How Airplanes Fly: The Basics of Aerodynamics

The Four Forces of Flight

Every aircraft in flight is acted upon by four fundamental forces: lift, weight, thrust, and drag. These forces define what the aircraft can do and how efficiently it does it. For straight and level flight at constant speed, these forces must be in balance — lift exactly equals weight, and thrust exactly equals drag. Any imbalance creates acceleration: more thrust than drag accelerates the aircraft forward; more lift than weight causes the aircraft to climb. Understanding how pilots and aircraft systems manipulate these four forces is the foundation of understanding flight.

Weight is the simplest of the four forces — it is the gravitational pull on the aircraft's total mass, acting downward through the center of gravity. A fully loaded Boeing 777-300ER has a maximum takeoff weight of 352,000 kilograms (775,000 pounds). Every kilogram of passengers, cargo, fuel, and aircraft structure must be lifted off the ground and held aloft against gravity for the duration of the flight. As fuel burns, the aircraft becomes lighter, which is why long-haul aircraft can cruise efficiently for 14+ hours even though they initially need enormous thrust to get airborne.

Thrust is generated by the aircraft's engines — turbofan jet engines on virtually all commercial airliners. A modern high-bypass turbofan like the GE90-115B (which powers the Boeing 777) produces up to 115,000 pounds (about 512,000 Newtons) of thrust per engine, with two engines giving the aircraft 230,000 pounds of thrust total at maximum power. For most cruise flight, the engines operate at 70–85% of maximum thrust, balancing fuel efficiency against the drag of flight at cruising altitude and speed.

How Wings Generate Lift

Lift is the upward force that counteracts weight, and it is generated primarily by the wings. The explanation most people learned in school — that air travels faster over the curved top of the wing due to the longer path, creating lower pressure above the wing — is partially correct but incomplete. A more accurate understanding involves the wing's angle of attack (the angle between the wing chord line and the incoming airflow) and the net downward deflection of air as it passes over the wing.

As a wing moves through air, it deflects air downward. By Newton's third law, deflecting air downward means the wing receives an equal and opposite upward force — lift. The Bernoulli effect (lower pressure above the curved upper surface) contributes to this, but the dominant mechanism at typical flight speeds is the circulation of air around the wing cross-section (the airfoil). Lift is proportional to the square of airspeed, the air density, the wing area, and a coefficient of lift that depends on wing shape and angle of attack.

Wing design is an engineering art with enormous implications for performance. The aspect ratio (wingspan squared divided by wing area) affects efficiency: a long, narrow wing like a glider's generates lift more efficiently than a short, stubby wing. Modern long-haul jets use high-aspect-ratio wings and winglets — the upturned tips visible on aircraft like the Boeing 737 MAX and Airbus A320neo. Winglets reduce wingtip vortices (swirling air at the wing tips that represents wasted energy), improving fuel efficiency by 3–5%. The Boeing 787's raked wingtips serve the same purpose through a different geometry, contributing to the 787's approximately 25% better fuel efficiency per passenger compared to the aircraft it replaced.

Control Surfaces: Steering Through the Air

An aircraft is controlled in three axes of rotation: pitch (nose up/down), roll (banking left/right), and yaw (nose left/right). Each axis is controlled by dedicated aerodynamic surfaces. The elevator (on the tail's horizontal stabilizer) controls pitch. Ailerons (hinged surfaces on the outer wing trailing edges) control roll. The rudder (on the vertical tail fin) controls yaw. Modern fly-by-wire aircraft like the Airbus A320, Boeing 787, and all Airbus aircraft since the A320 add computers between the pilot's inputs and the control surfaces, providing envelope protection and reducing pilot workload.

High-lift devices dramatically increase the wing's lift coefficient at low speeds, enabling aircraft to land and take off at manageable speeds. Flaps (hinged sections on the inner wing trailing edge) extend and droop to increase wing area and curvature, boosting lift by up to 70–80% at full extension. Slats (movable leading-edge sections) open a gap at the wing's front edge, allowing high-energy air from below the wing to re-energize the boundary layer above, delaying stall to higher angles of attack. Without flaps and slats, a Boeing 747 would need to land at around 250 knots (465 km/h) rather than the typical 145 knots (270 km/h) — making normal airports impossible to use.

Spoilers (panels that pop up from the upper wing surface on landing) serve two critical functions. In flight, they can be deployed asymmetrically to assist with roll control, particularly at high speeds where aileron effectiveness diminishes. After touchdown, all spoilers deploy simultaneously (called ground spoilers) to "dump" lift — rapidly reducing the wing's lifting force to firmly plant the aircraft on the runway and maximize the effectiveness of the brakes. Without ground spoilers, an aircraft's wings continue generating significant lift even after landing, reducing tire-to-runway friction.

Drag: The Enemy of Efficiency

Drag is the aerodynamic resistance the aircraft experiences as it pushes through the air. It comes in two primary forms. Parasite drag (also called form drag and friction drag) increases with the square of airspeed — double the speed, quadruple the drag. It includes pressure drag from the aircraft's frontal area pushing through the air and skin friction drag from air flowing over every surface. Induced drag arises as a byproduct of generating lift: the wing tip vortices represent energy wasted in rotating air rather than propelling the aircraft, and induced drag is greatest at low speeds and high angles of attack.

The total drag curve reaches a minimum at a specific speed that represents the aircraft's most aerodynamically efficient operating point. Commercial jets are designed so that this minimum-drag speed roughly corresponds to economical cruise conditions. The lift-to-drag ratio at this optimal point describes aerodynamic efficiency: modern airliners achieve L/D ratios of 17–20:1, meaning a Boeing 787 generates 17–20 pounds of lift for every pound of drag. A well-designed sailplane might achieve 50:1 or higher, but they trade off speed for that efficiency.

Cruise altitude selection involves balancing engine efficiency against aerodynamic drag. At higher altitudes, the air is thinner — lower air density means less drag, allowing the aircraft to fly faster for the same thrust. However, thinner air also means less lift per unit of wing area, forcing a higher true airspeed to generate sufficient lift. Jet engines are most efficient (lowest specific fuel consumption) at the altitudes between about 30,000 and 42,000 feet where the atmospheric conditions match their thermodynamic design points. The sweet spot — high enough for thin air and efficient engines, not so high that the aircraft struggles to generate adequate lift — is typically 33,000–41,000 feet for commercial jets.

Takeoff and Landing: Flight at the Extremes

Takeoff is the most power-intensive phase of flight. During the takeoff roll, the aircraft accelerates from zero to rotation speed (Vr) — typically 130–165 knots for large jets — while the engines produce maximum (or occasionally derated) thrust. At Vr, the pilot rotates the nose up to a pitch attitude of 10–15 degrees, increasing the angle of attack sufficiently for the wings to generate enough lift to become airborne. The aircraft then accelerates to V2 (takeoff safety speed) as it climbs, ensuring it can continue safely even if an engine fails.

Landing reverses most of the process. The aircraft configures for approach — flaps extended, slats deployed, landing gear down — progressively slowing while descending on a 3-degree glide path toward the runway threshold. At threshold crossing height (typically 50 feet), the aircraft should be at reference approach speed (Vref), usually about 1.3 times the stall speed at the landing configuration. Over the threshold, the pilot reduces thrust and flares — gently raises the nose — allowing the aircraft to slow and sink onto the runway. The ideal touchdown occurs at a rate of descent of about 100–200 feet per minute, gently enough for passenger comfort but firmly enough to break the ground effect.

Ground effect is an aerodynamic phenomenon that makes landing more challenging than it might appear. As the wing descends within one wingspan of the ground, the vortices and downwash pattern changes, causing the wing to generate more lift and less induced drag than at altitude. This temporarily "cushions" the aircraft on final approach, causing it to float beyond the intended touchdown point if the pilot does not correctly anticipate it. Experienced pilots learn to reduce power slightly earlier and allow the aircraft to settle through ground effect rather than fighting it. On particularly long, flat runways this is not critical, but on shorter runways or in gusty conditions, managing the flare correctly becomes essential.