Researchers have measured the aerodynamic forces of a bird flying inside a box.

Until now, the aerodynamic lift achieved by free-flying animals has only been estimated using models. David Lentink of Stanford University in California and his team built an enclosed device to directly measure forces generated by a bird's wings during flight. With each flap, moving air exerts a force on the walls of the box, which is captured by sensors. The signals were synchronized with those from a high-speed camera, which records a bird's flight from one side of the enclosure to the other.

The researchers confirmed previous findings that each downstroke of a bird's wings generates enough force to lift twice the animal's body weight into the air. The device could be used with other animals and free-flying robots, says the team.

J. R. Soc. Interface 12, 20141283 (2015)