Comment by porphyra
2 years ago
It's pretty interesting that many airfoils used in aircraft design were derived by NACA in the 1920s and 1930s [1]. 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. So nowadays if you want to design a plane you can just look up the desired NACA airfoil from a table based on the speed, air pressure, etc that you require.
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)
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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.
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
"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.
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.
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.
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++.
FORTRAN is easier to optimize than something like C so is much used for numerical computation
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
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.
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.
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.
Not really, they're a nice first step though, or if you require something "good enough".