The cleaner the airflow going into the propeller the better. When you start having turbulent airflow rolling off of one prop and into another, it is going to have some non-uniform effects and cause some problems. Also, when you have shorter arms on your multicopter, it causes the rotational moment of inertia of your vehicle to drop which increases it's sensitivity to anomalies in your propulsion systems.
Some studies (Leishman is one) show that 20-30% overlap actually has a beneficial effect.
Brad Hughey is a proponent of his work. I'll probably do some experiments.
Yes, I'm aware of the effect of the moment of inertia, I'm just wondering how much effect it will really have. The Octo would have a similar diameter to the 3DR quad, just more motors, and my 3DR quad is pretty stable.
Is there a paper or report that explains the plot?
Edit: Just kidding. Found these:
There is, I'll try to find it...
Some interesting discussion here:
Ah, here it is:
You'll notice that Chinook helicopters have just about this amount of overlap, but I don't think it's known if the designer knew this going in, or if it was simply happy chance.
This would be simple to test. Mount two motors on a thrust bench, with sliding mounts, and obviously staggered to ensure they won't hit. Then simply slide the motors in and out and see what happens.
I don't think overlaping props will work... One will be CW.. the other CCW.. has to be that way..
They will cross the same area going the same direction.. adding more swrill to the air...
Is that Right.. ?
Yeah, I dunno Eddie, but that's the way they do the test. Look at a Chinook. Counter-rotating.
I'll probably rig up a test before I build it and report back.
I thought I would post this before I run off for weekend. We just finished testing this morning and I processed all the data this afternoon. See the attached PDF for all of our test data. Cheers!
This is grate data that describes relation with thrust and propeller. Useful for selecting power unit for Quad.
@Adam: The Korean paper you reference makes the case that the Kov in forward flight has a unique relationship to advance ratio (related to blade pitch).
@all: The Wright Brothers' first powered airplane had twin propellers. Isn't it intriguing that more than a century later, we're still investigating how two propellers working together interact with each other?