Hi Bart,
I too have similar problem with coax prop setup.
On flat octo, Wkm, Graupner 11x5 were rock steady, but now what to start with on coax?
I have APC 14"x4,7 slow fly, to put on top. Question is what to use on lower motors?
13"x6,5 but they are not slow fly, they are thin electrics...