Gengis' mention of Lowry reminded me of the discussions he had with Denker on Usenet many years ago and his 'bootstrap' approach which apparently lets you calculate these sorts of things from an hour flight testing. Google found this link
http://www.allstar.fiu.edu/aero/AirPer-BA.htm I don't have a clue how it works, just mentioning it in case Timothy might find it useful and can work it out. Google 'google groups' for john lowry and you will find many of his posts.