Figure 1 Vectorial representation (not to scale) of thrust generation and aeroelastic response at low running speeds (bold symbols represent vectors). While running at the speed V_run (left), Archaeopteryx flaps its wings downward with a velocity V_flap and encounters the upcoming flow V at a steep angle with the horizon (wing path angle ). During the downstroke, the airfoil aligns itself to the upcoming airflow by rotating aeroelastically, trailing edge up, owing to the pressure build-up between the wing and the ground (elliptical dotted zone).

This aeroelastic response is possible because the feathered wing of Archaeopteryx is attached to the body only at the shoulder. This alignment is also expected as the result of wrist pronation along its semilunate carpal^9,18. Thus, the wing meets the incoming airflow V at an angle a and generates R, an aerodynamic force perpendicular to V. The horizontal and vertical projections of R are thrust T and lift L, respectively. As the running speed increases (right), the aerodynamic resultant R increases its magnitude and rotates counterclockwise. This rotating is due not to an induced drag increase but to an increase in the thrust and lift components. All forces shown are aerodynamic forces, not net forces on the bird.