alt Hacker NewsAny competitive sailor or foil-racer knows that the underwater surface has the least friction and best laminar flow when sanded with fine-grid sandpaper, around 1000 to 1500 grid.
It always surprised me that this was not true in air and airplane wings were supposedly best when glossy. So now it turns out that this is indeed not true, and airfoils also benefit from micro-roughness for lowest friction.
Now the surprising question to me is how is it possible that something so simple was not known in this very well-researched and well-funded field. It probably was known, just not by the paper-publishing researchers.
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as usual these things are presented as new and revolutionary but aren't actually.
the specific process and implemention however are usually newer or slightly different from before.
this is our sensationalistic based society - any iterative progress, or sometimes even copy, is explained as a revolution.
now show me a 737 using 40% less fuel - guess what - that wont happen - however, perhaps we'll get a slightly better process to create aircraft skins. keep in mind you cant re-sand a fuselage every week, it needs to work reliably with no maintenance.
Water is fairly viscous, and when you try to pull through too fast you completely change regime due to cavitation.
In comparison, from my days studying aerodynamics for RC soaring, air has a wider range of "viscosities" (represented by the Reynolds number) depending on the scale of your aeroplane and the speeds you intend to go through the atmosphere. The aerodynamic ideal or what count as useful tricks (winglets, dimples) can be fairly different for a a golf ball compared to a RC airplane compared to a commercial jet compared to a fighter jet...
I wonder how quickly airlines will adopt sanded/rough wings. It's also interesting that the efficiency of winglets were known for quite awhile but only somewhat recently have nearly all airliners adopted them.
I thought that shark skin foil was a thing for years. Where they tried to emulate the micro roughness of shark skin.
> and airfoils also benefit from micro-roughness for lowest friction.
I thought this was known to some extent that smooth surfaces are not always the best e.g. golf balls have dimples on them? No?
Yeah I'm pretty sure I remember reading something in a pop science magazine 20 or 30 years ago when MEMS nano structures were all the rage and how they were gonna use mass arrays of them on airplane wings to somehow increase flow
The core tenant of the paper is that roughness reduces drag IN the transition zone. A very small region of the total flow.
Thats the region between laminar and turbulent flow. Laminar flow is typically 5x less drag than turbulent, and will be encountered about a Reynolds number of 500K-1M (ratio of inertial flow to viscous flow).
Surfboards will have a Reynolds number of 10^7 which is entirely turbulent.
A Cessna aircraft will have a Reynolds number of 1-5x10^6.