Pilatus PC-12 has a constant speed propeller, but it does not have a prop control lever in cockpit, just a power lever and small fuel on/off lever.
So how does it determine the prop RPM? Does it simply maintain constant RPM at all times, or is there some predefined schedule depending on power setting and airspeed, and if yes, what?
You effectively have a two speed prop (kinda like the 2 speed Power Glide automatic transmission on my first car, a 1964 Pontiac - well, not really). It's controlled by software, and there is a 1700 rpm high rpm and 1550 rpm low rpm mode, and that's it.
The PC-12's PT-6's FADEC system controls the prop governor, which runs the prop at 1700 rpm for all settings by default. When you want to quiet things down, you push a button that engages a low rpm mode, where it runs the prop at 1550 rpm for all settings below Take Off power (if you are in low rpm mode and push the lever balls-to-the-wall, it automatically brings the prop up to 1700).
So, you take off, and the prop is at 1700. Come back to Max Continuous, it stays at 1700, come back more for cruise, and it still stays at 1700. Then when you want to slow it down, you push the PROP LOW SPEED switch/light just ahead of the power lever, and it drops to 1550 for all power settings, except if you go back to full power, where it runs back up to 1700.
Another innovation is a low, 1,550-rpm propeller setting. Down on the center pedestal is a push-button for shuttling between the electronic propeller governor’s normal, 1,700 propeller rpm and a lower, 1,550-rpm mode for a quieter cabin. Except for take-off—when the FADEC will automatically command 1,700 rpm—you can use low-speed mode at any time. In cruise, Pilatus says that the penalty is a paltry one to two knots.
