For a helicopter flies as does an airplane, by the movement of its wings through the air. These wings, of course, are whirling rotor blades. Here lies the similarity and, paradoxically, the difference between the two types of aircraft. In the most basic terms, a wing is a fixed surface that will produce a lifting force when it is moved through the air with enough speed or, as an engineer would put it, with sufficient velocity. To appreciate why a wing has this property, we need to consider the substance called air.
Air is a fluid that behaves just as do wet fluids such as water or oil.
If you move through a fluid slowly it will resist but give way as the water in a swimming pool clings and then flows around you as you walk or swim through it. And consequently, this property has resulted in the art of streamlining, the creation of shapes that offer the least resistance so that movement through fluids can be accomplished with the least expenditure of power. The means for accomplishing this was known for hundreds, even thousands of years before the first aircraft were flown. In more precise aeronautical terminology, the front or leading edge of the lifting surface had to be raised higher, relative to the air flowing against it, than the rear edge. In the same way, a lifting force results as air is pushed down by the slanting under-surface of a wing or rotor blade.