Honestly? All of it!!! This is especially true about larger scale models, and by "larger scale" I mean planes with wingspan of about 70" or more.

Do you have to LEARN ALL of it? No. But all the concepts about load, center of gravity, torque, drag and so on are fully applicable to the model aviation. Knowledge of the basics of aerodynamics will sure help you in this hobby.

Good luck in your knowledge quest!