Kinematics and hydrodynamics analyses of swimming penguins: wing bending improves propulsion performance

Abstract

Penguins are adapted to underwater life and have excellent swimming abilities. Although previous motion analyses revealed their basic swimming characteristics, the details of the 3-D wing kinematics, wing deformation, and thrust generation mechanism of penguins are still largely unknown. In this study, we recorded the forward and horizontal swimming of gentoo penguins Pygoscelis papua at an aquarium with multiple underwater action cameras and then performed a 3-D motion analysis. We also conducted a series of water tunnel experiments with a 3-D printed rigid wing to obtain the lift and drag coefficients in the gliding configuration. Using these coefficients, the thrust force during flapping was calculated in a quasi-steady manner, where the following two wing models were considered: (1) an “original” wing model reconstructed from 3-D motion analysis including bending deformation and (2) a “flat” wing model obtained by flattening the original wing model. The resultant body trajectory showed that the penguin accelerated forward during both upstroke and downstroke. The motion analysis of the two wing models revealed that considerable bending occurred in the original wing, which reduced its angle of attack during upstroke in particular. Consequently, the calculated stroke-averaged thrust was larger for the original wing than for the flat wing during upstroke. In addition, the original wing required less work for flapping, indicating more efficient propulsion. Our results unveil a detailed mechanism of lift-based propulsion in penguins and underscore the importance of wing bending.

Coracoid strength as an indicator of wing‐beat propulsion in birds

“…Reducing the moment arm of the hydrodynamic force and the wing beat frequency cause the reduction of the force required in the flapping muscles. According to the quasi‐steady calculation of the hydrodynamic force of the wing in swimming penguins (Harada et al, 2021), the hydrodynamic force of the wing is about 0.50 times body mass, which is much smaller than the aerodynamic force of the wing in ‘ flap‐flight ’ birds, such as pigeons (~4 times body mass, Ros et al, 2011) and Pacific parrotlets (~2 times body mass, Lentink et al, 2015). Therefore, it is likely that ‘ wp‐diving ’ birds are not required to exert much larger force than ‘ flap‐flight ’ birds to flap the wings underwater.…”

Experimental analysis of the sweepback angle effect on the thrust generation of a robotic penguin wing

“…However, the wing model that we fabricated for the previous study was a no sweepback wing, which means that the leading edge of the wing is set to be vertical to the longitudinal axis of the body. During our observations on the swimming of real penguins, it was found that the wings swept backward by a range from 24.6 • to 46.7 • relative to the forward direction [19]. This variation in the sweepback angle possibly affects the thrust and efficiency of the flapping-wing propulsion.…”

“…The cross-sectional profiles of the wing are obtained by scanning a real penguin wing and adjusting it to determine symmetric profiles along the thickness direction [20]. The details of the obtained profiles can be found in our previous paper [19]. According to our kinematic research on penguin swimming, the wing has an average sweepback angle of approximately 35 • relative to the feathering axis +Z W [19].…”

“…The details of the obtained profiles can be found in our previous paper [19]. According to our kinematic research on penguin swimming, the wing has an average sweepback angle of approximately 35 • relative to the feathering axis +Z W [19]. Hence, the sweepback angles studied in this paper are set from 0 • to 50 • at an interval of 10 • (figure 2).…”

“…These excellent swimming performances have motivated us to explore the relationship between wing motion and the generated propelling forces. In our previous work, the 3D wing motion of a real penguin during swimming was measured in an aquarium [19], and a robotic penguin wing was developed to mimic the motion [20]. Based on the measurements, we defined the wing motion and decomposed it into three degrees of freedom: flapping, feathering, and pitch of the flapping axis.…”