Monday, February 10, 2014

The Aerodynamics of Diving Peregrines

Falcons are well known for their speed and agility. They prey primarily on other birds and generally catch their prey on the wing. (Small falcons like kestrels often eat large insects or small rodents in addition to birds.) All falcons are sleek with pointed wings and capable of fast flight, but Peregrine Falcons in particular are noted for their high-speed dives. I have been fortunate to witness such dives on several occasions, and they remain impressive every time. A recent paper in PLoS ONE examines the aerodynamics of diving Peregrine Falcons through a series of experiments.

A typical sequence of images taken during a dive at the selected points. Source: PLoS ONE
Peregrines were trained by falconers to dive in front of the south wall of a dam to allow high-speed cameras to capture their flight shape and trajectory precisely. The resulting photographs were then used to build a model for wind tunnel testing.

Flow visualization on the surface of the falcon model (A) and the falcon at the same flight position (B). Source: PLoS ONE
At the top of the dive, the falcon initially accelerated and then flew at constant speed with its wings tucked into a diamond shape. During this dive, the falcon reached a maximum speed of about 50 mph. As it approached its intended prey, it spread its wings to generate lift and rapidly decelerated, finally pulling out of its dive and landing. The wind tunnel tests showed several regions where the airflow was uneven around the model. At one such region, near where the trailing edge of the wing is tucked close to the body, photographs showed feathers popped up to smooth the airflow. Benjamin Ponitz, Anke Schmitz, Dominik Fischer, Horst Bleckmann, & Christoph Brücker (2014). Diving-Flight Aerodynamics of a Peregrine Falcon (Falco peregrinus) PLoS ONE DOI: 10.1371/journal.pone.0086506