below flight idle , some call "beta" anything below flight idle and this mixes it sometimes up
.but there is beta
and reverse and both are different things.
technically ,when you taxi, and are in flight idle the turbine is running idle speed and the prop is pitched to flight idle pitch .going over the
gate engages beta but initially not reverse.
in beta the turbine is still fixed on idle speed but the pitch starts to go below flight idle pitch . forward thrust and power absorption reduce , with the same output from the idling turbine the prop speed start to rise. the rising speed of the blade tips as well interferences from the low pitch of the blades generate the sound you talk about.
pulling back further engages reverse. the main difference is that the governor kicks in again ( reverse is instead of beta governor controlled !) and the turbine spools up , power output increases but with thrust back since the blades are now negative.
braking "without reverse" means a taxi in beta but before the turbine spools up again.
With more or less +10°, it seems hard that the prop will have such a braking effect, that it can allow such dives, and in the PoH, it's well written the Bęta angle is -0°30', so, I assume achievable in-flight..
due to its design nature a turboprop generates thrust by moving a large amount of air with relative small velocity. thrust occours when there is a speed difference between air pushed by the props and the airspeed of the aircraft.
so in taxi, with an airspeed of virtually zero the thrust generated by the props is pretty high even in idle- thats the reason we need beta at all for taxi. in flight idle especially higher powered twins would need a constant brake input without beta.
in a dive with a high airspeed and pulled back power the speed of the air surrounding the aircraft starts to be greater than the speed of the air pushed by the props of an idling turbine , the thrust goes to zero and the props , now driven by the wind, start to act as an massive airbrake .
cheers