The next stop on our tour is my personal favorite: 3D inviscid tools.
You would think the lack of viscous effects—i.e., stall, skin friction drag, and so on—would be a significant drawback and make this category of tool almost useless. It’s pretty close to the opposite!
Because these tools don’t model boundary layers or incorporate turbulence, they can run pretty quickly and without needing much computational power. They’re not as fast as XFOIL, but you can get a decent angle of attack sweep within about five minutes.
To compare, 3D viscous simulations can take 30+ minutes to solve a single angle of attack data point, while running on a mini cluster of 240 cores (16 times more than your laptop). The time difference is substantial.
And we have good ways to make up for the lack of viscous effects. Airfoil data, like what XFOIL provides, gives us our stall angles. We can use our vehicle’s geometry and handbook methods to calculate our skin friction and other drag sources. These bits of info let us adjust the data our inviscid tool generates to reflect these viscous effects.
The quick computational speed and easily-fixed limitations make 3D inviscid tools truly excellent for the kind of fast, rough, expansive trade studies conducted at the start of almost all design programs.
Most of these tools are decently user friendly, making it straightforward to create a new aircraft geometry to test or tweak an existing one. And when you can run a single alpha sweep within a few minutes, you can really embrace the spaghetti method: try all sorts of adjustments and permutations, and see what looks best.
A lot of the more well-known tools use what’s called the “vortex lattice method” to calculate aerodynamics. A vehicle model is made up of a number of large trapezoidal panels that define the shape of the wings and tails. Each of these panels is split into even smaller panels, with a horseshoe-shaped vortex anchored to each mini-panel (picture an upside-down U shape, with airflow spilling over from the outside to the inside of the U).
Adding up all of these vortices allows for calculation of the forces and moments experienced by the vehicle model. The nice thing is that modern codes only require you to define the larger panels that make up the model geometry; a handful of settings determine the lattice of vortices automatically.
It’s this method of solving—splitting the model into sections, not the fluid volume around it—that gives these tools the colloquial name of “panel methods.” Whatever you call them, they’re genuinely more powerful than you might realize.
(For the record, my explanation of the vortex lattice method was incredibly simplified. Let me know if you’re interested in a slightly more detailed version!)