Four Fundamental Aerodynamic Forces
The age of modern aviation can be said to have begun on December 17, 1903 at Kitty Hawk, North Carolina. On that date, Wilbur and Orville Wright, two bicycle makers from Dayton, Ohio, flew the world's first powered aircraft, a biplane (double-winged aircraft) with a wing span of 40 ft 4 in (12 m 1.5 cm) and weighing 605 lb (275 kg). The plane remained in the air a total of only 12 seconds and covered only 120 ft (37 m). But both brothers knew, as Wilbur then said, that "the age of flight had come at last." The problems that the Wright brothers had to solve in the early 1900s were essentially the same as those confronting aeronautical engineers today. In order to make a heavier-than-air machine fly, four factors have to be taken into consideration: weight, lift, drag, and thrust.
Weight is, of course, caused by the pull of the Earth's gravitational field on the airplane itself, its passengers, and its cargo. The plane will never leave the ground unless some method is found for counteracting the gravitational effect of weight. The way in which weight is counterbalanced is by means of lift, a force equal to and opposite in direction to the pull of gravity. Lift is provided by means of the flow of air over the airplane's wings.
Imagine that a strong wind blows over the wings of an airplane parked on a landing strip. Airplane wings are always designed so that the flow of air across the top surface and across the bottom surface of the wing is not identical. For example, the upper surface of the wing might have a slightly curved shape, like half of a tear drop. The lower surface of the same wing might then have a perfectly flat shape.
When air passes over a wing of this design, it moves more quickly over the top surface than it does the bottom surface. The effect produced was first observed by Swiss mathematician Daniel Bernoulli in the 1730s and is now known by his name, the Bernoulli effect. Bernoulli discovered that the faster a fluid moves over a surface, the less pressure it exerts on that surface.
In the case of an airplane wing, air moving over the top of the wing travels faster and exerts less pressure than air moving over the bottom of the wing. Since the pressure under the wing is greater than the pressure on top of the wing, air tends to push upward on the wing, raising the airplane off the ground.
A number of factors determine the amount of lift a wing can provide an airplane. One factor is the size of the wing. The larger the wing, the greater the total force exerted on the bottom of the wing compared to the reduced pressure on top of the wing. A second factor is speed. The faster that air moves over a wing, the less pressure it exerts and the greater the lifting force it provides to the airplane. A third factor is the relative orientation of the wing to the air flow coming toward it. This factor is known as the angle of attack. In general, the greater the angle of attack, the greater the lifting force provided by the wing.
A pilot has control over all three of these factors in an airplane. Speed is controlled by increasing or decreasing the rate at which the engine turns, thereby changing the speed at which the propeller turns. Wing size is also under the pilot's control because of flaps that are attached to the following edge of a wing. These flaps can be extended during takeoff and landing to increase the total wing area and then retracted during level flights to reduce drag. Wing flaps and flap-like sections on the tail—the elevators—change the angle at which the wing or tail meets oncoming air and, thus, the airplane's angle of attack.