The physics and concepts always apply, regardless of scale. I'm more amused by the people that say they don't. You do however need to know how certain things are affected by scale. Weight and aerodynamics are good examples. A molecule of air is always the same size. A molecule of air is larger in proportion to a smaller object than it is to a larger object meaning a full sized airplane wing will have far more molecules of air travelling across of it as a model a fraction of it's size will. You just have to be aware of this. An rc car travelling at 50 mph is not the same from an air resistance standpoint as a full sized vehicle travelling at 500 mph. 50 mph is 50 mph. The key is to understand that you've got something a foot and a half long pushing through the wind at 50 mph as opposed to something 15 feet long pushing through the air at 50 mph. The concepts still apply but some of the variables have changed. Failing to understand that doesn't mean that they don't apply. Incidentally if you want to get a scale representation of weight, it isn't a linear relationship. Not even close.