1. A concise explanation of the problem you're experiencing.
I find that when I apply an orientation to an entity, the 3D model is rotated around a point which is not the origin of the 3D model.
2. A minimal code example. If you've found a bug, this helps us reproduce and repair it.
I modded the 3D Model sandcastle example to rotate the aircraft model around the roll axis. Here you can see that the aircraft is rolling around a point above the wings. However, the origin of the model is below the wing; so it is not rotating around the origin.
If you upload the Cesium_Air.glb here, and activate the RenderGrid, you can see that the model origin is below the wing.
3. Context. Why do you need to do this? We might know a better way to accomplish your goal.
Creating a flight simulation; need the model to rotate around its center of gravity.
4. The Cesium version you're using, your operating system and browser.
Latest as of today, Chrome browser.