I wrote something similar to this (well exactly but I am tired) in a post but thought it might be interesting enough to have a post of it's own.

A source of good information was a government document called DATCOM. I forget what the word means. It allows you to go through all of the steps to determine the airplane characteristics. I spent way too much time with it back in the '60s doing manual calculations.

An electronic version of it called Digital Datcom as well as other programs of interest can be obtained here for $295. I think there is a lower price for students.

http://www.pdas.com