Ok, just got done watching this video and now I want to extend the arms on my quad to 800mm (or thereabouts).
Just just posted:
So he gets a full 28 minutes in the air (ok...in loiter mode but still). Does this mean that longer arms is better than bigger battery power? I have a 450mm right now but an 800mm would probably do really well in the wind. Just thinking, because these 800 arms look really tempting! Thoughts?
Replies
Noted
Length of arms has the following affects:
o quieter (lower amplitude sound wave mixing from multiple propellers); further apart the rotors the quieter
o possible lower vibrations at the APM (but a trade with stiffness that decreases with length)
o not as portable.
o flies less (not longer) duration because apples to apples, longer means heavier.
o flies worse (not better) in the wind because the surface area of the ship increased (boom length and probably width and or thickness) and more of rotor power is trying to keep the heavier mass in the air.
But, in the video, it appears that carbon tubes were used for the arms. Even with more length, they will:
o weigh significantly less than their shorter plastic brothers.
o therefore the ship will fly longer
o will be quieter because the props are spaced more
o will have less vibration at the APM because of the spacing and because carbon is so much stiffer than plastic
o and if one of those tubes is continuous (goes all the way through the center section) it will be much stiffer and remove the need for another 40 grams of fasteners.
o will fly better in the wind because despite the fact that the arms have more length, they have less width and are round (friendlier for wind to go around) versus a flat surface that blocks wind.
So the longer plastic arms would create a quieter ship, but with several other penalties including handling in wind and less flight duration.
If you notice that arm length makes a handling difference (in the opposite direction that what has beenoutlined here) then you have a PID tuning issue that will solve the handling in wind issue. As you change the payload (batteries), the PIDs also need to change.
Longer arms means a larger moment generated about the centre of mass for the same thrust input... or conversely, for a given torque input required to stabilise the vehicle a smaller thrust force is required. This is somewhat countered by the increase in the moment of interia about any axis in the horizontal thrust plane arising from moving mass distribution away from the centre of mass, but with lightweight arms being a small fraction of the overall vehicle mass, the power efficiency gains would outweigh the angular acceleration losses. There will be diminishing returns though as the mass of the arms increase, as well as their length... ultimately, while you'll gain energy efficiency in stabilisation, you'll lose performance in terms of vehicle agility.