Originally Posted by Ricke90
My guess would be that at full throttle, the flaps go to full power setting. Under less throttle position, it switches stages as you are asking more of the engine.
the flaps are operated at rpm trigger points. The lower flap opens around 3000rpm, and the upper flap is around 4100 rpm or so, where both stay open to redline. They are both closed under 3000rpm. The flaps are either 100% open or 100% closed, there is no in-between setting. I think the difference people are feeling at part throttle is that the volume of air in the manifold is different, which has an effect on the resonance properties of the flaps opening and closing.