Vortices enable the complex aerobatics of peregrine falcons

Abstract

The peregrine falcon (Falco peregrinus) is known for its extremely high speeds during hunting dives or stoop. Here we demonstrate that the superior manoeuvrability of peregrine falcons during stoop is attributed to vortex-dominated flow promoted by their morphology, in the M-shape configuration adopted towards the end of dive. Both experiments and simulations on life-size models, derived from field observations, revealed the presence of vortices emanating from the frontal and dorsal region due to a strong spanwise flow promoted by the forward sweep of the radiale. These vortices enhance mixing for flow reattachment towards the tail. The stronger wing and tail vortices provide extra aerodynamic forces through vortex-induced lift for pitch and roll control. A vortex pair with a sense of rotation opposite to that from conventional planar wings interacts with the main wings vortex to reduce induced drag, which would otherwise decelerate the bird significantly during pull-out. These findings could help in improving aircraft performance and wing suits for human flights.

Investigation of Falcon Dive and Developing Kinematic Approach

“…Finally, during the M-shape configuration that is adopted towards the end of dive, the stoop was characterized by vortex dominated flow. [6] The experiments and simulations revealed that vortices rose from the frontal and dorsal regions due to strong spanwise flow promoted by the forward sweep, as shown in Fig. 8.…”

“…During this phase, the arm opens up further into the horizontal plane and the wings are aligned in a way that the bird appears to be in an M-shaped planform when seen from above. [6] In this phase, the trailing edge of wing detaches from the body, such that there is a forward sweep in the in-board section and aft sweep in the outboard section of the wing. [4] As the falcon goes deep into pull out maneuver, there is a reduction in sweep angle in the outboard section of the wing.…”

“…Without a doubt, the success of attacks can be largely associated with the manoeuvrability during these phases. [6] This paper aims at performing literature review in order to provide better intuition on the kinematics of falcon dive trajectory. In the past, various attempts have been made to study the falcon dive, from producing mathematical models to performing CFD analysis on different phases of the model during the dive.…”

“…The model failed to consider the lift generation during the transient phase (cupped and open cupped phase), since not enough was known to allow for the accurate calculation of the drag experienced by the falcon, wherein most of the lift is due to vortex generation and flow acceleration. [6] The model failed to account for additional lift during both, the transient phase, and the teardrop phase.…”

“…8. [6] These vortices enabled flow reattachment towards the tail. Moreover, the strong wing and tail vortices provided extra aerodynamic forces through vortex induced lift, also shown in Fig 9.…”