If the aircraft speed is doubled while weight and wingspan remain constant, how does the vorticity strength change?

• A. Double
• B. Halved
• C. Quadrupled
• D. Unchanged

Answer: (B)

The vorticity strength (circulation) around a wing is tied to the lift the wing must produce at a given speed. For a finite wing, the lift L is approximately L ≈ ρ V Γ b, where ρ is air density, V is flight speed, Γ is the bound circulation (vorticity strength), and b is the wingspan. If weight is constant, L stays the same. With density and wingspan held constant, increasing speed means the product ρ V increases, so Γ must decrease to keep L constant. Since V is doubled, Γ must be halved. In other words, the wingtip vortices become weaker because the required circulation to generate the same lift is reduced at higher speeds.