An airplane wing produces lift by interacting with the air around it. Two common explanations are the Bernoulli view, which focuses on pressure differences, and the Newton view, which focuses on changing the motion of air. These are not competing stories.
They are two valid descriptions of the same lift force acting upward on the wing.
In the Bernoulli view, faster airflow over part of a wing is associated with lower pressure, while slower or less accelerated airflow is associated with higher pressure. In the Newton view, the wing and its angle of attack turn air downward, and the air pushes the wing upward in response. A full explanation of lift uses both pressure fields and momentum changes.
Real wings also depend on shape, angle of attack, speed, air density, and avoiding stall.
Key Facts
- Lift is the upward aerodynamic force on a wing, usually written as L.
- Bernoulli relation along a streamline: P + 1/2 rho v^2 + rho g h = constant.
- Higher air speed in a flow region often corresponds to lower static pressure in that region.
- Newton view: a wing gives air downward momentum, and the air exerts an upward force on the wing.
- Newton's third law: F_wing on air = -F_air on wing.
- Lift equation: L = 1/2 rho v^2 A C_L, where rho is air density, v is airspeed, A is wing area, and C_L is lift coefficient.
Vocabulary
- Lift
- Lift is the aerodynamic force perpendicular to the incoming airflow that supports an aircraft in flight.
- Airfoil
- An airfoil is a wing-shaped surface designed to produce useful aerodynamic forces as air flows around it.
- Pressure difference
- A pressure difference is an imbalance in force per area between two regions, such as the lower and upper surfaces of a wing.
- Angle of attack
- Angle of attack is the angle between the wing's chord line and the direction of the oncoming air.
- Downwash
- Downwash is the downward motion of air behind a lifting wing caused by the wing turning the airflow.
Common Mistakes to Avoid
- Saying Bernoulli and Newton explanations are rivals is wrong because they describe the same force using pressure energy and momentum change.
- Assuming air must travel equal times over and under the wing is wrong because the equal transit time idea is not a law of fluid motion.
- Thinking a flat wing cannot produce lift is wrong because a flat plate at a positive angle of attack can turn air downward and create a pressure difference.
- Ignoring angle of attack is wrong because lift depends strongly on the wing's orientation to the incoming airflow and can drop suddenly during a stall.
Practice Questions
- 1 Using L = 1/2 rho v^2 A C_L, find the lift for rho = 1.2 kg/m^3, v = 50 m/s, A = 16 m^2, and C_L = 0.8.
- 2 A wing causes 30 kg/s of air to gain a downward speed of 12 m/s. Using F = mass flow rate times change in velocity, estimate the upward lift force on the wing.
- 3 A student says lift happens only because air over the top of the wing must meet air from the bottom at the trailing edge. Explain what is wrong with this statement using both the Bernoulli and Newton views.