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People have always wanted to fly. Leonardo Da Vinci, who often dreamed about flight, drew intricate designs for several different flying machines. Among the many scientists whose discoveries made human flight a reality are Sir Isaac Newton (1642-1727) and Daniel Bernoulli (1700-1782). Both are credited with developing the scientific principles that explain how an airplane flies. However, it was not until many years later, on December 17, 1903, that Orville and Wilbur Wright actually applied these principles to successful airplane flight. Their first flight lasted only 12 seconds, and they traveled only 120 feet. The entire trip could have taken place inside of a 747. Whether a single-seat propeller plane or a giant 747, there are four fundamental forces at work in airplanes. Thrust is the forward movement of the airplane, provided by the propellers or jet engines. Drag is the opposing force of the displaced air, the same force you feel walking into a strong wind. Gravity acts on the airplane, pulling it down toward the earth, and lift counteracts the force of gravity, keeping the airplane up in the air. The lift force is equal to the force of gravity when an airplane is in level flight. Bernoulli's principle helps to explain lift. Bernoulli demonstrated that the faster a fluid moves, the less pressure it exerts. An airplane wing is shaped so that air (which is a fluid) moves faster over the top of the wing than it moves under the bottom of the wing. As a result, there is greater pressure underneath the wing than there is on the top of the wing, resulting in a net force which "lifts" the wing. The angle at which the wing meets the air (the angle of attack) also contributes to lift. You can easily demonstrate this when you hold your hand out the window of a moving vehicle and change the angle at which your palm meets the oncoming air. If the surface of an airplane wing meets the air at an angle, the wing exerts a force on the air and the air exerts an equal force on the wing--an effect which Newton described in his third law of motion. There are many other factors that influence lift: the shape and area of the wing, the velocity of the airplane, and even the density of the air. Pilots and aeronautical engineers use the forces involved in lift to help them design and fly planes of all sizes and shapes. With the right lift, even a 870,000-pound jumbo jet can head for the sky.