The observed birds gliding flight performance studies indicate that their aerodynamic drag is lower than expected for their flight regime ^[1], and the best “sailing” birds (condor, vulture, albatross, etc.), have black feathers, a quite unattractive matting attribute...

Also, birds contour feathers barbs and barbules are covered by a nano- scale smooth layer of beta keratin, a substance with large and platelet-shaped molecules, (evolved from the reptiles and dinosaurs scales alpha keratin), and that in the black birds are superimposed over a eumelanin grain layer ^[2], that besides to provide UV rays protection, contribute to beards surface mechanical properties.

Already in a 1972 “OSTIV” paper ^[3], the author proposed that fish scales and bird feathers deserved more research, and in a 2018 SAE paper ^[4], that nanometer scale smooth surfaces, made with large atomic or molecular mass materials, and having low surface energy, by turning more elastic the gas molecules to surface collisions, could reduce gas flows friction drag by creating slip boundary layers, even for continuous flows (Knudsen numbers < 0.01), for which normally “slip” = 0.

Beta keratin possessing the above quoted properties, the present proposal is then a finish consisting of a nano scale smooth external surface nano layer of beta keratin, or with a high content of it, superimposed (or not) on a nano layer containing eumelanin grains, to be applied to surfaces or surface coverings, be then smooth or textured at millimeter or micrometer scale.

In level cruising conditions a subsonic aircraft friction drag represents 50% to 60% of its total drag, and by applying this proposal finish to 40 to 70% of its external surfaces, reductions of 25% to 50% of this friction, 5% to 21% in its fuel consumption and CO2 emissions, and 1% to 6% of its operating costs could be expected ^[4].

Fig 1 (not to scale), presents a schematic view of a practical example of this finish applied to a plastic or metallic surface coating to be glued onto surfaces that for various reasons such as: size, shape, maintenance and repair etc., a direct application would be difficult or expensive.

The beta keratin and eumelanin finish layers could be obtained through PVD, CVD, “Sputering”, “Langmuir/Blodgett”, Solvation, Self-assembly, etc, organic thin film manufacturing processes, with raw material obtained from the feathers (black ones for eumelanin) from poultry meat industries dejects ^[5].

This finish could also be applied to internal surfaces of ducts, tubes, and micro channels, to increase their gas transport efficiencies.

The market for the proposed finish is the worldwide aircraft manufacturers, and operators pressed by greenhouse gases emission restrictions.

Obs: “AeroShark”, a shark scales biomimetic surface plastic covering containing grooves (riblets), with millimeter or micrometer sizes is already being used by Lufthansa/Swiss jets with an announced reduction of 1% in fuel consumption and CO2 emissions.

References:

1. DOI:10.5028/jatm.v8i1.564
2. DOI: 10.1098/rspb.2010.1637
3. Some thoughts on nature and sailplane design | OSTIV Publications (sfu.ca)
4. https://saemobilus.sae.org/content/2018-36-0285/
5. DOI: 10.1007/s11356-023-25788-x