did some testing on NACAPROP, plotted AoA vs Lift and the mathematical formula is:
Lift = sin(-(90/55)*AngleOfAttack) . This means peak Lift at AoA=-55. It is dodgey however because you expect zero lift at AoA=90 but because of its sine wave nature, it drops to zero lift at AoA=-110, add another 55 deg and you get peak downforce at AoA=-165 and pretty much 90% downforce at -179. Looking at those numbers in the opposite direction is a mirror image with 90% lift at 179. The oddity in this scenario is that if your stationary on the runway with a tail wind, your measured angle of attack varies between -179 to 179 (like -1 to 1 if it was a headwind) and lift jumps between 90% of peak lift to 90% of peak downforce once the numbers tick over in the rotation from 179.9 to -179.9. where as same speed headwind, your basically at minute levels of lift/downforce.

Devs could fix this by keeping the Lift vs AoA curve a sine wave but make the expression: sin(-(90/45)*AngleOfAttack) instead. This would give peak lift at -45, zero at -90, peak downforce at -135, zero at 180/-180. To keep lift levels similar in the range of 0 to ~20 degrees AoA, lift scale would then need to be reduced to 82% (45/55).

Devs please fix NACAPROP Lift curve.

Thanks for blowing my mind for the day. :)

@OrderlyHippo I haven't done more research into it, but for your application, I'd use the flat bottom airfoil as it actually resembles a vortex lift vector for some reason lol.
Pretty sure adding sweep just changes the amount if lift and doesn't change the lift graph.

@jamesPLANESii Did you further investigate about wing sweep and lift? I'm trying to see how to make the F-35B handle better at low airspeeds and I read, vortex lift contributes a lot to its high AoA capabilities. I'm not sure which wing construction method to use...I'm actually simulating it's slow flight mode too and wing lift is so weird UGH! I’m referencing a comment made by xiaofootball on your post about wings. Even considering wing construction like this plane’s rear stabilizers where the wing’s hingeDistance is 0 on 2 planes put together as one. Do smaller wings have slower stall speeds?

@WNP78 (and CoolPeach) Thanks for chiming in with that. I will admit that I had a preconceived notion that SP was deciphering the values from the NACA code, using that to define the shape of the Airfoil, and letting the physics engine handle how it actually performed.
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As a result, admittedly, I didn't consider that there was lots of precompiled data the game was running off of. Definitely a failure on my part (for assuming it was done in some sort of real-time way). :} I suppose I was basing all of this on thinking that the wing physics were done in a same/similar way to the Drag Points (which are dynamic).
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That being said, if adding one or two more airfoils happens to not be a monumental task... I do think having one of the NACA/NASA Laminar Flow models, and an early 1900s model, would be a nice addition and give folk a bit more choice.
There are a bunch of Laminar airfoils and their performance data (a number of which are not NACA/NASA, if that matters) that can be found in this huge "Parametric Airfoil Catalog - Part II" PDF. (There's also Part I, but I haven't looked at it.)
Not sure if the data provided in there is at all what's needed, regarding what WNP78 said?

Bonus Question (open to anyone that knows): Does changing the Root and Tip thickness have any physics impact, or is it solely for aesthetics?

@Formula350 "straight forward to decipher" = it's straight forward to decode into parameters that define the shape of the airfoil, like the camber and thickness. It is not at all straight forward to get CL/CD characteristics for them, which is what is needed for simulating the wings in SP. For that, you're better off just getting a database of all of them with precomputed values.

Well implementing extra aerofoils would mean storing a lot more data for each aerofoil type and the Cl and Cd, or a lot more computation to calculate them. Realistic aerofoil shapes would be nice to have though. @Formula350

@CoolPeach Yea there's just those three NACA codes that work (plus the NACAPROP), which coincide with the actual 3 options we can select in the menu. I tried to include that in my EDIT2, but reached the 4000 char limit on posts... :}
I'm actually a bit surprised that they didn't implement that 4 & 5 code ability, given it's a straight forward equation to decipher them. ("straight forward" for a computer to work out, but even someone like me who can barely add numbers together can still make sense of it to some degree lol)
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Wouldn't even need to be tied to anything in the menu, to preserve the "Simple" nature of the game, only there as a hidden feature for those adventurous enough to dive into the more advanced stuff.
Personally it's all over my head, as I mentioned, but I'd still like the ability to try out different stuff! Specifically due to more recently having made some WWI era stuff and wishing I could be more accurate by using those very deep airfoils some of those planes had. (I've considered using very thin strips of Wing-3 back to back, angling each one a bit in order to recreate how some look, but I don't know if it'd yield the same results heh)
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I also think it'd be great if the wing mesh would mimic the airfoil we have selected, given the game technically has that ability with what Fuselage parts can do: Rise, Run, and Custom corners. So like a Flat-Bottom for corners would use 0,0,0,3, or Semi-Symm as 0,0,1,3.
...Though... I won't hold my breath on either feature coming anytime soon (or ever). :P

simpleplanes lore

Flat bottom is the best performing aerofoil, since it has the highest CL/CD ratio and in general produces a lot of lift for not that much drag (relatively). Likewise, it also doesn't stall, so you will generate lift at nearly any AoA.
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I've heard you cannot enter the codes as they are fixed, so the ones on the document are just the reference ones, but if you say that you can Id love to see an example.
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The NACAPROP aerofoil, which I didn't include, is quite linear in terms of its lift vs AoA, so it doesn't stall, but it will generate less lift the majority of the time than a symmetrical aerofoil. @Formula350

I'm a total idiot here in terms of the math/science, but for what it's worth, here's a couple things I found these last 2 days, as well as in general.
1) "Flat Bottom" flies the best. It results in better handling, the plane flies faster, and seems to be better (easier) for taking off.
--Example: This "Coanda 1910" I just built, with a legitimately modeled Turbine (though I have a feeling the "20inch" measurement I read about is a radius [a turbine blade's length], not the turbine's diameter, but I digress) with the main Propeller set at 50HP.
WING SET TO 'FLAT BOTTOM': Plane will automatically, and gently, take off on its own at 73MPH, and level out on its own with a flat (maybe 1deg) AoA, cruising at 72MPH.
WING SET TO 'SEMI-SYMMETRIC': Plane will reach 67MPH and never takes off. That's its max top speed. Note: In both cases, the plane's tail (the X fins are Symm, the wide fixed tail 'wing' is Flat-Bott) rises up around 30MPH and the plane is sitting at 0deg as it travels down the runway,
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2) Using the Hover Car, during ascent at a low speed (if you switch the BFE300 Input from VTOL to Throttle, at roughly 10%), transitioning the nacelles at various steps on the TRIM slider, produces a very sudden shift between the way the wings produce lift. Seems to be around the time they're at... hmm... 25-30deg (above horizon; not from their vertical orientation). At that point the whole thing pitches as though the game's calculations have switched gears,
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3) SEE EDIT2 The note on the SimpleCheats document about the Wing Airfoils, is a lie T_T
You cannot input the NACA values and apply them. At least not using the Overload mod's menu. It claims invalid input and "restores" a backup. I was wanting to try naca63415, and I attempted using it with lowercase, uppercase, inside quotes, just the numbers, and even the provided "NACA23015" that it claims the game uses.
Note: I could easily have misunderstood the document's note about that specific entry, and in fact it was -not- implying that we could input ANY valid NACA airfoil value...

airfoil string A name relating to the different aerofoil types. If you’re really into aerodynamics, here are the technical names for the choices:
“Flat Bottom” :“ NACA23016”

“Semi-Symmetric” :” NACA23015”

“Symmetric” : ”NACA0009”

These are all 4 or 5 digit NACA codes which you can read up on here.

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EDIT: I think the NACAPROP's superpower is.... for use as a Canard airfoil...
I tested that on my Coanda 1910, and it's top speed rocketed to 83MPH; however, it was unable to achieve takeoff at all, due to creating exceptional downforce. I attempted to counter it by changing the inverted setting (via Overload), but same results.
Then I tried it on my de Bruyere xC.2 (albeit with a Jet engine), which has fully moving canards (no rear ailerons or horizontal fins).
They both auto-takeoff at roughly 180MPH, but with the canard wings set to NACAPROP it gets there faster it seems.
----While set to Symmetrical, the nose will continue to pitch up until the entire plane is 99% stalled at 90deg) and then the nose finally crests and makes the loop. It attains roughly 360mph during its nose dive before crashing into the ocean at roughly 60deg still (it cannot self-recover).
----While set to NACAPROP, the nose continues to pitch up, but is arrested around 60deg. Which then, despite the plane not being stalled out, the nose starts to drop a little. Then once sufficient forward speed is attained, the cycle repeats, but no full loop occurs (or anywhere close).
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EDIT2: Ok it seems it DOES support the 3 NACA codes. I must've typoed earlier when trying.

Am confuse

idk man I haven’t build a plane in years @jamesPLANESii

@Chancey21 Flat bottom doesn't stall properly, so no unless you're using slats :P

According to edensk it doesn't, but I'm not sure. I have a feeling there won't be any change though, but I would need to test it to make sure. @jamesPLANESii

Nice!
A thing I'd like to see tested is how the lift changes when you give the wing a sweep. I started on this and what I found is adding a 45° sweep to the wing seems to have the same effect as doubling the wing area, though I didn't go into it much more because my mechanism was breaking.

@CoolPeach @edensk Makes sense. The physics is simplified after all

@spefyjerbf SP has very basic wing physics, each airfoil has fixed graphs for Cl and Cd that don't change with anything, not even sweep, ground effect or mach

Haven’t checked properly, but believe wing chord is around 1m. So that’s an Re of around 3 million (in that region). Not sure if SP models this though, I’m not sure changing the chord on this setup would have any effect (assuming we keep the area the same) But would have to test this to find out. @spefyjerbf

Length of wing (chord)? So Reynold's Number can be calculated (lift curves generally differ slightly for each Reynold's #).

Lift curves often vary at each Reynold's and/or Mach number

Basically use the flat bottom in every situation

Cheers for that, I'll add that in too. @edensk

The exact stall or maximum lift angles are 13.5, 16.5 and 27.5
And the maximum lift coefficients I found out were 1.38, 1.5, and 1.95 respectively. I used 1.225 Kg/m^3 as density at sea level though, so my results are slightly different