According to the OpenSim documentation here it uses a Simbody PointInPlane constraint for the normal contact. That is described here . I'm not sure why it wasn't implemented by a Simbody SphereOnPlaneContact constraint, but I believe it produces a similar result. Ajay Seth might have further insight...

The constraint is applied at the point of contact.
- Sherm

Thanks for your kind remarks -- I'm glad you're finding Simbody useful. Yes, minimizing PE only finds stable equilibria. You should be able to find everything you need in SimbodyMatterSubsystem . For example you can calculate accelerations udot directly with calcAccelerations() and you can convert t...

Yes, a Free (6dof) joint for the body and Pin (1dof revolute) joints for the propellers.

Hi, Nat. There is one ragdoll among the Simbody examples that I know of. It's called "Gazebo2Simbody" and is mostly about reading .sdf files. But the .sdf it reads by default is a ragdoll -- you can run it and see the ragdoll fall down.

Regards,
Sherm

Definitely possible since we don't currently build Simbody on arm64 Macs in CI. If you have access to one and are able to find the problem we would welcome a pull request so others can benefit!

Possible workaround: grab the latest Simbody source from GitHub and build it locally.

Agreed, there is no one-size-fits-all perfect solution. In Simbody I biased my choices to modeling some physical system at all times. That allows validation based on physical laws like f=ma, conservation of energy, etc. A comment on finding feasible solutions: Simbody has an Assembler study for find...

Another good question, thanks Kaiwen. I had to think about this one for a while since it has been a long time! This comes from Simbody's definition of the position constraint matrix P, based on the intended use in computation. You can see the definition in Doxygen here (scroll down to "Theory discus...

Thanks for this great question, Kaiwen. Here is our thinking behind this constraint: Simbody constraints always model some physically-possible system, always assuming that the position and velocity constraints can't be exactly satisfied (they never are in practice). It is easiest to think of this by...