Originally Posted by hugocraft
With helicopters I run <10% expo and 3d planes <45% expo. Since I'm low with helicopters maybe I need to try lower with the planks
Helicopter blades stay at relatively low angles of attack in their flow, so over the range of deflection the force generated is still fairly linear with deflection. Therefore the pitch/roll rate of the heli will be close to linearly proportional to the transmitter stick position. This lends itself well to low expo, since it behaves how you think it should. Airplane surfaces are not full moving like a heli blade, only part of the surface deflects, so you aren't changing the angle of attack of the surface, you are effectively changing the camber of the airfoil on that surface. This tends to result in pitch/roll responses that are not that close to linearly proportional to stick position, so some level of expo helps maintain that "linear" response feeling. Also with the massive deflections on most 3-d planes, the response is very non-linear since you will end up stalling the surface at some deflection, totally changing the slope of the response beyond that deflection.
I run low expo on helis (0-30%) and moderate on 3-d planes (30-50%). That is what "feels" the best to me personally. There is no right answer, though, you just have to fly what feels comfortable to you. I was surprised to learn that several of the IMAC unlimited pilots at the contest I went to use 50-60% expo on their low rates, it's all a personal preference and there is no right answer.