I got into Flight Simulator just for that. I had a handful of airplane designs and wanted to see what they looked like in Flight Simulator, how it would be like to fly them, etc.
I found I could setup aerodynamic figures in the air-files (config/air file set).
For architecture, it is also pretty amazing. You can setup ground polygons with photo scenery and build buildings on it that are advanced, etc, and see them in a world environment and even take photos of them with the sun and weather in different modes.
If you are engineering planes, you can make tweeks to the aerodynamic settings and get a feel as to how the planes act.
Some people do boats and vehicles as well for P3D (and FSX). It used to be that they didnt have a proper car config, but I think that changed with the AI they have now for traffic vehicles, so its probably more up to date.
Have fun!