Downwash & Induced Drag
The flow pattern at the tip of the finite wing generates a vortical flow field. The trailing vortices create a ‘downwash’ at the wing that rotates the freestream velocity downwards vectorially. These vortices make the resultant lift force vector rotate backward, which gives rise to a force that is in the direction of the flow. This is in addition to the ‘lift force’ (an upward force at right angles to the flow direction). This ‘drag force‘ is called the ‘induced drag’ as it is accompanied by the production of lift.
The above discussion regarding forces generated due to uniform motion of aerodynamic elements(airfoils/wings) does not necessarily apply to bird flights.
Birds generate both lift and thrust forces using the same aerodynamic surfaces, contrary to the mechanism of aerospace vehicles where the lift and thrust generation are decoupled.
Generation of the required lift forces by the flapping wing motion of the birds can be approximated as a linear combination of the up/down motion, with a twist of the wing surfaces at each extreme of oscillation and a simple constant angle of attack (Lighthill 1975).
During the up and down strokes, the wing surfaces experience a fixed inclination such that one component of force is always facing backward, which generates the thrust. The wing remains inclined at a small angle of attack during the downstroke and then twists into a nose-up position for the upstroke.
The forces producing thrust oppose the wing movements during both up and downstroke. However, in lift production, they oppose the wing movement downward but assist it during the downstroke. As a consequence, the wing is heavily loaded during the downstroke.
An important aspect here relates to the unsteady nature of flapping-wing aerodynamics of birds.
The complexity of the bird’s flight is associated with the interactive relationship between the wing’s motion and the aerodynamic and elastic forces, which are essentially nonlinear.
- Dhawan, S. (1991). Bird flight. Sadhana, 16(4), 275-352.