Most all RC transmitter antenna are linearly polarized so the optimum is with the receiving antenna in the same orientation as the transmitter antenna. However, since the plane moves over all 360 degrees in flight if you do any manuvering at all it is impossible to maintain the optimum orientation. If you have two antennae, then try to keep them at 90 degrees to one another for best possible reception at all attitudes. Also, the higher frequencies such as 2.4GHz can be easily shadowed (any object between the transmitter and receiver antennae such as a motor or battery) by conductive objects so, if using two antennae, keep them separated as far apart physically as possible as well as 90 degrees to each other for maximum reception.