I am experimenting with SITL of a quad-plane on XPlane 9 & 11, and I observe that while controlled flights are possible I would not qualify them as smooth or reliable ones.
For instance, my tilt-rotor quad-plane design can hover manually without any gyro aids within Xplane. Manually means with a joystick directly connected to Xplane and without any interaction with Ardupilot.
When I set up SITL with ArduPlane, the aircraft has difficulty with a stable hover. At times, the wings may rock left and right or the aircraft may oscillate pitching up and down. At times the aircraft stabilizes and the hover ls rock solid. Some oscillations are visible in conventional flights after a transition too. I ran many iterations of Q_AUTOTUNE, and AUTOTUNE, but I can not get rid off these fluctuations.
Suspecting data latency issues, I tried some test flights in MANUAL mode but with the joystick interfaced through the SITL/ Mission Planner. To my surprise, the aircraft was hardly controllable even in conventional flight. It was mega sensitive and could easily flip or roll with tiny joystick inputs. This is a huge inconsistency, considering conventional flight should be within realm of SITL with Xplane.
Now, my suspicion is data latency issues across SITL and Xplane perhaps particular to my computer or may be more general. I have read some claims that the data rate of Xplane is insufficient for SITL especially for quadplanes. Is it?
What are the minimal data rates for SITL involving conventional planes and multi-rotor VTOLs? How does each flight sim software rate against these rates - FlightGear/Jsbsim, Xplane and RealFlight in particular?
Any insight would be helpful in diagnosing the problem.