How much distance do I need between quadcopter propellers to avoid issues?

Question

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

Answer 1

Well, you gotta keep in mind that the airflow is mainly down, but there will always be a little flow side to side, (thus frames, also used for safer collisions,) so you should put them roughly 1/3 of the diameter of the rotors themselves away from each other. This provides enough space to keep your interference down, as the interference spreads out to about 1/4 of the radius of the rotor. I have not seen that the number of blades matters much, as long as you follow the 1/3 diameter rule. You want to avoid the counter- airflow that can make your craft less efficient or jack the blades. Note also that raising two of the blades would cause serious problems with your efficiency, because two rotors blowing downwards in a cone, unless very effectively controlled, will kill the stability, creating unnecessary strain, or in bad cases, frame torque. The two above would create interference for the lower. This could, however, be avoidable if the top two were about 3/4 rotor diameter away, and no higher than 1/4 a rotor diameter. But then you'd be flying a very odd and somewhat unbalanced copter.