Let’s see how each of these analysis tools fills a role in your design workflow. It’s great to know they all can do something, but it’s essential to also know how engineers can use them.
You’re designing a new UAV. Using a few initial requirements—like max takeoff weight and cruise airspeed—you find roughly how large the wing needs to be. From this wing area and a notional horizontal tail location, you can assume a tail volume coefficient and use that to get your starting tail size.
At this point, you’re still heavily iterating on your geometry. You need something that will let you easily adjust your dimensions and quickly run new configurations, but still gives reliable aero results. It would make sense to start using a 2D inviscid analysis tool, such as AVL.
After experimenting, you’ve found the right wing size and shape, as well as tail size and location, to have an aircraft that is stable in pitch. You also evaluated some airfoil options and found one you like. The mechanical and electrical engineering teams get to modeling this aircraft in CAD and designing the internal structure.
But then one of the mechanicals comes to you—the airfoil is too skinny! They can’t design a strong enough wing with the space they have. Can we change it?
Now you need a tool that takes the thickness of an airfoil into account. That’s a job for a 2D viscous tool. And because this aircraft flies at subsonic speeds, XFOIL will work perfectly.
You increase the airfoil’s thickness in XFOIL and quantify the increase in drag. It’s a bummer, but engineering is all about compromises.
As soon as you solve that problem, another one comes up. A standard pitot-static air data probe is suddenly no longer an option. This UAV will need to have static ports along the side of the fuselage.
Air accelerates when it moves over curved surfaces, so you know those static ports may not report the correct atmospheric pressure. To analyze this, you need a tool that calculates pressures across a surface. To get the proper behavior, you ought to include viscous effects. And you could make do with a 2D analysis, but since this impacts the air data system, you’d rather do a 3D simulation.
You make the case to use a 3D viscous tool for this analysis—you’re a responsible engineer, after all. Comparing the resulting pressure at the static ports to the actual ambient pressure, the CFD simulation lets you make a correction factor for the UAV autopilot to compensate for the static ports’ location. Now the plane should fly just fine.
This is, of course, an extremely simplified design process. You can’t exactly snap your fingers and end up with a stable aircraft configuration! And there are way more factors to include.
But hopefully this illustrates how you’d use each tool in the process. Every type of tool serves a purpose, and leveraging the strengths of each one is how you successfully make an airplane.