Is there any good database for aircraft technical specifications?
One word for the definitive authority on this subject: Jane's.  They'll get you anything you want to know - if the price is right.

Another good source, especially for military aircraft is The Military Analysis Network, run the Federation of American Scientists (FAS).  Great information database and it's free, too!

As with so many other subjects, Wikipedia remains an excellent source of information on aircraft.  It is not a recommended source to cite for writing technical publications of your own, but is a excellent internet portal to additional information and resources e.g. books, periodical articles, etc.