A man-made airplane flies according to the same principles of physics as does a bird: it must overcome gravitational forces to achieve lift and flight. An airplane's wings work to generate the lift, and they accomplish this by curving the flow of air around them. Without wings, an airplane is a mere automobile.
Aircraft—and birds—are able to fly because they balance out four forces: lift, weight, drag and thrust. An airplane takes off into the air when the lift—the force pushing upwards on the lower surface of its wings—exceeds the plane’s weight due to the force of gravity. Lift is created by the airflow around the plane, especially around the wings. Drag is the force of air resistance against the motion of the plane. This force increases with increased aircraft speed but decreases if the airplane has a smooth, or aerodynamic, shape. The airplane’s engine and propulsion system, either jet or propeller, generates a thrust force to overcome the drag.
Newton and Bernoulli
Two European scientists explained the principles of aircraft flight. The English physicist Isaac Newton (1642–1727) enumerated three laws of motion that are applicable to all moving objects. The first is that objects remain at rest or in uniform motion unless they are compelled to change by an external force. The second states that a force directed at an object causes it to accelerate in the direction of that force. The third states that for every force, there exists an equal and opposite force. The Swiss mathematician Daniel Bernoulli (1700–1782) was a pioneer in developing a mathematical explanation for fluid dynamics, the mechanics of how liquids and gases flow. His major finding, known as the Bernoulli principle, states that as the speed of air flow increases, its pressure decreases.
Angle of Attack
Airplane wings are designed to tilt slightly from the horizontal, also known as the path of flight. This tilt angle is called the angle of attack and is the most important variable in generating lift. An airplane starts moving when the pilot applies thrust from the engine to make the airplane travel forward on the ground. The pilot rotates the aircraft upward by lifting its nose to increase the angle of attack and achieve takeoff. However, too large of an angle of attack will stall the airplane.
Lift is generated by air curving around an airplane’s wings. As air flow hits the leading edge of a wing, it splits into two, some flowing along the upper surface and some flowing along the surface below. The shape of a wing is slightly asymmetrical, with a larger surface area on the top side. The airflow sticks to the upper surface as it moves between the wing’s leading and trailing edges, curving and lowering the pressure according to the Bernoulli principle. As the airplane gathers speed, the lift increases according to Newton’s second law of motion. This in turn increases the air curvature on the upper surface, forcing more air down from the wing's trailing edge. As the plane moves through the air, the wing's underside that faces the air flow at the angle of attack also deflects some air flow downwards. This downward air flow generates an equal and opposite reaction in an upward flow of high-pressure air (Newton’s third law), increasing the lift and keeping the plane airborne.