If I make a destructible mesh, put it in a level and damage it until it fragments, the resulting pieces fall correctly due to gravity until they reach a fixed terminal velocity that cannot be changed in the project settings or by a physics volume.
This unchangeable terminal velocity (about 4500 cps IIRC), doesn’t affect normal physics components, which do follow the project’s default value or physics volumes. I tested it setting up a scene with a destructible mesh and a static mesh with physics enabled side by side and wathcing them fall. Both fell correctly until the destructible mesh reached terminal velocity and fell behind. The static mesh reached the floor much earlier and faster.
I’m not seeing the same results on my end that I can tell.
I have a simple scene setup using the default scene. I used the cube mesh included in the engine content folder. I created a DM out of this cube mesh. I placed this DM and the cube next to each other and raised them off the floor mesh to ~100k units (1 kilometer). I set them to simulate physics and both hit the floor mesh at or indistinguishably close to the same time.
This obviously is not what you’re describing and based on your notes above I’m fairly certain I’ve set up my test correctly.
If you have any additional details to add I’m happy to look into this and see if I can reproduce on my end.
I set up a scene just like yours and I did not have the problem. It turns out there is a “chunk max speed” parameter in DMs and someone had set it in the DM I had been using. Setting this parameter to 0 solves the problem.
This parameter is always used, even when the DM is not fragmented, and I’m not sure it is the right behavior or even documented.
The behavior isn’t documented. There really isn’t a lot of in-depth documentation for destructibles at the moment, at least with the UE4 documentation. This may be in PhysX’s documentation though, but I’m not sure off hand.
I did a brief search through PhysXLab documentation for destructibles, but I didn’t see anything that stood out other than Max Chunk Speed is “scaled by the Global Scale Factor upon the destructible asset creation.” But there is no reference to Global Scale Factor that I could find by using a quick search. So it would seem by this that regardless of a destructible being fractured or not, the base chunk, along with any additional depths will be considered by this value.