Some friends and I were on the beach the other day, staring up at the sky watching pelicans and seagulls fly by in big Vs. “Did you hear that the military is considering having its planes fly in ad-hoc assembled V formation, no matter where any particular plane is supposed to land, to save fuel?” “That’s really cool! Why does flying in V formation save fuel?”
Downwash must be involved, I thought.
Downwash is the vortex of air that shoots straight for the ground on the back of a wing. Downwash actually exists in sort of a square behind any foil moving through the fluid: Imagine a square-shaped drain in a sink, with all the water pouring in over the edges. That’s what the air’s doing after a wing passes by — one rolling wave of air coming off behind the wing, two waves that extend backwards from the wingtips, and a fourth wave that counteracts the first one, sort of chasing after the plane. THESE, I thought, must have the greatest effect.
Turns out that’s right! In short; the lead bird makes some vortices that curl upwards on the outside. The next bird floats on this small updraft of air. This updraft lets the bird pitch more horizontally (or reduce its angle of attack), because it doesn’t have to work so hard to try and keep the same altitude. Reducing the angle of attack significantly decreases the drag on the bird. And, this is no extra work for the first bird, because it would be producing these vortices anyway! Hooray!
Next I wanna figure out why small birds like swarms better than flying in Vs.