Nerdy Turboprop question

[MayorQuimby]

Please excuse the nerdy question, but curiosity has got the better of me..

As a passenger in turboprop aircraft, such as the ATR, and given my limited GA knowledge of constant speed props, I often wonder why the prop RPM appears to to decrease initially from the idle setting as the spool up starts on take off. Possibilities that suggest themselves are some kind of initial pitch over compensation by the CSU as the power comes in, or maybe a transition of mode from beta?

You do understand that this isn't keeping me awake at night or anything, but I would like to know the answer. :)

[Tu.114]

I would think that Your explanation is spot on.

A turboprop engine is much more responsive to power lever movements and spools up rather more quickly than a jet engine, so a lot of torque can be delivered to the propeller within shortest time and the propeller has to be able to digest what is thrown at it. Increasing the blade angle in anticipation of an upcoming high power setting while accepting a minimal reduction in prop RPM is therefore appropriate. Not overspeeding the prop is of some importance, therefore on the DH8, there are 2 dedicated overspeed protection systems: the first guard increases the prop pitch passing 1071/min, the second one reduces fuel flow above 1173/min (nominal max prop RPM is at 1020/min; values shown are for the -400)

[MayorQuimby]

Thanks Tu.144.

I understand the overspeed protection you described. But, as regards the 'anticipation' you describe.... is this a management system detecting the commanded power increase and pre-emptively applying the increased pitch (rather than a CSU reacting to an increase of rpm over a set value)?

The case, for example, on takeoff in this video at 2:08, where the prop rpm doesn't appear to increase at all initially, but actually decreases below idle RPM:

https://www.youtube.com/watch?v=JOn-IvHtLXU