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Formation
Overlap and spacing set where each bird sits in the leaders' wake.
Aerodynamic model
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The science
Wingtip vortex pair
A bird of weight W flying at speed U sheds two trailing vortices at ±b0 = πb/4 with circulation Γ = W / (ρ U b0) (elliptical loading). Outside the pair the air moves up: w(y) = Γ/(2π) [1/(y - b0) - 1/(y + b0)], regularised with a vortex core and decaying with distance behind the leader.
Induced power
A solo bird has a mean induced downwash wi = 2W / (π ρ U b²). A follower whose wing sees a lift-weighted mean upwash w̄ needs only (1 - w̄/wi) of the induced power. The total power adds the unchanged profile and parasite share (slider), so whole-flight saving is smaller.
Honest simplifications: steady, time-averaged flight (no flapping phase), vortices stay straight with a simple decay, no wake roll-up or turbulence, and birds are treated as rigid elliptical wings. Real flocks save less than the ideal (about 10-20% measured on pelicans and ibises) because positions fluctuate and the phase of the wing beat matters.
What to try
- Switch to Directly behind: the bird sits in the downwash gap and pays extra power.
- Sweep the overlap slider and find the best lateral spacing in the bottom-right graph.
- Raise the wake persistence: later rows benefit from every bird ahead.