There are a lot of 3d game engines around, does anyone with experience with them have advice on which one would likely be able to handle these (arbitrary) requirements?
You can look at Ogre3D. It supports OpenGL/DirectX, Mac/Linux/Windows, written in C++. But it is not game engine, it is 3D rendering engine. You can integrate other networking/physics engines with Ogre.
I'd like to suggest the Torque Game Engine. Some of the things you're asking for aren't really possible (as others have pointed out), but are problems that have to be tackled in a round about way (like faking the airfoil physics). The Torque engine would be a good starting place for you. There's also a 2D version called Torque Game Builder which might be easier to start with.
A plug for openscenegraph, has all of that plus paeed terrains, level of detail LOD etc - it's used by flightgear flight simulator as well as a lot of commercial and military simulators