Bats leverage asymmetrical forces for nimble aerodynamic moves

The mechanics of powered flight are well understood. As a bird or bat flaps its wings, the net air flow is directed downward, which creates a net upward force. But the aerodynamics of complex movements that are required to catch prey and survive have remained difficult to measure directly. Now, with technological improvements in flow visualization, Per Henningsson of Lund University and his colleagues have shown that bats vary the magnitude of force during a wing flap to execute a turn in midflight.

The researchers trained two brown long-eared bats (Plecotus auritus) to fly in a wind tunnel. They motivated the bats to execute a turn during flight by moving a mealworm, which can be seen in this video. Henningsson and colleagues simultaneously filmed the bats repeating the flight pattern and used particle image velocimetry to collect high-resolution airflow data.

Before the experiments, the researchers predicted that the bats would generate more lift on one wing during the downstroke and complete the turn with a rolling motion. But that method was used in only 6 of the 30 cases. In 20 cases, the bats used the upstroke to perform a sideways turn, or yaw. The wing closer to the mealworm was positioned to increase drag while the outward wing was set to increase thrust. Because of the asymmetry of forces, the bats could complete the sideways turn. The bats used an additional thrust of the outward wing during the downstroke in the four other cases. Despite discovering that the bats predominately use a yaw during the upstroke to perform the maneuver, the researchers also found them to be versatile: During different phases of their wingbeats, the bats exploited variations in lift, drag, and thrust to execute the turn, sometimes through roll and sometimes through yaw. Future studies may show whether the bats prefer particular turning methods for different environmental conditions. (P. Henningsson, L. Jakobsen, A. Hedenström, J. R. Soc. Interface 15, 20180441, 2018 .)