Aerodynamics and the Forces of Flight

Aerodynamics is the study of how air flows around solid bodies and the forces that result. For an aircraft in steady flight, four forces act together: weight, the downward pull of gravity; lift, the upward force that supports the craft; thrust, the forward force produced by the engine or propeller; and drag, the resistance of the air opposing forward motion. When lift balances weight and thrust balances drag, the aircraft moves at constant speed and altitude; changing any of these forces alters the flight path.

Lift is generated mainly by the wings, whose cross-sectional shape is called an airfoil. As air flows over and under a wing, it is deflected downward, and by Newton's third law the wing experiences an equal upward reaction. The pressure difference between the faster-moving air over the curved upper surface and the slower air beneath, described in part by Bernoulli's principle, also contributes to the lift. The amount of lift depends on the wing's shape, its area, the air's density and speed, and the angle of attack at which it meets the oncoming flow.

Drag arises from several sources, including friction between the air and the surface of the body and the pressure differences created as the shape pushes air aside. Designers reduce drag by streamlining shapes so the airflow stays smooth and attached. Thrust to overcome this resistance comes from propellers, jet engines or rockets. Balancing these forces efficiently is central to the design of aircraft, and the same principles apply to vehicles, turbines and any object moving through air.

Frequently asked questions

What are the four forces acting on an aircraft?

They are lift, which holds it up; weight, which pulls it down; thrust, which drives it forward; and drag, which resists its motion.

How does a wing produce lift?

The airfoil deflects air downward and creates a pressure difference between its upper and lower surfaces, producing a net upward force that depends on shape, speed and angle of attack.

What causes drag?

Drag comes from friction between air and the body's surface and from pressure differences created as the body pushes through the air; streamlining reduces it.