Aerodynamics 101: The Physics of Flight

Pilots spend years mastering the science of flight, but as a passenger, you may be looking for a simple explanation. Here's what you need to know in five minutes.

An airplane in flight encounters four forces: lift, weight, thrust, and drag. These forces interact continuously, and stable flight results when they balance each other. This balancing act results from precise and constant manual or autopilot adjustments to the main controls of the airplane: the engine, that produces propulsion; the ailerons hinged to the wings that provide roll; the rudder hinged to the vertical stabilizer that adjusts yaw; and the elevators hinged to the horizontal stabilizer to make the airplane pitch up and down. 

Let's look at these forces in detail, starting with lift, the upward force on an airplane opposite to and countering the force of weight.

Lift

Lift is generated by air flow over the wings. Air flows faster over the curved upper wing surface creating a lower pressure, while the slower air flow below the wing creates higher pressure—producing lift. The extent of lift depends on a wing's length (span), sweep and curvature (camber).   

Lift in helicopters is achieved by the main rotor blades that essentially serve as rotating wings. In addition to rotating, the main rotor blades can change angles that provide lift and, in conjunction with the tail rotor, give the helicopter its unique ability to hover.

Weight 

Weight is the downward force on an airplane caused by the pull of gravity. It acts opposite lift. If this force is stronger than lift, the airplane will descend. And vice versa. Weight is always changing as fuel is consumed. Accordingly, the force of lift must be continually adjusted by the pilot's controls to counter this changing weight and sustain level flight. 

Thrust 

Thrust is the forward force produced by turbine engines, which expel air rearward to push the aircraft forward; or by propellers, which pull it forward. Thrust moves the airplane through the oncoming air overcoming drag to maintain or increase speed. The level of thrust required to create enough air flow over the wing to establish enough acceleration on takeoff and to climb to cruising altitude depends on the amount (pounds) of thrust produced, the aerodynamic efficiency of the wing and the effects of environmental factors, such as outside temperature, altitude, relative humidity, and wind velocity. 

These design and environmental elements determine how long a runway is required for take off. Because they need to build up to takeoff speed from a standing start, aircraft typically require more runway length to depart than to land.  

Drag 

Drag is the air resistance that opposes thrust. It's created primarily from airframe friction. Generally,  the more streamlined the aircraft, the less the drag. To make the airplane descend pilots reduce engine power so drag (and weight) overcome thrust and lift. 

To fly at a slower airspeeds and still overcome the forces of drag and weight, as on an approach to landing, the pilot lowers the flaps increasing the lift capability of the wing. In this configuration the airplane can slow to its landing speed yet have enough reserve thrust that if the landing suddenly must be aborted there is sufficient power to start an immediate climb for a go-around or flight to an alternate airport where a safe arrival is assured.