The airplane is a flying machine that uses lift, drag, weight and thrust to fly. Its wings give it lift and its engines provide thrust. For a plane to fly, all four forces must be balanced: lift must balance its weight and thrust must exceed drag. Airplanes can range from a small, light kite to large commercial passenger jets. Some are powered by screw propellers, others by high-velocity jets.
Throughout history, many people have dreamed of flight and developed different ways to achieve it. The first recorded human-powered aircraft was built by Otto Lilienthal in 1896, and the Wright brothers achieved controlled, powered mechanical flight just a few years later. Airplanes have revolutionized the way we travel and do business around the world. They help create jobs, boost the economy and provide easier cultural access. They also create less pollution than most other means of transportation.
Heavier-than-air flight is possible only if there is sufficient lift to overcome gravity and sufficient thrust to counteract the drag of the airflow over the surface of the wings. Airplanes need to generate lift and have to be designed with a sufficient amount of structural strength to support the flight weight. They need to be stable in flight and the wings must be capable of providing stability against roll, pitch and yaw.
Most fixed-wing aircraft have a wingspan that is longer than the fuselage, with a curved airfoil section which is often symmetrical. This allows the wings to generate lift and add to the stability of the aircraft. The wing surface may be flexible or rigid, and is often reinforced by an airframe. Those that are primarily flexible use a fabric covering. Airplanes that are predominantly rigid typically have a frame of metal or composite material which gives them the necessary strength.
For the best possible aerodynamics, the wing should have an airfoil shape that is well adapted to the speed and type of flight. As the speed of an aircraft increases, a more efficient wing is required to reduce its drag. An elliptical wing, for example, has better aerodynamics than a rectangular one at subsonic speeds, but it is unsuitable for supersonic speeds.
The wing deflects the air downward as it moves forward, producing lift and contributing to the overall lifting force of the aircraft. But if the angle of attack of the wing becomes too great, the flow is broken over the upper surface and lift is lost while induced drag increases; this condition is called a stall. The wing’s aerodynamic efficiency is improved by altering the angle of attack, but this can only be done to a limited degree.
Bernoulli’s principle and Newton’s third law explain why the air flowing over the wing’s bottom surface turns downward and is accelerated by the wing’s curvature, with an area of higher pressure below and lower pressure above. But neither of these laws explains why the parcels of air moving over the top of the wing must follow its downward curvature, as well.
