The figure (not to scale) shows a control volume to estimate the forces on the airfoil with elliptic cross-section. Surfaces 2 and 3 are streamlines. Velocity profiles are measured at the upstream end (surface 1) and at the downstream end (surface 4) of the control volume. The drag coefficient for the airfoil is defined as C_d = D/(1/2 * ρ * U_∞² * c), where D is the drag force on the airfoil per unit span and ρ is the density of the air. The static pressure, p_∞, is constant over the entire surface of the control volume. Assuming the flow to be incompressible, two-dimensional, and steady, the C_d for the airfoil is _______ (rounded off to 3 decimal places).