From an engineering viewpoint why isn't it feasible or practical to build a variable pitch design? helos have swash plates, could not an affair be built? why not for the large engines and multi blade props?
Now I may not be much of a flyer if at all but I am a fabricator and a machinist, if I had a 1/4 or 1/3 scale warbird like a Corsair, P-51 it would be interesting to have first a prop reduction unit and then a variable pitch control say either an independant servo or on engine speed.