Flight

Flight is the motion of an object through an atmosphere or the vacuum of space. This movement occurs without any contact with a planetary surface. It is a fundamental concept in both biology and engineering. Scientists study flight to understand how animals move and how machines travel. This field is divided into several specialized areas of study. Aeronautics focuses on vehicles traveling through the atmosphere. Astronautics is the study of vehicles traveling through space. Ballistics is the study of how projectiles fly.

To understand how things stay in the air, we must look at the forces involved. For an aerostat, the primary force is buoyancy. An aerostat is a system that stays aloft using buoyancy to match the density of the surrounding air. These include free balloons, airships, and moored balloons. The main part of an aerostat is the envelope. This is a lightweight skin that holds a lifting gas. Because the gas is lighter than the air, the craft floats without needing lateral movement.

Most other flying objects are called aerodynes. An aerodyne is heavier than the air it displaces. To stay up, an aerodyne must generate aerodynamic lift. This requires lateral movement through the air mass. Lift is the component of aerodynamic force that acts perpendicular to the flow of air. To move forward, an aerodyne needs propulsive thrust. This thrust overcomes drag, which is the resistance caused by air friction. For steady flight, thrust, lift, drag, and weight must all be balanced.

Aerodynamic flight can be unpowered or powered. Gliding is a form of unpowered flight where an object moves without generating its own thrust. Some animals, like the flying squirrel, use gliding to move between trees. Other animals, like raptors or man-made sailplanes, use soaring to climb by exploiting rising air. Powered flight requires a constant source of propulsion to climb and maintain speed. Most birds and all modern aircraft rely on powered flight to function effectively.

In the natural world, flight has evolved in many different ways. Insects were the first group of animals to evolve flight. Their wings may have evolved from structures that once served as gills. Among vertebrates, birds, bats, and extinct pterosaurs all developed flight. Bats are unique as the only mammals capable of sustained level flight. Many other animals use specialized body parts to glide instead. Flying fish use enlarged fins to glide for as long as 45 seconds.

Human history is filled with attempts to master the sky. Early cultures used stones, spears, boomerangs, and kites. In the 19th century, George Cayley began studying flight scientifically. Otto Lilienthal later performed over 200 gliding flights to gain scientific understanding. The Wright brothers built upon this work to achieve the first controlled, manned, powered flights. The 20th century brought the era of spaceflight. Researchers like Konstantin Tsiolkovsky and Robert H. Goddard provided the necessary breakthroughs. Yuri Gagarin became the first person to complete an orbital spaceflight in 1961.

Modern technology allows for extreme speeds and distances. Supersonic flight occurs when a vehicle travels faster than the speed of sound. This process creates shock waves that result in a loud sonic boom. Hypersonic flight is even faster and involves very high speeds. At these speeds, the heat from air compression causes chemical changes in the air. This type of flight is common during the reentry of spacecraft. Spaceflight uses rockets to provide the thrust needed to overcome Earth's gravity.