Birds use it. Bees use it. Boeing 747s use it. First devised by Daniel Bernoulli in 1732, the Bernoulli Principle would not be applied to flight for centuries, though it would become integral to understanding the force of lift and how things fly. Bernoulli, a Swiss mathematician, originally developed his principle in the field of fluid dynamics. He theorized that as fluid moves faster and its velocity increases, the pressure exerted by the fluid decreases.
Bernoulli’s Principle hinges on the conservation of energy principle, which states that energy can be neither created nor destroyed. Considering the kinetic energy and pressure energy of a fluid, Bernoulli theorized that as the fluid’s velocity increased (thus increasing its kinetic energy), the pressure energy of the fluid would have to decrease to keep the total energy of the fluid in balance.
Nearly two centuries after Bernoulli’s development of this principle and its application in fluid dynamics, it was applied directly to the concept of flight and the flow of air over a wing. In viewing air as a fluid, Bernoulli’s Principle asserts that faster moving air exerts lower pressure than slow moving air. The lower pressure energy of fast moving air means it has a lower gravitational pressure. Slow moving air feels gravity more acutely. Applied to flight, this concept came to be known as lift. Winged animals and aircraft stay aloft when the force of lift is greater than the gravitational pull to bring the wing back to earth.
Birds, bees, and Boeing aircraft are at the mercy of lift to stay aloft. When a wing is propelled forward, the air molecules around the wing are forced to move: some move above the wing and others move below. In a traditional aircraft wing model, the top of the wing is curved, and thus takes up more area than the bottom of the wing, which is flat. Because molecules want to return to their initial position, they must travel faster over the top of the wing and slower beneath it. According to Bernoulli’s Principle, molecules moving faster exert less gravitational pressure, while molecules moving more slowly exert more gravitational pressure. The greater pressure exerted beneath the wing allows it to stay aloft.
Since humans began flying, experiments have attempted to better utilize lift through different wing shapes and positioning. Helicopters, hang gliders, sail planes, fighter jets: all attempt to more efficiently and easily utilize lift to accomplish their goals as aircraft. And while there are actually four forces at work on an animal or machine in flight (lift, gravity, thrust, and drag), lift is the most essential: it keeps the aircraft airborne.
Daniel Bernoulli never saw humans in flight, and the Principle of Lift is attributed to him despite his death more than a century before it was illuminated. Nevertheless, his principle is integral to the understanding of fundamental aerodynamics and is the primary reason humans can take to the skies today.
