A large passenger jet can weigh more than 400 tons at takeoff, yet it rises gracefully into the sky and cruises for thousands of miles. Every day, tens of thousands of flights carry millions of people around the world. How can something so heavy stay in the air? The answer involves four forces, the shape and angle of the wings, and the way air behaves as it flows around objects. Even though flight is routine, the explanation is often oversimplified, and some popular versions are actually misleading. This explainer describes how airplanes fly, with a clear look at how wings really create lift.
The four forces of flight
Four forces act on an airplane in flight:
- Lift pushes the airplane upward. It is produced mainly by the wings.
- Weight pulls the airplane downward. It is caused by gravity acting on the mass of the airplane, its passengers, cargo and fuel.
- Thrust pushes the airplane forward. It is produced by engines, either jet engines or propellers.
- Drag resists the airplane's motion through the air. It is caused by air friction and pressure differences.
In steady, level flight at constant speed, lift equals weight and thrust equals drag. To climb, the airplane needs more lift than weight; to speed up, it needs more thrust than drag.
How wings create lift
The essence: turning air downward
A wing creates lift by deflecting air downward. According to Newton's third law of motion, for every action, there is an equal and opposite reaction. When a wing pushes a large mass of air downward, the air pushes the wing upward. You can feel this effect when you hold your hand out of a moving car's window and tilt it slightly upward: the air pushes your hand up.
Wings do this in two ways that work together:
- Angle of attack: Wings meet the oncoming air at a slight upward angle, called the angle of attack. This deflects air downward.
- Wing shape: Most wings are curved on top, a shape called an airfoil. The curved upper surface helps guide air smoothly downward behind the wing.
Pressure differences
The other way to describe lift is in terms of pressure. As air flows around a wing, the pressure above the wing becomes lower than the pressure below it. Air at higher pressure below pushes upward more strongly than air at lower pressure above pushes downward. The difference in pressure across the wing's large surface area produces lift.
This pressure difference and the downward deflection of air are two descriptions of the same physical phenomenon. The air speeds up as it flows over the curved upper surface, and according to Bernoulli's principle, faster-moving air has lower pressure. At the same time, the wing turns the flow downward.
The "equal transit time" myth
A popular explanation says that air flowing over the curved top of a wing must travel farther than air flowing underneath, so it speeds up to "meet" the air from below at the trailing edge, creating lower pressure. This explanation is incorrect. Experiments and simulations show that air over the top actually moves much faster than this theory suggests and reaches the trailing edge before the air from below. There is no rule that the two streams must meet. The myth also cannot explain how planes fly upside down or how flat wings, like those of paper airplanes, create lift.
The correct picture is that the wing's shape and angle of attack bend the airflow, creating a pressure difference and pushing air downward. Both Bernoulli's principle and Newton's laws are valid; they are two complementary ways of describing the same flow.
What affects the amount of lift
Lift depends on several factors:
- Air speed: Lift increases with the square of speed. Doubling the speed produces about four times the lift.
- Air density: Denser air produces more lift. That is why airplanes need longer runways at high-altitude airports or on hot days, when the air is less dense.
- Wing area: Larger wings produce more lift.
- Angle of attack: Increasing the angle increases lift, up to a point.
- Wing shape: The airfoil's curvature, called camber, affects lift.
Stalls
If the angle of attack becomes too large, typically around 15 to 20 degrees for many wings, the air can no longer follow the curved upper surface smoothly. It separates from the wing and becomes turbulent. Lift drops suddenly, and drag increases. This is called a stall. In aviation, a stall has nothing to do with the engine stopping; it is a loss of lift due to excessive angle of attack. Pilots are trained to recognize and recover from stalls by lowering the nose to reduce the angle of attack.
Flaps and slats
During takeoff and landing, airplanes fly slowly, so they need extra lift. Flaps at the back of the wings and slats at the front extend outward and downward, increasing the wing's curvature and area. This allows the airplane to fly safely at lower speeds. You can see flaps moving on the wings of passenger jets before landing.
Thrust: jet engines and propellers
Thrust also follows Newton's third law: an engine pushes air or exhaust gases backward, and the reaction pushes the airplane forward.
- Propellers are rotating wings that create lift in a forward direction, pulling or pushing the airplane through the air.
- Jet engines draw in air, compress it, mix it with fuel and ignite it. The hot, expanding gases rush out of the back of the engine at high speed, producing thrust. Most modern airliners use turbofan engines, in which a large fan at the front pushes a huge amount of air around the core of the engine. This bypass air produces most of the thrust and makes the engines quieter and more fuel efficient.
Drag
Drag is the force resisting motion. There are two main types:
- Parasitic drag comes from friction between the air and the airplane's surfaces and from the shape of the airplane pushing through the air. It increases with speed.
- Induced drag is a byproduct of creating lift. At the wingtips, high-pressure air from below the wing curls around to the low-pressure area above, forming swirling wingtip vortices. These vortices create drag. Many modern airplanes have upturned wingtips called winglets, which reduce these vortices and save fuel.
Airplanes are designed with smooth, streamlined shapes to reduce drag. At cruising altitude, around 35,000 feet (about 11 kilometers), the air is thinner, which reduces drag and improves fuel efficiency, while the engines can still produce enough thrust.
Steering an airplane
An airplane can rotate around three axes, and pilots control each using movable surfaces:
| Axis | Movement | Control surface |
|---|---|---|
| Pitch | nose up or down | elevators on the horizontal tail |
| Roll | wings tilt left or right | ailerons on the wings |
| Yaw | nose turns left or right | rudder on the vertical tail |
To turn, a pilot banks the airplane using the ailerons, tilting the lift force sideways, and coordinates the turn with the rudder. Many modern airliners use fly-by-wire systems, in which pilots' inputs are sent electronically to computers that move the control surfaces.
Turbulence
Turbulence is the bumpiness passengers sometimes feel. It is caused by irregular air movements, such as rising warm air, air flowing over mountains, jet streams or storms. Although it can be uncomfortable, airplanes are designed to withstand much stronger forces than they encounter in normal turbulence. The main risk is to unbuckled passengers and crew, which is why airlines recommend keeping seat belts fastened while seated.
Why flying is so safe
Commercial aviation is one of the safest forms of transport. Airplanes have redundant systems, so that if one component fails, another can take over. Most airliners can continue flying safely and land if one engine fails. Pilots train extensively in simulators, aircraft are inspected and maintained regularly, and air traffic control keeps airplanes safely separated.
Questions readers ask
What keeps an airplane in the air? Wings create lift by deflecting air downward and creating lower pressure above the wing than below. Lift balances the airplane's weight.
Is Bernoulli's principle wrong? No. Bernoulli's principle correctly relates air speed and pressure. What is wrong is the "equal transit time" explanation, which wrongly claims that air above and below the wing must meet at the trailing edge.
Can airplanes fly upside down? Yes, aerobatic airplanes can, by adjusting the angle of attack so that the wing still deflects air downward relative to the airplane's motion.
What is a stall? A sudden loss of lift when a wing's angle of attack is too large and airflow separates from the upper surface.
Why do airplanes fly so high? Thinner air at high altitude means less drag and better fuel efficiency, and flying above most weather makes the journey smoother.
In short
Airplanes fly by balancing four forces: lift, weight, thrust and drag. Wings create lift by turning air downward through their shape and angle of attack, creating lower pressure above the wing than below. The popular "equal transit time" explanation is a myth. Lift depends on speed, air density, wing area and angle of attack; too much angle causes a stall. Engines produce thrust by pushing air backward, streamlined shapes reduce drag, and control surfaces let pilots pitch, roll and yaw. It is physics that turns tons of metal into a graceful, safe way to cross the sky.