The principles of flight are the same for RC as it is for full scale aircraft. If you want to test the importance of the upper surface of a wing spray some wall texture (to simulate ice) on top of one of your models, let it dry and go fly it. My guess is it won't fly well due to airflow separation over the wing.

Another way to think of it is to take a simple flat bottom trainer wing. The bottom is flat and the top has a camber to it. The air passing below the wing is relatively unmodified whereas the air over the top is modified because it travels a greater distance causing higher velocity and lower pressure as a result. So clearly the wing is a lifting device, not a pushing up device.

Yet another example is is to blow over a sheet of paper while it lays flat on a table. As you blow over the sheet of paper you are creating a low pressure area and the paper will lift off the table. Nothing underneath it to push it up.