I'm trying to design my ultralight airplane. I know the basics of aerodynamics and airplane design, however, my question is if there are any tools (online, software, database ) that suggest the best set of airfoils with best possible Cl/Cd ratio based on a given Reynolds number?

I think that the database should have all airfoil graph data and not just the geometry information.

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    $\begingroup$ Did you try to search the net? Please let us know what is missing from what can easily be googled. $\endgroup$ – Peter Kämpf Dec 30 '19 at 16:17
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    $\begingroup$ @PeterKämpf as I mentioned I already did my primary design but I've never found any tools that have the capability to search inside all airfoil aerodynamics data. The online tools that already know, they just give the aerodynamics data for an airfoil that selected $\endgroup$ – Hamid s k Dec 31 '19 at 23:36
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    $\begingroup$ @Hamid s k May I suggest the choice of airfoils for a practical ultralight are limited and known, so you are best served by studying existing designs. Dimensions, weight and speed, there for Reynolds, will be in a fairly narrow range. $\endgroup$ – Robert DiGiovanni Jan 1 '20 at 1:23
  • $\begingroup$ The parameters of interest are too varied to make this a topic for a database search. I do not know of any airfoil database which would allow what you want. The most you can expect is to search for a given thickness and camber. Which thickness or camber would be best its up to you to decide – a database cannot do that. $\endgroup$ – Peter Kämpf Jan 1 '20 at 7:44
  • $\begingroup$ @RobertDiGiovanni, PeterKämpf, thanks for the advice, actually, I'm trying to make software design with python that its input is dynamic & geometry constraint and I look for airfoil aerodynamic repository data to estimate the best possible selection, but most the tools I checked, made the airfoil selection manually $\endgroup$ – Hamid s k Jan 2 '20 at 4:55

Reynolds Number is calculated as: velocity × chord/kinematic viscosity of air

velocity is meters/second, chord is meters, kinematic viscosity of air is around $1.48 × 10^-5$

Then go to Airfoil Tools. As you make your selections, you can see, in the polar diagrams, the effect Reynolds number has on the lift and drag properties of your wing.

A typical ultra light will have a Reynolds Number around 500,000 to 2 million. A possible path to a solution may be to study aircraft of similar size and speed range. These have been in existence for more than 100 years.

You may find a heavily cambered rectangular wing (Aspect ratio around 6-8) with a blunt rounded leading edge will give good low speed performance and very docile handling characteristics. Variations at this Reynolds range will be mainly in amount of undercamber you choose, depending on speed range you prefer and engine power.

The NACA 4418 may be a good place to start.

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    $\begingroup$ thanks for the answer, but my question was something else, I already calculate my Reynolds and look for a tools that look into the airfoils aerodynamics data graph and select a possible set for a given Reynolds, not show me aerodynamics data for what airfoil that I select $\endgroup$ – Hamid s k Dec 31 '19 at 23:18
  • $\begingroup$ @dalearn this response provides an answer in paragraph 4. Your experience as a "computer programmer interested in aviation" may benefit from following this advice. $\endgroup$ – Robert DiGiovanni Jan 31 '20 at 13:06

Airfoil choice is more than "best L/D" at a specific but unknown Reynolds number. Typical questions are:

  1. At which lift coefficient is the best L/D achieved?
  2. What is the maximum lift coefficient at the low speed Reynolds number? A higher lift coefficient allows for a smaller wing which produces less friction drag at high speed. The isolated value of best L/D might lead you to a higher drag overall.
  3. How does the airfoil behave in a stall? Abrupt lift loss would be a heavy price for a slightly better L/D, so a compromise must be reached.
  4. What is the minimum drag coefficient at low lift coefficients?
  5. What material and process is used for building the wing? Will it be smooth enough to reap the benefits of a long laminar boundary layer so the quoted L/D can actually be reached?
  6. What is the airfoil thickness? This determines structural mass and stiffness of the wing spar. Having a lighter wing might be worth a slightly worse L/D since less lift must be produced.

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