Damping properties and morphological characteristics of primary feathers

Flight provides to the birds efficient locomotion and occupation of various habitats. Wings and feathers sustain strong forces and vibration during flapping flight. Excessive vibration can lead to the stronger energy consumption, noise production and can therefore negatively impact the flight stability. Hence, it is reasonable to hypothesize that the primary feathers develop damping mechanisms to mitigate vibrations. Although feather microstructures have been well investigated, their damping properties still lack a systematic study. Here, this study shows that the primary flight feathers are capable of various damping mechanisms related to different structures.

In the shafts and barbs, the cortex and gradient foam support the mechanical bending and decay vibrations. The first indication of graded damping properties was related to the morphology of the shaft (foam cells size, area ratio of foam to cortex, and percentage of foam). In the vanes, there are three different types of structures related to the reduction of damping. First, we performed experiments to compare damping properties between feathers with vanes and the bare shaft without vanes. After trimming vanes, the damping ratio decreased by 38-60%. Second, the damping of zipped vanes and unzipped vanes was studied under normal air pressure. The results indicate that the zipped vanes have higher damping ratios. The planar surface of zipped vanes is responsible for aerodynamic damping, contributing 20% to 50% to the whole damping in a feather. Thirdly, the damping tests between zipped and unzipped vanes in vacuum were performed. Structural damping demonstrates 3.3 times (at vertical deflection) and 2.3 times (at horizontal deflection) difference in damping ratio between zipped and unzipped feathers in vacuum.