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MSFS Texture Distortion when Importing into MSFS

This is one of the several solutions I've accomplished. It's not a texture issue it's how I was applying it. But in Blender I'm not sure how to apply textures.
In Sketchup, I used the Thrupaint tool and it fixes that stretch or any misaligned, distorted texture. I thought it was a [face] issue with the structure. Just wanted to provide to you the fact that it's the way the material is applied.
Screenshot (91).png
Screenshot (93).png
I think the stretching you are referring to pertains to varying texel densities between UV islands or non-planar UV projections. Seeing that it is a flat face, the stretching can be solved by applying a uniform texel density to the blue frame area. This can be done by eyeing it using a UV grid texture, or by using a texel density tool for Blender.

To come back to the op's issue - I would cut the roof area with seams into smaller UV islands' seeing that you are using Substance Painter this should not be a problem. Unwrap the flat parts of the roof using view from above, and possible conformal for curved parts. In my experience with curved surfaces it is better to break it up into smaller parts that will not translate into curved lines when drawing straight lines through it. You almost have to approach it from an architectural/structural perspective on how it was constructed in the first place.

Warping happens when you are trying to squeeze too much flatness out of a curvy mesh. If you are trying to use a simple conformal unwrap approach or smart project unwrap it is like hitting it with a blunt hammer and expecting a clean result. Think of it as a pattern for clothing, except that the material doesn't like to stretch. Cut it into smaller pieces so that your curves can be better represented as flat islands.

Smart project unwrapping in Blender hides a lot of dragons if you don't cut sufficient number of seams into an object first.
This time, I remade the roof, and unwarpped it without using smart uv projection. The distortion definitely mitigated, however you can still see some minor distortion, as shown in this pic:

For reference, this is my UV layout. The stripes on the left are the edges.
You are not using any edge seams on your curved areas - I am not surprised that you are running into those issues.

Start perhaps by placing a seam along to centerline of the roof at least. With the number of vertices you are wasting on your curved roof you can more than half your vertex count and rather model the rooftile panel section in your mesh and unwrap it as its own island.
Admittedly it is not the easiest of shapes you are trying to model - I would still opt for a top down view and unwrapping from view for the roof area rather than smart project. In Substance I would also suggest painting with a UV alignment instead of Tangent | Wrap and see if that makes a difference.

Thanks! I think I've solved the issue. The topology in that area was kinda bad. So I fixed it, giving it a nice flow. Like this:

Then I just used normal unwrap, and the distortion disappeared:

The issue is totally related with topology. For example, I still get some small distortion in this area. But the distortion is pretty minor, so I might call it a day:
Very nice, glad you managed to resolve that. Would be cool to see more screenshots of this in future; looks like an awesome building!