# How do I choose the correct NACA airfoil for my requirements?

How do you find the correct NACA airfoil to use when you already know the requirements?

All of the research I have done on this has shown how to determine what kind of camber/chord/aspect ratio etc. only, but not how to actually pick the proper NACA airfoil. I would like to know if there is some website or program where you can easily find the correct airfoil given different circumstances. Then I would be more sure of having the correct airfoil when performing my tests. I understand I could just eyeball it, but I would like to be able to find the exact lift that my wings would produce, and that would only come from knowing the exact NACA airfoil.

It doesn't necessarily have to be a NACA airfoil, but it should be some type of airfoil where data can be taken from it.

I also could be approaching this from the wrong way, as I do not have an experienced mentor, so if using NACA is unnecessary or outdated please let me know because I really am at a loss of how to select the proper airfoil.

• So basically you want to search airfoils based on camber/chord/aspect ratio? – fooot Jan 12 '17 at 3:03
• I don't know of a reverse lookup program but if you have already decided on performance characteristics then there is a program that can modify airfoils to meet your characteristics using genetic algorithms. Google "xoptfoil". It uses xfoil as its back-end – slebetman Jan 12 '17 at 4:52
• Related questions: here, here, here. There are various links in those questions and their answers to tools, documents etc. – Pondlife Jan 12 '17 at 14:11

So you know your wing planform and look for the right airfoil.

You need to be more specific with your requirements, and yes, NACA airfoils will most likely not be the best choice. But since there is plenty of literature on them, they are a good start.

• What is the maximum airfoil thickness you can afford? A thicker airfoil makes for a lighter wing with more internal volume for flaps and fuel. However, if you want to operate your design at high subsonic speeds, thickness needs to be controlled.
• What is the lowest cruise lift coefficient and the highest lift coefficient for a clean wing and with flaps down? Does your design need flaps at all?
• If the aircraft is small: What Reynolds number is flown at which lift coefficient?
• What is the desired stall behaviour? Docile (as preferred for training aircraft) or snappy (as preferred for aerobatic aircraft)?
• What construction technique is used for the wing? Wooden frame and fabric? All metal with countersunk or exposed rivets? To be built on a jig or on a flat table? Composite, in a mold or positive over a foam core? All of them have subtle consequences for airfoil selection.

Generally, you look at all requirements and then select or design the airfoil according to the desired pressure distribution at different angles of attack and Mach numbers. Once you tell us more, I can become more specific.

• Hi, I have exactly the same question as OP and am building my airplane out of foam in order to experiment and have a better understanding of the different configurations and so. Reading aviation litterature helps a lot so far. I therefor answered the questions above and would love to know what you would suggest: 1) 3mm 2) 1.200kg (= +/- weight of aircraft without engine) -> lift coefficient $C_{L} = \frac{2mg}{\rho V^2 S} = 1.15$ - not sure flaps are needed, but I think adding flaps that can be oriented eg downwards helps during takeoff (that way at least chancs are higher my plane will fly) – LandonZeKepitelOfGreytBritn Oct 21 '17 at 11:19
• 3) yes is small - I think my Reynold's number is 211,164 assuming a velocity of 10m/s, chord with of 0.3m and (a guessed) kinematic viscuosity of 1.4207E-5 4) docile 5) foam - cut with light eg electric saw... - cut on flat table. – LandonZeKepitelOfGreytBritn Oct 21 '17 at 11:24
• Conclusion: I would choose an airfoil +/- like this: airfoiltools.com/images/airfoil/ag11-il_l.png or airfoiltools.com/images/airfoil/ag37-il_l.png Simply because the bottom seems to be flat and thus much easier for me to make in my home lab, but further than this I got no clue about what thickness I would need nor what minimal wingspan (maximum 1.3 meter) I would need. I am learning this on my own by reading various books about aircraft design, so I may have made some mistakes. Feel free to correct or tell me if something of what I said looks weird or incorrect. – LandonZeKepitelOfGreytBritn Oct 21 '17 at 11:31
• @LandonZeKepitelOfGreytBritn: So your relative thickness is 10%. You will need more camber than the two airfoils you selected if you want to fly at a cl of 1.15 at such low Re numbers. A quick search shows the S3410 to be quite good (see here: airfoiltools.com/airfoil/details?airfoil=s4310-il#polars). – Peter Kämpf Oct 21 '17 at 12:12
• @LandonZeKepitelOfGreytBritn: A higher Re does not imply less camber; I choose the higher camber because of the high lift coefficient. To increase Re you either fly faster or increase chord. I selected the S3410 due to the almost straight bottom; if you build it straight you will be close enough. – Peter Kämpf Oct 21 '17 at 15:17