It's true i tells yah,It's true!
RWM and Mad (flt) Scientist you are absolutely correct.In the RAF, we were taught the terms 'first phase fan flutter' to describe the initial vibration you hear on engine start up before self sustaining speed is reached. Can't remember it being mentioned on any other licence courses though. You hear it particularly well on the old Nimrod and Buccaneer Speys and extremely well on the Tristar RB211s. Just because some of you guys were not told about the condition it does not mean it doesn't exist! The blade dampers fitted to the root shrouding on CFM 56 engines as well as the mid-span shrouds help to disguise the condition. Its really about fine tuning the lenghts, widths and shapes of the fans......