In the 450 i have been working on, i have kept the blade too tight, so much that with a flick i am unable to move the blades. The heli spools up very nicly, with 0 vibrations and flyies very nicely.

The quesiton is what could go wrong with tight blades? Why do people say that the blades should be loose enough to move with hand?

Good question. Even I want to know the answer. Let the experts share their knowledge.

Mail blades should be fairly tight.  They should be kept movable with the application of some amount of *deliberate* force.  With one hand holding the head/blade-holder, the other hand should be able to move the blade with some force. 

This helps with initial spool up, final spool down, and most importantly, when you start doing autorotations.

The tail blades on the other hand, are best kept fairly loose.  Loose enough so that they can fall down on their own weight.

That helps Anwar, thanks for this detail, with main blades loose enough to swing with fairly gentle flick makes them strike each other or heli while spooling up, tight blades have only helped.

Yes now it makes sense why it has to be fairly tight. If very loose, I guess the heli will vibrate once the initial spooling happens.