How do the size affect the performance of a hexa or octacopter?
I suspect longer arms equals more stable platform, but is there a limit for the length of the arms? Recon longer arms will result in more slow response but for a camera platform i find that ideal. My plan is to ramp my hexa up to an heavy lift octa platform with 12" props which means its starting to get seriously big.
Yes longer arms do provide greater stability. You will want to have your arms long enough to clear the neighboring propellers prop washer. If you are going to use the aircraft for a camera platform, then it's better to keep the arm length to a minimum and tune the aircrafts stability to have a softer response.
How would you tune for a softer response? Decreased gain and therfore decreased performance and increased robustness? Or do you mean decrease some rate limiter?
It seems like decreasing the arm length would give you more chance for unwanted prop interaction at heavy weights and decreased control power, but it also keeps the weight of the arms and airframe inertia down. I guess there are no free lunches:)
I´ve found decreased gain suits me for now. But figured with a hexa x configuration you should double the roll gain for pitch gain? At least it seams to fly better that way... For the copter I´ve copied this maniac´s config: http://www.youtube.com/watch?v=DTE-kPQbOsA Same motors and props, just bigger esc´s to be safe. Frame 78 cm motor to motor. Should be plenty to lift a 7D plus gimbal :)
larger the props, the more efficient. Wow, x6 12 in. props must sound pretty cool. Let us know how much weight you can lift (and for how long) once you get it tested.