All radio waves travel the same whether AM, FM, PCM or ????. If your fuselage were made of carbon, was covered in metal or some other material that blocks RF, you would have a problem with the antenna inside the fuselage. However, most models consist of wood, non conductive covering, nonmetalic paint etc. so are not a problem or an obstruction to radio waves. You will be fine in all but the most specific case by putting your antenna inside the fuselage. The one place you could get into trouble is if you have metal push rods or pull-pull lines running parallel and close to the antenna.