A friend and I were building a Y6 Arducopter(APM1) on a 'Scorpion frame'. I understand that each arm of the Y has a pair of counter-rotating props that are developing thrust in the same direction(downwards). My question : If the lower prop is in the downwash of the upper one, wont it function at a substantially reduced aerodynamic performance, given the much lowered pressure above its blades due to the effect of the upper props? In that case, would'nt a hexacopter exhibit better performance( lifting capability / lower power consumption for similar loads etc.) since all the props see relatively clean air(above them).
Apparently this is not the case as they found out with the contra rotating props in the later part of WW2, when they were used on some aircraft to soak up the power of bigger and bigger engines.
I was following a build of a large Fairey Gannet with a rear driven and front free wheeling prop, where it was found, much to every ones surprise, that better performance was achieved, probably due to the air flow onto the second prop being at a better angle of attack.