Building a Pixhawk Hex -- Long or Minimal-Length Arms?

I am double-posting this in here and in the miscellaneous forum, but doing so because the 3DR guys are good to get back to me with helpful information. I’m building a hex with a second Pixhawk (yes, repeat customer) and all of the 3DR components (Radios, PPM Encoder, Power Module). It should arrive Wednesday.

I know prop-wash near the body of the craft (or anything else stationary) creates control/stability issues. But is it important for in-plane props to be as far away as feasibly possible?

In my design, at one arm length I can provide 2+ inches between the props. At the other arm length I can provide around 1/2 of an inch between the props.

The wingspan of the device is irrelevant to me. Overall size doesn’t matter. Which arm length would you guys recommend to maximize stability and agility?

I think the default PID’s are best for a 450 sized quad…

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I would manually (or auto) tune the PIDs. I’m not that worried about finding the correct settings.

Here are my two theories:

  1. Planar props (not co-axial – I know that has issues) can spin as closely together as they want as long as they don’t hit (even if the arm is flexing). A little “dirty” air from the side is no big deal since the props are the same size and on matched motors.


  1. Props work “better” (more stable control, more predictable air, more efficient) when they are separated by several (or more) inches. We push props closer together purely due to size limitations and in a perfect world, each prop could be so far away from the other that they’d never see each others’ air.

Now which one is true?