Apologies in advance as this problem is sort of hard to describe. Basically, when I turn my head in VR, my position in the world shifts slightly. This breaks immersion (and even causes a little dizziness) because my brain has an expectation about how the world should move as my head moves, but in addition to that amount of movement, there's some additional movement.
Put another way, when I'm in the Vive loading space before FlyInside/P3D loads, I can lay my controllers on the desk (so that they are completely stationary), and as I move my head around, they remain fixed in place - in the sense that where they are drawn on the display changes (of course), but their new location is offset by the change in my head position, such that to my brain they seem to be in the correct spot in 3D space. Then, when the FlyInside intro screen loads in VR, the same is true of the giant calibration window displayed off in the distance - as I move my head, it continues to be in the correct spot in the 3D, such that its position appears to be fixed in the 3D world. At this point, however, the controllers (now rendered as the blue hands), do *not* appear to be fixed in place - as I move my head, they seem to shift in position quite a bit - as if their location in 3D space is tied to my HMD in some way. Then, when I'm in the sim itself, this problem with the controllers continues to happen, but it also happens with the plane too - as I move my head, the cockpit shifts a little too, such that it doesn't seem fixed in place.
I'll try to think of a better way to describe the problem, but hopefully that gives you some idea.
I'm using Windows 10 64-bit, a Vive, and P3Dv4 (although I had the same problem on P3Dv3). I'm running the latest versions of my graphics drivers, Vive drivers, FlyInside (1.9), and P3Dv4. I've verified that my Vive room calibration is good, although I don't think that's an issue anyway since no other VR apps I have exhibit this problem, and the calibration billboard when FI doesn't have this problem.
Thanks!