5 ms·
Any 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
by Maarten88 4mo ago
Any 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.
- deleted 4mo ago[deleted]
- aeternum 4mo agoI 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.
- rogerrogerr 4mo agoIt’s probably operationally easier to keep surfaces smooth than to keep them a specific amount of roughness.
- greggsy 4mo agoIt’s presumably easier to keep a smooth surface clear of bugs, dust and ice too.
- swasheck 4mo agoyeah. what are the effects of too much roughness? may be safer and easier to maintain at smooth than at a specific roughness spec
- delecti 4mo agoAlso matters a bit what happens to a surface that they don't do anything to. Does a precisely rough surface get too rough or too smooth? Does a precisely smooth surface get rougher in a way that's beneficial? Could be the case that in-practice this means they just worry less as their perfectly smooth planes get a bit rough.
- stackghost 4mo agoModifications to an approved type design, especially for commercial passenger aircraft, are an intensely bureaucratic and thus very expensive process. This is part of the reason why product cycles are long.
- PaulRobinson 4mo agoAt least a decade. I remember people could smoke on planes. On some airlines seat backs and bathrooms had cigarette ashtrays in them. Smoking was phased out between 1988 and 2000, with most airlines being smoke free in the mid-1990s. But the ashtrays persisted well into the 2000s. Two reasons: they needed to refresh the cabins, which is on a longer maintenance cycle done every few years, and before that, they needed replacement seats and bathroom fittings without the ashtrays. That meant tests, regulatory approval, all sorts. For ashtrays being removed. Winglets are a similar story. They're an addition, but they needed test flying and type approval before they could be added to the maintenance cycle rotation and get added to aircraft. This is a bigger change. Boeing and Airbus (and others), are going to need to design it, push it through CFD, build different variants, test fly them, get them through regulatory approval and then... well, existing aircraft are probably not going to get these. Too expensive, too hard. What's going to make more sense is a new aircraft - even if it's a variant type like the 737-MAX or the A320-Neo or whatever - where they approve the type modification as a whole, but it's not a retrofit to an existing airframe, will help manufactures sell more aircraft, airlines don't need to ground existing fleet and over time the fuel efficiencies get involved.
- bitdivision 4mo agoThe FAA still requires ashtrays in bathrooms interestingly. To avoid those who do smoke there using the trash and causing a fire: Regardless of whether smoking is allowed in any other part of the airplane, lavatories must have self-contained, removable ashtrays located conspicuously on or near the entry side of each lavatory door, except that one ashtray may serve more than one lavatory door if the ashtray can be seen readily from the cabin side of each lavatory served https://www.law.cornell.edu/cfr/text/14/25.853 https://www.law.cornell.edu/cfr/text/14/25.853
- thaumasiotes 4mo ago> For ashtrays being removed. I don't think it's safe to generalize from this to a functional aspect of the plane. Removing the ashtrays serves no purpose, so there's no cost to letting it wait for a decade or two. Improving the aerodynamics does serve a purpose and might be done faster.
- colordrops 4mo agoYeah 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
- greggsy 4mo agoNot uncommon to hear bold claims with every new and emerging technology that isn’t well understood by the media or general public. The excitement over nanobots seems to have run its course (for now?). Blockchain managed to find its way into every market imaginable. Battery technologies have consistently delivered bold claims on an almost yearly cycle, but we have at least seen incremental improvements. AI is obviously the worst offender in the current timeline.
- otterdude 4mo agoThe 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.
- mike_hock 4mo ago> core tenant And Lady Mondegreen.
- deleted 4mo ago[deleted]
- bonsai_spool 4mo agoTenet is the word you mean
- Maarten88 4mo ago> Surfboards will have a Reynolds number of 10^7 which is entirely turbulent. A fin, foil or daggerboard below the board/boat is operating well within the range of Reynolds' numbers where laminar flow is relevant.
- dilawar 4mo ago> 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?
- larusso 4mo agoI thought that shark skin foil was a thing for years. Where they tried to emulate the micro roughness of shark skin.
- plorg 4mo agoThe article says the investigators identify this as something fundamentally different than the shark skin effect.
- lonely_wanderer 4mo agoThe article contrasts it with a manufacturing process known as “shark skin.” I wonder if it’s entirely identical to the skin of a shark or a marketing term?
- larusso 4mo agoSorry the article is paywalled and I reacted to the comment about sanded surfaces. I remember seeing documentation or other nature documentation’s about the shark skin effect. I had the chance to touch a shark and a ray in an aquarium but never felt the shark skin foil. I assume the properties would be the same though.
- Arodex 4mo agoWater 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...
- naasking 4mo agoWater is also largely incompressible. The fluid dynamics are just too dissimilar to air to carry over simplistic assumptions.
- warumdarum 4mo agoWasnt there something about building abblative vortexes that convert the friction into rotation and are then discarded at the edge of the surface?
- stasomatic 4mo agoAsking as a complete neophyte - how does this reconcile with modern war planes being inherently unstable as far they flight dynamics go, without their enormous thrust capabilities? I’m just curious, I know nothing about the subject, but it seems that the solution we came up with is thrust, baby.
- bombcar 4mo agoUnstable fighters gives them much more maneuverability at the cost of “not returning to straight and level flight” that normal planes have. It’s not directly related to how the wind goes over the wings.
- stasomatic 4mo agoI am curious about humans operating machinery outside of our inherent performance envelopes. Do we have enough runway beyond switching arrow tips from flint to SS?
- zobzu 4mo agoas 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.
- phonon 4mo ago"... this flying wing will burn 50% less fuel than today's jets..."[1] [1]https://time.com/7292452/jetzero-low-carbon-air-travel/ https://time.com/7292452/jetzero-low-carbon-air-travel/
- rayiner 4mo ago> It always surprised me that this was not true in air and airplane wings were supposedly best when glossy. I was an AE major and I don’t recall ever learning that airplane wings were best when perfectly smooth, even as a simplification in undergraduate courses. We were taught that drag is reduced by maintaining an attached laminar flow. Airplane wings are glossy because they’re metal (or CFRP) and painted for durability and corrosion and UV resistance.
- baybal2 4mo ago[dead]
- freetinker 4mo agoWe’ve known about this for a while I feel from dimples on a golf ball.
- biinjo 4mo agoSo you’re telling me I should sand my carbon fiber wing foil mast and wing with 1000-1500 grid sand paper for better gliding results? I’m fairly new to the sport and never heard of this yet.