It looks like the problem is not with ACflyt, but with the way I implemented the pitch animation. I have now loaded a version of the demo with the pitch animation disabled so you can see how ACflyt computes pitch: ACflytModelDemoXP
In ACflyt, you are not inputting pitch directly. This is one of the unique aspects of the program. What you are inputting is the coefficient of Lift (cfLift), which affects the lift vector. ACflyt computes the change in the flight path caused by the Lift (and Gravity) and then, by default, points the aircraft in the direction of flight. (So when you start out on the ground, the aircraft is pointed level since that is your initial direction of flight.)
The conversion of mouse movements into changes in cfLift is handled by the demo program. I was concerned that the program might be converting mouse Y movements into cfLift directly, e.g. cfLift = mouseYdiff * factor. However, the equation I am using is cfLift = cfLift + mouseYdiff * factor. This should be okay…
So that leaves the pitch animation as the source of the problem. In Blender, I created an animation that allows you to pitch the aircraft back and forth - a deflection from the direction of flight computed by ACflyt. Since clLift is a direct result of this deflection, I can use cfLift to back into the required deflection. It could be that I was keying this animation based on instantaneous changes in cfLift rather than on the actual change in cfLift. I will investigate.
I couldn’t tell from your video - was your frame rate over 60fps?