แรงยก (Lift)

คืออะไร Lift?

Lift is the aerodynamic force that acts perpendicular to the relative airflow and supports an aircraft in flight. It is the fundamental force that opposes gravity and allows heavier-than-air machines to become airborne. Without sufficient lift, no aircraft — from a Cessna 172 to an Airbus A380 — can remain aloft.

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Lift is generated primarily by the wings as air flows over and under them. Two complementary principles explain its generation:

• Bernoulli's Principle: The wing's cambered (curved) upper surface forces air to travel a longer path than the flatter lower surface. This accelerates the upper airflow, reducing pressure above the wing. The higher pressure beneath the wing pushes upward, producing lift.
• Newton's Third Law: The wing deflects incoming air downward. The reaction force pushes the wing — and the aircraft — upward.

Lift (L) is calculated as: L = ½ × ρ × V² × S × CL, where ρ is air density, V is airspeed, S is wing area, and CL is the coefficient of lift. This formula shows that doubling airspeed quadruples lift — a critical relationship for pilots during takeoff and approach.

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Lift determines an aircraft's maximum takeoff weight, cruise altitude, and approach speed. Wing design — including aspect ratio, camber, and sweep — is optimized to maximize lift while minimizing drag. High-lift devices such as flaps and slats increase the wing's CL during low-speed flight, allowing aircraft to operate at manageable V-speeds on shorter runways.

The angle of attack directly controls lift up to the critical AoA, beyond which a stall occurs. Pilots must manage this relationship constantly, especially during takeoff rotation and final approach.

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The Boeing 747-400 generates approximately 900,000 lbf of lift at maximum takeoff weight — enough to keep 412 tonnes airborne across the Pacific. The Airbus A380's 845 m² wing area is specifically designed to generate adequate lift at relatively low approach speeds, reducing stress on airframe and runway. In contrast, the Concorde used a delta wing that generated lift through vortex flow rather than classical Bernoulli effects, allowing it to cruise efficiently at Mach 2.