E18 Aerodynamics Training

Topic

The physics of formation flight describes how the mere presence of other birds flying nearby alters the aerodynamic flow field, and how a following bird can capitalize on this change to lower its own energy expenditure.

The key mechanism behind this energy saving is the wingtip vortex: any wing generating lift produces a helical vortex at its tip, featuring an upward airflow on its outer side and a downward airflow on its inner side.

A bird flying in the correct position relative to the bird ahead can position itself within that outer updraft of the leading bird's wingtip vortex; by doing so, it reduces the induced angle of attack required to maintain lift, thereby decreasing the induced drag associated with that angle.

This energy saving in V-formation flight has been directly measured in two species: pelicans, using implanted heart-rate monitors; and northern bald ibises, using high-precision GPS and accelerometers to track flight path and wingbeat frequency. In both cases, individuals flying in formation—within the updraft created by the bird ahead—exhibit significantly lower heart rates and wingbeat frequencies than those leading the formation.

The maximum energy saving predicted by wingtip vortex theory—ranging from 20% to 30% compared to the cost of leading the formation—aligns with the actual measurements of heart rate and wingbeat frequency recorded in pelicans and northern bald ibises. This very mechanism of the wingtip vortex is precisely what a follower bird suddenly loses when an external disturbance knocks it out of its proper position within a collective flight formation: the energy savings of between twenty and thirty percent afforded by the correct position turn, at that very instant, into a required power increase of the same magnitude, because it is precisely that same wingtip vortex upwash that the bird ceases to utilize.

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