For a given propeller size (say 5"), what is the minimum clearance needed between propellers to ensure that not only do they not hit each other (easy to calculate), they also don't interfere too much with each other (i.e. tip vortices, airflow, etc.)
For example consider the following design: an X-shaped quad-copter where the propellers are at the end of each tip of the X - and the X across the diagonal (max distance from the center of the propellers) is 180mm - then 5" propellers will only have .279mm of clearance between them - now assuming that mechanically there are no vibrations/imperfections/etc. that cause them to run into each other, what else is there to avoid in terms of airflow? if they were raised such that two diagonal propellers are lower/higher than the others, would there be any issues? does it depend on the propeller pitch, or if it's a 2-blade or 3-blade propeller?
P.S. - I am unsure of the tags, please adjust as necessary