For nearly a century Ludwig Prandtl’s lifting-line theory initially points to the elliptical spanload as the most efficient wing choice, and it has become the standard in aviation. Research in bird flight has increasingly generated data in disagreement with the elliptical spanload.
In 1933 Prandtl published a little-known paper presenting a superior spanload. We argue this second spanload is the correct model for bird flight data and we present a unifying theory for superior efficiency and coordinated control in a single solution. Specifically, Prandtl’s second spanload offers the only solution to three aspects of bird flight: how birds are able to turn without a vertical tail; why bird formations have wingtips overlapped; and why narrow wingtips do not wingtip stall.
We performed research using two experimental aircraft designed in accordance with the fundamentals of Prandtl’s second paper, applying recent developments, to validate the new spanload. We propose Prandtl’s second spanload: as an alternative for avian researchers, to demonstrate the concept of proverse yaw, and to offer a new method of aircraft control and efficiency.