Turn the mix on with the gear switch. I do the same thing with my jets. Assign the mix to the gear switch (E) and chose the On position to be the same as when the gear is down. By default the switch is "Null"

One other trick, I assign the rudder trim (T4) to the channel that the nose steering is using. I "Null" the trim for the rudder channel. When was the last time you needed rudder trim in a jet?