A fundamental principle of aeronautical engineering has been overturned
wired.comAny 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.
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.
> core tenant
And Lady Mondegreen.
Tenet is the word you mean
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.
At 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.
The FAA still requires ashtrays in bathrooms interestingly. To avoid those who do smoke there using the trash and causing a fire:
https://www.law.cornell.edu/cfr/text/14/25.853Regardless 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
It’s probably operationally easier to keep surfaces smooth than to keep them a specific amount of roughness.
It’s presumably easier to keep a smooth surface clear of bugs, dust and ice too.
yeah. what are the effects of too much roughness? may be safer and easier to maintain at smooth than at a specific roughness spec
Modifications 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.
I thought that shark skin foil was a thing for years. Where they tried to emulate the micro roughness of shark skin.
The article says the investigators identify this as something fundamentally different than the shark skin effect.
> 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?
Never mind. I didn't read the article (paywalled) and someone in the comments below answered this exact point.
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
Not 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.
> It's long been accepted that the smoother the surface, the lower the aerodynamic drag. That turns out not always to be the case.
Huh... I'd always heard that a golf ball's dimples help reduce drag?
From the article:
>This principle is fundamentally different from the effect of dimples on golf balls. Dimples reduce pressure resistance by intentionally turbulizing the airflow and suppressing backward separation. DMR, on the other hand, delays the transition, thereby suppressing not pressure resistance but the wall friction itself. They are opposite mechanisms.
mlmonkey did not say that this new observation was the same phenomenon as golf ball dimples, just golf ball dimples already disproved the "long accepted" belief that "smoother the surface, the lower the aerodynamic drag".
Exactly; golf balls are one example of it not being accepted that smooth surfaces are always best for drag, regardless of how the new result works.
TFA makes it clear that this is a very different phenomenon than golf ball dimples, and even goes as far as to say they are opposing.
However the TFA doesn't make clear that this: "It's long been accepted that the smoother the surface, the lower the aerodynamic drag. That turns out not always to be the case" was already known to NOT always be the case (e.g. in golf balls).
> Huh... I'd always heard that a golf ball's dimples help reduce drag?
Yep also vortex generators in cars have become common. So common that they've filtered down to after market parts you can put on a honda civic
Vortexes break up large air pockets and reduce drag.
Is that what those things are on random civics? Do they make any difference for regular street cars?
I put some (actual, as in from an airplane parts catalog) vortex generators on my hybrid. It slightly increased gas mileage and slightly reduced noise.
The less aerodynamic the vehicle, the more noticeable the result will probably be.
>It slightly increased gas mileage and slightly reduced noise.
Between the day to day road conditions and different routes noise and the modest effect, I'd say this would be impossible to tell.
I read somewhere that it depends... Different shaped objects benefit from different surface effects. A rounded surface like ball benefits from dimples where as more straight surface like arrow would not. I have no idea but I could also guess that speed affects things.
Read the article….this is a completely different effect.
Tough behind a paywall.
And the Mig-29 too but according to the reply that's different
yep
and a lot of "smooth" aerodynamic surfaces have "microscopic"/"very small" surface patterns to make the surface less perfect smooth as if it is too perfect smooth the air kinda "sticks" to it increasing drag (to say it in a very unscientific way)
It's almost certainly my adblocker playing poorly with their "subscribe to read" stuff, but I had to lol at the failure mode. When I load the page, I get the splash image/headline, and below it:
> Subscribe to listen [9 minutes]
> Aerodynamic drag is a major “barrier” in high-speed airplanes, automobiles, and bullet trains. This is because a design with less aerodynamic drag allows the aircraft to move at higher speeds with less energy.
And then just comments and links to other articles. No indication at all that there's more to the article beyond (apparently) an audio recording.
This might explain some of the "didn't read the article" comments? Not that it doesn't happen anyway tho.
Same happened to me but I opened it in reader view in Firefox and it's fine!
Same stuff! I’d rather prefer some archive link or something. Some websites are a bit aggressive these days.
If you happened to know Japanese it’s much easier to read the original article on wiredjp than .com https://wired.jp/article/distributed-micro-roughness-aerodyn...
If the application method is as rudimentary as sandblasting, it sounds rather simple to retrofit to existing aircraft. If it works as they state it does, it's a virtually free same-day fuel efficiency boost.
However, I did not see what the actual net improvement was. When they talk percentages, they are talking only about "in the transition zone". They say the coefficient improves throughout, but in theory, it could be almost irrelevant if the overall improvement throughout the profile is close to 0. It also sounds like a very difficult level of precise degradation to maintain for any period of time in real world conditions, since it would be easy to clog or abrade further.
… theoretically meets reality pretty quick in aviation. You’ll likely find a lot of red tape to modifying any particular aircraft until it has been tested or certified. Well, for certified aircraft anyway. Even in the experimental world you might find some (excuse the pun) resistance to sand blasting someone’s wing.
Based on the mechanism of flow attachment in the transition zone it seems like the overall airfoil profile would likely have to change to take full advantage of the reduced friction. I think its much more likely to see this technique played with somewhere like Formula 1, if it hasnt been already.
> "...like Formula 1"
Or projectiles like bullets and missiles. A sniper bullet with nanoscale textured surface that's able to go x% farther due to reduced drag seems plausible.
On a metal as soft as copper I imagine that texture'll last about 30 minutes after it's issued to the soldier.
Once the box magazine is loaded, it's not like the individual cartridges are being handled heavily.
Deer season involves a lot of loading and unloading a rifle (for those of use without removable magazines who are also bad at finding deer), and the bullets don't come out of it appreciably worse for wear. And that's for lead-free soft annealed copper ammunition. If you aren't being aggressively careless with your ammunition, it's not getting a lot of friction and scratches.
Doesn't the barrel remove this texture completely with the spin groves? Seems more of an issue than rough handeling.
What I've seen is a more structured texture applied with plastic films. https://www.lufthansa-technik.com/en/aeroshark One company claims up to 4% less fuel use. https://mako.aero/insights/delta-partners-with-mako-to-test-...
They allude to this alternative tech in the article, and I think it will stay the dominant approach because the far finer dimensions of the new tech talked about in the article, even if integrated into a film using glass beads as they also did, appears to be intrinsically much more susceptible to rapid functional degradation. It's about or less than the thickness of dirt / grime / bug goo. But tests will tell.
Paint and finish on an airplane has to account for a lot more than aerodynamics. So you need to build it from the ground up as that coating could be the difference between the surviving daily temperature fluctuation for 10000 trip vs 1000 trips
The physics of travelling at 600mph+ would affect the rough surface differently than at 60mph. Airplane wings experience erosion due to the high speed combined with particles in the air - dust, ice, volcanic ash, and rain/water. The erosion is a problem that sees significant mitigation. If the surface were made to be rough I'd expect some unexpected results, and it may even become a bigger problem. I do think the technique should be tested though.
Interesting finding, but hardly fundamental. My fluids lectures taught that there's form drag ("pressure drag" in the article) and skin friction drag. The two trade off with each other depending on Reynolds number. Keeping the flow laminar reduces skin friction drag (suggesting smooth skin), but keeping the flow attached for longer (e.g. by inducing turbulence, or injecting air...) reduces form drag (at a cost of increased skin friction due to turbulence).
Reads like they've discovered a neat way to delay flow separation while maintaining laminar flow, but the underlying principles have not changed. "Smooth thing low drag" was never a rule and only works at certain scales.
I'll await the experimental measurements of fuel efficiency using real aircraft.
Me too. The number of "revolutionary" designs that are announced but disappear makes me cynical. Looking wings on real aircraft, unless freshly painted, they're pretty close to finely sanded :) If the airlines and engineers saw a significant performance degradation with wear, they'd be out there polishing and repainting wings.
On a similar note - How many times have you seen announcements about someones blended wing that is going to save 50% fuel? But there are very few blended wings in nature (eg. rays), and those are in a very slow-speed regime.
The real obstacle to blended wing designs, I imagine, is more boring: airports are likely to be difficult to retrofit to support those, well for cargo anyways, and for passengers there's probably less appetite to board such a plane
Is this not useful in the speed regime of automobiles?
> The ... magnetic support balance system ... can levitate a streamlined model ... inside a wind tunnel without contact using electromagnetic force.
fyi: the paper cited in the wired article is at https://arxiv.org/abs/2603.23843
Klaus Savier is a longtime efficiency experimentalist, and opted for unpolished paint circa ~1990. His initial goal was weight reduction but numbers showed the finish had aerodynamic benefits.
I'm intrigued by the methodology of the wind tunnel: using magnets to more precisely measure and to avoid interference from guy wires...
This reminds me of the Dimple Car Experiment from Mythbusters.
Aren’t Turbulators doing similar thing i.e. its keeps the boundary layer for longer before it totally turns into turbulent layer?
> You’ve read your last free article.
You can worked around that in Firefox by switching to focused reading
Fascinating.
I wonder what the implications for radar-absorbing finishes are. Could they be more aerodynamic already?
Tell that to the ice build up on the wing.
I was thinking the same. Ice build upp will probably increase with a bigger and rougher surface area
Does this same principle make the moon orbit a little faster?
What order of aerodynamic drag does our moon in orbit experience?
This article and thread has got some major Tai’s Model vibes [1]
Uhh. I was taught that in university in the late 80s. Some surfaces have a lot of friction and if you add surface imperfections the turbulent airflow actually reduces drag.
You learned something different then because this finding is that some kinds of additional roughness delay the transition to turbulent flow which is pretty clear in the article.
https://phys.org/news/2014-01-smooth-rough-surfaces.html
A quick search looks to show the same general topic from more than a decade ago. I too have a recollection of this being discussed in the late 80s or early 90s. Maybe some folk wisdom that's just now getting quantified.
Thanks for clarifying.
I wrote about this ages ago, in that shark skin is an evolutionary adaptation worth study because water is thicker than air, but when air compounds, blah blah blah. Basically think of making a composite mold with directional tiny tiny dorsal fin looking surface. If you rub your hand on it the wrong way it cuts you open. Could even be scaled for large cargo ship hulls.
Next up: my personal wing invention which uses leading edges modeled on humpback whale fins, because the use case / stall profile is better.
Sigh, I’m going to have a great time in Heaven chatting with Leonardo da Vinci…
From the featured article:
> This technology is fundamentally different from the “rivulet (shark skin) process,” which is known as a typical aerodynamic drag reduction technology. The rivulet process mimics the fine longitudinal grooves in shark skin, and by carving grooves approximately 0.1 mm wide along the direction of airflow, it aligns the vortices that occur near the wall surface of turbulent airflow areas. DMR, on the other hand, delays the switch from laminar to turbulent flow by means of random and minute irregularities. The flow zones it affects and the mechanisms it employs are based on completely different concepts.
>humpback whale fins
you might find this video interesting then, the fastest rc drone in the world and it uses humpback inspired props.
Why wait for heaven. There probably are mods for Kerbal Space Program with exactly that parts. Create your wingsuit there.
Golf balls.
This article is kind of false. Keeping an object's boundary layer attached is known to reduce drag, even if the flow is turbulent. Golf ball dimples are a successful attempt to keep boundary layers attached.
The headline is perhaps overstating things a bit but they do discuss how this is different than e.g. rivulets
''' This technology is fundamentally different from the “rivulet (shark skin) process,” which is known as a typical aerodynamic drag reduction technology. The rivulet process mimics the fine longitudinal grooves in shark skin, and by carving grooves approximately 0.1 mm wide along the direction of airflow, it aligns the vortices that occur near the wall surface of turbulent airflow areas. DMR, on the other hand, delays the switch from laminar to turbulent flow by means of random and minute irregularities. The flow zones it affects and the mechanisms it employs are based on completely different concepts. '''
Yes, but this is not that.
Golf ball dimples are about 4 mm across and 0.2mm or 200μm (micrometers).
These features are several orders of magnitude smaller at 38 to 53μm diameter.
>>the first in the world to demonstrate that aerodynamic drag can be reduced by up to 43.6 percent simply by applying distributed micro-roughness (DMR), a surface roughness so fine and irregular that it cannot be distinguished by the naked eye. [...] Two types of DMRs were used in this experiment: A convex pattern made of glass beads with diameters ranging from 38 to 53 micrometers (μm) and a concave pattern applied by sandblasting. The height of the DMR coating is only 1 percent of the thickness of the boundary layer and is classified as a “smooth surface” from a hydrodynamic point of view.
Not to be that guy, but 38-53um is 1 order of magnitude smaller than 200um
Not to be that guy ;-) , but the diameter of the golf ball dimples is ~4 mm or about 4,000 μm, whilst the diameter of the spheres is 38-53 μm, or about 0.04 mm.
Diameter-to-diameter seems like about 100x or two orders of magnitude?
Similarly, 200 μm is the golf ball dimple depth (oops, just noticed I dropped that key word), and they didn't give us a measurement of the depth of the dents caused by the spheres or sandblasting, but it would likely be significantly less than half the radius of the spheres?
Sorry about misleading with dropping the "depth" word.
"We apologize for the mistake in overturning a fundamental principle of aeronautical engineering, those responsible have now been sacked."