Mechanics of bird flight: the power curve of a pigeon by C. J. Pennycuick.

The ability of certain animal groups to fly by themselves has always stirred our imaginations. Even in the 15th century Leonardo da Vinci was famously inspired to try to build bird-like ornithopters. However, it was not until the 19th century that the nature of aerodynamic lift was understood, and it is a little more than 100 years since it was successfully applied to achieve flight by an aircraft. The key to success, as previous attempts to mimic animal flapping flight had failed (sometimes fatally), was to separate lift and thrust generation, so that aircraft wings provide lift while a propeller generates thrust. But animals generate lift and thrust by flapping their wings, which continuously change shape and deform elastically throughout the wingstroke. An analytical solution of Navier–Stokes equations (the general differential equations arising from applying Newton’s second law to viscous fluid motion), which describe the aerodynamic forces that keep fliers aloft, would, in principle, solve the problem of how birds fly, but a solution to these equations defies scientists to this day. However, there is some light at the end of the tunnel. In a landmark paper from 1968 published in The Journal of Experimental Biology (Pennycuick, 1968b), Colin Pennycuick combined aerodynamic (helicopter) theory with ingenious wind tunnel experiments using a trained pigeon Columba livia to derive a quantitatively accurate mechanical model of bird flight. In a companion paper Pennycuick also estimated some basic properties for the bird in steady gliding flight in a tilted wind tunnel (Pennycuick, 1968a), including how the profile drag coefficient varies in relation to the lift coefficient and the magnitude of the parasite drag coefficient of a bird. Pennycuick used this information about wing lift and drag from the body and wings to develop his classic ‘momentum jet’ model of flapping flight mechanics (Pennycuick, 1968b).