There are two main tools available for two-dimensional aero analysis.
XFOIL is by far the tool I’d recommend using in nearly all situations. It’s very easy to acquire—you can download it from the official website for the low, low price of free.
It’s also relatively quick to learn, especially since it has excellent documentation available to walk you through all its various features and functions. This flows down into it being a very speedy analysis tool: you can open the program, load in an airfoil, set a Reynolds number, and complete an angle of attack sweep within maybe a minute or two.
XFOIL is also nice because it has a simple graphical interface that shows the streamlines around your airfoil at its current angle of attack, as well as a pressure coefficient plot. Few other tools show visuals like this by default, and they add an extra layer of understanding.
Of course, nothing is perfect, and XFOIL has a few limitations. The most notable is that it can only handle airflows where the entire airfoil sees subsonic flow (i.e., below the speed of sound); it doesn’t play nice if a supersonic bubble shows up. For the vast majority of UAV design programs, though, you’re not anywhere near supersonic airspeeds anyways so this is basically a non-issue.
But if you are expecting any supersonic flow, you’ll likely want to use the other well-known 2D tool: MSES. This tool is specifically intended for high transonic and supersonic flows. It’s also uniquely able to handle multi-element geometries, like flaps with multiple connected segments.
Unlike XFOIL, MSES needs you to first define and initialize a mesh of fluid elements in order to run a simulation. You’re essentially taking the fluid volume around your airfoil and chopping it into a bunch of tiny segments. The physics of each segment are calculated in sequence, so that the “upwind” segments’ properties are propagated to the “downwind” segments around the airfoil. (I’ll explain this in more detail when we get to CFD tools.)
This setup and solving method makes MSES more challenging to learn, but like with XFOIL it does have thorough documentation.
Possibly the biggest differentiator from XFOIL is that MSES is substantially harder to access. You can get a free license for academic use, and can also get access if you’re a government contractor. But if you are neither of the two, you’ll need to shell out for a commercial license.
If you’re designing a UAV that is intended to fly at high transonic or supersonic speeds, the effort to obtain and learn MSES is probably plenty worth it.
But outside of that very specific use case, XFOIL has practically all you need. It’s what I’d suggest using for almost any 2D analysis.