Which aerodynamic change occurs when a soaring bird extends its alula feathers during slow flight and steep climbs?

💡 Explanation

When a bird extends its alula feathers, a small group of feathers on the 'thumb', it creates a slot that re-energizes the boundary layer air over the wing because this generates a miniature leading-edge vortex. Therefore delayed stall angle results, rather than drag reduction which is related to aspect ratio, boundary layer turbulence related to skin friction, or vortex shedding frequency reduction which relates to sharp edges.

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