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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?

Propeller Arrangement

P.S. - I am unsure of the tags, please adjust as necessary

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    $\begingroup$ I'm voting to close this question as off-topic (and too broad). It belongs to a RC site, as already pointed out. $\endgroup$
    – mins
    Oct 21, 2015 at 22:57
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    $\begingroup$ I agree that this would be better answered on a site more devoted to RC. But spacing of full-sized props would be on-topic here, and that is just a matter of scale. $\endgroup$
    – fooot ♦
    Oct 22, 2015 at 0:21
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    $\begingroup$ I think this is a valid question for this site. The fact that another better forum might exist by itself does not seem reason enough to close it on here. $\endgroup$
    – curious_cat
    Oct 22, 2015 at 1:24
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    $\begingroup$ IANAAE: QC blades spin so fast that I simply cannot fathom any tip vortice issues at all, nor much, if any, interference between the rotor disks. $\endgroup$
    – CGCampbell
    Oct 22, 2015 at 1:53
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    $\begingroup$ what else is there to avoid in terms of airflow.... On-topic. $\endgroup$
    – digitgopher
    Oct 22, 2015 at 18:55

1 Answer 1

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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.

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    $\begingroup$ Care to share your calculations (where all those 1/3 and 1/4 comes from), and if you could expand on the "counter-airflow" bit - are you referring to the rotational direction of the blades? (The green ones are indicated as ClockWise, whereas the other two are marked CounterClockWise in my diagram) - anyways, point of different heights was in the case of the propellers otherwise overlapping (or being very close to overlapping and hitting each other if they were on the same plane) - I don't see much benefit if they are already spaced out. $\endgroup$
    – user2813274
    Oct 23, 2015 at 2:25
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    $\begingroup$ Would it matter at all if they were shrouded, I.e. there was a wall between the propellers so air could not move diagonally from one to the other? $\endgroup$
    – user2813274
    Dec 15, 2017 at 5:53

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