Trying to take advantage of the LOS 9 miles for Xbee 900, is it really possible?

Sure it is. You just have to have your antennas about 50’ above all obstructiuons to do it.

See http://www.digi.com/support/kbase/kbaseresultdetl?id=2051

I see then that is what I thought when I was performing a range test between two modules at about the top floor of a parking garage that is where I got the best signal strength over distance . Let’s say however I want to be able to have a an Xbee on a UAV and then be able to incorporate mesh capability with two ground stations (modules) would this scenario give me about the same result ?

If you puit them on polls and held them above all of the obstructions, then it would be but if you just place them on the floor of the guarge, it would do little to anything being at the top floor for range.

The reference you suggested help alot in the design of the RF communication that will need to be made in order to obtain the longest range possible. Thinking about using the yagi band antenna for the module no mounted on the UAV