Just to cover something that wasn't already covered in this post: Emergencys with no atc (on unicom which i beleive is 122.8)
I was flying a flight on Vatsim on the POSKY CRJ-200 out of BOS, (just some background info: if you've flown the posky crj, you'll know that while taking off, it has really sudden and jerky movements which have caused ALOT of unexpected tail strikes for me) I had one other guy at BOS and not atc, so i tookoff with the Eastern Virtual guy holding behind me, when on my t/o roll i had a tail strike and after taking off declared an emegency over unicom and got a timely response from the guy behind me who was lined up that hes vacating the rwy. (unless there in a bad mood, just about everyone on vatsim is very helpfull and freindly, whether they've been on vatsim for a few days or a few years) So i turned 180 degrees and just barley managed to land, then i taxied off the rwy, apoligized and thanked the Eastern guy and logged off.
So really, when theres no atc its ok to practice/decrlare emergencys as long as your not delaying any one else or ruining someones flight on VATSIM.
my two cents.