How Do Aircraft Fly?

Every day millions of people board aircraft headed for destinations around the world. It's puzzling how an aircraft made of metal, much heavier than air, can safely and effortlessly lift hundreds of passengers miles into the sky and propel them at speeds over 500 knots to points thousands of miles away. In this article we will briefly examine the laws of nature that allow this to be accomplished.

Every student pilot is first taught about the four forces acting upon an aircraft in flight: weight, lift, drag and thrust. The downward force of weight must be overcome by an upward acting aerodynamic force called lift.

The wing lifts the aircraft by producing an air pressure above the wing which is lower than that below the wing. This lower pressure pulls or lifts the aircraft upward. In the 1700s a Swiss mathematician named Daniel Bernoulli discovered the principle of a venturi, or that as the velocity of a fluid increases, the pressure it exerts on its surroundings decreases. The basis for this is in the conservation of energy of an airstream. The potential energy of a gas is proportional to its pressure, just like a compressed spring. As the speed of the air is increased the kinetic energy increases, resulting in a potential energy or pressure decrease to conserve the total energy.

A wing has a curved upper surface referred to as camber. Air molecules traveling over the top of the wing are forced to speed up by this camber in the same way air molecules in a venturi are sped up. The increased airstream speed over the top of the wing decreases the air pressure and produces the lift necessary to overcome weight.

In order for the airstream to be pushed over the wing, the aircraft must be pushed through the atmosphere. On the takeoff roll, the aircraft accelerates until reaching a speed with sufficient airflow over the wings to produce the required lift.

Hopefully this cursory explanation has answered some basic questions about how an aircraft flies. But don't let us pilots keep all the fun to ourselves -- take a flying lesson!

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