II.

Lift

One of the fundamental forces studied in aerodynamics is lift, or the force that keeps an airplane in the air. Airplanes fly because they push air down. The leading edge of an airplane wing is slightly higher than the trailing edge when the plane is maintaining altitude. As the wing moves through the air, it deflects the air that flows underneath it downward. Air flowing over the top of the wing follows the surface of the wing and is also deflected downward. The third law of motion formulated by English physicist Sir Isaac Newton states that every action causes an equal and opposite reaction (see Mechanics: The Third Law). As the wing pushes the air down, the air pushes the wing up. Lift is also often explained using Bernoulli’s principle, which relates an increase in the velocity of a flow of fluid (such as air) to a decrease in pressure and vice versa. The pressure on the upper side of an airplane wing is lower than that on the lower side, and many engineers use equations derived from Bernoulli’s principle to design aircraft.

Another important aspect of aerodynamics is the drag, or resistance, acting on solid bodies moving through air. The drag forces exerted by the air flowing over the airplane, for example, must be overcome by the thrust force developed by either the jet engine or the propellers (see Propeller). These drag forces can be significantly reduced by streamlining the body. For bodies that are not fully streamlined, the drag force increases approximately with the square of the speed as they move rapidly through the air. The power required, for example, to drive an automobile steadily at medium or high speeds is primarily absorbed in overcoming air resistance.