Bird Flight – Extreme Aerodynamics

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.

Linearized Theory

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.

Lift and thrust generation in flapping flight (Lighthill 1974)

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