Go here and post the question.

http://www.rcuniverse.com/forum/forumid_76/tt.htm

The theoretical answer to your question will be intresting, but it will not answer the real world question you have with any certainty.

The zinger pro and MA Scimitar wood propellers are very good for the ST90. They spin up quick because they are light and the have efficient airfoils(they put out more work or thrust per H.P used) 14 x6, 15 x4, 15x5, 15 x6-- Depends on specific ST.91motor, prop, engine, fuel, atmospheric conditions, flying style.