I don't have my slide rule but -20ºC is like -4ºF in real temperatures. You got a ways to go yet. The airfolis (including the prop) love cold air, as it is denser. You can richen the carb settings and get away with it. The batteries and Monokote, on the other hand, start to complain at -10º or so (-23ºC, aye?, in Canadian, aye?).

When the Monokote starts spontaneously splitting with a "BANG!" on landing, then it's time to pack it in and get some coco in fromt of the wood stove.