Airfoil Lift: Bernoulli's Principle Over a Wing
1 · Predict
Air flows faster over the curved top of an airfoil than underneath it. According to Bernoulli's principle, which surface has lower pressure — and what does that mean for the wing?
- The top surface has lower pressure, creating a net upward force (lift).
- The bottom surface has lower pressure, pushing the wing down.
- Both surfaces have equal pressure, so there's no net force.
2 · Set Up
- Open the fluid-wing preset and press Reset. Air flows over an airfoil cross-section; the top-surface path is narrower than the bottom, so air speeds up over the top.
- Enable the top-surface-speed and pressure-difference readouts.
- Set the bottom and top cross-sectional areas and the inlet air speed for each trial, and record the top speed and pressure-difference readings.
3 · Collect Data
| Bottom-path area A₁ (m²) | Top-path area A₂ (m²) | Inlet air speed v₁ (m/s) | Top-surface speed v₂ (m/s) | Pressure difference ΔP (bottom − top) (kPa) |
|---|---|---|---|---|
| 0.0006 | 0.0003 | 22 | ||
| 0.0006 | 0.0003 | 30 | ||
| 0.0006 | 0.0004 | 25 |
Plot the pressure difference ΔP (y-axis) against the top-surface speed v₂ (x-axis) for your three trials.
4 · Analyze
- For one trial, compute v₂ = A₁v₁/A₂ from continuity, then ΔP = ½ρ_air(v₂² − v₁²) using air density 1.225 kg/m³. Compare both to the readings.
- The pressure difference here is much smaller in Pa than in the fluid-venturi experiment for similar speeds. Explain why, using the density value in Bernoulli's equation.
5 · Extend
- This model (continuity + Bernoulli) explains part of how wings generate lift, but real aerodynamic lift also depends heavily on the wing's angle of attack, and does not require air parcels above and below the wing to arrive at the trailing edge together. Why might a simple 'equal transit time' story about lift be misleading?
- Air density drops at high altitude. Using ΔP = ½ρ_air(v₂² − v₁²), explain why a wing needs to move faster through thin high-altitude air to generate the same lift it would at sea level.
The Physics Behind This Experiment
Continuity Equation
For an incompressible fluid, the flow rate (area × speed) is conserved along a streamline: A₁v₁ = A₂v₂. The narrower path over the top of the airfoil forces the air there to move faster.
Bernoulli's Equation (air)
Along a horizontal streamline, P + ½ρv² stays constant. Air moving faster over the top of the wing has lower pressure there than the slower air underneath — the pressure difference that contributes to lift.