For thin airfoils, what is the typical lift-curve slope value?

For thin airfoils, what is the typical lift-curve slope value?

• A. Approximately 0
• B. Approximately π per rad
• C. Approximately 4π per rad
• D. Approximately 2π per rad

Answer: (D)

In thin airfoil theory, the lift coefficient for a slender airfoil in incompressible, inviscid flow increases nearly linearly with angle of attack for small angles. The classic result is CL ≈ 2π α, where α is in radians. This means the lift-curve slope—the change in lift per unit change in angle of attack—is approximately 2π per rad (about 6.28 per rad). The other values don’t match this fundamental relationship: π per rad is too small, 4π per rad is too large, and zero would imply no lift change with angle. So the typical lift-curve slope is approximately 2π per rad.

In thin airfoil theory, the lift coefficient for a slender airfoil in incompressible, inviscid flow increases nearly linearly with angle of attack for small angles. The classic result is CL ≈ 2π α, where α is in radians. This means the lift-curve slope—the change in lift per unit change in angle of attack—is approximately 2π per rad (about 6.28 per rad). The other values don’t match this fundamental relationship: π per rad is too small, 4π per rad is too large, and zero would imply no lift change with angle. So the typical lift-curve slope is approximately 2π per rad.