NACA airfoils aren’t so much a numbered set of standard, tested designs as a useful set of mathematical curve formulae for making airfoil-like shapes, and describing them using parameters.

NACA published empirically determined wind tunnel performance numbers for selected parameters, which was useful research but not a declaration of ‘these are the good values, you should only use these’.

It’s a bit like saying all satellites follow TLE orbits derived by NASA/NORAD in the 1950s - they do, but only because that’s just a standard way of writing down the orbital elements that describe a particular ellipse, not a catalog of ‘known good’ orbits.

my recollection was that the P51 wing used a NACA airfoil and it had low drag properties not commonplace at the time

Not quite true! Modern airplanes are way more complex. First of all, all modern airplanes have supercritical airfoils which go back to the 60s and 70s. Secondly, the airfoil of the wing root is typically different than the wing tip. Finally, new composite wings are adaptive during flight. They change their shape slightly to maximize efficiency.

Case in point would be modern gliders (sailplanes). One simple parameter that describes their aerodynamic performance is the maximum achievable Lift/Drag ratio, and that dimension-less ratio has climbed from ~30 in the 1960s to as high as 75 today. That means modern gliders can, using the same altitude/energy, go over 2 times further horizontally. The L/D is not the ultimate decider of performance but it is quite representative of the aerodynamic performance improvements.

BTW, all lift based flying objects have an L/D ratio (which depends mainly on the airspeed), this includes birds, fighter jets, commercial airliners; and the discrepancies can be pretty interesting. For example if one looks at the L/D of the Concorde vs a subsonic jet it becomes clear why it was so damn expensive to operate. Or why the U-2 looks like a glider :). I cannot find any aerodynamic performance data on any famous long endurance (>24h) unmanned drone, but I bet it's rather high as well.

> Concorde

Another good example is the space shuttle. It does actually glide back down. But it glides like a brick at first (1:1 during its initial braking into the atmosphere), and then like a less dense brick (2:1 while it's still supersonic), and then like a brick with shitty wings (a whopping 4:1 or whatever on final approach). Which is about what the Concorde is during landing, 4:1, yea.

Pretty crazy stuff

(Obviously the space shuttle was a tradeoff for, you know, getting it into orbit via rocket)

Your numbers are right but your analogies are misleading. I get “glides like a brick” is hyperbole, but you’ve added enough detail I can see people taking it seriously.

A brick’s L/D is much worse than 1:1. I’m seeing people say 1:10 online, but I can’t find a source and I think that’s incredibly high. A real brick is going to tumble and essentially not make any lift.

A less dense brick will have the same L/D. L/D is about the shape, not the mass.

I’m trying to get my head around the L/D being independent of mass. Does lift scale with airspeed at the same rate as drag? Or is L/D only considering lift-induced drag (whatever the term is) and not total drag including parasitic drag?

The way I've had it described is that when two objects of the same shape pitch for optimal glide (i.e. highest L/D) then the heavier one will reach the ground sooner (go faster), but both will take the exact same path and land the same distance away. In other words, same L/D.

This is not the explanation you are looking for, but "aha the heavier object takes the same path but drops faster" was what made me okay with L/D not depending on weight.

Roughly speaking, both lift and drag are proportional to v^2 for a given geometry.

Neither lift nor drag has anything to do with mass. They are entirely determined by the surface of the object, and are not affected at all by the interior properties, including density.

L/D changes with angle of attack. You can have a different airspeed at the same angle of attack and the ratio does stay the same. I think the Wikipedia page gives good descriptions.

Something a bit misleading done generally is aircraft don’t have one L/D, they have many, depending on angle of attack. When you see one number, it’s usually the best one.

Aren't they comparing high-supersonic to supersonic to subsonic?

I thought the point was that aerodynamics change from one domain to the next as shockwaves cause flow separation or eddies on or behind surfaces.

Space Shuttle pilots themselves referred to it as “The Flying Brick,” I think that is mostly what they were referencing. It was a term of endearment :)

Yea sorry, this was it. I didn't mean it literally has the glide ratio of a brick

I mean it looks like a brick and it flies

Despite knowing about its nickname… I found myself picturing a paraphrasing of the classic physicist’s “spherical cow in a vacuum”…

“Steerable brick in an atmosphere”… or the slightly more accurate “orientable brick in an atmospheric reentry regime”…

If you want a real "orientable brick in atmospheric reentry regime", check out how they steered the Apollo capsules back down. Kinda bonkers, but worked - they made it asymmetric and then rotated it depending on if they wanted to go down faster, slower, left, or right. Obviously it can't fly, and its glide ratio is actually brick-like unlike my facetious description of the shuttle. But it worked enough.

Kinda like a single control plane missile that spins (rolling airframe?), except... without the control plane lol.

"The F4 Phantom...America's proof to the world that with a big enough engine, even a brick can fly"

https://www.youtube.com/watch?v=XqUgUgiToNs

I guess you get maximum L/D out of a brick by giving it some serious backspin (which is a stable configuration so it might be able to maintain it on the way down) to set up the lifting circulation around it. But would this count?

>> tumble and essentially not make any lift

Tumbling itself can produce lift. The difference in drag between one side and the other can result in net pressure differences for a moving object. This is the basis of many baseball pitches. Spin a brick fast enough and it might just be able to climb if thrown horizontally.

If static airfoils are complicated, try looking into airfoils that rotate or otherwise move in relation to airflows. A Russian engineer once said that all problems in aerospace are placed on the tip of every helicopter blade.

The Magnus effect is super confusing to think about. Basically, to provide lift, the brick or ball or whatever would need to be "rolling" backards, like a wheel. In baseball, e.g. fastballs are usually thrown in such a way as to "roll" backwards, which causes them to climb. Curveballs are accomplished via topspin. Sinkers roll forward. You can even tune this behavior by making the surface of inconsistently "sticky" to the air, so flow of air is more or less affected by the objects rotation. This is why licking the baseball is against the rules.

Curiously, the rotation can also lend the ball's path greater stability against changing air currents/densities and crosswinds. Knuckleballs are famously hard to throw because they have very little spin, but they are also notoriously hard to hit because the trajectory is so subject to the vagaries of airflow between pitcher and batter.

This is to say nothing about when the axis of rotation is predominantly parallel to the direction of travel (e.g. rifle bullets and American footballs), where the Magnus effect effects the rotating objects ability to continue to rotate parallel to the direction of travel. Get it right and the spin makes the path more stable, but get it wrong, it becomes less stable. The hows and whys of that are beyond my understanding of fluid dynamics, but its fun to think about how complicated it can get.

edit: got my spin directions confused

I saw the space shuttle land once. From my perspective, it seemed to drop like a rock (fast!) and then as it got closer to the ground, it started "flying". I'd never seen anything like it.

For whatever reason the way I think about this phenomenon is like autorotation in a rotary craft.

The flare-out is really difficult to get right, from what I’ve read. As soon as you start leveling off, air speed is going to drop really fast, and you have very little time to get that bugger on the ground before you get in a stall. They used to practice using a modified jet, flying from high altitude to a landing with thrust reversers engaged all the way!

I personally give the space shuttle a little slack on the aerodynamics department when I remember that it enters the atmosphere at nearly Mach 25, and is big enough to produce two separate sonic booms. It was probably a very wasteful design (burdened by the military requirements), but it's like nothing else we've ever built!

The case with gliders and U2 and the max L/D is due to the wing aspect ratio (look up the formula for drag polar). Modern aircraft have much higher L/D because they have long skinny wings and these wings are possible because we moved from aluminum to carbon reinforced composites.

"better" airfoils are used in experimental craft design. for example mark drela wrote and used xfoil to design wings for mit's project daedalus, a human powered long distance flight aircraft. this is the case where, like sibling commenter stated, you need that extra % to get better performance characteristics. you can still run xfoil, it's a delightfully oldschol fortran program.

I have a friend whose PhD is in computational flow dynamics, as applied to airfoil design. He works almost exclusively in fortran (which is wild to me, for someone under 35, but I guess its the "industry" standard). I just asked him about xfoil and he observed that there are more modern programs for (as he put it) more "realistic/complex" designs, but said it was a good starting place.

oh i'm sure state of the art has advanced since then. i have the necessary physics background, but it's not otherwise my domain: i once used xfoil to design an airfoil for an autonomous model glider as a hackerspace project back when i had free time for things like that many years ago. the glider was also loaded into x-plane to develop and test the autonomous part. so whenever various experimental aircraft projects popup, i'm likely to look into them, and then also notice the peculiar foils they use.

Because Fortran was the industry standard for any heavy scientific calculation (like aerodynamics or nuclear bombs), the Fortran compiler has been optimised to death... And thus Fortran is still the industry standard.

FORTRAN is easier to optimize than something like C so is much used for numerical computation

It really depends. Up until the 90s Fortran was completely dominant. These days a lot of people have moved to C++. Important open source codes such as OpenFOAM and SU2 are in c++.

NACA and the other published airfoils[1] are generally a good starting point for hobbyist/RC folks. However if you want to eke out that last 5% bit of performance (ie. you are a company/institution), you would start with one of the above airfoils and optimize them to fit your flight envelope & mission profile. Here's a neat video of optimizing a round profile into an airfoil optimized for supersonic speeds [2].

[1] http://airfoiltools.com [2] https://www.youtube.com/watch?v=FHYTBguMfWc

Not really, they're a nice first step though, or if you require something "good enough".

The big variation now though is that the airfoil shape varies quite a bit from one end of the wing to the other.

Different airfoil shapes for the root and tip were common already in WWII era planes.

Eh, it's more like you can get to 90% of where you want to be with the 100 year old airfoils (though several of the other series are quite a bit newer).

https://aviation.stackexchange.com/questions/20798/are-naca-...

>You'd think that with modern computer software it would be possible to design better airfoils, but apparently, those shapes have already been mathematically perfected by hand and by experiment.

No, modern computer software indeed does better, but there's not a whole lot of room to do better, small changes to bump performance a percentage point or two. These are optimizations which can be (and are sometimes) skipped for many commercial projects.

For gliders the naca airfoils have been abandoned around 1970, when the first glasfiber composite gliders were made. We mostly use airfoils from German professor Wortmann (FX) , Quabeck (HQ) or Boermans (DU). The naca airfoils are still used in wind turbines though.