An airplane wing makes lift by changing the motion and pressure of the air around it. Its airfoil shape has a curved upper surface and a flatter lower surface, which helps air move differently above and below the wing. Lift matters because it is the upward force that supports an aircraft in flight.
Understanding lift connects ideas from pressure, motion, and Newton's laws in one real-world system.
As air flows over a wing, the streamlines above the curved top usually speed up, creating lower pressure there compared with the air below the wing. At the same time, the wing turns some of the airflow downward, called downwash, and the air pushes back upward on the wing. These two explanations work together rather than competing with each other.
Pilots can change lift by changing speed, wing area, air density, or angle of attack.
Key Facts
- Lift is the upward aerodynamic force produced by a wing moving through air.
- Lift equation: L = 1/2 rho v^2 A CL
- Faster airflow over the top of an airfoil usually means lower pressure above the wing.
- Newton's third law explains lift as the wing pushes air downward and the air pushes the wing upward.
- Angle of attack is the angle between the chord line and the oncoming airflow.
- Too large an angle of attack can cause a stall because airflow separates from the upper surface.
Vocabulary
- Airfoil
- An airfoil is a shaped surface, such as a wing cross-section, designed to produce lift as air flows around it.
- Chord line
- The chord line is the straight line from the leading edge to the trailing edge of an airfoil.
- Angle of attack
- Angle of attack is the angle between the chord line of the wing and the direction of the incoming air.
- Downwash
- Downwash is the downward deflection of air behind a wing that helps create an upward reaction force.
- Stall
- A stall occurs when airflow separates from the wing enough that lift drops sharply.
Common Mistakes to Avoid
- Thinking lift happens only because the top path is longer, which is wrong because air above and below the wing does not have to meet at the trailing edge at the same time.
- Ignoring Newton's third law, which is wrong because a wing gains upward lift partly by pushing air downward.
- Confusing lift with thrust, which is wrong because lift acts mostly upward while thrust pushes the aircraft forward.
- Assuming a bigger angle of attack always gives more lift, which is wrong because too large an angle can cause flow separation and a stall.
Practice Questions
- 1 A small wing has air density rho = 1.2 kg/m^3, speed v = 30 m/s, area A = 2.0 m^2, and lift coefficient CL = 0.80. Use L = 1/2 rho v^2 A CL to find the lift.
- 2 If an airplane doubles its speed while rho, A, and CL stay the same, by what factor does the lift change?
- 3 Explain how the pressure-difference view and the Newton downwash view can both describe the same lift-producing wing.